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  • 251.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Materials Center Leoben Forschung GmbH, Austria.
    First-principles modeling of the Invar effect in Fe65Ni35 by the spin-wave method2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 17, article id 174432Article in journal (Refereed)
    Abstract [en]

    Thermal lattice expansion of the Invar Fe0.65Ni0.35 alloy is investigated in first-principles calculations using the spin-wave method, which is generalized here for the ferromagnetic state with short-range order. It is shown that magnetic short-range order effects make a substantial contribution to the equilibrium lattice constant and cannot be neglected in the accurate ab initio modeling of the thermal expansion in Fe-Ni alloys. We also demonstrate that at high temperatures, close to and above the magnetic transition, magnetic entropy associated with transverse and longitudinal spin fluctuations yields a noticeable contribution to the equilibrium lattice constant. The obtained theoretical results for the temperature dependent lattice constant are in semiquantitative agreement with the experimental data apart from the region close the magnetic transition.

  • 252.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures.
    Qualitative ab initio theory of magnetic and atomic ordering in FeNi2024In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 109, no 9, article id 094108Article in journal (Refereed)
    Abstract [en]

    Magnetic and atomic ordering in equiatomic FeNi alloy is studied by different ab initio techniques and methods based on density functional theory in order to clarify the main driving forces and their interplay behind these transitions and possibility of their accurate description within standard density functional theory calculations. The Curie temperature is obtained in Monte Carlo simulations using magnetic exchange interactions obtained by applying the magnetic force theorem within multiple scattering theory for different magnetic and atomic configurational states, including account for the thermal atomic displacements and exchange-correlation potential. The calculations show a very strong sensitivity of the results upon exchange-correlation potential, atomic order, and thermal atomic displacements. The calculated Curie temperature of a completely random alloy with the account of thermal lattice displacement is at least about 200 K below the known experimental data (780-800 K) depending on the above mentioned factors. The atomic order-disorder transition temperature is determined from effective chemical interactions, which apart from the chemical contribution (on the ideal fcc lattice) include contributions from lattice thermal vibrations and local lattice relaxations. The effective chemical interactions are strongly affected by the magnetic state, so the order-disorder transition temperature changes between 1000 and 140 K in the fully ordered ferromagnetic and paramagnetic states, respectively. For the reduced magnetization 0.7 (close to the experimental order-disorder transition temperature at 600 K), the order-disorder transition temperature varies between 550 and 700 K depending mostly on the exchange-correlation potential. The latter effect is the uncertainty in the choice of the exchange-correlation approximation in density functional theory calculations.

  • 253.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM).
    Dehghani, M.
    Atomic configuration and properties of austenitic steels at finite temperature: Effect of longitudinal spin fluctuations2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 10, article id 104111Article in journal (Refereed)
    Abstract [en]

    High-temperature atomic configurations of fcc Fe-Cr-Ni alloys with alloy composition close to austenitic steel are studied in statistical thermodynamic simulations with effective interactions obtained in ab initio calculations. The latter are done taking longitudinal spin fluctuations (LSF) into consideration within a quasiclassical phenomenological model. It is demonstrated that the magnetic state affects greatly the alloy properties, and in particular, it is shown that the LSF substantially modify the bonding and interatomic interactions of fcc Fe-Cr-Ni alloys even at ambient conditions. The calculated atomic short-range order is in reasonable agreement with existing experimental data for Fe0.56Cr0.21Ni0.23, which has strong preference for the (001)-type ordering between Ni and Cr atoms. A similar ordering tendency is found for the Fe0.75Cr0.17Ni0.08 alloy composition, which approximately corresponds to the widely used 304 and 316 austenitic steel grades.

  • 254.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Materials Center Leoben Forschung GmbH, Leoben, Austria.
    Peil, O. E.
    Mat Ctr Leoben Forsch GmbH, A-8700 Leoben, Austria..
    Impact of thermal atomic displacements on the Curie temperature of 3d transition metals2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 17, article id 174426Article in journal (Refereed)
    Abstract [en]

    It is demonstrated that thermally induced atomic displacements from ideal lattice positions can produce considerable effect on magnetic exchange interactions and, consequently, on the Curie temperature of Fe. Thermal lattice distortion should, therefore, be accounted for in quantitatively accurate theoretical modeling of the magnetic phase transition. At the same time, this effect seems to be not very important for magnetic exchange interactions and the Curie temperature of Co and Ni.

  • 255.
    Rybakov, Filipp N.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Garaud, Julien
    Univ Orleans, Univ Tours, CNRS, UMR 7013,Inst Denis Poisson, Parc Grandmont, F-37200 Tours, France..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Stable Hopf-Skyrme topological excitations in the superconducting state2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 9, article id 094515Article in journal (Refereed)
    Abstract [en]

    At large scales, magnetostatics of superconductors is described by a massive vector field theory: the London model. The magnetic field cannot penetrate into the bulk unless quantum vortices are formed. These are topological excitations characterized by an invariant: the phase winding. The London model dictates that loops of such vortices are not stable because the kinetic energy of superflow and the magnetic energy are smaller, the smaller vortex loops are. We demonstrate that in two-component superconductors, under certain conditions, such as the proximity to pair-density-wave instabilities, the hydromagnetostatics of the superconducting state and topological excitation changes dramatically: the excitations acquire the form of stable vortex loops and knots characterized by the different topological invariant: the Hopf index and hence termed hopfions. This demonstrates that magnetic properties in a superconducting state can be dramatically different from those of a London's massive vector field theory.

  • 256.
    Rybakov, Filipp N.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Kiselev, Nikolai S.
    Forschungszentrum Julich, Peter Grunberg Inst, D-52425 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52425 Julich, Germany.;JARA, D-52425 Julich, Germany..
    Chiral magnetic skyrmions with arbitrary topological charge2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 6, article id 064437Article in journal (Refereed)
    Abstract [en]

    We show that continuous and spin-lattice models of chiral ferro- and antiferromagnets provide the existence of an infinite number of stable soliton solutions of any integer topological charge. A detailed description of the morphology of new skyrmions and the corresponding energy dependencies are provided. The considered model is general, and is expected to predict a plethora of particlelike states which may occur in various chiral magnets including ultrathin films, e.g., PdFe/Ir(111), rhombohedral GaV4S8 semiconductor, B20-type alloys as Mn1-xFexGe, Mn1-xFexSi, Fe1-xCoxSi, Cu2OSeO3, and acentric tetragonal Heusler compounds.

  • 257.
    Rybakov, Filipp N.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Pervishko, Anastasiia
    Skolkovo Inst Sci & Technol, Moscow 121205, Russia..
    Eriksson, Olle
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Antichiral ferromagnetism2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 2, article id L020406Article in journal (Refereed)
    Abstract [en]

    Here, by combining a symmetry-based analysis with numerical computations, we predict a different kind of magnetic ordering-antichiral ferromagnetism. This term aims to reflect that spontaneous modulation of the magnetization direction m(r) appears in a way that both types of chirality (handedness) exist simultaneously, and alternate in space. Without loss of generality, we focus our investigation on crystals with full tetrahedral symmetry where chiral interaction terms-Lifshitz invariants-are forbidden by symmetry. However, we demonstrate that the leading chirality-related term leads to nontrivial smooth magnetic textures exhibiting antichirality. In addition to the unconventional ground state, the revealed ordering gives rise to rich phenomena such as unique magnetic domains and skyrmions.

  • 258.
    Sadhukhan, Banasree
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Bergman, Anders
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Kvashnin, Yaroslav O.
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Hellsvik, Johan
    KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for High Performance Computing, PDC. NORDITA, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden.;Stockholm Univ, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden..
    Delin, Anna
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Spin-lattice couplings in two-dimensional CrI3 from first-principles computations2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 10, article id 104418Article in journal (Refereed)
    Abstract [en]

    Since thermal fluctuations become more important as dimensions shrink, it is expected that low-dimensional magnets are more sensitive to atomic displacement and phonons than bulk systems are. Here we present a fully relativistic first-principles study on the spin-lattice coupling, i.e., how the magnetic interactions depend on atomic displacement, of the prototypical two-dimensional ferromagnet CrI3. We extract an effective measure of the spin-lattice coupling in CrI3, which is up to ten times larger than what is found for bcc Fe. The magnetic exchange interactions, including Heisenberg and relativistic Dzyaloshinskii-Moriya interactions, are sensitive both to the in-plane motion of Cr atoms and out-of-plane motion of ligand atoms. We find that significant magnetic pair interactions change sign from ferromagnetic (FM) to antiferromagnetic (AFM) for atomic displacements larger than 0.16 (0.18) angstrom for Cr (I) atoms. We explain the observed strong spin-lattice coupling by analyzing the orbital decomposition of isotropic exchange interactions, involving different crystal-field-split Cr-3d orbitals. The competition between the AFM t(2g)-t(2g) and FM t(2g)-e(g) contributions depends on the bond angle formed by Cr and I atoms as well as Cr-Cr distance. In particular, if a Cr atom is displaced, the FM-AFM sign changes when the I-Cr-I bond angle approaches 90 degrees. The obtained spin-lattice coupling constants, along with the microscopic orbital analysis, can act as a guiding principle for further studies of the thermodynamic properties and combined magnon-phonon excitations in two-dimensional magnets.

