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  • 1. Abanin, D. A.
    et al.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    De Tomasi, G.
    Gopalakrishnan, S.
    Khemani, V
    Parameswaran, S. A.
    Pollmann, F.
    Potter, A. C.
    Serbyn, M.
    Vasseur, R.
    Distinguishing localization from chaos: Challenges in finite-size systems2021In: Annals of Physics, ISSN 0003-4916, E-ISSN 1096-035X, Vol. 427, article id 168415Article in journal (Refereed)
    Abstract [en]

    We re-examine attempts to study the many-body localization transition using measures that are physically natural on the ergodic/quantum chaotic regime of the phase diagram. Using simple scaling arguments and an analysis of various models for which rigorous results are available, we find that these measures can be particularly adversely affected by the strong finite-size effects observed in nearly all numerical studies of many-body localization. This severely impacts their utility in probing the transition and the localized phase. In light of this analysis, we discuss a recent study (?untajs et al., 2020) of the behaviour of the Thouless energy and level repulsion in disordered spin

  • 2.
    Aksoy, Omer M.
    et al.
    Paul Scherrer Inst, Condensed Matter Theory Grp, CH-5232 Villigen, Switzerland..
    Chandrasekaran, Anirudh
    Loughborough Univ, Dept Phys, Loughborough LE11 3TU, England..
    Tiwari, Apoorv
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Neupert, Titus
    Univ Zurich, Winterthurerstr 190, CH-8057 Zurich, Switzerland..
    Chamon, Claudio
    Boston Univ, Dept Phys, Boston, MA 02215 USA..
    Mudry, Christopher
    Paul Scherrer Inst, Condensed Matter Theory Grp, CH-5232 Villigen, Switzerland.;EPF Lausanne, Inst Phys, CH-1015 Lausanne, Switzerland..
    Single monkey-saddle singularity of a Fermi surface and its instabilities2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 20, article id 205129Article in journal (Refereed)
    Abstract [en]

    Fermi surfaces can undergo sharp transitions under smooth changes of parameters. Such transitions can have a topological character, as is the case when a higher-order singularity, one that requires cubic or higher-order terms to describe the electronic dispersion near the singularity, develops at the transition. When time-reversal and inversion symmetries are present, odd singularities can only appear in pairs within the Brillouin zone. In this case, the combination of the enhanced density of states that accompanies these singularities and the nesting between the pairs of singularities leads to interaction-driven instabilities. We present examples of single n = 3 (monkeysaddle) singularities when time-reversal and inversion symmetries are broken. We then turn to the question of what instabilities are possible when the singularities are isolated. For spinful electrons, we find that the inclusion of repulsive interactions destroys any isolated monkey-saddle singularity present in the noninteracting spectrum by developing Stoner or Lifshitz instabilities. In contrast, for spinless electrons and at the mean-field level, we show that an isolated monkey-saddle singularity can be stabilized in the presence of short-range repulsive interactions.

  • 3.
    Artiaco, Claudia
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Nava, Andrea
    Univ Calabria, Dipartimento Fis, I-87036 Cosenza, Italy.;Ist Nazl Fis Nucl, Grp collegato Cosenza, I-87036 Cosenza, Italy..
    Fabrizio, Michele
    Scuola Int Super Studi Avanzati SISSA, Via Bonomea 265, I-34136 Trieste, Italy..
    Wetting critical behavior in the quantum Ising model within the framework of Lindblad dissipative dynamics2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 10, article id 104201Article in journal (Refereed)
    Abstract [en]

    We investigate the critical behavior, both in space and time, of the wetting interface within the coexistence region around the first-order phase transition of a fully connected quantum Ising model in slab geometry. For that, we employ the Lindblad master equation formalism in which temperature is inherited by the coupling to a dissipative bath, rather than being a functional parameter as in the conventional Cahn's free energy. Lindblad's approach gives not only access to the dissipative dynamics and steady-state configuration of the quantum wetting interface throughout the whole phase diagram but also shows that the wetting critical behavior can be successfully exploited to characterize the phase diagram as an alternative to the direct evaluation of the free energies of the competing phases.

  • 4.
    Artiaco, Claudia
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Rojas, Rafael Diaz Hernandez
    Sapienza Univ Roma, Dipartimento Fis, Piazzale Aldo Moro 5, I-00185 Rome, Italy..
    Parisi, Giorgio
    Sapienza Univ Roma, Dipartimento Fis, Piazzale Aldo Moro 5, I-00185 Rome, Italy.;INFN, Sez Roma 1, Piazzale Aldo Moro 5, I-00185 Rome, Italy.;CNR Nanotec, unita Roma, Piazzale Aldo Moro 5, I-00185 Rome, Italy..
    Ricci-Tersenghi, Federico
    Sapienza Univ Roma, Dipartimento Fis, Piazzale Aldo Moro 5, I-00185 Rome, Italy.;INFN, Sez Roma 1, Piazzale Aldo Moro 5, I-00185 Rome, Italy.;CNR Nanotec, unita Roma, Piazzale Aldo Moro 5, I-00185 Rome, Italy..
    Hard-sphere jamming through the lens of linear optimization2022In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 106, no 5, article id 055310Article in journal (Refereed)
    Abstract [en]

    The jamming transition is ubiquitous. It is present in granular matter, foams, colloids, structural glasses, and many other systems. Yet, it defines a critical point whose properties still need to be fully understood. Recently, a major breakthrough came about when the replica formalism was extended to build a mean-field theory that provides an exact description of the jamming transition of spherical particles in the infinite-dimensional limit. While such theory explains the jamming critical behavior of both soft and hard spheres, investigating the transition in finite-dimensional systems poses very difficult and different problems, in particular from the numerical point of view. Soft particles are modeled by continuous potentials; thus, their jamming point can be reached through efficient energy minimization algorithms. In contrast, the latter methods are inapplicable to hard-sphere (HS) systems since the interaction energy among the particles is always zero by construction. To overcome these difficulties, here we recast the jamming of hard spheres as a constrained optimization problem and introduce the CALiPPSO algorithm, capable of readily producing jammed HS packings without including any effective potential. This algorithm brings a HS configuration of arbitrary dimensions to its jamming point by solving a chain of linear optimization problems. We show that there is a strict correspondence between the force balance conditions of jammed packings and the properties of the optimal solutions of CALiPPSO, whence we prove analytically that our packings are always isostatic and in mechanical equilibrium. Furthermore, using extensive numerical simulations, we show that our algorithm is able to probe the complex structure of the free-energy landscape, finding qualitative agreement with mean-field predictions. We also characterize the algorithmic complexity of CALiPPSO and provide an open-source implementation of it.

  • 5. Atai, Farrokh
    et al.
    Hallnäs, Martin
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Super-Macdonald Polynomials: Orthogonality and Hilbert Space Interpretation2021In: Communications in Mathematical Physics, ISSN 0010-3616, E-ISSN 1432-0916, Vol. 388, no 1, p. 435-468Article in journal (Refereed)
  • 6. Atai, Farrokh
    et al.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Exact solutions by integrals of the non-stationary elliptic Calogero–Sutherland equation2020In: Journal of Integrable Systems, ISSN 2058-5985, Vol. 5, no 1Article in journal (Refereed)
    Abstract [en]

    We use generalized kernel functions to construct explicit solutions by integrals of the non-stationary Schrödinger equation for the Hamiltonian of the elliptic Calogero–Sutherland model (also known as elliptic Knizhnik–Zamolodchikov–Bernard equation). Our solutions provide integral representations of elliptic generalizations of the Jack polynomials.

