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  • 51.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    On the upper critical field of anisotropic superconductors1991In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 73, p. 347-356Article in journal (Refereed)
  • 52.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    QCD(1+1) with massless quarks and gauge covariant Sugawara construction1994In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 341, p. 195-204Article in journal (Refereed)
  • 53.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Quantum Gauge Theories and Noncommutative Geometry1996In: Acta Physica Polonica B, ISSN 0587-4254, E-ISSN 1509-5770, Vol. 27, p. 2477-2496Article in journal (Refereed)
  • 54.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Scattering matrix in external field problems1996In: Journal of Mathematical Physics, ISSN 0022-2488, E-ISSN 1089-7658, Vol. 37, p. 3933-3953Article in journal (Refereed)
  • 55.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Strong coupling gauge theory, quantum spin systems and the spontaneous breaking of chiral symmetry1992In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 297, p. 175-180Article in journal (Refereed)
  • 56.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    SU (N) antiferromagnets and strongly coupled QED: effective field theory for Josephson junctions arrays1993In: Nuclear physics B, Proceedings supplements, ISSN 0920-5632, E-ISSN 1873-3832, Vol. 33, p. 192-208Article in journal (Refereed)
  • 57.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Supersymmetry breaking and the Jaynes-Cummings model1993In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 176, no 5, p. 307-312Article in journal (Refereed)
  • 58.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    The critical temperature of superconductors with a cylinder symmetrical Fermi surface1989In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 157, no 3, p. 415-424Article in journal (Refereed)
  • 59.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    The geometric phase and the Schwinger term in some models1992In: International Journal of Modern Physics A, ISSN 0217-751X, E-ISSN 1793-656X, Vol. 7, no 21, p. 5045-5083Article in journal (Refereed)
  • 60.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    The Luttinger-Schwinger Model1997In: Annals of Physics, ISSN 0003-4916, E-ISSN 1096-035X, Vol. 253, p. 310-331Article in journal (Refereed)
  • 61.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    The Superfluidity and Experimental Properties of Odd-Energy-Gap Superconductors1993In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 26, no 2Article in journal (Refereed)
  • 62.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Theory of the upper critical magnetic field without local approximation1989In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 59, p. 561-569Article in journal (Refereed)
  • 63.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Towards a string representation of infrared SU(2) Yang-Mills theory1999In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 463, p. 252-256Article in journal (Refereed)
  • 64.
    Langmann, Edwin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Moosavi, Per
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Diffusive Heat Waves in Random Conformal Field Theory2019In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 2, article id 020201Article in journal (Refereed)
    Abstract [en]

    We propose and study a conformal field theory (CFT) model with random position-dependent velocity that, as we argue, naturally emerges as an effective description of heat transport in one-dimensional quantum many-body systems with certain static random impurities. We present exact analytical results that elucidate how purely ballistic heat waves in standard CFT can acquire normal and anomalous diffusive contributions due to our impurities. Our results include impurity-averaged Green's functions describing the time evolution of the energy density and the heat current, and an explicit formula for the thermal conductivity that, in addition to a universal Drude peak, has a nontrivial real regular contribution that depends on details of the impurities.

  • 65.
    Langmann, Edwin
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Univ Claude Bernard, Univ Lyon, ENS Lyon, CNRS,Lab Phys, F-69342 Lyon, France..
    Moosavi, Per
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Finite-Time Universality in Nonequilibrium CFT2018In: Journal of statistical physics, ISSN 0022-4715, E-ISSN 1572-9613, Vol. 172, no 2, p. 353-378Article in journal (Refereed)
    Abstract [en]

    Recently, remarkably simple exact results were presented about the dynamics of heat transport in the local Luttinger model for nonequilibrium initial states defined by position-dependent temperature profiles. We present mathematical details on how these results were obtained. We also give an alternative derivation using only algebraic relations involving the energy-momentum tensor which hold true in any unitary conformal field theory (CFT). This establishes a simple universal correspondence between initial temperature profiles and the resulting heat-wave propagation in CFT. We extend these results to larger classes of nonequilibrium states. It is proposed that such universal CFT relations provide benchmarks to identify nonuniversal properties of nonequilibrium dynamics in other models.