  • 259.
    Sadhukhan, Banasree
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova. Leibniz Inst Solid State & Mat Res IFW Dresden, Helmholtzstr 20, D-01069 Dresden, Germany..
    Nag, Tanay
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden.;Rhein Westfal TH Aachen, Inst Theorie Stat Phys, D-52056 Aachen, Germany..
    Effect of chirality imbalance on Hall transport of PrRhC22023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 8, article id L081110Article in journal (Refereed)
    Abstract [en]

    Much has been learned about the topological transport in real materials. We investigate the interplay between magnetism and topology in the magnetotransport of PrRhC2. The fourfold degeneracy reduces to twofold followed by nondegenerate Weyl nodes when the orientation of the magnetic quantization axis is changed from easy axis to body diagonal through face diagonal. This engenders chirality imbalance between positive and negative chirality Weyl nodes around the Fermi energy. We observe a significant enhancement in the chiral anomaly mediated response such as planar Hall conductivity and longitudinal magnetoconductivity, due to the emergence of chirality imbalance upon orienting the magnetic quantization axis to body diagonal. The angular variations of the above quantities for different magnetic quantization axes clearly refer to the typical signature of planar Hall effect in Weyl semimetals. We further investigate the profiles of anomalous Hall conductivities as a function of Fermi energy to explore the effects of symmetries as well as chirality imbalance on Berry curvature.

  • 260.
    Sadhukhan, Banasree
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. IFW Dresden, Leibniz Inst Solid State & Mat Res, Helmholtzstr 20, D-01069 Dresden, Germany..
    Nag, Tanay
    Rhein Westfal TH Aachen, Inst Theorie Stat Phys, D-52056 Aachen, Germany..
    Electronic structure and unconventional nonlinear response in double Weyl semimetal SrSi22021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 24, article id 245122Article in journal (Refereed)
    Abstract [en]

    Considering a noncentrosymmetric, nonmagnetic double Weyl semimetal (WSM) SrSi2, we investigate the electron and hole pockets in bulk Fermi surface behavior that enables us to characterize the material as a type-I WSM. We study the structural handedness of the material and correlate it with the distinct surface Fermi surface at two opposite surfaces following an energy evolution. The Fermi arc singlet becomes doublet with the onset of spin orbit coupling that is in accordance with the topological charge of the Weyl nodes (WNs). A finite energy separation between WNs of opposite chirality in SrSi2 allows us to compute circular photogalvanic effect (CPGE). Followed by the three band formula, we show that CPGE is only quantized for Fermi level chosen in the vicinity of WN residing at a higher value of energy. Surprisingly, for the other WN of opposite chirality in the lower value of energy, CPGE is not found to be quantized. Such a behavior of CPGE is in complete contrast to the time reversal breaking WSM where CPGE is quantized to two opposite plateau depending on the topological charge of the activated WN. We further analyze our finding by examining the momentum resolved CPGE. Finally we show that two band formula for CPGE is not able to capture the quantization that is apprehended by the three band formula.

  • 261.
    Sadhukhan, Banasree
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Nag, Tanay
    SISSA, Via Bonomea 265, I-34136 Trieste, Italy..
    Role of time reversal symmetry and tilting in circular photogalvanic responses2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 103, no 14, article id 144308Article in journal (Refereed)
    Abstract [en]

    We study the role of time reversal symmetry (TRS) in the circular photogalvanic (CPG) responses considering a chiral Weyl semimetal (WSM), while a quantized CPG response is guaranteed by both the broken inversion symmetry and broken mirror symmetries. The TRS broken WSM yields one left and one right chiral Weyl node (WN), while there are two left and right chiral WNs for a TRS invariant WSM. We show that these features can potentially cause the quantization of a CPG response at higher values compared to the topological charge of the underlying WSM. This is further supported by the fact that Berry curvature and velocity behave differently depending on whether the system preserves or breaks the TRS. We find the CPG responses for a TRS invariant type-II WSM to be quantized at two and four times the topological charge of the activated WNs while the chemical potentials are, respectively, chosen in the vicinity of the energies associated with the left and right chiral WNs. By contrast, irrespective of the above choice of the chemical potential, the quantization in the CPG response is directly given by the topological charge of the activated WNs for the TRS broken case. Interestingly, we notice a nonquantized peak in the CPG response when the energies of the WNs associated with opposite chiralities are close to each other, as is the case for the TRS invariant type-I WSM considered here. Moreover, we show that the tilt can significantly modify the CPG response as the velocity in the tilt direction changes, which enters into the CPG tensor through the Fermi distribution function. Given these exciting outcomes, the second-order CPG response emerges as a useful indicator to characterize the system under consideration. Furthermore, we investigate the momentum resolved structure of the CPG response to relate with the final results and strengthen our analysis from the perspective of the lattice models.

  • 262.
    Sadhukhan, Surasree
    et al.
    Indian Inst Technol Goa, Sch Phys Sci, Ponda 403401, India..
    Sadhukhan, Banasree
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. IFW Dresden, Inst Theoret Solid State Phys, Helmholtz str 20, D-01069 Dresden, Germany..
    Kanungo, Sudipta
    Indian Inst Technol Goa, Sch Phys Sci, Ponda 403401, India..
    Pressure-driven tunable properties of the small-gap chalcopyrite topological quantum material ZnGeSb2: A first-principles study2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 12, article id 125112Article in journal (Refereed)
    Abstract [en]

    Search for new topological quantum materials is the demand of time and the theoretical prediction plays a crucial role besides the obvious experimental verification. Divination of topological properties in already well-known narrow gap semiconductors is a flourishing area in quantum material. In this view we revisited the semiconductor compound in the chalcopyrite series, with a very small gap near the Fermi energy. Using the density functional theory-based first-principles calculations, we report a strong topologically nontrivial phase in chalcopyrite ZnGeSb2, which can act as a model system of strained HgTe. The calculations reveal the nonzero topological invariant (Z2), the presence of Dirac cone crossing in the surface spectral functions with spin-momentum locked spin texture. We also study the interplay between the structural parameters and electronic properties, and report the tunable topological properties due to a very small band gap, from nontrivial to trivial phase under the application of moderate hydrostatic pressure within approximate to 7 GPa. A small modification of a lattice parameter is enough to achieve this topological phase transition which is easily accomplished in an experimental laboratory. The calculations show that a discontinuity in the tetragonal distortion of noncentrosymmetric ZnGeSb2 plays a crucial role in driving this topological phase transition. Our results are further collaborated with a low energy k center dot p model Hamiltonian to validate our abinitio findings. We showed that the evaluation of the model band energy dispersion under the hydrostatic pressure is consistent with the obtained results.

  • 263.
    Salamone, Tancredi
    et al.
    Norwegian Univ Sci & Technol, Dept Phys, Ctr Quantum Spintron, NTNU, NO-7491 Trondheim, Norway..
    Hugdal, Henning G.
    Norwegian Univ Sci & Technol, Dept Phys, Ctr Quantum Spintron, NTNU, NO-7491 Trondheim, Norway..
    Amundsen, Morten
    Nordita SU; Nordita, Stockholm Univ, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden..
    Jacobsen, Sol H.
    Norwegian Univ Sci & Technol, Dept Phys, Ctr Quantum Spintron, NTNU, NO-7491 Trondheim, Norway..
    Curvature control of the superconducting proximity effect in diffusive ferromagnetic nanowires2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 13, article id 134511Article in journal (Refereed)
    Abstract [en]

    Coupling a conventional s-wave superconductor to a ferromagnet allows us, via the proximity effect, to generate superconducting triplet correlations. This feature can be employed to achieve a superconducting triplet spin-valve effect in superconductor-ferromagnet (SF) hybrid structures, for example by switching the magnetizations of the ferromagnets between parallel and antiparallel configurations in F1SF2 and SF1F2 trilayers, or in SF bilayers with both Rashba and Dresselhaus spin-orbit coupling (SOC). It was recently reported that geometric curvature can control the generation of long-ranged triplets. We use this property to show that the superconducting critical temperature of an SF hybrid nanowire can be tuned by varying the curvature of the ferromagnetic side alone, with no need of another ferromagnet or SOC. We show that the variation of the critical temperature as a function of the curvature can be exploited to obtain a robust, curvature-controlled, superconducting triplet spin-valve effect. Furthermore, we perform an analysis with the inclusion of spin-orbit coupling and explain how it modifies the spin-valve effect both quantitatively and qualitatively.