  • 7.
    Banerjee, Saikat
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Interacting Dirac Matter2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The discovery of graphene in 2004 has led to a surge of activities focused on the theoretical and experimental studies of materials hosting linearly dispersive quasiparticles during the last decade. Rapid expansion in the list of materials having similar properties to graphene has led to the emergence of a new class of materials known as the Dirac materials. The low energy quasiparticles in this class of materials are described by a Dirac-like equation in contrast to the Schrödinger equation which governs the low energy dynamics in any conventional materials such as metals. The Dirac fermions, as we call these low-energy quasiparticles, in a wide range of materials ranging from the d-wave superconductors, graphene to the surface states of topological insulators share the common property. The particles move around as if they have lost their mass. This feature results in a completely new set of physical effects consisting of various transport and thermodynamic quantities, that are absent in conventional metals.

    This thesis is devoted to studying the properties of bosonic analogs of the commonly known Dirac materials where the quasiparticle are fermionic. In chapter one, we discuss the microscopic origin of the Dirac equation in several fermionic and bosonic systems. We observe identical features of the Dirac materials with quasiparticles of either statistics when the interparticle interaction is absent. Dirac materials with both types of quasiparticles possess the nodal excitations that are described by an effective Dirac-like equation. The possible physical effects due to the linear dispersions in fermionic and bosonic Dirac materials are also outlined.

    In chapter two, we propose a system of superconducting grains arranged in honeycomb lattice as a realization for Bosonic Dirac Materials (BDM). The underlying microscopic dynamics, which give rise to the emergence of Dirac structure in the spectrum of the collective phase oscillations, is discussed in detail. Similarities and differences of BDM systems to the conventional Dirac materials with fermionic quasiparticles are also mentioned. Chapter three is dedicated to the detailed analysis of the interaction effects on the stability and renormalization of the conical Dirac band structure. We find that the type of interaction dictates the possible fate of renormalized Dirac cone in both fermionic and bosonic Dirac materials. We study interaction effects in four different individual systems : (a) Dirac fermions in graphene interacting via Coulomb interactions, (b) Dirac fermions subjected to an onsite Hubbard repulsion, (c) Coulomb repulsion in charged Cooper pairs in honeycomb lattice and (d) Dirac magnons interacting via Heisenberg exchange interaction. The possibility of interaction induced gap opening at the Dirac nodal point described is also discussed in these cases.

    Chapter four mainly concerns the study of a related topic of the synthetic gauge fields. We discuss the possibility of Landau quantization in neutral particles. Possible experimental evidence in toroidal cold atomic traps is also mentioned. A connection to Landau levels in case of magnons is also described. We finally conclude our thesis in chapter five and discuss the possible future directions that can be taken as an extension for our works in interacting Dirac materials.

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    PhD-Thesis-Banerjee
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  • 8.
    Bardarson, Jens H.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Ilan, R.
    Transport in Topological Insulator Nanowires2018In: Springer Series in Solid-State Sciences, Springer Nature , 2018, p. 93-114Chapter in book (Refereed)
    Abstract [en]

    In this chapter, we review our work on the theory of quantum transport in topological insulator nanowires. We discuss both normal state properties and superconducting proximity effects, including the effects of magnetic fields and disorder. Throughout we assume that the bulk is insulating and inert, and work with a surface-only theory. The essential transport properties are understood in terms of three special modes: in the normal state, half a flux quantum along the length of the wire induces a perfectly transmitted mode protected by an effective time-reversal symmetry; a transverse magnetic field induces chiral modes at the sides of the wire, with different chiralities residing on different sides protecting them from backscattering; and finally, Majorana zero modes are obtained at the ends of a wire in a proximity to a superconductor, when combined with a flux along the wire. Some parts of our discussion have a small overlap with the discussion in the review [1]. We do not aim to give a complete review of the published literature, instead the focus is mainly on our own and directly related work.

  • 9.
    Barkman, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Superconducting surfaces, solitons and skyrmions2024Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis focuses on superconductivity, a field within condensed matter physics which since its experimental discovery roughly a century ago, not only has lead to significant contributions revealing the fundamental theories of physics, but also to practical applications. This includes for example quantum vortices, which play paramount roles both in other condensed matter settings, but also in high-energy physics. The dissipationless currents in superconductors are essential to achieve the strong magnetic fields necessary when performing Magnetic Resonant Imaging (MRI). 

    My research on superconductors spans across three topics: superconducting surfaces, multiband superconductivity and inhomogeneous states formed in imbalanced superfluids. A brief introduction and summary of the scientific contribution of this thesis to each of these topics is given below. 

    Bardeen-Cooper-Schrieffer (BCS) theory tells us that from a microscopic perspective, superconductivity is the phenomenon of condensation of bound electron pairs, so-called Cooper pairs. The superconducting state is described by a complex-valued field known as the superconducting gap parameter. In the most simple superconducting materials, where there is only one electronic band, only one complex field is necessary to describe the superconducting state, which spontaneously breaks U(1)-symmetry. In other superconducting materials, such as the iron-based superconductor Ba1−xKxFe2As2, the band structure is more complicated and multiple electronic bands are present. Such multiband superconductors may require multiple complex fields to describe the superconducting state, which can spontaneously break other symmetries, such as time-reversal symmetry, in addition to U(1)-symmetry. 

    Two proposed pairing symmetries for spontaneous time-reversal symmetry breaking (TRSB) spin-singlet superconductors are s+is and s+id. In Paper IV, we demonstrate how magnetic features of pinned domain walls in anisotropic TRSB superconductors can be used to distinguish between s+is and s+id pairing. 

    Classifying topological excitations in superconductors is crucial to understand the superconducting state. For example, quantum vortices are key in understanding the magnetic response of type-II superconductors, and the thermal fluctuations-induced phase transitions in superconductors and superfluids. It has been hypothesized that multiband superconductors, which are described by multiple complex fields, can host topological excitations which are different from the ordinary quantum vortices. Understanding the properties of these new topological excitations carries similar importance to that of ordinary quantum vortices. In Paper VII and Paper VIII, we provide the first microscopic demonstration of multiband fractional vortices and CP2-skyrmions using fully self-consistent Bogoliubov-de Gennes (BdG) theory. Previous demonstrations of such topological excitations have been done using classical field theory approaches, such as Ginzburg-Landau (GL) theory. Our BdG calculations maintain microscopic degrees of freedom which are neglected using GL and quasiclassical theories of superconductivity. 

    The most well-known inhomogeneous superconducting phase is the Abrikosov vortex lattice, which forms in the presence of an external magnetic field in type-II superconductors. Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) proposed another type of inhomogeneous superconducting state, which may form in the presence of a sufficiently large population imbalance between spin up and spin down electrons. The origin of this supersolid state is the formation of Cooper pairs with non-zero net momentum due to spin-dependent Fermi surfaces. In Paper V, we show that spin-imbalanced superfluids can host a unique type of solitons, even before the FFLO regime is entered. These solitons are not present in ordinary uniform superconducting states, and can therefore act as identifiable traces of the FFLO state. 