  • 66.
    Lezama, Talia L. M.
    et al.
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Bera, Soumya
    Indian Inst Technol, Dept Phys, Mumbai 400076, Maharashtra, India..
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Apparent slow dynamics in the ergodic phase of a driven many-body localized system without extensive conserved quantities2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 16, article id 161106Article in journal (Refereed)
    Abstract [en]

    We numerically study the dynamics on the ergodic side of the many-body localization transition in a periodically driven Floquet model with no global conservation laws. We describe and employ a numerical technique based on the fast Walsh-Hadamard transform that allows us to perform an exact time evolution for large systems and long times. As in models with conserved quantities (e.g., energy and/or particle number) we observe a slowing down of the dynamics as the transition into the many-body localized phase is approached. More specifically, our data are consistent with a subballistic spread of entanglement and a stretched-exponential decay of an autocorrelation function, with their associated exponents reflecting slow dynamics near the transition for a fixed system size. However, with access to larger system sizes, we observe a clear flow of the exponents towards faster dynamics and cannot rule out that the slow dynamics is a finite-size effect. Furthermore, we observe examples of nonmonotonic dependence of the exponents with time, with the dynamics initially slowing down but accelerating again at even larger times, consistent with the slow dynamics being a crossover phenomenon with a localized critical point.

  • 67. Mattesini, Maurizio
    et al.
    Belonoshko, Anatoly
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Tkalcic, Hrvoje
    Polymorphic Nature of Iron and Degree of Lattice Preferred Orientation Beneath the Earth's Inner Core Boundary2018In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 19, no 1, p. 292-304Article in journal (Refereed)
    Abstract [en]

    Deciphering the polymorphic nature and the degree of iron lattice-preferred orientation in the Earth's inner core holds a key to understanding the present status and evolution of the inner core. A multiphase lattice-preferred orientation pattern is obtained for the top 350 km of the inner core by means of the ab initio based Candy Wrapper Velocity Model coupled to a Monte Carlo phase discrimination scheme. The achieved geographic distribution of lattice alignment is characterized by two regions of freezing, namely within South America and the Western Central Pacific, that exhibit an uncommon high degree of lattice orientation. In contrast, widespread regions of melting of relatively weak lattice ordering permeate the rest of the inner core. The obtained multiphase lattice-preferred orientation pattern is in line with mantle-constrained geodynamo simulations and allows to setup an ad hoc mineral physics scenario for the complex Earth's inner core. It is found that the cubic phase of iron is the dominating iron polymorph in the outermost part of the inner core.

  • 68.
    Pozo, Oscar
    et al.
    Inst Ciencia Mat Madrid, Madrid 28049, Spain.;CSIC, Madrid 28049, Spain..
    Ferreiros, Yago
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Vozmediano, Maria A. H.
    Inst Ciencia Mat Madrid, Madrid 28049, Spain.;CSIC, Madrid 28049, Spain..
    Anisotropic fixed points in Dirac and Weyl semimetals2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 11, article id 115122Article in journal (Refereed)
    Abstract [en]

    The effective low energy description of interacting Dirac and Weyl semimetals is that of massless quantum electrodynamics with several Lorentz breaking material parameters. We perform a renormalization group analysis of Coulomb interaction in anisotropic Dirac and Weyl semimetals and show that the anisotropy persists in the material systems at the infrared fixed point. In addition, a tilt of the fermion cones breaking inversion symmetry induces a magnetoelectric term in the electrodynamics of the material whose magnitude runs to match that of the electronic tilt at the fixed point.

  • 69.
    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.

  • 70.
    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.

  • 71.
    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.

  • 72.
    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.

