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  • 1. Alidoust, M.
    et al.
    Halterman, K.
    Zyuzin, A. A.
    KTH, School of Engineering Sciences (SCI), Physics.
    Superconductivity in type-II Weyl semimetals2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 15, article id 155124Article in journal (Refereed)
    Abstract [en]

    We study superconductivity in a Weyl semimetal with a tilted dispersion around two Weyl points of opposite chirality. In the absence of any tilt, the state with zero momentum pairing between two Fermi sheets enclosing each Weyl point has four point nodes in the superconducting gap function. Moreover, the surface of the superconductor hosts Fermi arc states and Majorana flat bands. We show that a quantum phase transition occurs at a critical value of the tilt, at which two gap nodes disappear by merging at the center of the first Brillouin zone, or by escaping at its edges, depending on the direction of the tilt. The region in the momentum space that the Majorana flat band occupies is found to increase as the tilt parameter is made larger. Additionally, the superconducting critical temperature and electronic specific heat can be enhanced in the vicinity of the quantum phase transition due to the singularity in the electronic density of states.

  • 2. Alidoust, Mohammad
    et al.
    Zyuzin, Alexander
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Halterman, Klaus
    Pure odd-frequency superconductivity at the cores of proximity vortices2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 4, article id 045115Article in journal (Refereed)
    Abstract [en]

    After more than a decade, direct observation of the odd frequency triplet pairing state in superconducting hybrid structures remains elusive. We propose an experimentally feasible setup that can unambiguously reveal the zero energy peak due to proximity-induced equal spin superconducting triplet correlations. We theoretically investigate a two-dimensional Josephson junction in the diffusive regime. The nanostructure consists of a normal metal sandwiched between two ferromagnetic layers with spiral magnetization patterns. By applying an external magnetic field perpendicular to the junction plane, vortices nucleate in the normal metal. The calculated energy and spatially resolved density of states, along with the pair potential, reveal that remarkably, only triplet Cooper pairs survive in the vortex cores. These isolated odd frequency triplet correlations result in well defined zero energy peaks in the local density of states that can be identified through tunneling spectroscopy experiments. Moreover, the diffusive regime considered here rules out the possibility of Andreev bound states in the vortex core as contributors to the zero energy peaks.

  • 3.
    Andresen, Juan Carlos
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Katzgraber, Helmut G.
    Schechter, Moshe
    Random-field-induced disordering mechanism in a disordered ferromagnet: Between the Imry-Ma and the standard disordering mechanism2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 21, article id 214414Article in journal (Refereed)
    Abstract [en]

    Random fields disorder Ising ferromagnets by aligning single spins in the direction of the random field in three space dimensions, or by flipping large ferromagnetic domains at dimensions two and below. While the former requires random fields of typical magnitude similar to the interaction strength, the latter Imry-Ma mechanism only requires infinitesimal random fields. Recently, it has been shown that for dilute anisotropic dipolar systems a third mechanism exists, where the ferromagnetic phase is disordered by finite-size glassy domains at a random field of finite magnitude that is considerably smaller than the typical interaction strength. Using large-scale Monte Carlo simulations and zero-temperature numerical approaches, we show that this mechanism applies to disordered ferromagnets with competing short-range ferromagnetic and antiferromagnetic interactions, suggesting its generality in ferromagnetic systems with competing interactions and an underlying spin-glass phase. A finite-size-scaling analysis of the magnetization distribution suggests that the transition might be first order.

  • 4. Atteia, J.
    et al.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cayssol, J.
    Ballistic transport through irradiated graphene2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 245404Article in journal (Refereed)
    Abstract [en]

    The coherent charge transport through an illuminated graphene ribbon is studied as a function of electronic doping and characteristics of the electromagnetic driving, for monochromatic circularly polarized light. We focus on the DC current carried by 2D bulk carriers which is dominant (over edge transport) for short and wide enough samples. We investigate how the ballistic conductance suppression, due to photon resonances between the valence and conduction bands, evolves when the experimentally tunable parameters are varied. The residual conductance can be associated with evanescent states and related to dynamical gaps in the Floquet quasienergy spectrum.

  • 5. Bagschik, Kai
    et al.
    Froemter, Robert
    Bach, Judith
    Beyersdorff, Bjoern
    Mueller, Leonard
    Schleitzer, Stefan
    Berntsen, Magnus H.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. Deutsch Elekt Synchrotron DESY.
    Weier, Christian
    Adam, Roman
    Viefhaus, Jens
    Schneider, Claus Michael
    Gruebel, Gerhard
    Oepen, Hans Peter
    Employing soft x-ray resonant magnetic scattering to study domain sizes and anisotropy in Co/Pd multilayers2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 13, article id 134413Article in journal (Refereed)
    Abstract [en]

    It is demonstrated that themagnetic diffraction pattern of the isotropic disorderedmaze pattern is well described utilizing a gamma distribution of domain sizes in a one-dimensional model. From the analysis, the mean domain size and the shape parameter of the distribution are obtained. The model reveals an average domain size that is significantly different from the value that is determined from the peak position of the structure factor in reciprocal space. As a proof of principle, a wedge-shaped (Co-t angstrom/Pd-10 angstrom) 8 multilayer film, that covers the thickness range of the spin-reorientation transition, has been used. By means of soft x-ray resonant magnetic scattering (XRMS) and imaging techniques the thickness-driven evolution of the magnetic properties of the cobalt layers is explored. It is shown that minute changes of the domain pattern concerning domain size and geometry can be investigated and analyzed due to the high sensitivity and lateral resolution of the XRMS technique. The latter allows for the determination of the magnetic anisotropies of the cobalt layers within a thickness range of a few angstroms.

  • 6.
    Banerjee, R.
    et al.
    Nanyang Technol Univ, Sch Phys & Math Sci, Div Phys & Appl Phys, Singapore 637371, Singapore..
    Liew, T. C. H.
    Nanyang Technol Univ, Sch Phys & Math Sci, Div Phys & Appl Phys, Singapore 637371, Singapore..
    Kyriienko, O.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Realization of Hofstadter's butterfly and a one-way edge mode in a polaritonic system2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 7, article id 075412Article in journal (Refereed)
    Abstract [en]

    We present a scheme to generate an artificial gauge field for the system of neutral bosons, represented by polaritons in micropillars arranged into a square lattice. The splitting between the two polarizations of the micropillars breaks the time-reversal symmetry (TRS) and results in the effective phase-dependent hopping between cavities. This can allow for engineering a nonzero flux on the plaquette, corresponding to an artificial magnetic field. Changing the phase, we observe a characteristic Hofstadter's butterfly pattern and the appearance of chiral edge states for a finite-size structure. For long-lived polaritons, we show that the propagation of wave packets at the edge is robust against disorder. Moreover, given the inherent driven-dissipative nature of polariton lattices, we find that the system can exhibit topological lasing, recently discovered for active ring cavity arrays. The results point to a static way to realize artificial magnetic field in neutral spinful systems, avoiding the periodic modulation of the parameters or strong spin-orbit interaction. Ultimately, the described system can allow for high-power topological single-mode lasing which is robust to imperfections.

  • 7.
    Barkman, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Zyuzin, Alexander A.
    Aalto Univ, Dept Appl Phys, POB 15100, FI-00076 Aalto, Finland.;Ioffe Phys Tech Inst, St Petersburg 194021, Russia..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Antichiral and nematicity-wave superconductivity2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 22, article id 220508Article in journal (Refereed)
    Abstract [en]

    Larkin-Ovchinnikov superconducting state has spontaneous modulation of Cooper pair density, while Fulde-Ferrell state has a spontaneous modulation in the phase of the order parameter. We report that a quasi-two-dimensional Dirac metal, under certain conditions has principally different inhomogeneous superconducting states that by contrast have spontaneous modulation in a submanifold of a multiple-symmetries-breaking order parameter. The first state we find can be viewed as a nematic superconductor where the nematicity vector spontaneously breaks rotational and translational symmetries due to spatial modulation. The other demonstrated state is a chiral superconductor with spontaneously broken time-reversal and translational symmetries. It is characterized by an order parameter, which forms a lattice pattern of alternating chiralities.