  • 264.
    Salamone, Tancredi
    et al.
    Center for Quantum Spintronics, Department of Physics, NTNU, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
    Hugdal, Henning G.
    Center for Quantum Spintronics, Department of Physics, NTNU, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
    Jacobsen, Sol H.
    Center for Quantum Spintronics, Department of Physics, NTNU, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
    Amundsen, Morten
    Nordita SU; Stockholm University, Hannes Alfvéns väg 12, SE-106 91 Stockholm, Sweden, Hannes Alfvéns väg 12.
    High magnetic field superconductivity in a two-band superconductor2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 17, article id 174516Article in journal (Refereed)
    Abstract [en]

    We present a mechanism allowing for superconductivity at high magnetic fields, beyond the Pauli-Chandrasekhar-Clogston limit. We consider spin splitting induced by an in-plane external magnetic field in a superconductor with two relevant bands close to the Fermi level. The magnetic field therefore controls which bands are available for Cooper pair formation. The presence of interband superconducting pairing, i.e., Cooper pairs formed by electrons with different band indices, produces high-field reentrant superconducting domains, whose critical magnetic field violates the Pauli-Chandrasekhar-Clogston limit. We analyze how the interband superconducting domains are influenced by the band parameters, and show that, for a certain range of parameters, the system presents two separate superconducting regions, for low and high magnetic field.

  • 265.
    Salamone, Tancredi
    et al.
    Center for Quantum Spintronics, Department of Physics, NTNU, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
    Skjærpe, Magnus
    Center for Quantum Spintronics, Department of Physics, NTNU, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
    Hugdal, Henning G.
    Center for Quantum Spintronics, Department of Physics, NTNU, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
    Amundsen, Morten
    Nordita SU.
    Jacobsen, Sol H.
    Center for Quantum Spintronics, Department of Physics, NTNU, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
    Interface probe for antiferromagnets using geometric curvature2024In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 109, no 9, article id 094508Article in journal (Refereed)
    Abstract [en]

    We propose that geometric curvature and torsion may be used to probe the quality of an uncompensated antiferromagnetic interface, using the proximity effect. We study a helix of antiferromagnetic wire coupled to a conventional superconductor, and show that a density of states measurement can give information about the quality of an uncompensated interface, crucial for many recently predicted antiferromagnetic proximity effects. Furthermore, we show that geometric curvature alone can result in long-ranged superconducting triplet correlations in the antiferromagnet, and we discuss the impact curvature and torsion can have on the future development of superconducting spintronic devices.

  • 266.
    Salamone, Tancredi
    et al.
    NTNU Norwegian Univ Sci & Technol, Dept Phys, Ctr Quantum Spintron, NO-7491 Trondheim, Norway..
    Svendsen, Mathias B. M.
    NTNU Norwegian Univ Sci & Technol, Dept Phys, Ctr Quantum Spintron, NO-7491 Trondheim, Norway..
    Amundsen, Morten
    Nordita SU.
    Jacobsen, Sol
    NTNU Norwegian Univ Sci & Technol, Dept Phys, Ctr Quantum Spintron, NO-7491 Trondheim, Norway..
    Curvature-induced long-range supercurrents in diffusive superconductor-ferromagnet-superconductor Josephson junctions with a dynamic 0-pi transition2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 6, article id L060505Article in journal (Refereed)
    Abstract [en]

    We report that spin supercurrents can be induced in diffusive superconductor-ferromagnet-superconductor Josephson junctions without any magnetic misalignment or intrinsic spin-orbit coupling. Instead, the pathway to spin-triplet generation is provided via geometric curvature, and results in a long-range Josephson effect. In addition, the curvature is shown to induce a dynamically tunable 0-pi transition in the junction. We provide the analytic framework and discuss potential experimental and innovation implications.

  • 267.
    Samoilenka, Albert
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Boundary states with elevated critical temperatures in Bardeen-Cooper-Schrieffer superconductors2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 13, article id 134512Article in journal (Refereed)
    Abstract [en]

    Bardeen-Cooper-Schrieffer (BCS) theory describes a superconducting transition as a single critical point where the gap function or, equivalently, the order parameter vanishes uniformly in the entire system. We demonstrate that in superconductors described by standard BCS models, the superconducting gap survives near the sample boundaries at higher temperatures than superconductivity in the bulk. Therefore, conventional superconductors have multiple critical points associated with separate phase transitions at the boundary and in the bulk. We show this by revising the Caroli-De Gennes-Matricon theory of a superconductor-vacuum boundary and finding inhomogeneous solutions of the BCS gap equation near the boundary, which asymptotically decay in the bulk. This is demonstrated for a BCS model of almost free fermions and for lattice fermions in a tight-binding approximation. The analytical results are confirmed by numerical solutions of the microscopic model. The existence of these boundary states can manifest itself as discrepancies between the critical temperatures observed in calorimetry and transport probes.

  • 268.
    Samoilenka, Albert
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Ground-state fractal crystals2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 18, article id 184104Article in journal (Refereed)
    Abstract [en]

    We propose a generalization of the crystalline order: the ground-state fractal crystal. We demonstrate that by deriving a simple continuous-space-discrete-field model whose ground state is a crystal where each unit cell is a fractal.

  • 269.
    Samoilenka, Albert
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Microscopic derivation of superconductor-insulator boundary conditions for Ginzburg-Landau theory revisited: Enhanced superconductivity at boundaries with and without magnetic field2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 103, no 22, article id 224516Article in journal (Refereed)
    Abstract [en]

    Using the standard Bardeen-Cooper-Schrieffer (BCS) theory, we revise microscopic derivation of the superconductor-insulator boundary conditions for the Ginzburg-Landau (GL) model. We obtain a negative contribution to free energy in the form of surface integral. Boundary conditions for the conventional superconductor have the form n .del psi = const psi. These are shown to follow from considering the order parameter reflected in the boundary. The boundary conditions are also derived for more general GL models with higher-order derivatives and pair-density-wave states. It shows that the boundary states with higher critical temperature and the boundary gap enhancement, found recently in BCS theory, are also present in microscopically derived GL theory. In the case of an applied external field, we show that the third critical magnetic-field value H-c3 is higher than what follows from the de Gennes boundary conditions and is also significant in type-I regime.

  • 270.
    Samoilenka, Albert
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Spiral magnetic field and bound states of vortices in noncentrosymmetric superconductors2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 18, article id 184517Article in journal (Refereed)
    Abstract [en]

    We discuss the unconventional magnetic response and vortex states arising in noncentrosymmetric superconductors with chiral octahedral and tetrahedral (O or T) symmetry. We microscopically derive Ginzburg-Landau free energy. It is shown that due to spin-orbit and Zeeman coupling magnetic response of the system can change very significantly with temperature. For sufficiently strong coupling this leads to a crossover from type-1 superconductivity at elevated temperature to vortex states at lower temperature. The external magnetic field decay in such superconductors does not have the simple exponential law. We show that in the London limit, magnetic field can be solved in terms of complex force-free fields (W) over right arrow, which are defined by del x (W) over right arrow = const (W) over right arrow. Using that we demonstrate that the magnetic field of a vortex decays in spirals. Because of such behavior of the magnetic field, the intervortex and vortex-boundary interaction becomes nonmonotonic with multiple minima. This implies that vortices form bound states with other vortices, antivortices, and boundaries.

  • 271.
    Samoilenka, Albert
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Barkman, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Benfenati, Andrea
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Pair-density-wave superconductivity of faces, edges, and vertices in systems with imbalanced fermions2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 5, article id 054506Article in journal (Refereed)
    Abstract [en]

    We describe boundary effects in superconducting systems with Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting instability, using Bogoliubov-de-Gennes and Ginzburg-Landau (GL) formalisms. First, we show that in dimensions larger than one the standard GL functional formalism for FFLO superconductors is unbounded from below. This is demonstrated by finding solutions with zero Laplacian terms near boundaries. We generalize the GL formalism for these systems by retaining higher order terms. Next, we demonstrate that a cuboid sample of a superconductor with imbalanced fermions at a mean-field level has a sequence of the phase transitions. At low temperatures it forms Larkin-Ovchinnikov state in the bulk but has a different modulation pattern close to the boundaries. When temperature is increased the first phase transition occurs when the bulk of the material becomes normal while the faces remain superconducting. The second transition occurs at higher temperature where the system retains superconductivity on the edges. The third transition is associated with the loss of edge superconductivity while retaining superconducting gap in the vertices. We obtain the same sequence of phase transition by numerically solving the Bogoliubov-de Gennes model.