  • 73.
    Twengström, Mikael
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Department of physics, KTH.
    Spin ice and demagnetising theory2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Frustration, or the inability to simultaneously minimise all local interactions is, a phenomenon occurring in a broad number of physical systems. We will in this thesis focus on a class of frustrated ferromagnetic materials called spin ices and how both numerical and experimental techniques can be used to understand their properties. Spin ices show a number of peculiar properties such as low temperature residual entropy and magnetic monopole excitations. 

    Considering a dipolar Hamiltonian model with exchange interactions we verify a qualitative and previously established agreement with experimental data of the quantity χT/C , where χ is the magnetic susceptibility, T  the temperature and C the Curie parameter. We find a quantitative agreement by identifying that further near-neighbour interactions are sensitive probes of χT/C and the neutron structure factor, in particular its zone boundary scattering and relative peak intensities. 

    In systems passing from being governed by ferromagnetic interactions into potentially ordered anti-ferromagnets at low temperature we define special temperatures in close relation with real gases. These temperatures enable a new classication of "inverting" magnets of which spin ice is a member. 

    Due to rich complex long-range interactions in spin ice and a high sensitivity of the quantity χT/C, we identify demagnetising corrections to be crucial in extracting the correct physics. Apart from previously reported results we find the demagnetising factor to be clearly temperature and lattice structure dependent and not just shape dependent. The large moment of the Dy ions in Dy2Ti2O7  thus implies that an incorrect demagnetising treatment can shift the important features in χT/C  outside of the relevant temperature range considered. Employing our refined demagnetising theory we obtain good agreement with experiments down to sub-kelvin temperatures. 

    The magnetic ions in spin ice enable neutron scattering as an excellent tool to study spin ice. A massively parallel computer code is developed in order to obtain high resolution neutron scattering factors in Fourier space. These high resolution charts are in good agreement with carefully verified experimental data down to 350 mK.

  • 74.
    Twengström, Mikael
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Bovo, L.
    Gingras, M. J. P.
    Bramwell, S. T.
    Henelius, Patrik
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Microscopic aspects of magnetic lattice demagnetizing factors2017In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 4, article id 044406Article in journal (Refereed)
    Abstract [en]

    The demagnetizing factor N is of both conceptual interest and practical importance. Considering localized magnetic moments on a lattice, we show that for nonellipsoidal samples, N depends on the spin dimensionality (Ising, XY, or Heisenberg) and orientation, as well as the sample shape and susceptibility. The generality of this result is demonstrated by means of a recursive analytic calculation as well as detailed Monte Carlo simulations of realistic model spin Hamiltonians. As an important check and application, we also make an accurate experimental determination of N for a representative collective paramagnet (i.e., the Dy2Ti2O7 spin ice compound) and show that the temperature dependence of the experimentally determined N agrees closely with our theoretical calculations. Our conclusion is that the well-established practice of approximating the true sample shape with "corresponding ellipsoids" for systems with long-range interactions will in many cases overlook important effects stemming from the microscopic aspects of the system under consideration.

  • 75.
    Wang, Wenlong
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Diaz-Mendez, Rogelio
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Capdevila, Raudys
    Univ Politecn Valencia, Dept Appl Math, E-46022 Valencia, Spain..
    Solving the one-dimensional Ising chain via mathematical induction: an intuitive approach to the transfer matrix2019In: European journal of physics, ISSN 0143-0807, E-ISSN 1361-6404, Vol. 40, no 6, article id 065102Article in journal (Refereed)
    Abstract [en]

    The aim of this work is to present a formulation to solve the one-dimensional Ising model using the elementary technique of mathematical induction. This formulation is physically clear and leads to the same partition function form as the transfer matrix method, which is a common subject in the introductory courses of statistical mechanics. In this way our formulation is a useful tool to complement the traditional more abstract transfer matrix method. The method can be straightforwardly generalised to other short-range chains, coupled chains and is also computationally friendly. These two approaches provide a more complete understanding of the system, and therefore our work can be of broad interest for undergraduate teaching in statistical mechanics.

