arXiv subjects

Egor Babaev

Publications and source records attributed to Egor Babaev.

At least 37 records · Page 2Linked to original sources

arXiv2024

Microscopic solutions for vortex clustering in two-band type-1.5 superconductors

Two-band superconductors exhibit a distinct phase characterized by two correlation lengths, one smaller and the other larger than the magnetic field penetration length. This regime was coined type-1.5 superconductivity, with several unconventional properties, such as vortex clustering. However, a fully microscopic solution for vortex clusters has remained challenging due to computational complexities beyond quasiclassical models. This work presents numerical solutions obtained in a fully self-consistent two-band Bogoliubov-de Gennes model. We show the presence of discrepant correlation lengths leading to vortex clustering in two-band superconductors.

cond-mat.supr-con↗

arXiv2024

Phase transitions and composite order in $\mathrm{U}(1)^N$ lattice London models

The phase diagrams and the nature of the phase transitions in multicomponent gauge theories with an Abelian gauge field are important topics with various physical applications. While an early renormalization-group-based study indicated that the direct transition from a fully ordered to a fully disordered state is continuous for $N = 1$ and $N > 183$, recently it was demonstrated that the transition is discontinuous for $N = 2$. We quantitatively study the dependence on $N$ of the degree of discontinuity of this transition. Our results suggest that the transition is discontinuous at least up to $N = 7$. Furthermore, we demonstrate that, at increased coupling strength, the phase transitions of the neutral and charged sectors of the model split, which for $N > 2$ yields a new phase with composite order. The transition from the composite-order phase to the fully disordered phase is then also discontinuous, at least for $N = 3$ and $N = 4$. Via a duality argument, this indicates that van der Waals-type interaction between directed loops may be responsible for the discontinuous phase transitions in these models.

cond-mat.str-el↗

arXiv2024

Hydrodynamics of Borromean Counterfluids

Counterflow superfluidity in a system with $N\geq 3$ components is distinctively different from the $N=2$ case. The key feature is the difference between the number ($N$) of elementary vortex excitations and the number ($N-1$) of independent branches of phonon modes, that is, the number of superfluid modes is larger than the number of ordered phase variables. We formulate a hydrodynamic theory of this state. We show how all the dynamical and statistical aspects of this (``Borromean") type of ordering are naturally described by effective $N$-component theory featuring compact-gauge invariance. We also discuss how off-diagonal intercomponent couplings convert the Borromean supercounterfluid into a Borromean insulator, with an emphasis on the properties of a non-trivial state with broken time-reversal symmetry.

cond-mat.str-el↗

arXiv2024

Fractional Skyrme lines in ferroelectric barium titanate

We predict a topological defect in ferroelectric barium titanate which we call a skyrme line. These are line-like objects characterized by skyrmionic topological charge. As well as configurations with integer charge, the charge density can split into well-localized fractional parts. We show that under certain conditions the fractional skyrme lines are stable. We discuss a mechanism to create fractional topological charge objects and investigate their stability.

cond-mat.mtrl-sci↗

arXiv2023

Magneto-Crystalline Composite Topological Defects and Half-Hopfions

We consider a new class of topological defects in chiral magnetic crystals such as FeGe and MnSi. These are composite topological defects that arise when skyrmions in the magnetic order intersect with twin boundaries in the underlying crystalline lattice. We show that the resulting stable configurations are a new type of defect that can be viewed as half-hopfions.

cond-mat.mes-hall↗

arXiv2023

Counterpart of the Chandrasekhar-Kendall state in noncentrosymmetric superconductors

We demonstrate that superconductors with broken inversion symmetry support a family of stable, spatially localized configurations of the self-knotted magnetic field. These solutions, that we term ``toroflux,'' are the superconducting counterparts of the Chandrasekhar-Kendall states (spheromaks) that appear in highly conducting, force-free astrophysical and nuclear-fusion plasmas. The superconducting torofluxes are solutions of superconducting models, in the presence of a parity-breaking Lifshitz invariant associated with the $O$ point-group symmetry. These solutions are characterized by a non-vanishing helicity of the magnetic field, and also by a toroidal dipole moment of the magnetic field. We demonstrate that a magnetic dipole or a ferromagnetic inclusion in the bulk of a noncentrosymmetric superconductor sources finite-energy toroflux solutions.

cond-mat.supr-con↗

arXiv2023

Stable Kink-Kink and Metastable Kink-Antikink Solutions

We construct and study two kink theories. One contains a static 2-kink configuration with controllable binding energy. The other contains a locally stable non-topological solution, which we call a lavion. The new models are 1D analogs of non-integrable systems in higher dimensions such as the Skyrme model and realistic vortex systems. To help construct the theories, we derive a simple expression for the interaction energy between two kinks.