  • 8. Behrends, Jan
    et al.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max Planck Institute, Germany.
    Strongly angle-dependent magnetoresistance in Weyl semimetals with long-range disorder2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 6, article id 060201Article in journal (Refereed)
    Abstract [en]

    The chiral anomaly in Weyl semimetals states that the left- and right-handed Weyl fermions, constituting the low energy description, are not individually conserved, resulting, for example, in a negative magnetoresistance in such materials. Recent experiments see strong indications of such an anomalous resistance response; however, with a response that at strong fields is more sharply peaked for parallel magnetic and electric fields than expected from simple theoretical considerations. Here, we uncover a mechanism, arising from the interplay between the angle-dependent Landau-level structure and long-range scalar disorder, that has the same phenomenology. In particular, we analytically show, and numerically confirm, that the internode scattering time decreases exponentially with the angle between the magnetic field and the Weyl node separation in the large field limit, while it is insensitive to this angle at weak magnetic fields. Since, in the simplest approximation, the internode scattering time is proportional to the anomaly-related conductivity, this feature may be related to the experimental observations of a sharply peaked magnetoresistance.

  • 9. Behrends, Jan
    et al.
    Rhim, Jun-Won
    Liu, Shang
    Grushin, Adolfo G.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max-Planck-Institut für Physik Komplexer Systeme, Germany.
    Nodal-line semimetals from Weyl superlattices2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 245101Article in journal (Refereed)
    Abstract [en]

    The existence and topological classification of lower-dimensional Fermi surfaces is often tied to the crystal symmetries of the underlying lattice systems. Artificially engineered lattices, such as heterostructures and other superlattices, provide promising avenues to realize desired crystal symmetries that protect lower-dimensional Fermi surfaces, such as nodal lines. In this work, we investigate a Weyl semimetal subjected to spatially periodic onsite potential, giving rise to several phases, including a nodal-line semimetal phase. In contrast to proposals that purely focus on lattice symmetries, the emergence of the nodal line in this setup does not require small spin-orbit coupling, but rather relies on its presence. We show that the stability of the nodal line is understood from reflection symmetry and a combination of a fractional lattice translation and charge-conjugation symmetry. Depending on the choice of parameters, this model exhibits drumhead surface states that are exponentially localized at the surface, or weakly localized surface states that decay into the bulk at all energies.

  • 10.
    Behrends, Jan
    et al.
    Max Planck Inst Phys Komplexer Systeme, D-01187 Dresden, Germany..
    Roy, Sthitadhi
    Max Planck Inst Phys Komplexer Systeme, D-01187 Dresden, Germany.;Univ Oxford, Phys & Theoret Chem, South Parks Rd, Oxford OX1 3QZ, England.;Univ Oxford, Rudolf Peierls Ctr Theoret Phys, Clarendon Lab, Parks Rd, Oxford OX1 3PU, England..
    Kolodrubetz, Michael H.
    Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA.;Lawrence Berkeley Natl Lab, Mat Sci Div, Berkeley, CA 94720 USA.;Univ Texas Dallas, Dept Phys, Richardson, TX 75080 USA..
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max Planck Inst Phys Komplexer Systeme, D-01187 Dresden, Germany.
    Grushin, Adolfo G.
    Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA.;CNRS, Inst Neel, F-38042 Grenoble, France.;Univ Grenoble Alpes, F-38042 Grenoble, France..
    Landau levels, Bardeen polynomials, and Fermi arcs in Weyl semimetals: Lattice-based approach to the chiral anomaly2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 14, article id 140201Article in journal (Refereed)
    Abstract [en]

    Condensed matter systems realizing Weyl fermions exhibit striking phenomenology derived from their topologically protected surface states as well as chiral anomalies induced by electromagnetic fields. More recently, inhomogeneous strain or magnetization were predicted to result in chiral electric E-5 and magnetic B-5 fields, which modify and enrich the chiral anomaly with additional terms. In this Rapid Communication, we develop a lattice-based approach to describe the chiral anomaly, which involves Landau and pseudo-Landau levels and treats all anomalous terms on equal footing, while naturally incorporating Fermi arcs. We exemplify its potential by physically interpreting the largely overlooked role of Fermi arcs in the covariant (Fermi level) contribution to the anomaly and revisiting the factor of 1/3 difference between the covariant and consistent (complete band) contributions to the E-5 . B-5 term in the anomaly. Our framework provides a versatile tool for the analysis of anomalies in realistic lattice models as well as a source of simple physical intuition for understanding strained and magnetized inhomogeneous Weyl semimetals.

  • 11. Bjornson, Kristofer
    et al.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. os Alamos National Laboratory, United States; ETH Institute for Theoretical Studies, Switzerland.
    Black-Schaffer, Annica M.
    Superconducting order parameter pi-phase shift in magnetic impurity wires2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 10, article id 104521Article in journal (Refereed)
    Abstract [en]

    It has previously been found that amagnetic impurity in a conventional s-wave superconductor can give rise to a local pi-phase shift of the superconducting order parameter. By studying a finite wire of ferromagnetic impurities, we are able to trace the origin of the pi-phase shift to a resonance condition for the Bogoliubov-de Gennes quasiparticle states. When nonresonating states localized at the impurity sites are pulled into the condensate for increasing magnetic strength, the superconducting order parameter is reduced in discrete steps, eventually resulting in a pi-phase shift. We also show that for a finite spin-orbit coupling, the pi-phase shift is preserved and occurs in a large portion of the topologically nontrivial phase.

  • 12.
    Bondarenko, Artem
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Holmgren, Erik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Li, Zhong Wei
    Ivanov, B. A.
    Institute of Magnetism, National Academy of Science, 03142 Kiev, Ukraine; National University of Science and Technology “MISiS”, Moscow 119049, Russian Federation..
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Chaotic dynamics in spin-vortex pairs2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, article id 054402Article in journal (Refereed)
    Abstract [en]

    We report on spin-vortex pair dynamics measured at temperatures low enough to suppress stochastic core motion, thereby uncovering the highly nonlinear intrinsic dynamics of the system. Our analysis shows that the decoupling of the two vortex cores is resonant and can be enhanced by dynamic chaos. We detail the regions of the relevant parameter space, in which the various mechanisms of the resonant core-core dynamics are activated. We show that the presence of chaos can reduce the thermally induced spread in the decoupling time by up to two orders of magnitude.

  • 13.
    Bondarenko, N.
    et al.
    Uppsala Univ, Dept Phys & Astron, Div Mat Theory, Box 516, SE-75121 Uppsala, Sweden..
    Chico, J.
    Uppsala Univ, Dept Phys & Astron, Div Mat Theory, Box 516, SE-75121 Uppsala, Sweden.;Forschungszentrum Julich, Peter Grunberg Inst, D-52428 Julich, Germany.;Forschungszentrum Julich, Inst Adv Simulat, D-52428 Julich, Germany.;JARA, D-52428 Julich, Germany..
    Kvashnin, Y. O.
    Uppsala Univ, Dept Phys & Astron, Div Mat Theory, Box 516, SE-75121 Uppsala, Sweden..
    Bergman, A.
    Uppsala Univ, Dept Phys & Astron, Div Mat Theory, Box 516, SE-75121 Uppsala, Sweden.;Univ Versailles, Univ Paris Sud, CNRS, Maison Simulat,CEA,INRIA,USR 3441, F-91191 Gif Sur Yvette, France.;CEA, INAC MEM, L Sim, F-38000 Grenoble, France..
    Skorodumova, Natalia
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling. Uppsala Univ, Dept Phys & Astron, Div Mat Theory, Box 516, SE-75121 Uppsala, Sweden.
    Eriksson, O.
    Uppsala Univ, Dept Phys & Astron, Div Mat Theory, Box 516, SE-75121 Uppsala, Sweden.;Orebro Univ, Sch Sci & Technol, SE-70182 Orebro, Sweden..
    Spin polaronics: Static and dynamic properties of spin polarons in La-doped CaMnO42019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 13, article id 134443Article in journal (Refereed)
    Abstract [en]