  • 76.
    Wang, Wenlong
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Díaz-Méndez, Rogelio
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Melting of a two-dimensional monodisperse cluster crystal to a cluster liquid2019In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 99, no 4, article id 042140Article in journal (Refereed)
    Abstract [en]

    Monodisperse ensembles of particles that have cluster crystalline phases at low temperatures can model a number of physical systems, such as vortices in type-1.5 superconductors, colloidal suspensions, and cold atoms. In this work, we study a two-dimensional cluster-forming particle system interacting via an ultrasoft potential. We present a simple mean-field characterization of the cluster-crystal ground state, corroborating with Monte Carlo simulations for a wide range of densities. The efficiency of several Monte Carlo algorithms is compared, and the challenges of thermal equilibrium sampling are identified. We demonstrate that the liquid to cluster-crystal phase transition is of first order and occurs in a single step, and the liquid phase is a cluster liquid. © 2019 American Physical Society.

  • 77.
    Wang, Wenlong
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Kevrekidis, P. G.
    Univ Massachusetts, Dept Math & Stat, Amherst, MA 01003 USA..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Ring dark solitons in three-dimensional Bose-Einstein condensates2019In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 100, no 5, article id 053621Article in journal (Refereed)
    Abstract [en]

    In this work we present a systematic study of the three-dimensional extension of the ring dark soliton, examining its existence, stability, and dynamics in isotropic harmonically trapped Bose-Einstein condensates. Detuning the chemical potential from the linear limit, the ring dark soliton becomes unstable immediately but can be fully stabilized by an external cylindrical potential. The ring has a large number of unstable modes which are analyzed through spectral stability analysis. Furthermore, a few typical destabilization dynamical scenarios are revealed with a number of interesting vortical structures emerging, such as the two or four coaxial parallel vortex rings. In the process of considering the stability of the structure, we also develop a modified version of the degenerate perturbation theory method for characterizing the spectra of the coherent structure. This semianalytical method can be reliably applied to any soliton with a linear limit to explore its spectral properties near this limit. The good agreement of the resulting spectrum is illustrated via a comparison with the full numerical Bogolyubov-de Gennes spectrum. The application of the method to the two-component ring dark-bright soliton is also discussed.

  • 78.
    Wang, Wenlong
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Meier, Hannes
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Three-dimensional universality class of the Ising model with power-law correlated critical disorder2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 14, article id 144204Article in journal (Refereed)
    Abstract [en]

    We use large-scale Monte Carlo simulations to test the Weinrib-Halperin criterion that predicts new universality classes in the presence of sufficiently slowly decaying power-law correlated quenched disorder. While new universality classes are reasonably well established, the predicted exponents are controversial. We propose a method of growing such correlated disorder using the three-dimensional Ising model as a benchmark system for both generating disorder and studying the resulting phase transition. Critical equilibrium configurations of a disorder-free system are used to define the two-value distributed random bonds with a small power-law exponent given by the pure Ising exponent. Finite-size scaling analysis shows a new universality class with a single phase transition, but the critical exponents nu(d) = 1.13(5), eta(d) = 0.48(3) differ significantly from theoretical predictions. We find that depending on the details of the disorder generation, disorder-averaged quantities can develop peaks at two temperatures for finite sizes. Finally, a layer model with the two values of bonds spatially separated in halves of the system genuinely has multiple phase transitions, and thermodynamic properties can be flexibly tuned by adjusting the model parameters.