hep-th↗

arXiv2023

Symmetries, Length Scales, Magnetic Response and Skyrmion Chains in Nematic Superconductors

Nematic systems are two component superconductors that break rotational symmetry, but exhibit a mixed symmetry that couples spatial rotations and phase difference rotations. We show that a consequence of this induced spatial anisotropy is mixed normal modes, that is the linear response to a small perturbation of the system about its ground state, generally couples magnetic and condensate degrees of freedom. We will study the effect of mode mixing on the magnetic response of a nematic system as the strength of applied field is increased. In general we show that the coupled modes generate magnetic field perpendicular to the applied field, causing the magnetic response to spontaneously twist direction. We will study this for the Meissner effect with weak fields and also for stronger applied fields, which produce a mixture of Skyrmions and composite vortices, forming orientation dependent bound states. We will also calculate the anisotropies of the resulting first and second critical fields $H_{c_1}$ and $H_{c_2}$. The Skyrmion lattices for $H_{c_1} \leq H \leq H_{c_2}$ in nematic superconductors are shown to be structurally complicated, in contrast to the triangular or square vortex lattices in conventional superconductors. For low fields the magnetic response of the system involves a loosely bound collection of parallel Skyrmion chains. As the external field is increased the chains attract one another, causing a transition where the unit cell becomes triangular for high applied fields. This unique Skyrmion lattice and the magnetic twisting are clear indicators that could be used experimentally to identify materials that exhibit nematic superconductivity. To obtain these results we develop and present a novel method to find the unit cell of a vortex lattice that can be applied to other kinds of superconducting systems.

cond-mat.supr-con↗

arXiv2023

Ground state fractal crystals

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

cond-mat.other↗

arXiv2023

Ferroelectric domain wall clusters in barium titanate

We study ferroelectric domain walls in barium titanate. We search for structurally nontrivial, so-called non-Ising domain walls, where the Polarisation is non-zero along the entire wall. Our approach enables us to find solutions for domain walls in any orientation, and the existence and energy of these walls depend on their particular orientation. We find that, across all phases of the material, there are orientations where the non-Ising walls have lower energy than Ising walls. The most interesting property of these domain walls is their non-monotonic interaction forces, allowing them to form stable domain-wall clusters rather than following standard behavior where domain walls annihilate or repel each other. We found the required external electric field to create the non-Ising configurations. Besides theoretical interest, this unconventional property of domain walls makes them a good candidate for memory application.

cond-mat.mtrl-sci↗

arXiv2023

Effects of intercomponent couplings on the appearance of time-reversal symmetry breaking fermion quadrupling state in two-component London models

A detection of bosonic metallic state that breaks the $Z_2$ time-reversal symmetry has been recently reported in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ with a doping level $x \approx 0.8$. This is a metallic state of fermionic quadruplets that breaks time-reversal symmetry. As such, it has no condensate of Cooper pairs but has a long-range order between fermionic quartets. In the present manuscript, we investigate the emergence of this phase in a two-component London model via Monte Carlo simulations as a function of various intercomponent couplings.

cond-mat.supr-con↗

arXiv2023

Prediction of time-reversal-symmetry breaking fermionic quadrupling condensate in twisted bilayer graphene

Recent mean-field calculations suggest that the superconducting state of twisted bilayer graphene exhibits either a nematic order or a spontaneous breakdown of the time-reversal symmetry. The two-dimensional character of the material and the large critical temperature relative to the Fermi energy dictate that the material should have significant fluctuations. We study the effects of these fluctuations using Monte Carlo simulations. We show that in a model proposed earlier for twisted bilayer graphene there is a fluctuation-induced phase with quadrupling fermionic order for all considered parameters. This four-electron condensate, instead of superconductivity, shows a spontaneous breaking of time-reversal symmetry. Our results suggest that twisted bilayer graphene is an especially promising platform to study different types of condensates, beyond the pair-condensate paradigm.

cond-mat.str-el↗

arXiv2023

Magnetic response of nematic superconductors: skyrmion stripes and their signatures in muon spin relaxation experiments

We investigate the magnetic response of nematic superconductors, presenting a new approach to find vortex and skyrmion structures beyond symmetry-constraining ansätze. Using this approach we show that nematic superconductors form distinctive skyrmion stripes. Our approach lends itself to accurate determination of the field distribution for muon spin rotation probes. We use this to show that the skyrmion structure manifests as a double peak in the field distribution, markedly different from the signal of standard vortex lattices.