    The static and dynamic properties of spin polarons in La-doped CaMnO3 are explored theoretically, by means of an effective low-energy Hamiltonian. All parameters of the effective Hamiltonian are evaluated from first-principles theory. The Hamiltonian is used to investigate the temperature stability as well as the response to an external applied electric field, for spin polarons in bulk, surface, and as single two-dimensional layers. Technically this involves atomistic spin-dynamics simulations in combination with kinetic Monte Carlo simulations. Where a comparison can be made, our simulations exhibit excellent agreement with available experimental data and previous theory. Remarkably, we find that excellent control of the mobility of spin polarons in this material can be achieved, and that the critical parameters deciding this are the temperature and strength of the applied electrical field. We outline different technological implications of spin polarons, and point to spin polaronics as an emerging subfield of nanotechnology. In particular, we demonstrate that it is feasible to write and erase information on an atomic scale, by use of spin polarons in CaMnO3.

  • 14. Bondarenko, N.
    et al.
    Kvashnin, Y.
    Chico, J.
    Bergman, A.
    Eriksson, O.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling. Uppsala Univ, Sweden.
    Spin-polaron formation and magnetic state diagram in La-doped CaMnO32017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 22, article id 220401Article in journal (Refereed)
    Abstract [en]

    LaxCa1-xMnO3 (LCMO) has been studied in the framework of density functional theory (DFT) using Hubbard-U correction. We show that the formation of spin polarons of different configurations is possible in the G-type antiferromagnetic phase. We also show that the spin-polaron (SP) solutions are stabilized due to an interplay of magnetic and lattice effects at lower La concentrations and mostly due to the lattice contribution at larger concentrations. Our results indicate that the development of SPs is unfavorable in the C- and A-type antiferromagnetic phases. The theoretically obtained magnetic state diagram is in good agreement with previously reported experimental results.

  • 15. Borlenghi, Simone
    et al.
    Mahani, M. R.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Fangohr, Hans
    Franchin, M.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Sweden.
    Fransson, Jonas
    Micromagnetic simulations of spin-torque driven magnetization dynamics with spatially resolved spin transport and magnetization texture2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 9, article id 094428Article in journal (Refereed)
    Abstract [en]

    We present a simple and fast method to simulate spin-torque driven magnetization dynamics in nanopillar spin-valve structures. The approach is based on the coupling between a spin transport code based on random matrix theory and a micromagnetics finite-elements software. In this way the spatial dependence of both spin transport and magnetization dynamics is properly taken into account. Our results are compared with experiments. The excitation of the spin-wave modes, including the threshold current for steady-state magnetization precession and the nonlinear frequency shift of the modes are reproduced correctly. The giant magneto resistance effect and the magnetization switching also agree with experiment. The similarities with recently described spin-caloritronics devices are also discussed.

  • 16. Borlenghi, Simone
    et al.
    Mahani, M. Reza
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Delin, Anna
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Fransson, Jonas
    Nanoscale control of heat and spin conduction in artificial spin chains2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 13, article id 134419Article in journal (Refereed)
    Abstract [en]

    We describe a mechanism to control the energy and magnetization currents in an artificial spin chain, consisting of an array of permalloy nanodisks coupled through a magnetodipolar interaction. The chain is kept out of equilibrium by two thermal baths with different temperatures connected to its ends, which control the current propagation. Transport is enhanced by applying a uniform radio-frequency pump field resonating with some of the spin-wave modes of the chain. Moreover, the two currents can be controlled independently by tuning the static field applied on the chain. Thus we describe two effective means for the independent control of coupled currents and the enhancement of thermal and spin-wave conductivity in a realistic magnonics device, suggesting that similar effects could be observed in a large class of nonlinear oscillating systems.

  • 17. Bostrom, Mathias
    et al.
    Malyi, Oleksandr I.
    Parashar, Prachi
    Shajesh, K. V.
    Thiyam, Priyadarshini
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Milton, Kimball A.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Oslo, Norway.
    Parsons, Drew F.
    Brevik, Iver
    Lifshitz interaction can promote ice growth at water-silica interfaces2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 15, article id 155422-1Article in journal (Refereed)
    Abstract [en]

    At air-water interfaces, the Lifshitz interaction by itself does not promote ice growth. On the contrary, we find that the Lifshitz force promotes the growth of an ice film, up to 1-8 nm thickness, near silica-water interfaces at the triple point of water. This is achieved in a system where the combined effect of the retardation and the zero frequency mode influences the short-range interactions at low temperatures, contrary to common understanding. Cancellation between the positive and negative contributions in the Lifshitz spectral function is reversed in silica with high porosity. Our results provide a model for how water freezes on glass and other surfaces.

  • 18. Boyko, D.
    et al.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Los Alamos National Laboratory, United States.
    Haraldsen, J. T.
    Evolution of magnetic Dirac bosons in a honeycomb lattice2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 1, article id 014433Article in journal (Refereed)
    Abstract [en]

    We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the collinear phase diagram as well as the change in the spin dynamics with various exchange interactions. We show that the ferromagnetic structure produces bosonic Dirac and Weyl points due to the competition between the interactions. Furthermore, it is shown that the criteria for magnetic Dirac nodes are coupled to the magnetic structure and not the overall crystal symmetry, where the breaking of inversion symmetry greatly affects the antiferromagnetic configurations. The tunability of the nodal points through variation of the exchange parameters leads to the possibility of controlling Dirac symmetries through an external manipulation of the orbital interactions.

  • 19.
    Calugaru, Dumitru
    et al.
    Max Planck Inst Phys Komplexer Syst, Nothnitzer Str 38, D-01187 Dresden, Germany.;Univ Cambridge, Cavendish Lab, JJ Thomson Ave, Cambridge CB3 0HE, England..
    Juricic, Vladimir
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Roy, Bitan
    Max Planck Inst Phys Komplexer Syst, Nothnitzer Str 38, D-01187 Dresden, Germany..
    Higher-order topological phases: A general principle of construction2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 4, article id 041301Article in journal (Refereed)
    Abstract [en]

    We propose a general principle for constructing higher-order topological (HOT) phases. We argue that if a D-dimensional first-order or regular topological phase involves m Hermitian matrices that anticommute with additional p - 1 mutually anticommuting matrices, it is conceivable to realize an nth-order HOT phase, where n = 1, ..., p, with appropriate combinations of discrete symmetry-breaking Wilsonian masses. An nth-order HOT phase accommodates zero modes on a surface with codimension n. We exemplify these scenarios for prototypical three-dimensional gapless systems, such as a nodal-loop semimetal possessing SU(2) spin-rotational symmetry, and Dirac semimetals, transforming under (pseudo)spin-1/2 or 1 representations. The former system permits an unprecedented realization of a fourth-order phase, without any surface zero modes. Our construction can be generalized to HOT insulators and superconductors in any dimension and symmetry class.