  • 79.
    Weston, Daniel
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Phase transitions and phase frustration in multicomponent superconductors2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Multicomponent superconductors described by several complex matter fields have properties radically different from those of their single-component counterparts. Examples include partially ordered phases and spontaneous breaking of time-reversal symmetry due to frustration between Josephson-coupled components. Recent experimental results make such symmetry breaking a topic of central interest in superconductivity. Multicomponent gauge theories appear as effective theories e.g. for quantum antiferromagnets, and are thus of interest well beyond superconductivity. The nature of the phase transitions in these models is of great importance in modern physics, and yet remains poorly understood. These models and phenomena are studied theoretically in this thesis, mainly using large-scale Monte Carlo simulations.Superconducting s+is states have recently been described for superconductors with N = 3 components. The novelty of these states is that they break time-reversal symmetry due to frustration of interband couplings. In the first paper, we consider whether there can be new states in N-component Ginzburg-Landau models with bilinear Josephson couplings when N >= 4. We find that these models have new states associated with accidental continuous ground-state degeneracies. Also, we show that the possible combinations of signs of the couplings can for any N be divided into equivalence classes in a way that is related to the graph-theoretical concept of Seidel switching.In the second paper, we consider fluctuation effects in models of SU(N) symmetric superconductors. We demonstrate that there is a novel type of paired phase that is given by proliferation of non-topological vortices for N = 3 and 4, and that despite the absence of topologically stable vortices these systems form vortex lattices in external magnetic field; these lattices involve structures that are not simply hexagonal and differ between components.In the third paper, we consider fluctuation effects in London models of U(1)^N symmetric superconductors. These models are of central interest due to the theory of deconfined quantum criticality, according to which such gauge theories may describe phase transitions beyond the Ginzburg-Landau-Wilson paradigm. The direct transitions from fully ordered to fully disordered phases have been reported to be continuous for N = 1 and N = 183, and discontinuous for N = 2. The nature of the phase transitions for small N is an outstanding open question. We demonstrate that the degree of discontinuity of the direct transitions increases with N, at least for small N, and that the transitions from paired phases to fully disordered phases can be discontinuous. Both these results are in contrast to previous expectations.In the fourth and final paper, we report the first experimental observation of a state of matter that has an order parameter that is fourth order in fermionic fields: a bosonic Z_2 metal, in which time-reversal symmetry is broken due to partial ordering of Cooper pairs despite superconducting order being absent. By considering fluctuation effects in phase-frustrated three-component Ginzburg-Landau models, we place constraints on the models used to describe the material in question. Also, we give an example of this anomalous state occurring in a type-2 Ginzburg-Landau model in external magnetic field, despite it not occurring in this model in the absence of external field.

  • 80.
    Weston, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Vortices and composite order in SU(N) theories coupled to Abelian gauge fieldManuscript (preprint) (Other academic)
  • 81.
    Weston, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Sellin, Karl
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Phase transitions in U(1)N lattice London modelsManuscript (preprint) (Other academic)
  • 82.
    Xypakis, Emmanouil
    et al.
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Rhim, Jun-Won
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany.;Inst for Basic Sci Korea, Ctr Correlated Electron Syst, Seoul 08826, South Korea.;Seoul Natl Univ, Dept Phys & Astron, Seoul 08826, South Korea..
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany.
    Ilan, Roni
    Tel Aviv Univ, Raymond & Beverly Sackler Sch Phys & Astron, IL-69978 Tel Aviv, Israel..
    Perfect transmission and Aharanov-Bohm oscillations in topological insulator nanowires with nonuniform cross section2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 4, article id 045401Article in journal (Refereed)
    Abstract [en]

    Topological insulator nanowires with uniform cross section, combined with a magnetic flux, can host both a perfectly transmitted mode and Majorana zero modes. Here we consider nanowires with rippled surfaces- specifically, wires with a circular cross section with a radius varying along its axis-and we calculate their transport properties. At zero doping, chiral symmetry places the clean wires (no impurities) in the AIII symmetry class, which results in a Z topological classification. A magnetic flux threading the wire tunes between the topologically distinct insulating phases, with perfect transmission obtained at the phase transition. We derive an analytical expression for the exact flux value at the transition. Both doping and disorder break the chiral symmetry and the perfect transmission. At finite doping, the interplay of surface ripples and disorder with the magnetic flux modifies quantum interference such that the amplitude of Aharonov-Bohm oscillations reduces with increasing flux, in contrast to wires with uniform surfaces where it is flux-independent.

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