  • 272.
    Sandberg, Lise Orduk
    et al.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Edberg, Richard
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Bakke, Ingrid-Marie Berg
    Univ Oslo, Ctr Mat Sci & Nanotechnol, NO-0315 Oslo, Norway..
    Pedersen, Kasper S.
    Tech Univ Denmark, Dept Chem, DK-2800 Lyngby, Denmark..
    Hatnean, Monica Ciomaga
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England..
    Balakrishnan, Geetha
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England..
    Mangin-Thro, Lucile
    Inst Laue Langevin, 71 Ave Martyrs,CS 20156, F-38042 Grenoble 9, France..
    Wildes, Andrew
    Inst Laue Langevin, 71 Ave Martyrs,CS 20156, F-38042 Grenoble 9, France..
    Fak, B.
    Inst Laue Langevin, 71 Ave Martyrs,CS 20156, F-38042 Grenoble 9, France..
    Ehlers, Georg
    Oak Ridge Natl Lab, Neutron Technol Div, Oak Ridge, TN 37831 USA..
    Sala, Gabriele
    Oak Ridge Natl Lab, Neutron Technol Div, Oak Ridge, TN 37831 USA..
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Åbo Akad Univ, Fac Sci & Engn, Turku, Finland..
    Lefmann, Kim
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Deen, Pascale P.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark.;European Spallat Source ERIC, S-22363 Lund, Sweden..
    Emergent magnetic behavior in the frustrated Yb3Ga5O12 garnet2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 6, article id 064425Article in journal (Refereed)
    Abstract [en]

    We report neutron scattering, magnetic susceptibility and Monte Carlo theoretical analysis to verify the short-range nature of the magnetic structure and spin-spin correlations in a Yb3Ga5O12 single crystal. The quantum spin state of Yb3+ in Yb3Ga5O12 is verified. The quantum spins organize into a short-ranged emergent director state for T < 0.6 K derived from anisotropy and near-neighbor exchange. We derive the magnitude of the nearneighbor exchange interactions 0.6 < J(1) < 0.7 K, J(2) = 0.12 K and the magnitude of the dipolar exchange interaction, D, in the range 0.18 < D < 0.21 K. Certain aspects of the broad experimental dataset can be modeled using a J(1)D model with ferromagnetic near-neighbor spin-spin correlations while other aspects of the data can be accurately reproduced using a J(1)J(2)D model with antiferromagnetic near-neighbor spin-spin correlation. As such, although we do not quantify all the relevant exchange interactions, we nevertheless provide a strong basis for the understanding of the complex Hamiltonian required to fully describe the magnetic state of Yb3Ga5O12.

  • 273.
    Sarkar, Madhumita
    et al.
    Jožef Stefan Institute, SI-1000 Ljubljana, Slovenia.
    Ghosh, Roopayan
    Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom, Gower Street.
    Khaymovich, Ivan M.
    Nordita SU; Institute for Physics of Microstructures, Russian Academy of Sciences, 603950 Nizhny Novgorod, GSP-105, Russia, GSP-105.
    Tuning the phase diagram of a Rosenzweig-Porter model with fractal disorder2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 108, no 6, article id L060203Article in journal (Refereed)
    Abstract [en]

    The Rosenzweig-Porter (RP) model has garnered much attention in the last decade, as it is a simple analytically tractable model showing both ergodic-nonergodic extended and Anderson localization transitions. Thus, it is a good toy model to understand the Hilbert-space structure of many-body localization phenomenon. In our Letter, we present analytical evidence, supported by exact numerics, that demonstrates the controllable tuning of the phase diagram in the RP model by employing on-site potentials with a nontrivial fractal dimension instead of the conventional random disorder. We demonstrate that such disorder extends the fractal phase and creates an unusual dependence of fractal dimensions of the eigenfunctions. Furthermore, we study the fate of level statistics in such a system to understand how these changes are reflected in the eigenvalue statistics.

  • 274.
    Schossler, Matheus
    et al.
    Washington Univ St Louis, Dept Phys, 1 Brookings Dr, St Louis, MO 63130 USA..
    Bandyopadhyay, Sumanta
    Nordita SU; Stockholm University, Sweden.
    Seidel, Alexander
    Washington Univ St Louis, Dept Phys, 1 Brookings Dr, St Louis, MO 63130 USA..
    Inner workings of fractional quantum Hall parent Hamiltonians: A matrix product state point of view2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 15, article id 155124Article in journal (Refereed)
    Abstract [en]

    We study frustration-free Hamiltonians of fractional quantum Hall (FQH) states from the point of view of the matrix product state (MPS) representation of their ground and excited states. There is a wealth of solvable models relating to FQH physics, which, however, is mostly derived and analyzed from the vantage point of first-quantized ???analytic clustering properties.??? In contrast, one obtains long-ranged frustration-free lattice models when these Hamiltonians are studied in an orbital basis, which is the natural basis for the MPS representation of FQH states. The connection between MPS-like states and frustration-free parent Hamiltonians is the central guiding principle in the construction of solvable lattice models, but thus far, only for short-range Hamiltonians and MPSs of finite bond dimension. The situation in the FQH context is fundamentally different. Here we expose the direct link between the infinite-bond-dimension MPS structure of Laughlin???conformal field theory (CFT) states and their parent Hamiltonians. While focusing on the Laughlin state, generalizations to other CFT-MPSs will become transparent.

  • 275. Schuler, Thomas
    et al.
    Lopes, Denise Adorno
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Claisse, Antoine
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Transport properties of C and O in UN fuels2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 9, article id 094117Article in journal (Refereed)
    Abstract [en]

    Uranium nitride fuel is considered for fast reactors (GEN-IV generation and space reactors) and for light water reactors as a high-density fuel option. Despite this large interest, there is a lack of information about its behavior for in-pile and out-of-pile conditions. From the present literature, it is known that C and O impurities have significant influence on the fuel performance. Here we perform a systematic study of these impurities in the UN matrix using electronic-structure calculations of solute-defect interactions and microscopic jump frequencies. These quantities were calculated in the DFT+U approximation combined with the occupation matrix control scheme, to avoid convergence to metastable states for the 5f levels. The transport coefficients of the system were evaluated with the self-consistent mean-field theory. It is demonstrated that carbon and oxygen impurities have different diffusion properties in the UN matrix, with O atoms having a higher mobility, and C atoms showing a strong flux coupling anisotropy. The kinetic interplay between solutes and vacancies is expected to be the main cause for surface segregation, as incorporation energies show no strong thermodynamic segregation preference for (001) surfaces compared with the bulk.

  • 276.
    Schönecker, Stephan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Richter, Manuel
    IFW Dresden, D-01069 Dresden, Germany.;Dresden Ctr Computat Mat Sci, D-01069 Dresden, Germany..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, School of Engineering Sciences (SCI), Applied Physics.
    Lattice dynamics and metastability of fcc metals in the hcp structure and the crucial role of spin-orbit coupling in platinum2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 22, article id 224305Article in journal (Refereed)
    Abstract [en]

    We investigate the lattice dynamical properties of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au in the nonequilibrium hcp structure by means of density-functional simulations, wherein spin-orbit coupling (SOC) was considered for Ir, Pt, and Au. The determined dynamical properties reveal that all eight elements possess a metastable hcp phase at zero temperature and pressure. The hcp Ni, Cu, Rh, Pd, and Au previously observed in nanostructures support this finding. We make evident that the inclusion of SOC is mandatory for an accurate description of the phonon dispersion relations and dynamical stability of hcp Pt. The underlying sensitivity of the interatomic force constants is ascribed to a SOC-induced splitting of degenerate band states accompanied by a pronounced reduction of electronic density of states at the Fermi level. To give further insight into the importance of SOC in Pt, we (i) focus on phase stability and examine a lattice transformation related to optical phonons in the hcp phase and (ii) focus on the generalized stacking fault energy (GSFE) of the fcc phase pertinent to crystal plasticity. We show that the intrinsic stable and unstable fault energies of the GSFE scale as in other common fcc metals, provided that the spin-orbit interaction is taken into account.