    The Fulde-Ferrell state and the Larkin-Ovchinnikov state are characterized respectively by modulation in the phase and the density of the superconducting gap parameter. In Paper II, we explored the possibility of other types of inhomogeneous states caused by imbalance in multiband superconductors. Using GL theory, we demonstrated two new types of inhomogeneous states, characterized by spatially alternating chirality and nematicity. 

    Understanding the superconducting properties of surfaces and boundaries is important, both fundamentally to the theory of superconductivity and practically in the construction of superconducting devices. In Paper I and Paper III we demonstrate using both GL and BdG theory that pair-density-wave superconductors support superconducting surface states with critical temperatures larger than the bulk critical temperature. In Paper VI we show increased critical temperatures of superconductor-insulator interfaces. The increase in critical temperature occurs without locally increasing the superconducting pairing strength near the boundaries, or without the introduction of modified surface phonons. 

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    kappa
  • 10.
    Barkman, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Benfenati, Andrea
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Samoilenka, Albert
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Comment on "Surface Pair-Density-Wave Superconducting and Superfluid States" Reply2021In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 126, no 17, article id 179603Article in journal (Other academic)
  • 11.
    Barkman, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Samoilenka, Albert
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Benfenati, Andrea
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Elevated critical temperature at BCS superconductor-band insulator interfaces2022In: Article in journal (Other academic)
    Abstract [en]

    We consider the interface between a BCS superconductor and non-superconducting band insulator. In the simplest example of a one-dimensional lattice model, we show that, under certain conditions, such interfaces can have an elevated superconducting critical temperature, without increasing the strength of pairing interaction at the interface. We identify the regimes where the interface critical temperature exceeds the critical temperature associated with a superconductor-vacuum interface.

  • 12.
    Barkman, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Samoilenka, Albert
    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.
    Elevated critical temperature at BCS superconductor-band insulator interfaces2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 22, article id 224518Article in journal (Refereed)
    Abstract [en]

    We consider the interface between a Bardeen-Cooper-Schrieffer superconductor and nonsuperconducting band insulator. We show that under certain conditions, such interfaces can have an elevated superconducting critical temperature, without increasing the strength of the pairing interaction at the interface. We identify the regimes where the interface critical temperature exceeds the critical temperature associated with a superconductor vacuum interface.

  • 13.
    Barkman, Mats
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Samoilenka, Albert
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Winyard, Thomas
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Ring solitons and soliton sacks in imbalanced fermionic systems2020In: Physical Review Research, E-ISSN 2643-1564, Vol. 2, no 4, article id 043282Article in journal (Refereed)
    Abstract [en]

    We show that in superfluids with fermionic imbalance and uniform ground state, there are stable solitons. These solutions are formed of radial density modulations resulting in nodal rings. We demonstrate that these solitons exhibit nontrivial soliton-soliton and soliton-vortex interactions and can form complicated bound states in the form of "soliton sacks." In a phase-modulating (Fulde-Ferrell) background, we find different solitonic states, in the form of stable vortex-antivortex pairs.

  • 14.
    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.

  • 15.
    Beatrez, William
    et al.
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    Fleckenstein, Christoph
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Pillai, Arjun
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    de Leon Sanchez, Erica
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    Akkiraju, Amala
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    Diaz Alcala, Jesus
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    Conti, Sophie
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    Reshetikhin, Paul
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    Druga, Emanuel
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA.
    Bukov, Marin
    Department of Physics, St Kliment Ohridski University of Sofia, Sofia, Bulgaria; Max Planck Institute for the Physics of Complex Systems, Dresden, Germany.
    Ajoy, Ashok
    Department of Chemistry, University of California, Berkeley, Berkeley, CA, USA; Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
    Critical prethermal discrete time crystal created by two-frequency driving2023In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 19, no 3, p. 407-413Article in journal (Refereed)
    Abstract [en]

    Discrete time crystals are non-equilibrium many-body phases of matter characterized by spontaneously broken discrete time-translation symmetry under periodic driving. At sufficiently high driving frequencies, the system enters the Floquet prethermalization regime, in which the periodically driven many-body state has a lifetime vastly exceeding the intrinsic decay time of the system. Here, we report the observation of long-lived prethermal discrete time-crystalline order in a three-dimensional (3D) lattice of 13C nuclei in diamond at room temperature. We demonstrate a two-frequency driving protocol, involving an interleaved application of slow and fast drives that simultaneously prethermalize the spins with an emergent quasi-conserved magnetization along the x̂ axis. This enables continuous and highly resolved observation of their dynamic evolution. We obtain videos of the time-crystalline response with a clarity and throughput orders of magnitude greater than previous experiments. Parametric control over the drive frequencies allows us to reach time-crystal lifetimes of up to 396 Floquet cycles, which we measure in a single-shot experiment. Such rapid measurement enables detailed characterization of the entire phase diagram, highlighting the role of prethermalization in stabilizing the time-crystal response. The two-frequency drive approach expands the toolkit for investigating non-equilibrium phases of matter stabilized by emergent quasi-conservation laws.

  • 16. Behrends, J.
    et al.
    Ilan, R.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Anomalous conductance scaling in strained Weyl semimetals2019In: Physical Review Research, E-ISSN 2643-1564, Vol. 1, no 3, article id 032028Article in journal (Refereed)
    Abstract [en]

    Magnetotransport provides key experimental signatures in Weyl semimetals. The longitudinal magnetoresistance is linked to the chiral anomaly and the transversal magnetoresistance to the dominant charge relaxation mechanism. Axial magnetic fields that act with opposite sign on opposite chiralities facilitate new transport experiments that probe the low-energy Weyl nodes. As recently realized, these axial fields can be achieved by straining samples or adding inhomogeneities to them. Here, we identify a robust signature of axial magnetic fields: an anomalous scaling of the conductance in the diffusive ultraquantum regime. In particular, we demonstrate that the longitudinal conductivity in the ultraquantum regime of a disordered Weyl semimetal subjected to an axial magnetic field increases with both the field strength and sample width due to a spatial separation of charge carriers. We contrast axial magnetic with real magnetic fields to clearly distinguish the different behavior of the conductance. Our results rely on numerical tight-binding simulations and are supported by analytical arguments. We argue that the spatial separation of charge carriers can be used for directed currents in microstructured electronic devices.

  • 17. 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.

  • 18.
    Behrends, Jan
    et al.
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany.;Univ Cambridge, Cavendish Lab, TCM Grp, JJ Thomson Ave, Cambridge CB3 0HE, England..
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Beri, Benjamin
    Univ Cambridge, Cavendish Lab, TCM Grp, JJ Thomson Ave, Cambridge CB3 0HE, England.;Univ Cambridge, DAMTP, Wilberforce Rd, Cambridge CB3 0WA, England..
    Tenfold way and many-body zero modes in the Sachdev-Ye-Kitaev model2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 19, article id 195123Article in journal (Refereed)
    Abstract [en]

    The Sachdev-Ye-Kitaev (SYK) model, in its simplest form, describes k Majorana fermions with random all-to-all four-body interactions. We consider the SYK model in the framework of a many-body Altland-Zirnbauer classification that sees the system as belonging to one of eight (real) symmetry classes depending on the value of k mod 8. We show that, depending on the symmetry class, the system may support exact many-body zero modes with the symmetries also dictating whether these may have a nonzero contribution to Majorana fermions, i.e., single-particle weight. These zero modes appear in all but two of the symmetry classes. When present, they leave clear signatures in physical observables that go beyond the threefold (Wigner-Dyson) possibilities for level spacing statistics studied earlier. Signatures we discover include a zero-energy peak or hole in the single-particle spectral function, depending on whether symmetries allow or forbid zero modes to have single-particle weight. The zero modes are also shown to influence the many-body dynamics, where signatures include a nonzero long-time limit for the out-of-time-order correlation function. Furthermore, we show that the extension of the four-body SYK model by quadratic terms can be interpreted as realizing the remaining two complex symmetry classes; we thus demonstrate how the entire tenfold Altland-Zirnbauer classification may emerge in the SYK model.

