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  • 1. Babaev, Egor
    et al.
    Carlstrom, J.
    Silaev, Mihail
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Speight, J. M.
    Type-1.5 superconductivity in multicomponent systems2017In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 533, p. 20-35Article in journal (Refereed)
    Abstract [en]

    In general a superconducting state breaks multiple symmetries and, therefore, is characterized by several different coherence lengths i = 1,..., N. Moreover in multiband material even superconducting states that break only a single symmetry are nonetheless described, under certain conditions by multi component theories with multiple coherence lengths. As a result of that there can appear a state where some coherence lengths are smaller and some are larger than the magnetic field penetration length A: xi(1) <= xi(2)...<root 2 lambda < xi(M) <=... (N). That state was recently termed "type-1.5" superconductivity. This breakdown of type-1/type-2 dichotomy is rather generic near a phase transition between superconducting states with different symmetries. The examples include the transitions between U(1) and U(1) x U(1) states or between U(1) and U(1) x Z(2) states. The later example is realized in systems that feature transition between s-wave and s + is states. The extra fundamental length scales have many physical consequences. In particular in these regimes vortices can attract one another at long range but repel at shorter ranges. Such a system can form vortex clusters in low magnetic fields. The vortex clustering in the type 1.5 regime gives rise to many physical effects, ranging from macroscopic phase separation in domains of different broken symmetries, to unusual transport properties. Prepared for the proceedings of Vortex IX conference, Rhodes 12-17 September 2015.

  • 2.
    Babaev, Egor
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Carlström, J.
    Silaev, Mihail
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Speight, J. M.
    Type-1.5 superconductivity2017In: Superconductors at the Nanoscale: From Basic Research to Applications, Walter de Gruyter GmbH , 2017, p. 133-164Chapter in book (Other academic)
  • 3.
    Barkman, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Samoilenka, Albert
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Surface Pair-Density-Wave Superconducting and Superfluid States2019In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 16, article id 165302Article in journal (Refereed)
    Abstract [en]

    Fulde, Ferrell, Larkin, and Ovchinnikov (FFLO) predicted inhomogeneous superconducting and superfluid ground states, spontaneously breaking translation symmetries. In this Letter, we demonstrate that the transition from the FFLO to the normal state as a function of temperature or increased Fermi surface splitting is not a direct one. Instead, the system has an additional phase transition to a different state where pair-density-wave superconductivity (or superfluidity) exists only on the boundaries of the system, while the bulk of the system is normal. The surface pair-density-wave state is very robust and exists for much larger fields and temperatures than the FFLO state.

  • 4.
    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.

  • 5.
    Bommer, Jouri D. S.
    et al.
    Delft Univ Technol, QuTech, NL-2600 GA Delft, Netherlands.;Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands..
    Zhang, Hao
    Delft Univ Technol, QuTech, NL-2600 GA Delft, Netherlands.;Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands.;Tsinghua Univ, State Key Lab Low Dimens Quantum Phys, Dept Phys, Beijing 100084, Peoples R China..
    Gul, Onder
    Delft Univ Technol, QuTech, NL-2600 GA Delft, Netherlands.;Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands..
    Nijholt, Bas
    Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands..
    Wimmer, Michael
    Delft Univ Technol, QuTech, NL-2600 GA Delft, Netherlands.;Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands..
    Rybakov, Filipp N.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Garaud, Julien
    Univ Tours, Lab Math & Phys Theor CNRS UMR 7350, Inst Denis Poisson FR2964, Parc Grandmt, F-37200 Tours, France..
    Rodic, Donjan
    Swiss Fed Inst Technol, Inst Theoret Phys, CH-8093 Zurich, Switzerland..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics. KTH Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden..
    Troyer, Matthias
    Swiss Fed Inst Technol, Inst Theoret Phys, CH-8093 Zurich, Switzerland.;Microsoft Quantum, Redmond, WA 98052 USA..
    Car, Diana
    Eindhoven Univ Technol, Dept Appl Phys, NL-5600 MB Eindhoven, Netherlands..
    Plissard, Sebastien R.
    Eindhoven Univ Technol, Dept Appl Phys, NL-5600 MB Eindhoven, Netherlands..
    Bakkers, Erik P. A. M.
    Delft Univ Technol, QuTech, NL-2600 GA Delft, Netherlands.;Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands.;Eindhoven Univ Technol, Dept Appl Phys, NL-5600 MB Eindhoven, Netherlands..
    Watanabe, Kenji
    Natl Inst Mat Sci, Adv Mat Lab, 1-1 Namiki, Tsukuba, Ibaraki 3050044, Japan..
    Taniguchi, Takashi
    Natl Inst Mat Sci, Adv Mat Lab, 1-1 Namiki, Tsukuba, Ibaraki 3050044, Japan..
    Kouwenhoven, Leo P.
    Delft Univ Technol, QuTech, NL-2600 GA Delft, Netherlands.;Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands.;Microsoft Stn Q Delft, NL-2600 GA Delft, Netherlands..
    Spin-Orbit Protection of Induced Superconductivity in Majorana Nanowires2019In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 18, article id 187702Article in journal (Refereed)
    Abstract [en]