  • 277. Schönfeld, B.
    et al.
    Sax, C. R.
    Zemp, J.
    Engelke, M.
    Boesecke, P.
    Kresse, T.
    Boll, T.
    Al-Kassab, T.
    Peil, O. E.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Local order in Cr-Fe-Co-Ni: Experiment and electronic structure calculations2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 1, article id 014206Article in journal (Refereed)
    Abstract [en]

    A quenched-in state of thermal equilibrium (at 723 K) in a single crystal of Cr-Fe-Co-Ni close to equal atomic percent was studied. Atom probe tomography revealed a single-phase state with no signs of long-range order. The presence of short-range order (SRO) was established by diffuse x-ray scattering exploiting the variation in scattering contrast close to the absorption edges of the constituents: At the incoming photon energies of 5969, 7092, and 8313 eV, SRO maxima that result from the linear superposition of the six partial SRO scattering patterns, were always found at X position. Electronic structure calculations showed that this type of maximum stems from the strong Cr-Ni and Cr-Co pair correlations, that are furthermore connected with the largest scattering contrast at 5969 eV. The calculated effective pair interaction parameters revealed an order-disorder transition at approximately 500 K to a L12-type (Fe,Co,Ni)3Cr structure. The calculated magnetic exchange interactions were dominantly of the antiferromagnetic type between Cr and any other alloy component and ferromagnetic between Fe, Co, and Ni. They yielded a Curie temperature (TC) of 120 K, close to experimental findings. Despite the low value of TC, the global magnetic state strongly affects chemical and elastic interactions in this system. In particular, it significantly increases the ordering tendency in the ferromagnetic state compared to the paramagnetic one.

  • 278.
    Sellin, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Superfluid drag in the two-component Bose-Hubbard model2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094517Article in journal (Refereed)
    Abstract [en]

    In multicomponent superfluids and superconductors, co- and counterflows of components have, in general, different properties. A. F. Andreev and E. P. Bashkin [Sov. Phys. JETP 42, 164 (1975)] discussed, in the context of He-3/He-4 superfluid mixtures, that interparticle interactions produce a dissipationless drag. The drag can be understood as a superflow of one component induced by phase gradients of the other component. Importantly, the drag can be both positive (entrainment) and negative (counterflow). The effect is known to have crucial importance for many properties of diverse physical systems ranging from the dynamics of neutron stars and rotational responses of Bose mixtures of ultracold atoms to magnetic responses of multicomponent superconductors. Although substantial literature exists that includes the drag interaction phenomenologically, only a few regimes are covered by quantitative studies of the microscopic origin of the drag and its dependence on microscopic parameters. Here we study the microscopic origin and strength of the drag interaction in a quantum system of two-component bosons on a latticewith short-range interaction. By performing quantum Monte Carlo simulations of a two-component Bose-Hubbard model we obtain dependencies of the drag strength on the boson-boson interactions and properties of the optical lattice. Of particular interest are the strongly correlated regimes where the ratio of coflow and counterflow superfluid stiffnesses can diverge, corresponding to the case of saturated drag.

  • 279.
    Shahnazaryan, V
    et al.
    ITMO Univ, St Petersburg 197101, Russia..
    Kyriienko, Oleksandr
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. ITMO Univ, St Petersburg 197101, Russia.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Univ Exeter, Dept Phys & Astron, Stocker Rd, Exeter EX4 4QL, Devon, England..
    Rostami, Habibi
    Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Nordita SU.
    Exciton routing in the heterostructure of a transition metal dichalcogenide monolayer on a paraelectric substrate2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 16, article id 165303Article in journal (Refereed)
    Abstract [en]

    We propose a scheme for the spatial exciton energy control and exciton routing in a transition-metal dichalcogenide (TMD) monolayer which lies on a quantum paraelectric substrate. It relies on the ultrasensitive response of the substrate dielectric permittivity to temperature changes, allowing for spatially inhomogeneous screening of Coulomb interaction in a monolayer. As an example, we consider the heterostructure of TMD and strontium titanate oxide SrTiO3, where large dielectric screening can be attained. We study the impact of substrate temperature on the characteristic electronic features of TMD monolayers such as the particle band gap and exciton binding energy, Bohr radius, and nonlinearity (an exciton-exciton interaction). The combination of particle band gap and exciton binding energy modulation results in the shift of the exciton resonance energy. Applying local heating, we create spatial patterns with varying exciton resonant energy and an exciton flow toward the energetically lower region of the sample.

  • 280.
    Shahnazaryan, Vanik
    et al.
    ITMO Univ, St Petersburg 197101, Russia..
    Rostami, Habib
    Stockholm Univ, Hannes Alfvens Vag 12, S-10691 Stockholm, Sweden.;Nordita SU.
    Nonlinear exciton drift in piezoelectric two-dimensional materials2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 8, article id 085405Article in journal (Refereed)
    Abstract [en]

    The noncentrosymmetric nature of single-layer (SL) transition-metal dichalcogenides (TMD) manifests itself in the finite piezoelectricity and valley-Zeeman coupling. We microscopically model nonlinear exciton transport in nanobubbles of SL TMDs. Thanks to the giant piezoelectric effect, we obtain an enormous internal electric field, E-piezo similar to 10(7) V/m, resulting in a built-in dipole moment of excitons. We demonstrate that the piezo-induced dipole-dipole interaction provides a novel channel for the nonlinear exciton transport distinct from the conventional isotropic funneling of excitons and leads to the formation of a hexagon-shaped exciton droplet on the top of circularly symmetric nanobubbles. Moreover, we found that the hexagonal distribution of exciton density is preserved even for strongly elliptic nanobubbles. The effect is tunable via the bubble-size dependence of the piezoelectric field E-piezo similar to h(max)(2)/R-3 with h(max) and R being the bubble height and radius, respectively.

  • 281. Sharma, Raghav
    et al.
    Sisodia, Naveen
    Durrenfeld, Philipp
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Gothenburg, Sweden.
    Muduli, Pranaba Kishor
    Time-domain stability of parametric synchronization in a spin-torque nano-oscillator based on a magnetic tunnel junction2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 2, article id 024427Article in journal (Refereed)
    Abstract [en]

    We report on time-domain stability of the parametric synchronization in a spin-torque nano-oscillator (STNO) based on a magnetic tunnel junction. Time-domain measurements of the instantaneous frequency (f(i)) of a parametrically synchronized STNO showrandom short-term unlocking of the STNO signal for low injected radio-frequency (RF) power, which cannot be revealed in time-averaged frequency domain measurements. Macrospin simulations reproduce the experimental results and reveal that the random unlocking during synchronization is driven by thermal fluctuations. We show that by using a high injected RF power, random unlocking of the STNO can be avoided. However, a perfect synchronization characterized by complete suppression of phase noise, so-called phase noise squeezing, can be obtained only at a significantly higher RF power. Our macrospin simulations suggest that a lower temperature and a higher positive ratio of the fieldlike torque to the spin transfer torque reduce the threshold RF power required for phase noise squeezing under parametric synchronization.

  • 282. Shiranzaei, M.
    et al.
    Fransson, J.
    Cheraghchi, H.
    Parhizgar, Fariborz
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Nonlinear spin susceptibility in topological insulators2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 18, article id 180402Article in journal (Refereed)
    Abstract [en]

    We revise the theory of the indirect exchange interaction between magnetic impurities beyond the linear response theory to establish the effect of impurity resonances in the surface states of a three-dimensional topological insulator. The interaction is composed of isotropic Heisenberg, anisotropic Ising, and Dzyaloshinskii-Moriya types of couplings. We find that all three contributions are finite at the Dirac point, which is in stark contrast to the linear response theory which predicts a vanishing Dzyaloshinskii-Moriya-type contribution. We show that the spin-independent component of the impurity scattering can generate large values of the Dzyaloshinskii-Moriya-type coupling in comparison with the Heisenberg and Ising types of couplings, while these latter contributions drastically reduce in magnitude and undergo sign changes. As a result, both collinear and noncollinear configurations are allowed magnetic configurations of the impurities.