  • 19. 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.

  • 20.
    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.

  • 21.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Lukinov, Timofei
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Fu, Jie
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Zhao, Jijun
    Davis, Sergio
    Simak, Sergei I.
    Stabilization of body-centred cubic iron under inner-core conditions2017In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 10, no 4, p. 312-+Article in journal (Refereed)
    Abstract [en]

    The Earth's solid core is mostly composed of iron. However, despite being central to our understanding of core properties, the stable phase of iron under inner-core conditions remains uncertain. The two leading candidates are hexagonal close-packed and body-centred cubic (bcc) crystal structures, but the dynamic and thermodynamic stability of bcc iron under inner-core conditions has been challenged. Here we demonstrate the stability of the bcc phase of iron under conditions consistent with the centre of the core using ab initio molecular dynamics simulations. We find that the bcc phase is stabilized at high temperatures by a diffusion mechanism that arises due to the dynamical instability of the phase at lower temperatures. On the basis of our simulations, we reinterpret experimental data as support for the stability of bcc iron under inner-core conditions. We suggest that the diffusion of iron atoms in solid state may explain both the anisotropy and the low shear modulus of the inner core.

  • 22.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Fu, Jie
    Ningbo Univ, Dept Phys, Fac Sci, Ningbo 315211, Zhejiang, Peoples R China..
    Bryk, Taras
    Natl Acad Sci Ukraine, Inst Condensed Matter Phys, UA-79011 Lvov, Ukraine..
    Simak, Sergei, I
    Linkoping Univ, Dept Phys Chem & Biol IFM, SE-58183 Linkoping, Sweden..
    Mattesini, Maurizio
    Univ Complutense Madrid, Dept Earths Phys & Astrophys, E-28040 Madrid, Spain.;UCM, CSIC, Fac Ciencias Fis, Inst Geociencias, Plaza Ciencias 1, Madrid 28040, Spain..
    Low viscosity of the Earth's inner core2019In: Nature Communications, E-ISSN 2041-1723, Vol. 10, article id 2483Article in journal (Refereed)
    Abstract [en]

    The Earth's solid inner core is a highly attenuating medium. It consists mainly of iron. The high attenuation of sound wave propagation in the inner core is at odds with the widely accepted paradigm of hexagonal close-packed phase stability under inner core conditions, because sound waves propagate through the hexagonal iron without energy dissipation. Here we show by first-principles molecular dynamics that the body-centered cubic phase of iron, recently demonstrated to be thermodynamically stable under the inner core conditions, is considerably less elastic than the hexagonal phase. Being a crystalline phase, the body-centered cubic phase of iron possesses the viscosity close to that of a liquid iron. The high attenuation of sound in the inner core is due to the unique diffusion characteristic of the body-centered cubic phase. The low viscosity of iron in the inner core enables the convection and resolves a number of controversies.

  • 23.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Natl Res Univ Higher Sch Econ, Moscow 123458, Russia..
    Fu, Jie
    Ningbo Univ, Sch Phys Sci & Technol, Dept Phys, Ningbo 315211, Peoples R China..
    Smirnov, Grigory
    Natl Res Univ Higher Sch Econ, Moscow 123458, Russia.;Russian Acad Sci, Joint Inst High Temp, Moscow 125412, Russia..
    Free energies of iron phases at high pressure and temperature: Molecular dynamics study2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 10, article id 104103Article in journal (Refereed)
    Abstract [en]

    The crystal structure of iron, the major component of the Earth's inner core (IC), is unknown under the IC high pressure (P) (3.3-3.6 Mbar) and temperature (T) (5000-7000 K). Experimental and theoretical data on the phase diagram of iron at these extreme PT conditions are contradictory. Applying quasi-ab initio and ab initio molecular dynamics we computed free energies of the body-centered cubic (bcc), hexagonal close-packed (hcp), and liquid phases. The ionic free energies, computed for the embedded-atom model, were corrected for electronic entropy. Such correction brings the melting temperatures of the hcp iron in very good agreement with previous ab initio data. This validates the calculation of the bcc phase, where fully ab initio treatment is not technically possible due to large sizes required for convergence. The resulting phase diagram shows stabilization of the bcc phase prior to melting in the pressure range of the IC. The melting temperature of the bcc phase is equal to 7190 K at the pressure 360 GPa.

  • 24.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Simak, S. I.
    Olovsson, W.
    Vekilova, O. Y.
    Elastic properties of body-centered cubic iron in Earth's inner core2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 18, article id L180102Article in journal (Refereed)
    Abstract [en]

    The solid Earth's inner core (IC) is a sphere with a radius of about 1300 km in the center of the Earth. The information about the IC comes mainly from seismic studies. The composition of the IC is obtained by matching the seismic data and properties of candidate phases subjected to high pressure (P) and temperature (T). The close match between the density of the IC and iron suggests that the main constituent of the IC is iron. However, the stable phase of iron is still a subject of debate. One such iron phase, the body-centered cubic phase (bcc), is dynamically unstable at pressures of the IC (330-364 GPa) and low T but gets stabilized at high T characteristic of the IC (5000-7000 K). So far, ab initio molecular dynamics (AIMD) studies attempted to compute the bcc elastic properties for a small (order of 102) number of atoms. The mechanism of the bcc stabilization cannot be enabled in such cells and that has led to erroneous results. Here we apply AIMD to compute elastic moduli and sound velocities of the Fe bcc phase for a 2000 Fe atom computational cell, which is a cell of unprecedented size for ab initio calculations of iron. Unlike in previous ab initio calculations, both the longitudinal and the shear sound velocities of the Fe bcc phase closely match the properties of the IC material at P = 360 GPa and T = 6600 K, likely the PT conditions in the IC. The calculated density of the bcc iron at these PT conditions is just 3% higher than the density of the IC material according to the Preliminary Earth Model. This suggests that the widely assumed amount of light elements in the IC may need a reconsideration. The anisotropy of the bcc phase is an exact match to the most recent seismic studies. 