  • 20. Castin, N.
    et al.
    Messina, L.
    Domain, C.
    Pasianot, R. C.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Improved atomistic Monte Carlo models based on ab-initio -trained neural networks: Application to FeCu and FeCr alloys2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 21, article id 214117Article in journal (Refereed)
    Abstract [en]

    We significantly improve the physical models underlying atomistic Monte Carlo (MC) simulations, through the use of ab initio fitted high-dimensional neural network potentials (NNPs). In this way, we can incorporate energetics derived from density functional theory (DFT) in MC, and avoid using empirical potentials that are very challenging to design for complex alloys. We take significant steps forward from a recent work where artificial neural networks (ANNs), exclusively trained on DFT vacancy migration energies, were used to perform kinetic MC simulations of Cu precipitation in Fe. Here, a more extensive transfer of knowledge from DFT to our cohesive model is achieved via the fitting of NNPs, aimed at accurately mimicking the most important aspects of the ab initio predictions. Rigid-lattice potentials are designed to monitor the evolution during the simulation of the system energy, thus taking care of the thermodynamic aspects of the model. In addition, other ANNs are designed to evaluate the activation energies associated with the MC events (migration towards first-nearest-neighbor positions of single point defects), thereby providing an accurate kinetic modeling. Because our methodology inherently requires the calculation of a substantial amount of reference data, we design as well lattice-free potentials, aimed at replacing the very costly DFT method with an approximate, yet accurate and considerably more computationally efficient, potential. The binary FeCu and FeCr alloys are taken as sample applications considering the extensive literature covering these systems.

  • 21.
    Claisse, Antoine
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Schuler, Thomas
    Lopes, Denise Adorno
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Transport properties in dilute UN(X) solid solutions (X = Xe, Kr)2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 17, article id 174302Article in journal (Refereed)
    Abstract [en]

    Uranium nitride (UN) is a candidate fuel for current GEN III fission reactors, for which it is investigated as an accident-tolerant fuel, as well as for future GEN IV reactors. In this study, we investigate the kinetic properties of gas fission products (Xe and Kr) in UN. Binding and migration energies are obtained using density functional theory, with an added Hubbard correlation to model f electrons, and the occupation matrix control scheme to avoid metastable states. These energies are then used as input for the self-consistent mean field method which enables to determine transport coefficients for vacancy-mediated diffusion of Xe and Kr on the U sublattice. The magnetic ordering of the UN structure is explicitly taken into account, for both energetic and transport properties. Solute diffusivities are compared with experimental measurements and the effect of various parameters on the theoretical model is carefully investigated. We find that kinetic correlations are very strong in this system, and that despite atomic migration anisotropy, macroscopic solute diffusivities show limited anisotropy. Our model indicates that the discrepancy between experimental measurements probably results from different irradiation conditions, and hence different defect concentrations.

  • 22. Commeau, Benjamin
    et al.
    Geilhufe, R. Matthias
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Fernando, Gayanath W.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Structural and electronic properties of alpha-(BEDT-TTF)(2)I-3, ss-(BEDT-TTF)(2)I-3, and kappa-(BEDT-TTF)(2)X-3 (X = I, F, Br, Cl) organic charge transfer salts2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 12, article id 125135Article in journal (Refereed)
    Abstract [en]

    (BEDT-TFF)(2)I-3 charge transfer salts are reported to show superconductivity and pressure-induced quasi-twodimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases alpha, beta, and kappa(.) We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and the origin of band crossings. Additionally, we discuss the chemically induced strain in kappa-(BEDT-TTF)(2)I-3 achieved by replacing the iodine layer with other halogens: fluorine, bromine, and chlorine. In the case of kappa-(BEDT-TTF)(2)F-3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the nonsymmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).

  • 23. Csire, Gabor
    et al.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ujfalussy, Balazs
    First-principles approach to thin superconducting slabs and heterostructures2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 14, article id 140502Article in journal (Refereed)
    Abstract [en]

    We present a fully first-principles method for superconducting thin films. The layer dependent phonon spectrum is calculated to determine the layer dependence of the electron-phonon coupling for such systems, which is coupled to the Kohn-Sham-Bogoliubov-de Gennes equations, and it is solved in a parameter-free way. The theory is then applied to different surface facets of niobium slabs and to niobium-gold heterostructures. We investigate the dependence of the transition temperature on the thickness of the slabs and the inverse proximity effect observed in thin superconducting heterostructures.

  • 24.
    Dai, Jin
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Ding, Fei
    Bozhevolnyi, Sergey I.
    Yan, Min
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Ultrabroadband super-Planckian radiative heat transfer with artificial continuum cavity states in patterned hyperbolic metamaterials2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 24, article id 245405Article in journal (Refereed)
    Abstract [en]

    Localized cavity resonances due to nanostructures at material surfaces can greatly enhance radiative heat transfer (RHT) between two closely placed bodies owing to stretching of cavity states in momentum space beyond the light line. Based on such understanding, we numerically demonstrate the possibility of ultrabroadband super-Planckian RHT between two plates patterned with trapezoidal-shaped hyperbolic metamaterial (HMM) arrays. The phenomenon is rooted not only in HMM's high effective index for creating subwavelength resonators but also its extremely anisotropic isofrequency contour. The two properties enable one to create photonic bands with a high spectral density to populate a desired thermal radiation window. At submicron gap sizes between such two plates, the artificial continuum states extend outside the light cone, tremendously increasing overall RHT. Our study reveals that structured HMM offers unprecedented potential in achieving a controllable super-Planckian radiative heat transfer for thermal management at nanoscale.

  • 25.
    Dai, Jin
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.
    Dyakov, Sergey A.
    Bozhevolnyi, Sergey I.
    Yan, Min
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.
    Near-field radiative heat transfer between metasurfaces: A full-wave study based on two-dimensional grooved metal plates2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 12, article id 125431Article in journal (Refereed)
    Abstract [en]

    Metamaterials possess artificial bulk and surface electromagnetic states. Tamed dispersion properties of surface waves allow one to achieve a controllable super-Planckian radiative heat transfer (RHT) process between two closely spaced objects. We numerically demonstrate enhanced RHT between two two-dimensional grooved metal plates by a full-wave scattering approach. The enhancement originates from both transverse-magnetic spoof surface-plasmon polaritons and a series of transverse-electric bonding-and anti-bonding-waveguide modes at surfaces. The RHT spectrum is frequency selective and highly geometrically tailorable. Our simulation also reveals thermally excited nonresonant surface waves in constituent metallic materials may play a prevailing role for RHT at an extremely small separation between two metal plates, rendering metamaterial modes insignificant for the energy-transfer process.

  • 26.
    D'Ambrosio, Federico
    et al.
    Univ Utrecht, Dept Informat & Comp Sci, Princetonpl 5, NL-3584 CC Utrecht, Netherlands..
    Juricic, Vladimir
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Barkema, Gerard T.
    Univ Utrecht, Dept Informat & Comp Sci, Princetonpl 5, NL-3584 CC Utrecht, Netherlands..
    Discontinuous evolution of the structure of stretching polycrystalline graphene2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 16, article id 161402Article in journal (Refereed)
    Abstract [en]

    Polycrystalline graphene has an inherent tendency to buckle, i.e., develop out-of-plane, three-dimensional structure. A force applied to stretch a piece of polycrystalline graphene influences the out-of-plane structure. Even if the graphene is well relaxed, this happens in nonlinear fashion: Occasionally, a tiny increase in stretching force induces a significant displacement, in close analogy to avalanches, which in turn can create vibrations in the surrounding medium. We establish this effect in computer simulations: By continuously changing the strain, we follow the displacements of the carbon atoms that turn out to exhibit a discontinuous evolution. Furthermore, the displacements exhibit a hysteretic behavior upon the change from low to high stress and back. These behaviors open up another direction in studying dynamical elasticity of polycrystalline quasi-two-dimensional systems, and in particular the implications on their mechanical and thermal properties.

  • 27.
    De Luca, Eleonora
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Electronics and Quantum Optics, QEO.
    Sanatinia, Reza
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Mensi, Mounir
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Anand, Srinivasan
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Swillo, Marcin
    KTH, School of Engineering Sciences (SCI), Applied Physics, Quantum Electronics and Quantum Optics, QEO.
    Modal phase matching in nanostructured zinc-blende semiconductors for second-order nonlinear optical interactions2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 7, article id 075303Article in journal (Refereed)
    Abstract [en]

    We demonstrate enhanced second-harmonic generation in arrays of nanowaveguides satisfying modal-phase-matching condition, both theoretically and experimentally. The overlap of interacting fields defined by the fundamental mode of the pump and the second-order mode of the second-harmonic wave is enhanced by the longitudinal component of the nonlinear polarization density. For guided modes with significant longitudinal electric field components, the overlap of fields is comparable to that obtained in the quasi-phase-matching technique leading to higher conversion efficiencies. Thus, the presented method is preferable to achieve higher conversion efficiency in second-order nonlinear processes in nanowaveguides.