  • 283.
    Shirinyan, Albert A.
    et al.
    TTMO Univ, St Petersburg 197101, Russia..
    Kozin, Valerii K.
    TTMO Univ, St Petersburg 197101, Russia.;Univ Iceland, Sci Inst, Dunhagi 3, IS-107 Reykjavik, Iceland..
    Hellsvik, Johan
    KTH, School of Engineering Sciences (SCI), Physics.
    Pereiro, Manuel
    Uppsala Univ, Dept Phys & Astron, Mat Theory Div, Box 516, SE-75120 Uppsala, Sweden..
    Eriksson, Olle
    Uppsala Univ, Dept Phys & Astron, Mat Theory Div, Box 516, SE-75120 Uppsala, Sweden.;Orebro Univ, Sch Sci & Technol, SE-70182 Orebro, Sweden..
    Yudin, Dmitry
    Skolkovo Inst Sci & Technol, Deep Quantum Labs, Moscow 121205, Russia..
    Self-organizing maps as a method for detecting phase transitions and phase identification2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 4, article id 041108Article in journal (Refereed)
    Abstract [en]

    Originating from image recognition, methods of machine learning allow for effective feature extraction and dimensionality reduction in multidimensional datasets, thereby providing an extraordinary tool to deal with classical and quantum models in many-body physics. In this study, we employ a specific unsupervised machine learning technique-self-organizing maps-to create a low-dimensional representation of microscopic states, relevant for macroscopic phase identification and detecting phase transitions. We explore the properties of spin Hamiltonians of two archetype model systems: a two-dimensional Heisenberg ferromagnet and a three-dimensional crystal, Fe in the body-centered-cubic structure. The method of self-organizing maps, which is known to conserve connectivity of the initial dataset, is compared to the cumulant method theory and is shown to be as accurate while being computationally more efficient in determining a phase transition temperature. We argue that the method proposed here can be applied to explore a broad class of second-order phase-transition systems, not only magnetic systems but also, for example, order-disorder transitions in alloys.

  • 284. Shubnic, A.
    et al.
    Shahnazaryan, V.
    Shelykh, I. A.
    Rostami, Habib
    Nordita SU.
    Exciton spin Hall effect in arc-shaped strained WSe22024In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 109, no 20, article id L201409Article in journal (Refereed)
    Abstract [en]

    Generating a pure spin current using electrons, which have degrees of freedom beyond spin, such as electric charge and valley index, presents challenges. In response, we propose a mechanism based on intervalley exciton dynamics in arc-shaped strained transition metal dichalcogenides (TMDs) to achieve the exciton spin Hall effect in an electrically insulating regime, without the need for an external electric field. The interplay between strain gradients and strain-induced pseudomagnetic fields results in a net Lorentz force on long-lived intervalley excitons in WSe2, carrying nonzero spin angular momentum. This process generates an exciton-mediated pure spin Hall current, resulting in opposite-sign spin accumulations and local magnetization on the two sides of the single-layer arc-shaped TMD. We demonstrate that the magnetic field induced by spin accumulation, at approximately ∼mT, can be detected using techniques such as superconducting quantum interference magnetometry or spatially resolved magneto-optical Faraday and Kerr rotations.

  • 285.
    Sidorova, M.
    et al.
    Humboldt Univ, Dept Phys, Newtonstr 15, D-12489 Berlin, Germany.;German Aerosp Ctr, Inst Opt Sensor Syst, Rutherfordstr 2, D-12489 Berlin, Germany..
    Semenov, A. D.
    German Aerosp Ctr, Inst Opt Sensor Syst, Rutherfordstr 2, D-12489 Berlin, Germany..
    Huebers, H-W
    Humboldt Univ, Dept Phys, Newtonstr 15, D-12489 Berlin, Germany.;German Aerosp Ctr, Inst Opt Sensor Syst, Rutherfordstr 2, D-12489 Berlin, Germany..
    Gyger, Samuel
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Steinhauer, Stephan
    KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum and Biophotonics.
    Zhang, X.
    Chinese Acad Sci, Shanghai Inst Microsyst & Informat Technol, State Key Lab Funct Mat Informat, Shanghai 200050, Peoples R China.;CAS Ctr Excellence Superconducting Elect, Shanghai 200050, Peoples R China..
    Schilling, A.
    Univ Zurich, Phys Inst, Winterthurerstr 190, CH-8057 Zurich, Switzerland..
    Magnetoconductance and photoresponse properties of disordered NbTiN films2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 18, article id 184514Article in journal (Refereed)
    Abstract [en]

    We report on the experimental study of phonon properties and electron-phonon scattering in thin superconducting NbTiN films, which are intensively exploited in various applications. Studied NbTiN films with sub-10-nm thicknesses are disordered with respect to electron transport, the Ioffe-Regel parameter of kFle = 2.5-3.0 (kF is the Fermi wave vector, and le is the electron mean free path), the inelastic electron-phonon interaction, and the product qTle << 1 (qT is the wave vector of a thermal phonon). By means of magnetoconductance and photoresponse techniques, we derive the inelastic electron-phonon scattering rate 1/tau e-ph and determine sound velocities and phonon heat capacities. In the temperature range from 12 to 20 K, the scattering rate varies with temperature as 1/tau e-ph proportional to T 3.45 +/- 0.05; its value extrapolated to 10 K amounts to approximately 1/16 ps. Making a comparative analysis of our films and other films used in superconducting devices, such as polycrystalline granular NbN and amorphous WSi, we find a systematic reduction of the sound velocity in all these films by about 50% compared to the corresponding bulk crystalline materials. A corresponding increase in the phonon heat capacities in all these films is, however, less than the Debye model predicts. We attribute these findings to reduced film dimensionality and film morphology.

  • 286.
    Silaev, Mihail
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Garaud, Julien
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics. KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Phase diagram of dirty two-band superconductors and observability of impurity-induced s plus i s state2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 2, article id 024517Article in journal (Refereed)
    Abstract [en]

    We investigate the phase diagram of dirty two-band superconductors. This paper primarily focuses on the properties and observability of the time-reversal symmetry-breaking s + is superconducting states, which can be generated in two-band superconductors by interband impurity scattering. We show that such states can appear in two distinct ways. First, according to a previously discussed scenario, the s + is state can form as an intermediate phase at the impurity-driven crossover between s(+/-) and s(++) states. We show that there is a second scenario where domains of the s + is state exists in the form of an isolated dome inside the s(+/-) domain, completely detached from the transition between s(+/-) and s(++) states. We demonstrate that in both cases the s + is state generated by impurity scattering exists in an extremely small interval of impurity concentrations. Although this likely precludes direct experimental observation of the s + is state formation due to this mechanism, this physics leads to the appearance of a region inside both the s(+/-) and s(++) domains with unusual properties due to softening of normal modes.

  • 287.
    Silaev, Mihail
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Vargunin, Artjom
    Vortex motion and flux-flow resistivity in dirty multiband superconductors2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 22, article id 224506Article in journal (Refereed)
    Abstract [en]

    The conductivity of vortex lattices in multiband superconductors with high concentration of impurities is calculated based on microscopic kinetic theory at temperatures significantly smaller than the critical one. Both the limits of high and low fields are considered, when the magnetic induction is close to or much smaller than the critical field strength IIc2, respectively. It is shown that in contrast to single-band superconductors, the resistive properties are not universal but depend on the pairing constants and ratios of diffusivities in different bands. The low-field magnetoresistance can strongly exceed the Bardeen-Stephen estimation in a quantitative agreement with experimental data for the two-band superconductor MgB2.

  • 288.
    Silaev, Mihail
    et al.
    Univ Jyvaskyla, Dept Phys, POB 35 YFL, FI-40014 Jyvaskyla, Finland.;Univ Jyvaskyla, Nanosci Ctr, POB 35 YFL, FI-40014 Jyvaskyla, Finland..
    Winyard, Thomas
    KTH, School of Engineering Sciences (SCI), Physics. Univ Leeds.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Non-London electrodynamics in a multiband London model: Anisotropy-induced nonlocalities and multiple magnetic field penetration lengths2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 17, article id 174504Article in journal (Refereed)
    Abstract [en]

    The London model describes strongly type-2 superconductors as massive vector field theories, where the magnetic field decays exponentially at the length scale of the London penetration length. This also holds for isotropic multiband extensions, where the presence of multiple bands merely renormalizes the London penetration length. We show that, by contrast, the magnetic properties of anisotropic multiband London models are not this simple, and the anisotropy leads to the interband phase differences becoming coupled to the magnetic field. This results in the magnetic field in such systems having N + 1 penetration lengths, where N is the number of field components or bands. That is, in a given direction, the magnetic field decay is described by N + 1 modes with different amplitudes and different decay length scales. For certain anisotropies we obtain magnetic modes with complex masses. That means that magnetic field decay is not described by a monotonic exponential increment set by a real penetration length but instead is oscillating. Some of the penetration lengths are shown to diverge away from the superconducting phase transition when the mass of the phase-difference mode vanishes. Finally the anisotropy-driven hybridization of the London mode with the Leggett modes can provide an effectively nonlocal magnetic response in the nominally local London model. Focusing on the two-component model, we discuss the magnetic field inversion that results from the effective nonlocality, both near the surface of the superconductor and around vortices. In the regime where the magnetic field decay becomes nonmonotonic, the multiband London superconductor is shown to form weakly-bound states of vortices.