  • 25.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Nanjing Univ, Frontiers Sci Ctr Crit Earth Mat Cycling, Sch Earth Sci & Engn, Nanjing 210023, Peoples R China; HSE Univ, Moscow Inst Elect & Math, Int Lab Supercomp Atomist Modelling & Multiscale A, Moscow 123458, Russia.; Univ S Florida, Dept Phys, Tampa, FL 33620 USA..
    Smirnov, Grigory S. S.
    HSE Univ, Moscow Inst Elect & Math, Int Lab Supercomp Atomist Modelling & Multiscale A, Moscow 123458, Russia.;Russian Acad Sci, Joint Inst High Temp, Moscow 125412, Russia..
    A Comparison of Experimental and Ab Initio Structural Data on Fe under Extreme Conditions2023In: Metals, ISSN 2075-4701, Vol. 13, no 6, article id 1096Article in journal (Refereed)
    Abstract [en]

    Iron is the major element of the Earth's core and the cores of Earth-like exoplanets. The crystal structure of iron, the major component of the Earth's solid inner core (IC), is unknown under the high pressures (P) (3.3-3.6 Mbar) and temperatures (T) (5000-7000 K) and conditions of the IC and exoplanetary cores. Experimental and theoretical data on the phase diagram of iron at these extreme PT conditions are contradictory. Though some of the large-scale ab initio molecular dynamics (AIMD) simulations point to the stability of the body-centered cubic (bcc) phase, the latest experimental data are often interpreted as evidence for the stability of the hexagonal close-packed (hcp) phase. Applying large-scale AIMD, we computed the properties of iron phases at the experimental pressures and temperatures reported in the experimental papers. The use of large-scale AIMD is critical since the use of small bcc computational cells (less than approximately 1000 atoms) leads to the collapse of the bcc structure. Large-scale AIMD allowed us to compare the measured and computed coordination numbers as well as the measured and computed structural factors. This comparison, in turn, allowed us to suggest that the computed density, coordination number, and structural factors of the bcc phase are in agreement with those observed in experiments, which were previously assigned either to the liquid or hcp phase.

  • 26.
    Benfenati, Andrea
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Spontaneous edge and corner currents in s+is superconductors and time reversal symmetry breaking surface states2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 13, p. 134518-, article id PhysRevB.105.134518Article in journal (Refereed)
    Abstract [en]

    We present a study of the basic microscopic model of a s-wave superconductor with frustrated interbandinteraction. When frustration is strong, such an interaction gives raise to a s + is state. This is a s-wave superconductor that spontaneously breaks time reversal symmetry. We show that in addition to the known s + is state,there is additional phase where the system’s bulk is a conventional s-wave state, but superconducting surfacestates break time reversal symmetry. Furthermore, we show that s + is superconductors can have spontaneousboundary currents and spontaneous magnetic fields. These arise at lower-dimensional boundaries, namely, thecorners in two-dimensional samples. This demonstrates that boundary currents effects in superconductors canarise in states which are not topological and not chiral according to the modern classification.

  • 27.
    Benfenati, Andrea
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Barkman, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Demonstration of CP2 skyrmions in three-band superconductors by self-consistent solutions of a Bogoliubov-de Gennes model2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 9, article id 094503Article in journal (Refereed)
    Abstract [en]

    Topological defects, such as magnetic-flux-carrying quantum vortices, determine the magnetic response of superconductors and hence are of fundamental importance. Here, we show that stable CP2 skyrmions exist in three-band s + is superconductors as fully self-consistent solutions to a microscopic Bogoliubov-de Gennes model. This allows us to calculate microscopically the magnetic signatures of CP2 skyrmions and their footprint in the local density of states.

  • 28.
    Benfenati, Andrea
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Barkman, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Demonstration of CP2 skyrmions in three-band superconductors by self-consistent solutions to a Bogoliubov-de Gennes model2022In: Article in journal (Other academic)
    Abstract [en]

    Topological defects, such as magnetic-flux-carrying quantum vortices determine the magnetic response of superconductors and hence are of fundamental importance. Here, we show that stable CP2 skyrmions exist in three-band s+is superconductors as fully self-consistent solutions to a microscopic Bogoluibov-de Gennes model. This allows us to calculate microscopically the magnetic signatures of CP2 skyrmions and their footprint in the local density of states.

  • 29.
    Benfenati, Andrea
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Barkman, Mats
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Winyard, Thomas
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Wormald, Alex
    School of Mathematics, University of Leeds, Leeds, LS2 9JT, United Kingdom.
    Speight, Martin
    Univ Leeds, Sch Math, Leeds LS2 9JT, W Yorkshire, England..
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Magnetic signatures of domain walls in s plus is and s plus id superconductors: Observability and what that can tell us about the superconducting order parameter2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 5, article id 054507Article in journal (Refereed)
    Abstract [en]

    One of the defining features of spontaneously broken time-reversal symmetry (BTRS) is the existence of domain walls, the detection of which would be strong evidence for such systems. There is keen interest in BTRS currently, in part, due to recent muon spin rotation experiments, which have pointed towards Ba1-xKxFe2As2 exhibiting a remarkable case of s-wave superconductivity with spontaneously broken time-reversal symmetry. A key question, however, is how to differentiate between the different theoretical models which describe such a state. Two particularly popular choices of model are s + is and s + id superconducting states. In this paper, we obtain solutions for domain walls in s + is and s + id systems, including the effects of lattice anisotropies. We show that, in general, both models exhibit spontaneous magnetic fields that extend along the entire length of the domain wall. We demonstrate the qualitative difference between the magnetic signatures of s + is and s + id domain walls and propose a procedure to extract the superconducting pairing symmetry from the magnetic-field response of domain walls.

  • 30.
    Benfenati, Andrea Ludovico
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Numerical solutions to non-linear inhomogeneous problems in Superconductivity: From sphalerons to multi-band boundary states and spontaneous magnetic fields2022Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is a compilation of theoretical works focused on simulating and studying open questions regarding single and multiband superconductivity. In the last decades, with the discovery of multiband superconductors, the spectrum of potential applications has greatly widened. Superconductors are not only employed to realize dissipationless current carrying devices, but are used to construct quantum-based measurement instruments, such as single photon detectors as well as superconducting qubits. The properties of superconductors, as critical temperatures and vortex nucleation barriers are of key importance for applications, and still poorly understood. They are strongly affected by the physics of the boundaries, as well as by the sample's geometry and by the presence of impurities. The open questions can be answered with new theoretical methods, which can then guide and optimize the construction process of superconducting devices, which constitutes a crucial challenge today. 

    There are several models that can be utilized to describe superconductors, from the microscopic Bardeen Cooper Schrieffer theory, up to the macroscopic Ginzburg Landau models. Each of these theories carries advantages and limitations, making it impossible to rely only on a specific model. In this thesis we utilize microscopic and macroscopic models to answer the following questions:

    • How can we determine the free energy barriers to vortex nucleation in single band and multiband superconductors without relying on uncontrolled approximations?
    • What are the properties of the superconducting states which spontaneously break time reversal symmetry?
    • How do boundaries and interfaces influence the critical temperatures of superconductors?

    We answer these questions in eight papers, which we shortly summarize in the following. 

    In Paper 1, Magnetic signatures of domain walls in s+is and s+id superconductors: Observability and what that can tell us about the superconducting order parameter, we consider an effective two-band anisotropic Ginzburg Landau model, describing a superconductor breaking time reversal symmetry. There is high interest on spontaneous time reversal symmetry breaking due to recent muon-spin rotation experiments, claiming to measure spontaneous magnetic field in Fe-based superconductors such as Ba1-xKxFe2As2. However, the symmetry of the superconducting order parameters remains undetermined, and the most promising candidates are s+is and s+id states. In the work, we obtain solutions for domain walls within the Ginzburg Landau model.  By studying the spontaneous magnetic signatures of domain walls, having different orientations with respect to the crystalline axes, for both s+is and s+id order parameters, we demonstrate their differences and propose a procedure to infer the order parameter's symmetry from magnetic field measurements.