  • 28.
    Delacretaz, Luca, V
    et al.
    Stanford Univ, Dept Phys, Stanford, CA 94305 USA..
    Gouteraux, Blaise
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Paris Saclay, Ctr Theoret Phys, Ecole Polytech, CNRS UMR 7644, F-91128 Palaiseau, France.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Hartno, Sean A.
    Stanford Univ, Dept Phys, Stanford, CA 94305 USA.;SLAC Natl Accelerator Lab, Stanford Inst Mat & Energy Sci, 2575 Sand Hill Rd, Menlo Pk, CA 94025 USA..
    Karlsson, Anna
    Princeton Univ Sch Nat Sci, Inst Adv Study, 1 Einstein Dr, Princeton, NJ 08540 USA.;Chalmers Univ Technol, Dept Phys, Div Theoret Phys, SE-41296 Gothenburg, Sweden..
    Theory of collective magnetophonon resonance and melting of a field-induced Wigner solid2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 8, article id 085140Article in journal (Refereed)
    Abstract [en]

    Electron solid phases of matter are revealed by characteristic vibrational resonances. Sufficiently large magnetic fields can overcome the effects of disorder, leading to a weakly pinned collective mode called the magnetophonon. Consequently, in this regime it is possible to develop a tightly constrained hydrodynamic theory of pinned magnetophonons. The behavior of the magnetophonon resonance across thermal and quantum melting transitions has been experimentally characterized in two-dimensional electron systems. Applying our theory to these transitions we explain several key features of the data. Firstly, violation of the Fukuyama-Lee sum rule as the transition is approached is shown to be a consequence of the non-Lorentzian form taken by the resonance. Secondly, this non-Lorentzian shape is shown to be caused by dissipative channels that become especially important close to melting: proliferating dislocations and uncondensed charge carriers.

  • 29. Delacretaz, Luca V.
    et al.
    Goutéraux, Blaise
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stanford University, United States.
    Hartnoll, Sean A.
    Karlsson, Anna
    Theory of hydrodynamic transport in fluctuating electronic charge density wave states2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 19, article id 195128Article in journal (Refereed)
    Abstract [en]

    We describe the collective hydrodynamic motion of an incommensurate charge density wave state in a clean electronic system. Our description simultaneously incorporates the effects of both pinning due to weak disorder and also phase relaxation due to proliferating dislocations. We show that the interplay between these two phenomena has important consequences for charge and momentum transport. For instance, it can lead to metal-insulator transitions. We furthermore identify signatures of fluctuating density waves in frequency and spatially resolved conductivities. Phase disordering is well known to lead to a large viscosity. We derive a precise formula for the phase relaxation rate in terms of the viscosity in the dislocation cores. We thereby determine the viscosity of the superconducting state of BSCCO from the observed melting dynamics of Abrikosov lattices and show that the result is consistent with dissipation into Bogoliubov quasiparticles.

  • 30.
    Dong, Zhihua
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Chen, Dengfu
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Long, Mujun
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden; Research Institute for Solid State Physics and Optics, Wigner Research Center for Physics, Budapest, Hungary.
    Elastic properties of paramagnetic austenitic steel at finite temperature: Longitudinal spin fluctuations in multicomponent alloys2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 17, article id 174415Article in journal (Refereed)
    Abstract [en]

    We propose a first-principles framework for longitudinal spin fluctuations (LSFs) in disordered paramagnetic (PM) multicomponent alloy systems and apply it to investigate the influence of LSFs on the temperature dependence of two elastic constants of PM austenitic stainless steel Fe15Cr15Ni. The magnetic model considers individual fluctuating moments in a static PM medium with first-principles-derived LSF energetics in conjunction with describing chemical disorder and randomness of the transverse magnetic component in the single-site alloy formalism and disordered local moment (DLM) picture. A temperature-sensitive mean magnetic moment is adopted to accurately represent the LSF state in the elastic-constant calculations. We make evident that magnetic interactions between an LSF impurity and the PM medium are weak in the present steel alloy. This allows gaining accurate LSF energetics and mean magnetic moments already through a perturbation from the static DLM moments instead of a tedious self-consistent procedure. We find that LSFs systematically lower the cubic shear elastic constants c' and c(44) by similar to 6 GPa in the temperature interval 300-1600 K, whereas the predominant mechanism for the softening of both elastic constants with temperature is the magneto-volume coupling due to thermal lattice expansion. We find that non-negligible local magnetic moments of Cr and Ni are thermally induced by LSFs, but they exert only a small influence on the elastic properties. The proposed framework exhibits high flexibility in accurately accounting for finite-temperature magnetism and its impact on the mechanical properties of PM multicomponent alloys.

  • 31.
    Duan, Yu-Xia
    et al.
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Zhang, Cheng
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Rusz, Jan
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden..
    Oppeneer, Peter M.
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden..
    Durakiewicz, Tomasz
    Marie Curie Sklodowska Univ, Lnstitute Phys, PL-20031 Lublin, Poland..
    Sassa, Yasmine
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden.;Chalmers Univ Technol, Dept Phys, S-41296 Gothenburg, Sweden..
    Tjernberg, Oscar
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Månsson, Martin
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Berntsen, Magnus H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Wu, Fan-Ying
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Zhao, Yin-Zou
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Song, Jiao-Jiao
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Wu, Qi-Yi
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Luo, Yang
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China..
    Bauer, Eric D.
    Los Alamos Natl Lab, Condensed Matter & Magnet Sci Grp, Los Alamos, NM 87545 USA..
    Thompson, Joe D.
    Los Alamos Natl Lab, Condensed Matter & Magnet Sci Grp, Los Alamos, NM 87545 USA..
    Meng, Jian-Qiao
    Cent S Univ, Sch Phys & Elect, Changsha 410083, Hunan, Peoples R China.;Hunan Normal Univ, SICQEA, Changsha 410081, Hunan, Peoples R China..
    Crystal electric field splitting and f-electron hybridization in heavy-fermion CePt2In72019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 8, article id 085141Article in journal (Refereed)
    Abstract [en]

    We use high-resolution angle-resolved photoemission spectroscopy to investigate the electronic structure of the antiferromagnetic heavy fermion compound CePt2In7, which is amember of the CeIn3-derived heavy fermion material family. Weak hybridization among 4f electron states and conduction bands was identified in CePt2In7 at low temperature much weaker than that in the other heavy fermion compounds like CeIrIn5 and CeRhIn5. The Ce 4f spectrum shows fine structures near the Fermi energy, reflecting the crystal electric field splitting of the 4f(5/2)(1) and 4f(7/2)(1) states. Also, we find that the Fermi surface has a strongly three-dimensional topology, in agreement with density-functional theory calculations.

  • 32. Dufouleur, J.
    et al.
    Xypakis, E.
    Buechner, B.
    Giraud, R.
    Bardarson, Jens Hjörleifur
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max Planck Institute for Physics - Max-Planck-Gesellschaft, Germany.
    Suppression of scattering in quantum confined 2D helical Dirac systems2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 7, article id 075401Article in journal (Refereed)
    Abstract [en]

    Transport properties of helical Dirac fermions in disordered quantum wires are investigated in the large energy limit. In the quasiballistic regime, the conductance and the Fano factor are sensitive to disorder only when the Fermi energy is close to an opening of a transverse mode. In the limit of a large number of transverse modes, transport properties are insensitive to the geometry of the nanowire or the nature and strength of the disorder but, instead, are dominated by the properties of the interface between the ohmic contact and the nanowire. In the case of a heavily doped Dirac metallic contact, the conductance is proportional to the energy with an average transmission T = pi/4 and a Fano factor of F similar or equal to 0.13. Those results can be generalized to a much broader class of contacts, the exact values of T and F depending on the model used for the contacts. The energy dependence of Aharonov-Bohm oscillations is determined, revealing a damped oscillatory behavior and phase shifts due to the one-dimensional subband quantization and which are not the signature of the nontrivial topology.