cond-mat.supr-con↗

arXiv2023

Observation of superconducting vortices carrying a temperature-dependent fraction of the flux quantum

The magnetic response is a state-defining property of superconductors. The magnetic flux penetrates type-II bulk superconductors by forming quantum vortices when the enclosed magnetic flux is equal to the magnetic flux quantum. The flux quantum is the universal quantity that depends only on the ratio of fundamental constants: the electron charge and the Planck constant. This work investigates the vortex state in the hole-overdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by using scanning superconducting quantum interference device (SQUID) magnetometry. We observed quantum vortices that carry only a fraction of the flux quantum, which vary continuously with temperature. This finding establishes the phenomenon that superconductors support quantum vortices with non-universally quantized magnetic flux. Furthermore, the demonstrations of the mobility of the fractional vortices and the manipulability of their positions open up a route for future fluxonics applications.

cond-mat.supr-con↗

arXiv2022

A microscopic study of boundary superconducting states on a honeycomb lattice

We address the problem of boundary s-wave superconductivity on rectangular honeycomb lattices: nanoflakes, armchair and zigzag nanotubes. We discuss how the presence of edges and corners in these systems can significantly alter the superconducting correlations at a macroscopic length scale, leading to either nontrivial enhancement or suppression of the superconducting gap value near the boundaries. This in turn results in different critical temperatures of the gap closure at boundaries compared to the bulk gap. The effects are macroscopic but strongly depend on the atomic-level structure of the boundaries.

cond-mat.supr-con↗

arXiv2022

Calorimetric evidence for two phase transitions in Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_{2}$As$_{2}$ with fermion pairing and quadrupling states

Theoretically, materials that break multiple symmetries allow, under certain conditions, the formation of four-fermion condensates above the superconducting critical temperature. Such states can be stabilized by phase fluctuations. Recently a fermionic quadrupling condensate that breaks the $Z_2$ time-reversal symmetry was reported in Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_{2}$As$_{2}$ [V. Grinenko et al., Nat. Phys. 17, 1254 (2021)]. Evidence for the new state of matter comes from muon-spin rotation, transport, thermoelectric, and ultrasound experiments. Observing a specific heat anomaly is a very important signature of a transition to a new state of matter. However, a fluctuation-induced specific heat singularity is usually very challenging to resolve from a background of other contributions. Here, we report on detecting two anomalies in the specific heat of Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_{2}$As$_{2}$ at zero magnetic field. The anomaly at the higher temperature is accompanied by the appearance of a spontaneous Nernst effect, indicating broken time-reversal ($Z_2$) symmetry. The second anomaly at the lower temperature coincides with the transition to a zero resistance state, indicating superconductivity breaking the $U(1)$ gauge symmetry. Our data provide calorimetric evidence for the $Z_2$ phase formation above the superconducting phase transition.

cond-mat.supr-con↗

arXiv2022

Drag-induced dynamical formation of dark solitons in Bose mixture on a ring

Andreev-Bashkin drag plays a very important role in multiple areas like superfluid mixtures, superconductors and dense nuclear matter. Here, we point out that the drag phenomenon can be also important in physics of solitons, ubiquitous objects arising in a wide array of fields ranging from tsunami waves and fiber-optic communication to biological systems. So far, fruitful studies were conducted in ultracold atomic systems where nontrivial soliton dynamics occurred due to inter-component density-density interaction. In this work we show that current-current coupling between components (Andreev-Bashkin drag) can lead to a substantially different kind of effects, unsupported by density-density interactions, such as a drag-induced dark soliton generation. This also points out that soliton dynamics can be used as a tool to experimentally study the dissipationless drag effect.

cond-mat.quant-gas↗

arXiv2022

Wave functions and edge states in rectangular honeycomb lattices revisited: nanoflakes, armchair and zigzag nanoribbons and nanotubes

Properties of bulk and boundaries of materials can, in general, be quite different, both for topological and non-topological reasons. One of the simplest boundary problems to pose is the tight-binding problem of noninteracting electrons on a finite honeycomb lattice. Despite its simplicity, the problem is quite rich and directly related to the physics of graphene. We revisit this long-studied problem and present an analytical derivation of the electron spectrum and wave functions for graphene rectangular derivatives. We provide an exact analytical description of extended and localized states, the transition between them, and a special case of a localized state when the wave function is nonzero only at the edge sites. The later state has zero energy, we discuss its existence in zigzag nanoribbons, zigzag nanotubes with number of sites along a zigzag edge divisible by 4, and rectangular graphene nanoflakes with an odd number of sites along both zigzag and armchair edges.

cond-mat.mes-hall↗