    In Paper 2, Vortex nucleation barrier in superconductors beyond the Bean-Livingston approximation: A numerical approach for the sphaleron problem in a gauge theory, we address the long standing problem of calculating the energy barriers for the vortex nucleation in a superconductor. The only available tool to do so, was the Bean-Livingston theory, which relies on uncontrollable approximations. This does not allow to take into account the non-linear nature of the Ginzburg Landau model, or the presence of impurities and surface roughness. Therefore, we develop the gauged string method, a gauge invariant numerical framework, based on the simplified string method, which enables us to accurately compute the minimum free energy path for the vortex nucleation. Moreover, we present a study of how the nucleation energy barrier changes in the presence of impurities and surface roughness. 

    In Paper 3, Vortex nucleation barriers and stable fractional vortices near boundaries in multicomponent superconductors, we extend the gauged string method to multiband superconductors, where the energy landscape is much broader than in the single band case, and the number of possible processes is higher. In multiband superconductors the topological excitations are fractional vortices, which once bounded, form composite vortices. Fractional vortices are energetically unfavorable, as they are associated to an energy cost which scales logarithmically with the system size. Once they bind and form a composite vortex, the extra energy cost is canceled. However, it was previously shown in the London model that fractional vortices can be stabilized near boundaries. In this paper, we study the energy barriers for the nucleation of fractional vortices, and for the formation composite vortices. Moreover, we show how the presence of anisotropies can influence such barriers. Then we study how the same processes are influenced by the interband Josephson interactions. By using the gauged string method, we demonstrate how the fractionalized nucleation process results in multiple saddle points and intermediate metastable configurations.

    In Paper 4, Boundary effects in two-band superconductors, we study microscopically the behavior of the superconducting order parameters near the boundaries of a two-band s-wave superconductor. We describe the system using a tight binding Bardeen Cooper Schrieffer model with interband interaction. We show the existence of surface states, and calculate how the difference between bulk and surface critical temperatures depends on the strength of the interband coupling. Then, we focus the analysis on weak interband interactions to show, at the level of a fully microscopic theory, how the variations of the gaps near the boundaries occur with multiple length scales. 

    In Paper 5, Spontaneous edge and corner currents in s+is superconductors and time-reversal-symmetry-breaking surface states, we consider a three band superconductor, described with a microscopic tight binding Bardeen Cooper Schrieffer model with interband interaction. In the current classification scheme, an s+is state is a non-topological and non-chiral state, which does not exhibit topological surface states and therefore no spontaneous surface currents. In the article, we consider a system where the three bands have slightly different intraband pairing potential but the same interband coupling, resulting in slightly asymmetric bands. We show that, as the temperature is increased, the state which spontaneously break time reversal symmetry becomes localized near the sample boundaries, and generate spontaneous magnetic signatures. Finally, we show how, by changing the sample geometry, the magnetic signatures can be enhanced. We underline that, this phenomenon is not a general property of time reversal symmetry breaking states, but can account for the presence of spontaneous magnetic fields in s+is superconductors and cannot be predicted using the macroscopic Ginzburg Landau theory. Moreover, the paper shows that spontaneous surface currents can arise for non-topological reasons.

    In Paper 6, Demonstration of CP2 skyrmions in three-band superconductors by self-consistent solutions to a Bogoliubov-de-Gennes model, we continue the study of three component s+is superconductors, described using a microscopic tight binding Bardeen Cooper Schrieffer model. In this work, we consider three symmetric bands, and we study the CP2 skyrmionic topological excitations of the system. We present not only the configurations of the superconducting order parameters, but also the respective magnetic field and density of states. Moreover, the simulation of CP2 skyrmions in superconductors, described a with fully microscopic model, had not been done before. In the context of superconductivity, CP2 skyrmion solutions were previously described only within the phenomenological macroscopic Ginzburg-Landau theory.

    In Paper 7, Pair-density-wave superconductivity of faces, edges, and vertices in systems with imbalanced fermions we analyze the boundary effects in superconductors exhibiting Fulde-Ferrell-Larkin-Ovchinnikov states. We do so by employing and comparing Bogoliubov-de-Gennes and Ginzburg Landau formalisms. We show that, within the Ginzburg Landau theory, in a three dimensional superconductor, there is a sequence of phase transitions as the temperature increases. Then, we perform the same sequence of simulations for two dimensional samples described using the Bogoliubov-de-Gennes formalism, showing the same sequence of phase transitions.

    In Paper 8, Elevated critical temperature at BCS superconductor-band insulator interfaces, we study the physics of interfaces between a superconductor, described using a tight-binding mean field Hamiltonian, and a band insulator. We limit the study to one-dimensional samples and demonstrate that, within certain parameter ranges, it is indeed possible to enhance the critical temperature in the vicinity of the interface. This occurs without changing the strength of the superconducting-pairing interaction. Then we present the parameters regimes in which the near-interface critical temperature exceeds the critical temperature of a conventional superconductor-vacuum interface.

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  • 31.
    Benfenati, Andrea
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Maiani, Andrea
    KTH, School of Engineering Sciences (SCI).
    Rybakov, Filipp N.
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Statistical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Vortex nucleation barrier in superconductors beyond the Bean-Livingston approximation: A numerical approach for the sphaleron problem in a gauge theory2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 22, article id 220505Article in journal (Refereed)
    Abstract [en]

    The knowledge of vortex nucleation barriers is crucial for applications of superconductors, such as single-photon detectors and superconductor-based qubits. Contrarily to the problem of finding energy minima and critical fields, there are no controllable methods to explore the energy landscape, identify saddle points, and compute associated barriers. Similar problems exist in high-energy physics where the saddle-point configurations are called sphalerons. Here, we present a generalization of the string method to gauge field theories, which allows the calculation of energy barriers in superconductors. We solve the problem of vortex nucleation, assessing the effects of the nonlinearity of the model, complicated geometry, surface roughness, and pinning.

  • 32.
    Benfenati, Andrea
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Samoilenka, Albert
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Boundary effects in two-band superconductors2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 103, no 14, article id 144512Article in journal (Refereed)
    Abstract [en]

    We present a microscopic study of the behavior of the order parameters near the boundaries of a two-band superconducting material, described by the standard tight-binding Bardeen-Cooper-Schrieffer model. We find superconducting surface states. The relative difference between bulk and surface critical temperatures is a nontrivial function of the interband coupling strength. For superconductors with weak interband coupling, boundaries induce variations of the gaps with the presence of multiple length scales, despite nonzero interband Josephson coupling.

  • 33. Bera, Soumya
    et al.
    Martynec, Thomas
    Schomerus, Henning
    Heidrich-Meisner, Fabian
    Hjörleifur Bardarson, Jens
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max-Planck-Institut für Physik komplexer Systeme, Germany.
    One-particle density matrix characterization of many-body localization2017In: Annalen der Physik, ISSN 0003-3804, E-ISSN 1521-3889, Vol. 529, no 7, article id 1600356Article in journal (Refereed)
    Abstract [en]

    We study interacting fermions in one dimension subject to random, uncorrelated onsite disorder, a paradigmatic model of many-body localization (MBL). This model realizes an interaction-driven quantum phase transition between an ergodic and a many-body localized phase, with the transition occurring in the many-body eigenstates. We propose a single-particle framework to characterize these phases by the eigenstates (the natural orbitals) and the eigenvalues (the occupation spectrum) of the one-particle density matrix (OPDM) in individual many-body eigenstates. As a main result, we find that the natural orbitals are localized in the MBL phase, but delocalized in the ergodic phase. This qualitative change in these single-particle states is a many-body effect, since without interactions the single-particle energy eigenstates are all localized. The occupation spectrum in the ergodic phase is thermal in agreement with the eigenstate thermalization hypothesis, while in the MBL phase the occupations preserve a discontinuity at an emergent Fermi edge. This suggests that the MBL eigenstates are weakly dressed Slater determinants, with the eigenstates of the underlying Anderson problem as reference states. We discuss the statistical properties of the natural orbitals and of the occupation spectrum in the two phases and as the transition is approached. Our results are consistent with the existing picture of emergent integrability and localized integrals of motion, or quasiparticles, in the MBL phase. We emphasize the close analogy of the MBL phase to a zero-temperature Fermi liquid: in the studied model, the MBL phase is adiabatically connected to the Anderson insulator and the occupation-spectrum discontinuity directly indicates the presence of quasiparticles localized in real space. Finally, we show that the same picture emerges for interacting fermions in the presence of an experimentally-relevant bichromatic lattice and thereby demonstrate that our findings are not limited to a specific model.