  • 33.
    Dunnett, K.
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Ferrier, A.
    UCL, Dept Phys & Astron, Gower St, London WC1E 6BT, England..
    Zamora, A.
    UCL, Dept Phys & Astron, Gower St, London WC1E 6BT, England..
    Dagvadorj, G.
    UCL, Dept Phys & Astron, Gower St, London WC1E 6BT, England.;Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England..
    Szymanska, M. H.
    UCL, Dept Phys & Astron, Gower St, London WC1E 6BT, England..
    Properties of the signal mode in the polariton optical parametric oscillator regime2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 16, article id 165307Article in journal (Refereed)
    Abstract [en]

    Theoretical analyses of the polariton optical parametric oscillator (OPO) regime often rely on a mean-field approach based on the complex Gross-Pitaevskii equations in a three-mode approximation, where only three momentum states, the signal, pump, and idler, are assumed to be significantly occupied. This approximation, however, lacks a constraint to uniquely determine the signal and idler momenta. In contrast, multimode numerical simulations and experiments show a unique momentum structure for the OPO states. In this work we show that an estimate for the signal momentum chosen by the system can be found from a simple analysis of the pump-only configuration. We use this estimate to investigate how the chosen signal momentum depends on the properties of the drive.

  • 34.
    Dunnett, K.
    et al.
    Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Narayan, Awadhesh
    Swiss Fed Inst Technol, Mat Theory, Wolfgang Pauli Str 27, CH-8093 Zurich, Switzerland..
    Spaldin, N. A.
    Swiss Fed Inst Technol, Mat Theory, Wolfgang Pauli Str 27, CH-8093 Zurich, Switzerland..
    Balatsky, Alexander
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Los Alamos Natl Lab, Inst Mat Sci, Los Alamos, NM 87545 USA.;Univ Connecticut, Dept Phys, Storrs, CT 06269 USA..
    Strain and ferroelectric soft-mode induced superconductivity in strontium titanate2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 14, article id 144506Article in journal (Refereed)
    Abstract [en]

    We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviors under uniaxial and biaxial strain can be understood.

  • 35. Durrenfeld, Philipp
    et al.
    Xu, Yongbing
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Department of Physics, University of Gothenburg, .
    Zhou, Yan
    Controlled skyrmion nucleation in extended magnetic layers using a nanocontact geometry2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 5, article id 054430Article in journal (Refereed)
    Abstract [en]

    We propose and numerically simulate a spintronic device layout consisting of a nanocontact on top of an extended Co/Pt bilayer. The interfacial Dzyaloshinskii-Moriya interaction in such bilayer systems can lead to the possible existence of metastable skyrmions. A small dc current injected through the nanocontact enables the manipulation of the size as well as the annihilation of an initially present skyrmion, while ps-long current pulses allow for the controlled nucleation of single skyrmions underneath the nanocontact. The results are obtained from micromagnetic simulations and can be potentially used for future magnetic storage implementations.

  • 36.
    Edberg, Richard
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Sandberg, L. Orduk
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Bakke, I. M. Bergh
    Univ Oslo, Ctr Mat Sci & Nanotechnol, N-0315 Oslo, Norway..
    Haubro, M. L.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Folkers, L. C.
    Lund Univ, Dept Chem, Lund, Sweden..
    Mangin-Thro, L.
    Inst Laue Langevin, F-38042 Grenoble, France..
    Wildes, A.
    Inst Laue Langevin, F-38042 Grenoble, France..
    Zaharko, O.
    Paul Scherrer Inst, Lab Neutron Scattering, CH-5232 Villigen, Switzerland..
    Guthrie, M.
    European Spallat Source ERIC, S-22363 Lund, Sweden.;Univ Edinburgh, Sch Phys & Astron, Edinburgh, Midlothian, Scotland.;Univ Edinburgh, Ctr Sci Extreme Condit, Edinburgh, Midlothian, Scotland..
    Holmes, A. T.
    European Spallat Source ERIC, S-22363 Lund, Sweden..
    Sorby, M. H.
    Inst Energy Technol, Dept Neutron Mat Characterizat, POB 40, N-2027 Kjeller, Norway..
    Lefmann, K.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark..
    Deen, P. P.
    Univ Copenhagen, Nanosci Ctr, Niels Bohr Inst, Univ Pk 5, DK-2100 Copenhagen O, Denmark.;European Spallat Source ERIC, S-22363 Lund, Sweden..
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Abo Akad Univ, Fac Sci & Engn, Turku, Finland..
    Dipolar spin ice under uniaxial pressure2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 14, article id 144436Article in journal (Refereed)
    Abstract [en]

    The magnetically frustrated spin ice family of materials is host to numerous exotic phenomena such as magnetic monopole excitations and macroscopic residual entropy extending to low temperature. A finite-temperature ordering transition in the absence of applied fields has not been experimentally observed in the classical spin ice materials Dy2Ti2O7 and Ho2Ti2O7. Such a transition could be induced by the application of pressure, and in this work we consider the effects of uniaxial pressure on classical spin ice systems. Theoretically, we find that the pressure-induced ordering transition in Dy2Ti2O7 is strongly affected by the dipolar interaction. We also report measurements of the neutron structure factor of Ho2Ti2O7 under pressure and compare the experimental results to the predictions of our theoretical model.

  • 37.
    Ehteshami, Hossein
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. National University of Science and Technology, Russia.
    Thermophysical properties of paramagnetic Fe from first principles2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 22, article id 224406Article in journal (Refereed)
    Abstract [en]

    A computationally efficient, yet general, free-energy modeling scheme is developed based on first-principles calculations. Finite-temperature disorder associated with the fast (electronic and magnetic) degrees of freedom is directly included in the electronic structure calculations, whereas the vibrational free energy is evaluated by a proposed model that uses elastic constants to calculate average sound velocity of the quasiharmonic Debye model. The proposed scheme is tested by calculating the lattice parameter, heat capacity, and single-crystal elastic constants of alpha-, gamma-, and delta-iron as functions of temperature in the range 1000-1800 K. The calculations accurately reproduce the well-established experimental data on thermal expansion and heat capacity of gamma- and delta-iron. Electronic and magnetic excitations are shown to account for about 20% of the heat capacity for the two phases. Nonphonon contributions to thermal expansion are 12% and 10% for alpha- and delta-Fe and about 30% for gamma-Fe. The elastic properties predicted by the model are in good agreement with those obtained in previous theoretical treatments of paramagnetic phases of iron, as well as with the bulk moduli derived from isothermal compressibility measurements [N. Tsujino et al., Earth Planet. Sci. Lett. 375, 244 (2013)]. Less agreement is found between theoretically calculated and experimentally derived single-crystal elastic constants of gamma- and delta-iron.