    Spin-orbit interaction (SOI) plays a key role in creating Majorana zero modes in semiconductor nanowires proximity coupled to a superconductor. We track the evolution of the induced superconducting gap in InSb nanowires coupled to a NbTiN superconductor in a large range of magnetic field strengths and orientations. Based on realistic simulations of our devices, we reveal SOI with a strength of 0.15-0.35 eV angstrom. Our approach identifies the direction of the spin-orbit field, which is strongly affected by the superconductor geometry and electrostatic gates.

  • 6.
    Diaz-Mendez, Rogelio
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Pupillo, Guido
    Univ Strasbourg, IPCMS, UMR 7504, IcFRC,ISIS,UMR 7006, F-67000 Strasbourg, France.;CNRS, F-67000 Strasbourg, France..
    Mezzacapo, Fabio
    ENS Lyon, CNRS, UMR 5672, Lab Phys, F-69364 Lyon 07, France..
    Wallin, Mats
    KTH, School of Engineering Sciences (SCI), Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Phase-change switching in 2D via soft interactions2019In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 15, no 3, p. 355-358Article in journal (Refereed)
    Abstract [en]

    We present a new type of phase-change behavior relevant for information storage applications, that can be observed in 2D systems with cluster-forming ability. The temperature-based control of the ordering in 2D particle systems depends on the existence of a crystal-to-glass transition. We perform molecular dynamics simulations of models with soft interactions, demonstrating that the crystalline and amorphous structures can be easily tuned by heat pulses. The physical mechanism responsible for this behavior is a self-assembled polydispersity, that depends on the cluster-forming ability of the interactions. Therefore, the range of real materials that can perform such a transition is very wide in nature, ranging from colloidal suspensions to vortex matter. The state of the art in soft matter experimental setups, controlling interactions, polydispersity and dimensionality, makes it a very fertile ground for practical applications.

  • 7.
    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.

  • 8.
    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.

  • 9. Garaud, Julien
    et al.
    Silaev, Mihail
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Microscopically derived multi-component Ginzburg-Landau theories for s plus is superconducting state2017In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 533, p. 63-73Article in journal (Refereed)
    Abstract [en]

    Starting with the generic Ginzburg-Landau expansion from a microscopic N-band model, we focus on the case of a 3-band model which was suggested to be relevant to describe some iron-based superconductors. This can lead to the so-called s + is superconducting state that breaks time-reversal symmetry due to the competition between different pairing channels. Of particular interest in that context, is the case of an interband dominated pairing with repulsion between different bands. For that case we consider in detail the relevant reduced two-component Ginzburg-Landau theory. We provide detailed analysis of the ground state, length scales and topological properties of that model. Prepared for the proceedings of Vortex IX conference in Rhodes (Sept. 2015).

  • 10.
    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.