  • 34.
    Bergqvist, Lars
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, Olle
    Bergman, Anders
    Hellsvik, Johan
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Atomistic Spin Dynamics: Foundations and Applications2017Book (Other academic)
  • 35.
    Berntson, Bjorn K.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Fagerlund, Alexander
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    A focusing–defocusing intermediate nonlinear Schrödinger system2023In: Physica D: Non-linear phenomena, ISSN 0167-2789, E-ISSN 1872-8022, Vol. 451, article id 133762Article in journal (Refereed)
    Abstract [en]

    We introduce and study a system of coupled nonlocal nonlinear Schrödinger equations that interpolates between the mixed, focusing–defocusing Manakov system on one hand and a limiting case of the intermediate nonlinear Schrödinger equation on the other. We show that this new system, which we call the intermediate mixed Manakov (IMM) system, admits multi-soliton solutions governed by a complexification of the hyperbolic Calogero–Moser (CM) system. Furthermore, we introduce a spatially periodic version of the IMM system, for which our main result is a class of exact solutions governed by a complexified elliptic CM system.

  • 36.
    Berntson, Bjorn K.
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.).
    Klabbers, Rob
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. ;Stockholm Univ, SE-10691 Stockholm, Sweden..
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Multi-solitons of the half-wave maps equation and Calogero-Moser spin-pole dynamics2020In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 53, no 50, article id 505702Article in journal (Refereed)
    Abstract [en]

    We consider the half-wave maps (HWM) equation which provides a continuum description of the classical Haldane-Shastry spin chain on the real line. We present exact multi-soliton solutions of this equation. Our solutions describe solitary spin excitations that can move with different velocities and interact in a non-trivial way. We make an ansatz for the solution allowing for an arbitrary number of solitons, each described by a pole in the complex plane and a complex spin variable, and we show that the HWM equation is satisfied if these poles and spins evolve according to the dynamics of an exactly solvable spin Calogero-Moser (CM) system with certain constraints on initial conditions. We also find first order equations providing a Backlund transformation of this spin CM system, generalize our results to the periodic HWM equation, and provide plots that visualize our soliton solutions.

  • 37.
    Berntson, Bjorn K.
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.).
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Lenells, Jonatan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematics (Div.).
    Nonchiral intermediate long-wave equation and interedge effects in narrow quantum Hall systems2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 15, article id 155308Article in journal (Refereed)
    Abstract [en]

    We present a nonchiral version of the intermediate long-wave (ILW) equation that can model nonlinear waves propagating on two opposite edges of a quantum Hall system, taking into account interedge interactions. We obtain exact soliton solutions governed by the hyperbolic Calogero-Moser-Sutherland (CMS) model, and we give a Lax pair, a Hirota form, and conservation laws for this new equation. We also present a periodic nonchiral ILW equation, together with its soliton solutions governed by the elliptic CMS model.

  • 38.
    Berntson, Björn K.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Stockholm Univ, S-10691 Stockholm, Sweden..
    Lenells, Jonatan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematics (Div.).
    Spin generalizations of the Benjamin-Ono equation2022In: Letters in Mathematical Physics, ISSN 0377-9017, E-ISSN 1573-0530, Vol. 112, no 3, article id 50Article in journal (Refereed)
    Abstract [en]

    We present new soliton equations related to the A-type spin Calogero-Moser (CM) systems introduced by Gibbons and Hermsen. These equations are spin generalizations of the Benjamin-Ono (BO) equation and the recently introduced non-chiral intermediate long-wave (ncILW) equation. We obtain multi-soliton solutions of these spin generalizations of the BO equation and the ncILW equation via a spin-pole ansatz where the spin-pole dynamics is governed by the spin CM system in the rational and hyperbolic cases, respectively. We also propose physics applications of the new equations, and we introduce a spin generalization of the standard intermediate long-wave equation which interpolates between the matrix Korteweg-de Vries equation, the Heisenberg ferromagnet equation, and the spin BO equation.

  • 39.
    Bhardwaj, Lakshya
    et al.
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK.
    Bottini, Lea E.
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK.
    Schäfer-Nameki, Sakura
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK.
    Tiwari, Apoorv
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Non-invertible higher-categorical symmetries2023In: SciPost Physics, E-ISSN 2542-4653, Vol. 14, no 1, article id 007Article in journal (Refereed)
    Abstract [en]

    We sketch a procedure to capture general non-invertible symmetries of a d-dimensional quantum field theory in the data of a higher-category, which captures the local properties of topological defects associated to the symmetries. We also discuss fusions of topological defects, which involve condensations/gaugings of higher-categorical symmetries localized on the worldvolumes of topological defects. Recently some fusions of topological defects were discussed in the literature where the dimension of topological defects seems to jump under fusion. This is not possible in the standard description of higher-categories. We explain that the dimension-changing fusions are understood as higher-morphisms of the higher-category describing the symmetry. We also discuss how a 0-form sub-symmetry of a higher-categorical symmetry can be gauged and describe the higher-categorical symmetry of the theory obtained after gauging. This provides a procedure for constructing non-invertible higher-categorical symmetries starting from invertible higher-form or higher-group symmetries and gauging a 0-form symmetry. We illustrate this procedure by constructing non-invertible 2-categorical symmetries in 4d gauge theories and non-invertible 3-categorical symmetries in 5d and 6d theories. We check some of the results obtained using our approach against the results obtained using a recently proposed approach based on’t Hooft anomalies.

  • 40.
    Bhardwaj, Lakshya
    et al.
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK, Andrew-Wiles Building, Woodstock Road.
    Bottini, Lea E.
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK, Andrew-Wiles Building, Woodstock Road.
    Schäfer-Nameki, Sakura
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK, Andrew-Wiles Building, Woodstock Road.
    Tiwari, Apoorv
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Non-invertible symmetry webs2023In: SciPost Physics, E-ISSN 2542-4653, Vol. 15, no 4, article id 160Article in journal (Refereed)
    Abstract [en]

    Non-invertible symmetries have by now seen numerous constructions in higher dimensional Quantum Field Theories (QFT). In this paper we provide an in depth study of gauging 0-form symmetries in the presence of non-invertible symmetries. The starting point of our analysis is a theory with G 0-form symmetry, and we propose a description of sequential partial gaugings of sub-symmetries. The gauging implements the theta-symmetry defects of the companion paper [1]. The resulting network of symmetry structures related by this gauging will be called a non-invertible symmetry web. Our formulation makes direct contact with fusion 2-categories, and we uncover numerous interesting structures such as symmetry fractionalization in this categorical setting. The complete symmetry web is derived for several groups G, and we propose extensions to higher dimensions. The highlight of this analysis is the complete categorical symmetry web, including non-invertible symmetries, for 3d pure gauge theories with orthogonal gauge groups and its extension to arbitrary dimensions.