  • 38. Ertan, Emelie
    et al.
    Kimberg, Victor
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Siberian Fed Univ, Russia.
    Gel'mukhanov, Faris
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Siberian Fed Univ, Russia.
    Hennies, Franz
    Rubensson, Jan-Erik
    Schmitt, Thorsten
    Strocov, Vladimir N.
    Zhou, Kejin
    Iannuzzi, Marcella
    Foehlisch, Alexander
    Odelius, Michael
    Pietzsch, Annette
    Theoretical simulations of oxygen K-edge resonant inelastic x-ray scattering of kaolinite2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 14, article id 144301Article in journal (Refereed)
    Abstract [en]

    Near-edge x-ray absorption fine structure (NEXAFS) and resonant inelastic x-ray scattering (RIXS) measurements at the oxygen K edge were combined with theoretical spectrum simulations, based on periodic density functional theory and nuclear quantum dynamics, to investigate the electronic structure and chemical bonding in kaolinite Al2Si2O5(OH)(4). We simulated NEXAFS spectra of all crystallographically inequivalent oxygen atoms in the crystal and RIXS spectra of the hydroxyl groups. Detailed insight into the ground-state potential energy surface of the electronic states involved in the RIXS process were accessed by analyzing the vibrational excitations, induced by the core excitation, in quasielastic scattering back to the electronic ground state. In particular, we find that the NEXAFS pre-edge is dominated by features related to OH groups within the silica and alumina sheets, and that the vibrational progression in RIXS can be used to selectively probe vibrational modes of this subclass of OH groups. The signal is dominated by the OH stretching mode, but also other lower vibrational degrees of freedom, mainly hindered rotational modes, contribute to the RIXS signal.

  • 39.
    Ferreiros, Yago
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Vozmediano, Maria A. H.
    Elastic gauge fields and Hall viscosity of Dirac magnons2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 5, article id 054404Article in journal (Refereed)
    Abstract [en]

    We analyze the coupling of elastic lattice deformations to the magnon degrees of freedom of magnon Dirac materials. For a honeycomb ferromagnet we find that, as happens in the case of graphene, elastic gauge fields appear coupled to the magnon pseudospinors. For deformations that induce constant pseudomagnetic fields, the spectrum around the Dirac nodes splits into pseudo-Landau levels. We show that when a Dzyaloshinskii-Moriya interaction is considered, a topological gap opens in the system and a Chern-Simons effective action for the elastic degrees of freedom is generated. Such a term encodes a phonon Hall viscosity response, entirely generated by quantum fluctuations of magnons living in the vicinity of the Dirac points. The magnon Hall viscosity vanishes at zero temperature, and grows as temperature is raised and the states around the Dirac points are increasingly populated.

  • 40.
    Ferreiros, Yago
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Zyuzin, A. A.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Bardarson, Jens H
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Anomalous Nernst and thermal Hall effects in tilted Weyl semimetals2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 11, article id 115202Article in journal (Refereed)
    Abstract [en]

    We study the anomalous Nernst and thermal Hall effects in a linearized low-energy model of a tilted Weyl semimetal, with two Weyl nodes separated in momentum space. For inversion symmetric tilt, we give analytic expressions in two opposite limits: For a small tilt, corresponding to a type-I Weyl semimetal, the Nernst conductivity is finite and independent of the Fermi level; for a large tilt, corresponding to a type-II Weyl semimetal, it acquires a contribution depending logarithmically on the Fermi energy. This result is in a sharp contrast to the nontilted case, where the Nernst response is known to be zero in the linear model. The thermal Hall conductivity similarly acquires Fermi surface contributions, which add to the Fermi level-independent, zero-tilt result, and is suppressed as one over the tilt parameter at half filling in the type-II phase. In the case of inversion-breaking tilt, with the tilting vector of equal modulus in the two Weyl cones, all Fermi surface contributions to both anomalous responses cancel out, resulting in zero Nernst conductivity. We discuss two possible experimental setups, representing open and closed thermoelectric circuits.

  • 41. Franco, A. F.
    et al.
    Gonzalez-Fuentes, C.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Garcia, C.
    Anisotropy constant and exchange coupling strength of perpendicularly magnetized CoFeB/Pd multilayers and exchange springs2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 14, article id 144417Article in journal (Refereed)
    Abstract [en]

    A model describing the ferromagnetic resonance of multilayer structures is used to characterize the interface anisotropy constant and interlayer exchange coupling strength associated to individual components of [CoFeB/Pd](n) multilayers with perpendicular magnetic anisotropy and [CoFeB/Pd](5)/(CoFeB or Co) exchange spring structures by comparing with ferromagnetic resonance behavior measurements. We find that the effective perpendicular anisotropy increases with the number of repetitions of the multilayer, which we could explain only after accounting for a different anisotropy at the bottom repetition of the multilayer with perpendicular anisotropy. Similarly, the characterization of the exchange coupling in our structures was only possible after accounting for individual components, thus portraying the importance of using a multilayer model to properly describe the magnetic behavior and properties of a multilayer structure. We find that the perpendicular anisotropy constant in amorphous Pd/CoFeB/Pd structures increases slightly from 0.295 to 0.315 mJ/m(2) when increasing the thickness of the CoFeB from 3 to 4 angstrom. Furthermore, we find that the exchange coupling in CoFeB/Pd(10 A degrees)/CoFeB structures decreases from 4.899 to 3.268 mJ/m(2) when increasing the thickness of the CoFeB from 3 to 4 angstrom. Finally, we find that the magnitude of the anisotropy at Co/Pd interfaces is 65% larger than at CoFeB/Pd interfaces, and the exchange coupling at CoFeB/Pd/Co interfaces decreases approximately 30% when compared to a CoFeB/Pd/CoFeB structure.

  • 42.
    Fu, Ying
    et al.
    KTH, Centres, Science for Life Laboratory, SciLifeLab.
    Jussi, Johnny Israelsson
    KTH, Centres, Science for Life Laboratory, SciLifeLab.
    Elmlund, Louise
    Swedish Natl Forens Ctr, SE-58194 Linkoping, Sweden..
    Dunne, Simon
    Swedish Natl Forens Ctr, SE-58194 Linkoping, Sweden..
    Wang, Qin
    RISE Res Inst Sweden AB, Box 1070, SE-16425 Kista, Sweden..
    Brismar, Hjalmar
    KTH, Centres, Science for Life Laboratory, SciLifeLab. KTH, School of Engineering Sciences (SCI), Applied Physics.
    Intrinsic blinking characteristics of single colloidal CdSe-CdS/ZnS core-multishell quantum dots2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 3, article id 035404Article in journal (Refereed)
    Abstract [en]

    Fluorescence blinking of single colloidal semiconductor quantum dots (QDs) has been extensively studied, and several sophisticated models have been proposed. In this work, we derive Heisenberg equations of motion to carefully study principal transition processes, i.e., photoexcitation, energy relaxation, impact ionization and Auger recombination, radiative and nonradiative recombinations, and tunneling between core states and surface states, of the electron-hole pair in single CdSe-CdS/ZnS core-multishell QDs and show that the on-state probability density distribution of the QD fluorescence obeys the random telegraph signal theory because of the random radiative recombination of the photoexcited electron-hole pair in the QD core, while the off-state probability density distribution obeys the inverse power law distribution due to the series of random walks of the photoexcited electron in the two-dimensional surface-state network after the electron tunnels from the QD core to the QD surface. These two different blinking characteristics of the single QD are resolved experimentally by properly adjusting the optical excitation power and the bin time.

  • 43.
    Gani, Yohanes S.
    et al.
    Coll William & Mary, Dept Phys, Williamsburg, VA 23187 USA..
    Abergel, D. S. L.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Nat Phys, 4 Crinan St, London N1 9XW, England..
    Rossi, Enrico
    Coll William & Mary, Dept Phys, Williamsburg, VA 23187 USA..
    Electronic structure of graphene nanoribbons on hexagonal boron nitride2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 20, article id 205415Article in journal (Refereed)
    Abstract [en]

    Hexagonal boron nitride is an ideal dielectric to form two-dimensional heterostructures due to the fact that it can be exfoliated to be just a few atoms thick and its very low density of defects. By placing graphene nanoribbons on high quality hexagonal boron nitride it is possible to create ideal quasi-one-dimensional systems with very high mobility. The availability of high quality one-dimensional electronic systems is of great interest also given that when in proximity to a superconductor they can be effectively engineered to realize Majorana bound states. In this work we study how a boron nitride substrate affects the electronic properties of graphene nanoribbons. We consider both armchair and zigzag nanoribbons. Our results show that for some stacking configurations the boron nitride can significantly affect the electronic structure of the ribbons. In particular, for zigzag nanoribbons, due to the lock between spin and sublattice degree of freedom at the edges, the hexagonal boron nitride can induce a very strong spin splitting of the spin-polarized, edge states. We find that such spin splitting can be as high as 40 meV.