  • 289.
    Singh, Deobrat
    et al.
    Uppsala Univ, Dept Phys & Astron, Mat Theory Div, Condensed Matter Theory Grp, Box 516, S-75120 Uppsala, Sweden..
    Shukla, Vivekanand
    Uppsala Univ, Dept Phys & Astron, Mat Theory Div, Condensed Matter Theory Grp, Box 516, S-75120 Uppsala, Sweden.;Chalmers Univ Technol, Dept Microtechnol & Nanosci MC2, SE-41296 Gothenburg, Sweden..
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties. Uppsala Univ, Dept Phys & Astron, Mat Theory Div, Condensed Matter Theory Grp, Box 516, S-75120 Uppsala, Sweden..
    Optical excitations and thermoelectric properties of two-dimensional holey graphene2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 7, article id 075444Article in journal (Refereed)
    Abstract [en]

    Recently, holey graphene (HG) was synthesized successfully at atomic precision with regard to hole size and shape, which indicates that HG has interesting physical and chemical properties for energy and environmental applications. The shaping of the pores also transforms semimetallic graphene into semiconductor HG, which opens new doors for its use in electronic applications. We investigated systematically the structural, electronic, optical, and thermoelectric properties of HG structure using first-principles calculations. HG was found to have a direct band gap of 0.65 eV (PBE functional) and 0.95 eV (HSE06 functional); the HSE06 functional is in good agreement with experimental results. For the optical properties, we used single-shot G(0)W(0) calculations by solving the Bethe-Salpeter equation to determine the intralayer excitonic effects. From the absorption spectrum, we obtained an optical gap of 1.28 eV and a weak excitonic binding energy of 80 meV. We found large values of thermopower of 1662.59 mu V/K and a better electronic figure of merit, ZT(e), of 1.13 from the investigated thermoelectric properties. Our investigations exhibit strong and broad optical absorption in the visible light region, which makes monolayer HG a promising candidate for optoelectronic and thermoelectric applications.

  • 290. Skoulatos, M.
    et al.
    Månsson, Martin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Paul Scherrer Institute, Switzerland; École Polytechnique Fédérale de Lausanne, Switzerland.
    Fiolka, C.
    Kramer, K. W.
    Schefer, J.
    White, J. S.
    Ruegg, Ch.
    Dimensional reduction by pressure in the magnetic framework material CuF2(D2O)(2)( pyz): From spin-wave to spinon excitations2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 2, article id 020414Article in journal (Refereed)
    Abstract [en]

    Metal organic magnets have enormous potential to host a variety of electronic and magnetic phases that originate from a strong interplay between the spin, orbital, and lattice degrees of freedom. We control this interplay in the quantum magnet CuF2(D2O)(2)( pyz) by using high pressure to drive the system through structural and magnetic phase transitions. Using neutron scattering, we show that the low pressure state, which hosts a two-dimensional square lattice with spin-wave excitations and a dominant exchange coupling of 0.89 meV, transforms at high pressure into a one-dimensional spin chain hallmarked by a spinon continuum and a reduced exchange interaction of 0.43 meV. This direct microscopic observation of a magnetic dimensional crossover as a function of pressure opens up new possibilities for studying the evolution of fractionalised excitations in low-dimensional quantum magnets and eventually pressure-controlled metal-insulator transitions.

  • 291. Slager, Robert-Jan
    et al.
    Juricic, Vladimir
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Roy, Bitan
    Dissolution of topological Fermi arcs in a dirty Weyl semimetal2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 20, article id 201401Article in journal (Refereed)
    Abstract [en]

    Weyl semimetals (WSMs) have recently attracted a great deal of attention as they provide a condensed matter realization of chiral anomaly, feature topologically protected Fermi arc surface states, and sustain sharp chiral Weyl quasiparticles up to a critical disorder at which a continuous quantum phase transition (QPT) drives the system into a metallic phase. We here numerically demonstrate that with increasing strength of disorder, the Fermi arc gradually loses its sharpness, and close to the WSM-metal QPT it completely dissolves into the metallic bath of the bulk. The predicted topological nature of the WSM-metal QPT and the resulting bulk-boundary correspondence across this transition can be directly observed in angle-resolved photoemission spectroscopy (ARPES) and Fourier transformed scanning tunneling microscopy (STM) measurements by following the continuous deformation of the Fermi arcs with increasing disorder in recently discovered Weyl materials.

  • 292.
    Smirnova, Ekaterina
    et al.
    Materials Modeling and Development Laboratory, National University of Science and Technology MISIS, 119049 Moscow, Russia.
    Nourazar, Mehdi
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Internal structure of metal vacancies in cubic carbides2024In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 109, no 6, article id L060103Article in journal (Refereed)
    Abstract [en]

    A combinatorial approach is employed to investigate the atomic and electronic structures of a metal vacancy in titanium carbide. It turns out that the usual relaxed geometry of the vacancy is just a metastable state representing a local energy minimum. Using ab initio calculations and by systematically searching through the configurational space of a Ti monovacancy, we identify a multitude of local minima with reconstructed geometry that are lower in energy. Among them, there is a planar configuration with two displaced carbons forming a dimer inside the vacancy. This structure has the optimal number and order of C-C bonds making it the global minimum. Further calculations show that this reconstructed geometry is also the ground state of metal vacancies in other carbides such as ZrC, HfC, and VC. The reconstructed metal vacancies are characterized by localized electron states due to the relatively short C-C bonds. The defect states lie just below the upper and lower valence bands. The existence of reconstructed vacancy configurations is essential for understanding the mechanism of metal self-diffusion in transition-metal carbides.

  • 293. Soda, Minoru
    et al.
    Hayashida, Shohei
    Roessli, Bertrand
    Månsson, Martin
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Paul Scherrer Inst.
    White, Jonathan S.
    Matsumoto, Masashige
    Shiina, Ryousuke
    Masuda, Takatsugu
    Continuous control of local magnetic moment by applied electric field in multiferroics Ba2CoGe2O72016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 9, article id 094418Article in journal (Refereed)
    Abstract [en]

    Ba2CoGe2O7 exhibits a collinear-antiferromagnetic structure with the easy axis along < 100 > directions and an antiferroelectric order with the polarization axis along the [001] direction. By applying the electric field the magnetic moment rotates from < 100 > to [110] directions and, simultaneously, the antiferroelectric state changes to the ferroelectric state gradually. This magnetoelectric effect, i.e., continuous control of the local magnetic moment by the electric field, is quantitatively explained by the Hamiltonian including the dielectric energy.

  • 294.
    Soldini, Martina O.
    et al.
    University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland, Winterthurerstrasse 190.
    Astrakhantsev, Nikita
    University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland, Winterthurerstrasse 190.
    Iraola, Mikel
    Donostia International Physics Center, 20018 Donostia-San Sebastian, Spain; Department of Physics, University of the Basque Country UPV/EHU, 48080 Bilbao, Spain.
    Tiwari, Apoorv
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Fischer, Mark H.
    University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland, Winterthurerstrasse 190.
    Valentí, Roser
    Institut für Theoretische Physik, Goethe-Universität Frankfurt, 60438 Frankfurt am Main, Germany.
    Vergniory, Maia G.
    Donostia International Physics Center, 20018 Donostia-San Sebastian, Spain; Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany.
    Wagner, Glenn
    University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland, Winterthurerstrasse 190.
    Neupert, Titus
    University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland, Winterthurerstrasse 190.
    Interacting topological quantum chemistry of Mott atomic limits2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 24, article id 245145Article in journal (Refereed)
    Abstract [en]

    Topological quantum chemistry (TQC) is a successful framework for identifying (noninteracting) topological materials. Based on the symmetry eigenvalues of Bloch eigenstates at maximal momenta, which are attainable from first principles calculations, a band structure can either be classified as an atomic limit, in other words adiabatically connected to independent electronic orbitals on the respective crystal lattice, or it is topological. For interacting systems, there is no single-particle band structure and hence, the TQC machinery grinds to a halt. We develop a framework analogous to TQC, but employing n-particle Green's function to classify interacting systems. Fundamentally, we define a class of interacting reference states that generalize the notion of atomic limits, which we call Mott atomic limits, and are symmetry protected topological states. Our formalism allows to fully classify these reference states (with n=2), which can themselves represent symmetry protected topological states. We present a comprehensive classification of such states in one dimension and provide numerical results on model systems. With this, we establish Mott atomic limit states as a generalization of the atomic limits to interacting systems.