  • 41.
    Bhardwaj, Lakshya
    et al.
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK, Andrew-Wiles Building, Woodstock Road.
    Schäfer-Nameki, Sakura
    Mathematical Institute, University of Oxford, Andrew-Wiles Building, Woodstock Road, Oxford, OX2 6GG, UK, Andrew-Wiles Building, Woodstock Road.
    Tiwari, Apoorv
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Unifying constructions of non-invertible symmetries2023In: SciPost Physics, E-ISSN 2542-4653, Vol. 15, no 3, article id 122Article in journal (Refereed)
    Abstract [en]

    In the past year several constructions of non-invertible symmetries in Quantum Field Theory in d ≥ 3 have appeared. In this paper we provide a unified perspective on these constructions. Central to this framework are so-called theta defects, which generalize the notion of theta-angles, and allow the construction of universal and non-universal topological symmetry defects. We complement this physical analysis by proposing a mathematical framework (based on higher-fusion categories) that converts the physical construction of non-invertible symmetries into a concrete computational scheme.

  • 42. Biswas, P. K.
    et al.
    Rybakov, Filipp N.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Singh, R. P.
    Mukherjee, Saumya
    Parzyk, N.
    Balakrishnan, G.
    Lees, M. R.
    Dewhurst, C. D.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics.
    Hillier, A. D.
    Paul, D. Mc K.
    Coexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 14, article id 144523Article in journal (Refereed)
    Abstract [en]

    Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example, at different temperatures, is rare in nature. We employed the muon spin rotation (mu SR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most important, we found a region showing unusual behavior where the usually mutually exclusive mu SR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.

  • 43. Blom, Jonas
    et al.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Finding and solving Calogero-Moser type systems using Yang-Mills gauge theories1999In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 563, p. 506-532Article in journal (Refereed)
  • 44. Blom, Jonas
    et al.
    Langmann, Edwin
    KTH, School of Engineering Sciences (SCI), Physics, Mathematical Physics. KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Novel integrable spin-particle models from gauge theories on a cylinder1998In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 429, p. 336-342Article in journal (Refereed)
  • 45.
    Blomquist, Emil
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Strong Correlation Effects in Bosonic and Fermionic Systems Through an Unbiased Quantum Monte Carlo Approach2021Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Strongly interacting many-body quantum systems constitute some of the most challenging problems in physics, and exact analytical solutions are rare. In this thesis, we will investigate the models of two such systems numerically using the unbiased worm-algorithm Monte Carlo method.

    The first model is the t-J model, which describes strongly correlated lattice electrons and is thought to capture the essential physics of high-temperature superconductors. Of particular interest to us is the emergent magnetic polaron studied in the first two papers. The microscopic pairing mechanism between these quasiparticles remains one of the most outstanding open questions in condensed matter physics.

    The second model is an extended multi-component Bose--Hubbard model, which describes lattice bosons of multiple species. This model is essential to the third paper, in which we demonstrate a novel superfluid phenomenon in the form of a dissipationless drag effect that couples the different superfluid components of a superfluid mixture in a noncollinear fashion. Dissipationless drag is a fundamental effect present in a diverse set of physical systems. In the fourth paper, we further use the Bose--Hubbard model to study other new superfluid phenomena that arise when the number of superfluid components is increased.

    A general introduction to the research field is first given in the thesis, after which some prerequisites are refreshed. Then essential concepts are introduced, and the numerical method is outlined and benchmarked. Following that is a summary of the research carried out.

    Download full text (pdf)
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  • 46.
    Blomquist, Emil
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Carlström, Johan
    Stockholm Univ, Dept Phys, SE-10691 Stockholm, Sweden..
    Evidence of attraction between charge carriers in a doped Mott insulator2021In: Physical Review Research, E-ISSN 2643-1564, Vol. 3, no 1, article id 013272Article in journal (Refereed)
    Abstract [en]

    Recent progress in optically trapped ultracold atomic gases is now making it possible to access microscopic observables in doped Mott insulators, which are the parent states of high-temperature superconductors. This makes it possible to address longstanding questions about the temperature scales at which attraction between charge carriers are present, and their mechanism. Controllable theoretical results for this problem are not available at low temperature due to the sign problem. In this work, we make important progress with this problem by employing worm-algorithm Monte Carlo, which allows us to obtain completely unbiased results for two charge carriers in a Mott insulator. Our method gives access to lower temperatures than what is currently possible in experiments, and provides evidence for attraction between dopants at a temperature scale that is now feasible in ultracold atomic systems. We also report on spin correlations in the presence of charge carriers, which are directly comparable to experiments.

  • 47.
    Blomquist, Emil
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Carlström, Johan
    Stockholm Univ, Dept Phys, S-10691 Stockholm, Sweden..
    Unbiased description of magnetic polarons in a Mott insulator2020In: Communications Physics, E-ISSN 2399-3650, Vol. 3, no 1, article id 172Article in journal (Refereed)
    Abstract [en]

    Polarons are elementary quasi-particles characterizing several interacting many-body quantum systems. The authors present an unbiased Quantum Monte Carlo simulation of a magnetic polaron in a t-J model at low-temperature, and find excellent agreement with a recent experimental realization in the framework of cold-atoms systems. Polarons are among the most elementary quasiparticles of interacting quantum matter, consisting of a charge carrier dressed by an excited background. In Mott insulators, they take the form of a dopant surrounded by a distorted spin-background. Despite the fundamental importance of polarons for the electronic structure of strongly correlated systems, access to their internal structure was only recently realized in experiments, while controllable theoretical results are still lacking due to the sign problem. Here we report unbiased high-precision data obtained from worm-algorithm Monte Carlo that reveal the real-space structure of a polaron in thet-Jmodel deep inside the region where the sign problem becomes significant. These results are directly comparable to recent quantum gas microscopy experiments, but give access to significantly lower temperatures.

  • 48.
    Blomquist, Emil
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Syrwid, Andrzej
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Borromean Supercounterfluidity2021In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 127, no 25, article id 255303Article in journal (Refereed)
    Abstract [en]

    We demonstrate microscopically the existence of a new superfluid state of matter in a three-component Bose mixture trapped in an optical lattice. The superfluid transport involving coflow of all three components is arrested in that state, while counterflows between any pair of components are dissipationless. The presence of three components allows for three different types of counterflows with only two independent superfluid degrees of freedom.

  • 49.
    Blomquist, Emil
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Syrwid, Andrzej
    KTH, School of Engineering Sciences (SCI), Physics.
    Babaev, Egor
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Borromean supercounterfluidityManuscript (preprint) (Other academic)
    Abstract [en]

    We demonstrate microscopically the existence of a new superfluid state of matter in a three-component Bose mixture trapped in an optical lattice. In that state, the superfluid transport involving co-flow of all three components is arrested, while counter-flows between any pair of components are dissipationless. Due to the presence of the third component, the types of counterpropagating components are allowed to fluctuate.

  • 50.
    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.
    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.

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