  • 11. Sellin, Karl
    et al.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Superfluid drag in the two-component Bose-Hubbard modelManuscript (preprint) (Other academic)
    Abstract [en]

    In multicomponent superfluids and superconductors, co- and counter-flows of components have in general different properties. It was discussed in 1975 by Andreev and Bashkin, in the context of He3/He4 superfluid mixtures, that inter-particle interactions produce a dissipationless drag. The drag can be understood as a superflow of one component induced by phase gradients of the other component. Importantly the drag can be both positive (entrainment) and negative (counter-flow). The effect is known to be of crucial importance for many properties of diverse physical systems ranging from the dynamics of neutron stars, rotational responses of Bose mixtures of ultra-cold atoms to magnetic responses of multicomponent superconductors. Although there exists a substantial literature that includes the drag interaction phenomenologically, much fewer regimes are covered by quantitative studies of the microscopic origin of the drag and its dependence on microscopic parameters. Here we study the microscopic origin and strength of the drag interaction in a quantum system of two-component bosons on a lattice with short-range interaction. By performing quantum Monte-Carlo simulations of a two-component Bose-Hubbard model we obtain dependencies of the drag strength on the boson-boson interactions and properties of the optical lattice. Of particular interest are the strongly-correlated regimes where the ratio of co-flow and counter-flow superfluid stiffnesses can diverge, corresponding to the case of saturated drag.

  • 12.
    Sellin, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Superfluid drag in the two-component Bose-Hubbard model2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094517Article in journal (Refereed)
    Abstract [en]

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

  • 13.
    Silaev, Mihail
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Garaud, Julien
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics. KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Phase diagram of dirty two-band superconductors and observability of impurity-induced s plus i s state2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 2, article id 024517Article in journal (Refereed)
    Abstract [en]

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

  • 14.
    Silaev, Mihail
    et al.
    Univ Jyvaskyla, Dept Phys, POB 35 YFL, FI-40014 Jyvaskyla, Finland.;Univ Jyvaskyla, Nanosci Ctr, POB 35 YFL, FI-40014 Jyvaskyla, Finland..
    Winyard, Thomas
    KTH, School of Engineering Sciences (SCI), Physics. Univ Leeds.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Non-London electrodynamics in a multiband London model: Anisotropy-induced nonlocalities and multiple magnetic field penetration lengths2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 17, article id 174504Article in journal (Refereed)
    Abstract [en]

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

  • 15.
    Svistunov, Boris
    et al.
    KTH.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Prokofev, N. V.
    KTH.
    Superfluid states of matter2015Book (Other academic)
    Abstract [en]

    Covers the State of the Art in Superfluidity and Superconductivity Superfluid States of Matter addresses the phenomenon of superfluidity/superconductivity through an emergent, topologically protected constant of motion and covers topics developed over the past 20 years. The approach is based on the idea of separating universal classical-field superfluid properties of matter from the underlying system’s “quanta.” The text begins by deriving the general physical principles behind superfluidity/superconductivity within the classical-field framework and provides a deep understanding of all key aspects in terms of the dynamics and statistics of a classical-field system. It proceeds by explaining how this framework emerges in realistic quantum systems, with examples that include liquid helium, high-temperature superconductors, ultra-cold atomic bosons and fermions, and nuclear matter. The book also offers several powerful modern approaches to the subject, such as functional and path integrals. Comprised of 15 chapters, this text: Establishes the fundamental macroscopic properties of superfluids and superconductors within the paradigm of the classical matter field Deals with a single-component neutral matter field Considers fundamentals and properties of superconductors Describes new physics of superfluidity and superconductivity that arises in multicomponent systems Presents the quantum-field perspective on the conditions under which classical-field description is relevant in bosonic and fermionic systems Introduces the path integral formalism Shows how Feynman path integrals can be efficiently simulated with the worm algorithm Explains why nonsuperfluid (insulating) ground states of regular and disordered bosons occur under appropriate conditions Explores superfluid solids (supersolids) Discusses the rich dynamics of vortices and various aspects of superfluid turbulence at T → 0 Provides account of BCS theory for the weakly interacting Fermi gas Highlights and analyzes the most crucial developments that has led to the current understanding of superfluidity and superconductivity Reviews the variety of superfluid and superconducting systems available today in nature and the laboratory, as well as the states that experimental realization is currently actively pursuing. 