  • 44. Gao, S.
    et al.
    Guratinder, K.
    Stuhr, U.
    White, J. S.
    Månsson, Martin
    KTH, School of Engineering Sciences (SCI), Applied Physics. Paul Scherrer Institute, Villigen, Switzerland.
    Roessli, B.
    Fennell, T.
    Tsurkan, V.
    Loidl, A.
    Ciomaga Hatnean, M.
    Balakrishnan, G.
    Raymond, S.
    Chapon, L.
    Garlea, V. O.
    Savici, A. T.
    Cervellino, A.
    Bombardi, A.
    Chernyshov, D.
    Rüegg, C.
    Haraldsen, J. T.
    Zaharko, O.
    Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr2 O42018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 13, article id 134430Article in journal (Refereed)
    Abstract [en]

    In spinels ACr2O4(A=Mg, Zn), realization of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et al. [Pow. Diff. 17, 230 (2002)PODIE20885-715610.1154/1.1479738], while other features depend on sample or cooling protocol. A complex, partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=(12120),k2=(1012) appear. The ordered moment reaches 1.94(3) μB/Cr3+ for k1 and 2.08(3) μB/Cr3+ for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1-10] diagonals and a smaller c component. We use inelastic neutron scattering to investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of clusterlike excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

  • 45.
    Garaud, Julien
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Bojesen, Troels Arnfred
    Sudbo, Asle
    Lattices of double-quanta vortices and chirality inversion in p(x) + i p(y) superconductors2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 10, article id 104509Article in journal (Refereed)
    Abstract [en]

    We investigate the magnetization processes of a standard Ginzburg-Landau model for chiral p-wave superconducting states in an applied magnetic field. We find that the phase diagram is dominated by triangular lattices of doubly quantized vortices. Only in close vicinity to the upper critical field the lattice starts to dissociate into a structure of single-quanta vortices. The degeneracy between states with opposite chirality is broken in a nonzero field. If the magnetization starts with an energetically unfavorable chirality, the process of chirality inversion induced by the external magnetic field results in the formation of a sequence of metastable states with characteristic magnetic signatures that can be probed by standard experimental techniques.

  • 46.
    Garaud, Julien
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Corticelli, Alberto
    KTH, School of Engineering Sciences (SCI), Physics.
    Silaev, Mihail
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Field-induced coexistence of s(++) and s(+/-) superconducting states in dirty multiband superconductors2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 5, article id 054520Article in journal (Refereed)
    Abstract [en]

    In multiband systems, such as iron-based superconductors, the superconducting states with locking and antilocking of the interband phase differences are usually considered as mutually exclusive. For example, a dirty two-band system with interband impurity scattering undergoes a sharp crossover between the s(+/-) state (which favors phase antilocking) and the s(++) state (which favors phase locking). We discuss here that the situation can be much more complex in the presence of an external field or superconducting currents. In an external applied magnetic field, dirty two-band superconductors do not feature a sharp s(perpendicular to) -> s(++) crossover but rather awashed-out crossover to a finite region in the parameter space where both s(+/-) and s(++) states can coexist for example as a lattice or a microemulsion of inclusions of different states. The current-carrying regions such as the regions near vortex cores can exhibit an s(+/-) state while it is the s(++) state that is favored in the bulk. This coexistence of both states can even be realized in the Meissner state at the domain's boundaries featuring Meissner currents. We demonstrate that there is a magnetic-field-driven crossover between the pure s(+/-) and the s(++) states.

  • 47.
    Garaud, Julien
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Corticelli, Alberto
    KTH, School of Engineering Sciences (SCI), Physics.
    Silaev, Mihail
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Properties of dirty two-band superconductors with repulsive interband interaction: Normal modes, length scales, vortices, and magnetic response2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 1, article id 014520Article in journal (Refereed)
    Abstract [en]

    Disorder in two-band superconductors with repulsive interband interaction induces a frustrated competition between the phase-locking preferences of the various potential and kinetic terms. This frustrated interaction can result in the formation of an s + is superconducting state that breaks the time-reversal symmetry. In this paper we study the normal modes and their associated coherence lengths in such materials. We especially focus on the consequences of the soft modes stemming from the frustration and time-reversal symmetry breakdown. We find that two-band superconductors with such impurity-induced frustrated interactions display a rich spectrum of physical properties that are absent in their clean counterparts. It features a mixing of Leggett's and Anderson-Higgs modes, and a soft mode with diverging coherence length at the impurity-induced second-order phase transition from s +/- / s ++ states to the s + is state. Such a soft mode generically results in long-range attractive intervortex forces that can trigger the formation of vortex clusters. We find that, if such clusters are formed, their size and internal flux density have a characteristic temperature dependence that could be probed in muon-spin-rotation experiments. We also comment on the appearance of spontaneous magnetic fields due to spatially varying impurities.

  • 48. Garaud, Julien
    et al.
    Silaev, Mihail
    Babaev, Egor
    Change of the vortex core structure in two-band superconductors at the impurity-scattering-driven s(+/-)/s(++) crossover2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 14, article id 140503Article in journal (Refereed)
  • 49.
    Geilhufe, Matthias
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Balatsky, Alexander V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, USA; Department of Physics, University of Connecticut, USA.
    Symmetry analysis of odd- and even-frequency superconducting gap symmetries for time-reversal symmetric interactions2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 2, article id 024507Article in journal (Refereed)
    Abstract [en]

    Odd-frequency superconductivity describes a class of superconducting states where the superconducting gap is an odd function in relative time and Matsubara frequency. We present a group theoretical analysis based on the linearized gap equation in terms of Shubnikov groups of the second kind. By discussing systems with spin-orbit coupling and an interaction kernel which is symmetric under the reversal of relative time, we show that both even-and odd-frequency gaps are allowed to occur. Specific examples are discussed for the square lattice, the octahedral lattice, and the tetragonal lattice. For irreducible representations that are even under the reversal of relative time the common combinations of s- and d-wave spin singlet and p-wave spin triplet gaps are revealed, irreducible representations that are odd under reversal of relative time give rise to s- and d-wave spin triplet and p-wave spin singlet gaps. Furthermore, we discuss the construction of a generalized Ginzburg-Landau theory in terms of the associated irreducible representations. The result complements the established classification of superconducting states of matter.

  • 50.
    Geilhufe, Matthias
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Guinea, Francisco
    Imdea Nanosci, Faraday 9, Madrid 28015, Spain.;Univ Manchester, Sch Phys & Astron, Manchester M13 9PY, Lancs, England..
    Juricic, Vladimir
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Hund nodal line semimetals: The case of a twisted magnetic phase in the double-exchange model2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 2, article id 020404Article in journal (Refereed)
    Abstract [en]

    We propose a class of topological metals, which we dub Hund nodal line semimetals, arising from the strong Coulomb interaction encoded in the Hund's coupling between itinerant electrons and localized spins. We here consider a particular twisted spin configuration, which is realized in the double-exchange model which describes the manganite oxides. The resulting effective tetragonal lattice of electrons with hoppings tied to the local spin features an antiunitary nonsymmorphic symmetry that, in turn, together with another nonsymmorphic but unitary glide-mirror symmetry, protects crossings of a double pair of bands along a high-symmetry line on the Brillouin zone boundary. We also discuss the stability of Hund nodal line semimetals with respect to symmetry breaking arising from various perturbations of the twisted phase. Our results motivate further studies of other realizations of this state of matter, for instance, in different spin backgrounds, properties of its drumhead surface states, as well as its stability to disorder and interactions among the itinerant electrons.

123 1 - 50 of 148
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