  • 295.
    Speight, Martin
    et al.
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Winyard, Thomas
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Chiral p-wave superconductors have complex coherence and magnetic field penetration lengths2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 17, article id 174514Article in journal (Refereed)
    Abstract [en]

    We show that in superconductors that break time-reversal symmetry and have anisotropy, such as p + ip materials, all order parameters and magnetic modes are mixed. Excitation of the gap fields produces an excitation of the magnetic field and vice versa. Correspondingly, the long-range decay of the magnetic field and order parameter is in general given by the same exponent. Thus, one cannot characterize p + ip superconductors by the usual coherence and magnetic field penetration lengths. Instead, the system has normal modes that are associated with linear combinations of magnetic fields, moduli of and phases of the order-parameter components. Each such normal mode has its own decay length that plays the role of a hybridized coherence/magnetic field penetration length. On a large part of the parameter space, these exponents are complex. Therefore, the system in general has damped oscillatory decay of the magnetic field accompanied by damped oscillatory variation of the order-parameter fields.

  • 296.
    Speight, Martin
    et al.
    School of Mathematics, University of Leeds, Leeds LS2 9JT, United Kingdom.
    Winyard, Thomas
    Maxwell Institute of Mathematical Sciences and School of Mathematics, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Symmetries, length scales, magnetic response, and skyrmion chains in nematic superconductors2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 19, article id 195204Article in journal (Refereed)
    Abstract [en]

    Nematic systems are two component superconductors that break rotational symmetry, but exhibit a mixed symmetry that couples spatial rotations and phase difference rotations. We show that a consequence of this induced spatial anisotropy is mixed normal modes, that is the linear response to a small perturbation of the system about its ground state, generally couples magnetic and condensate degrees of freedom. We will study the effect of mode mixing on the magnetic response of a nematic system as the strength of applied field is increased. In general we show that the coupled modes generate magnetic field perpendicular to the applied field, causing the magnetic response to spontaneously twist direction. We will study this for the Meissner effect with weak fields and also for stronger applied fields, which produce a mixture of skyrmions and composite vortices, forming orientation dependent bound states. We will also calculate the anisotropies of the resulting first and second critical fields Hc1 and Hc2. The skyrmion lattices for Hc1≤H≤Hc2 in nematic superconductors are shown to be structurally complicated, in contrast to the triangular or square vortex lattices in conventional superconductors. For low fields the magnetic response of the system involves a loosely bound collection of parallel skyrmion chains. As the external field is increased the chains attract one another, causing a transition where the unit cell becomes triangular for high applied fields. This unique skyrmion lattice and the magnetic twisting are clear indicators that could be used experimentally to identify materials that exhibit nematic superconductivity. To obtain these results we develop and present a method to find the unit cell of a vortex lattice that can be applied to other kinds of superconducting systems.

  • 297.
    Speight, Martin
    et al.
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Winyard, Thomas
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Wormald, Alex
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Magnetic field behavior in s plus is and s plus id superconductors: Twisting of applied and spontaneous fields2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 17, article id 174515Article in journal (Refereed)
    Abstract [en]

    We consider magnetic field screening and spontaneous magnetic fields in s + is and s + id superconductors both analytically and numerically. We show that in general, the linearized model couples the moduli of order parameters to the magnetic modes. This causes magnetic field screening that does not follow the standard exponential law and hence cannot be characterized by a single length scale: the London penetration length. We also demonstrate that the resulting linear mixed modes, correctly predict spontaneous fields and their orientation. We show that these mixed modes cause external fields to decay nonmonotonically in the bulk. This is observed as the magnetic field twisting direction, up to an angle of pi /2, as it decays in the nonlinear model. Finally, we demonstrate that there are two nondegenerate domain wall solutions for any given parameter set. These are distinguished by either clockwise or anticlockwise interpolation of the intercomponent phase difference, each producing a different solution for the other fields. However, only domain wall solutions ins + id systems exhibit magnetic field twisting.

  • 298.
    Spicer, T. M.
    et al.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Keatley, P. S.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Loughran, T. H. J.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Dvornik, M.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Awad, A. A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Durrenfeld, P.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Houshang, A.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Ranjbar, M.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Åkerman, Johan
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Kruglyak, V. V.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Hicken, R. J.
    Univ Exeter, Dept Phys & Astron, Exeter EX4 4QL, Devon, England..
    Spatial mapping of torques within a spin Hall nano-oscillator2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 21, article id 214438Article in journal (Refereed)
    Abstract [en]

    Time-resolved scanning Kerr microscopy (TRSKM) was used to study precessional magnetization dynamics induced by a radio frequency (RF) current within a Al2O3/Py(5 nm)/Pt(6 nm)/Au(150 nm) spin Hall nano-oscillator structure. The Au layer was formed into two needle-shaped electrical contacts that concentrated the current in the center of a Py/Pt mesa of 4 mu m diameter. Due to the spin Hall effect, current within the Pt layer drives a spin current into the Py layer, exerting a spin transfer torque (STT). By injecting RF current and exploiting the phase sensitivity of TRSKM and the symmetry of the device structure, the STT and Oersted field torques have been separated and spatially mapped. The STT and torque due to the in-plane Oersted field are observed to exhibit minima at the device center that is ascribed to spreading of RF current that is not observed for DC current. Torques associated with the RF current may destabilize the position of the self-localized bullet mode excited by the DC current and inhibit injection locking. The present study demonstrates the need to characterize both DC and RF current distributions carefully.

  • 299.
    Streib, Simon
    et al.
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Borisov, Vladislav
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Pereiro, Manuel
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Bergman, Anders
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Sjoqvist, Erik
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Delin, Anna
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Eriksson, Olle
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden..
    Thonig, Danny
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden..
    Equation of motion and the constraining field in ab initio spin dynamics2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 21, article id 214407Article in journal (Refereed)
    Abstract [en]

    It is generally accepted that the effective magnetic field acting on a magnetic moment is given by the gradient of the energy with respect to the magnetization. However, in ab initio spin dynamics within the adiabatic approximation, the effective field is also known to be exactly the negative of the constraining field, which acts as a Lagrange multiplier to stabilize an out-of-equilibrium, noncollinear magnetic configuration. We show that for Hamiltonians without mean-field parameters both of these fields are exactly equivalent, while there can be a finite difference for mean-field Hamiltonians. For density functional theory (DFT) calculations, the constraining field obtained from the auxiliary Kohn-Sham Hamiltonian is not exactly equivalent to the DFT energy gradient. This inequality is highly relevant for both ab initio spin dynamics and the ab initio calculation of exchange constants and effective magnetic Hamiltonians. We argue that the effective magnetic field and exchange constants have the highest accuracy in DFT when calculated from the energy gradient and not from the constraining field.

  • 300.
    Streib, Simon
    et al.
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Cardias, Ramon
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Pereiro, Manuel
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Bergman, Anders
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Sjöqvist, Erik
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Barreteau, Cyrille
    Univ Paris Saclay, CEA Saclay, CNRS, CEA,SPEC, F-91191 Gif Sur Yvette, France..
    Delin, Anna
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Eriksson, Olle
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden..
    Thonig, Danny
    Uppsala Univ, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden..
    Adiabatic spin dynamics and effective exchange interactions from constrained tight-binding electronic structure theory: Beyond the Heisenberg regime2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 22, article id 224408Article in journal (Refereed)
    Abstract [en]

    We consider an implementation of the adiabatic spin dynamics approach in a tight-binding description of the electronic structure. The adiabatic approximation for spin degrees of freedom assumes that the faster electronic degrees of freedom are always in a quasiequilibrium state, which significantly reduces the numerical complexity in comparison to the full electron dynamics. Noncollinear magnetic configurations are stabilized by a constraining field, which allows us to directly obtain the effective magnetic field from the negative of the constraining field. While the dynamics are shown to conserve energy, we demonstrate that adiabatic spin dynamics does not conserve the total spin angular momentum when the lengths of the magnetic moments are allowed to change, which is confirmed by numerical simulations. Furthermore, we develop a method to extract an effective two-spin exchange interaction from the energy curvature tensor of noncollinear states, which we calculate at each time step of the numerical simulations. We demonstrate the effect of noncollinearity on this effective exchange and limitations due to multispin interactions in strongly noncollinear configurations beyond the regime where the Heisenberg model is valid. The relevance of the results are discussed with respect to experimental pump-probe experiments that follow the ultrafast dynamics of magnetism.

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