  • 16.
    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.

  • 17.
    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)
  • 18.
    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)
  • 19.
    Winyard, Thomas
    et al.
    KTH, School of Engineering Sciences (SCI), Physics. University of Leeds, Leeds, LS2 9JT, United Kingdom.
    Silaev, Mihail
    Univ Jyvaskyla, Dept Phys, POB 35 YFL, FI-40014 Jyvaskyla, Finland.;Univ Jyvaskyla, Nanosci Ctr, POB 35 YFL, FI-40014 Jyvaskyla, Finland..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Hierarchies of length-scale based typology in anisotropic U(1) s-wave multiband superconductors2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 6, article id 064509Article in journal (Refereed)
    Abstract [en]

    Since Ginzburg and Landau's seminal work in 1950, superconducting states have been classified by the hierarchy of the fundamental length scales of the theory, the magnetic-field penetration lengths and coherence lengths. In the simplest single-component case they form a dimensionless ratio kappa. The model was generalized by Ginzburg for anisotropic materials in 1952. In this paper we expand the above length-scale analysis to anisotropic multicomponent superconductors that can have multiple coherence lengths as well as multiple magnetic-field penetration lengths, leading to unconventional length-scale hierarchies. We demonstrate that the anisotropies in multiband superconductors lead to new regimes with various mixed hierarchies in different directions. For example, a regime is possible, where for a field applied in a certain direction coherence lengths are smaller than the magnetic-field penetration lengths in one of the perpendicular directions, whereas the penetration lengths are larger in the other direction. Focusing on a model of a clean anisotropic multiband s-wave supercocoductors we show exampes of a new regime where vortex cores overlap in one direction, resulting in attractive core-core interaction, while in the orthogonal direction the magnetic-field penetration length exceeds the coherence lengths, leading to dominance of repulsive current-current interaction, resulting in an unconventional magnetic response.

  • 20.
    Winyard, Thomas
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Silaev, Mihail
    Univ Jyvaskyla, Dept Phys, POB 35 YFL, FI-40014 Jyvaskyla, Finland.;Univ Jyvaskyla, Nanosci Ctr, POB 35 YFL, FI-40014 Jyvaskyla, Finland..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics.
    Skyrmion formation due to unconventional magnetic modes in anisotropic multiband superconductors2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 2, article id 024501Article in journal (Refereed)
    Abstract [en]

    Multiband superconductors have a sufficient number of degrees of freedom to allow topological excitations characterized by skyrmionic topological invariants. In the most common, clean s-wave multiband systems, the interband Josephson and magnetic couplings favor composite vortex solutions, without a skyrmionic topological charge. It was discussed recently that certain kinds of anisotropies lead to hybridization of the interband phase difference (Leggett) mode with magnetic modes, dramatically changing the hydromagnetostatics of the system. Here we report this effect for a range of parameters that substantially alter the nature of the topological excitations, leading to solutions characterized by a nontrivial skyrmionic topological charge. The solutions have a form of a coreless texture formed of spatially separated but bound excitations in each band, namely fractional vortices, each carrying a fraction of the flux quantum. We demonstrate that in this regime there is a rich spectrum of skyrmion solutions, with various topological charges, that are robust with respect to changes of parameters of the system and present for a wide range of anisotropies.

  • 21.
    Zyuzin, Alexander
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    Garaud, Julien
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Nematic Skyrmions in Odd-Parity Superconductors2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 16, article id 167001Article in journal (Refereed)
    Abstract [en]

    We study topological excitations in two-component nematic superconductors, with a particular focus on CuxBi2Se3 as a candidate material. We find that the lowest-energy topological excitations are coreless vortices: a bound state of two spatially separated half-quantum vortices. These objects are nematic Skyrmions, since they are characterized by an additional topological charge. The inter-Skyrmion forces are dipolar in this model, i.e., attractive for certain relative orientations of the Skyrmions, hence forming multi-Skyrmion bound states.

1 - 21 of 21
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