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Egor Babaev

Publications and source records attributed to Egor Babaev.

At least 109 records · Page 6Linked to original sources
arXiv2015

Unconventional thermoelectric effect in superconductors that break time-reversal symmetry

We demonstrate that superconductors which break time-reversal symmetry can exhibit thermoelectric properties, which are entirely different from the Ginzburg mechanism. As an example, we show that in the $s+is$ superconducting state there is a reversible contribution to thermally induced supercurrent, whose direction is not invariant under time-reversal operation. Moreover in contrast to Ginzburg's mechanism it has a singular behavior near the time-reversal symmetry breaking phase transition. The effect can be used to confirm or rule out the $s+is$ state, which is widely expected to be realized in pnictide compounds Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ and stoichiometric LiFeAs.

cond-mat.supr-con↗
arXiv2015

Fluctuation-induced first order phase transitions in type-1.5 superconductors in zero external field

In a single-component Ginzburg-Landau model which possesses thermodynamically stable vortex excitations, the zero-field superconducting phase transition is second order even when fluctuations are included. Beyond the mean-field approximation the transition is described in terms of proliferation of vortex loops. Here we determine the order of the superconducting transition in an effective 3D vortex-loop model for the recently proposed multiband type-1.5 superconductors. The vortex interaction is nonmonotonic, i.e., exponentially screened and attractive at large separations, and short-range repulsive. We show that the details of the vortex interaction, despite its short-range nature, can lead to very different properties of the superconducting transition than found in type-1 and type-2 systems. Namely, the type-1.5 regime with nonmonotonic intervortex interaction can have a first-order vortex-driven phase transition not found in the single-band case.

cond-mat.supr-con↗
arXiv2015

Immiscibile two-component Bose Einstein condensates beyond mean-field approximation: phase transitions and rotational response

We consider a two-component immiscible Bose-Einstein condensate with dominating intra-species repulsive density-density interactions. In the ground-state phase of such a system only one condensates is present. This can be viewed as a spontaneous breakdown of $\mathbb{Z}_2$ symmetry. We study the phase diagram of the system at finite temperature beyond mean-field approximation. In the absence of rotation, we show that the system undergoes a first order phase transition from this ground state to a miscible two-component normal fluid as temperature is increased. In the presence of rotation, the system features a competition between vortex-vortex interaction and short range density-density interactions. This leads to a rotation-driven "mixing" phase transition in a spatially inhomogeneous state with additional broken $\mathrm{U}(1)$ symmetry. Thermal fluctuations in this state lead to nematic two-component sheets of vortex liquids. At sufficiently strong inter-component interaction, we find that the superfluid and $\mathbb{Z}_2$ phase transitions split. This results in the formation of an intermediate state which breaks only $\mathbb{Z}_2$ symmetry. It represents two phase separated normal fluids with density imbalance.

cond-mat.quant-gas↗
arXiv2015

Fluctuation effects in rotating Bose-Einstein condensates with broken $\mathrm{SU}(2)$ and $\mathrm{U}(1)\times \mathrm{U}(1)$ symmetries in the presence of intercomponent density-density interactions

Thermal fluctuations and melting transitions for rotating single-component superfluids have been intensively studied and are well understood. In contrast, the thermal effects on vortex states for two-component superfluids with density-density interaction, which have a much richer variety of vortex ground states, have been much less studied. Here, we investigate the thermal effects on vortex matter in superfluids with $\mathrm{U(1)}\times \mathrm{U(1)}$ broken symmetries and intercomponent density-density interactions, as well as the case with a larger $\mathrm{SU(2)}$ broken symmetry obtainable from the $\mathrm{U(1)}\times \mathrm{U(1)}$-symmetric case by tuning scattering lengths. In the former case we find that, in addition to first-order melting transitions, the system exhibits thermally driven phase transitions between square and hexagonal lattices. Our main result, however, concerns the case where the condensate exhibits $\mathrm{SU(2)}$-symmetry, and where vortices are not topological. At finite temperature, the system exhibits effects which do not have a counter-part in single component systems. Namely, it has a state where thermally averaged quantities show no regular vortex lattice, yet the system retains superfluid coherence along the axis of rotation. In such a state, the thermal fluctuations result in transitions between different (nearly)-degenerate vortex states without undergoing a melting transition. Our results apply to multi-component Bose-Einstein condensates, and we suggest how to experimentally detect some of these unusual effects in such systems.

cond-mat.quant-gas↗
arXiv2015

Vortex chains due to nonpairwise interactions and field-induced phase transitions between states with different broken symmetry in superconductors with competing order parameters

We study superconductors with two order components and phase separation driven by intercomponent density-density interaction, focusing on the phase where only one condensate has non-zero ground-state density and a competing order parameter exists only in vortex cores. We demonstrate there, that multi-body intervortex interactions can be strongly non-pairwise, leading to some unusual vortex patterns in an external field, such as vortex pairs and vortex chains. We demonstrate that, in external magnetic field, such a system undergoes a field-driven phase transition from (broken) $U(1)$ to (broken) $U(1)\times U(1)$ symmetries, when the subdominant order parameter in the vortex cores acquires global coherence. Observation of these characteristic ordering patterns in surface probes may signal the presence of a subdominant condensate in the vortex core.

cond-mat.supr-con↗
arXiv2015

Vortex matter in $U(1)\times U(1)\times\mathbb{Z}_2$ phase-separated superconducting condensates

We study the properties of vortex solutions and magnetic response of two-component $U(1)\times U(1)\times\mathbb{Z}_2$ superconductors, with phase separation driven by intercomponent density-density interaction. Such a theory can be viewed arising from the breakdown of $SU(2)$ symmetry by a biquadratic interaction between the components of the field. Depending on the symmetry-breaking term, there are two ground-state phases: one where both components of the doublet are equal (the miscible phase) and one where only one component assumes a non zero vacuum expectation value (the immiscible state). In the latter phase, the spectrum of topological excitations contains both domain walls and vortices. We show the existence of another kind of excitation that has properties of both topological excitations at the same time. They combine vorticity together with a circular domain wall, interpolating between inequivalent broken states, that shows up as a ring of localized magnetic flux. Asymptotically, this resembles a vortex carrying multiple flux quanta, but because the magnetic field is localized at a given distance from the center this looks like a pipe. The isolated multiquanta pipelike vortices can be either stable or metastable, even if the system is not type-1. We also discuss the response of such a system to an externally applied magnetic field.

cond-mat.supr-con↗
arXiv2014

Time reversal symmetry breakdown in normal and superconducting states in frustrated three-band systems

We discuss the phase diagram and phase transitions in $U(1)\times \groupZ{2}$ three-band superconductors with broken time reversal symmetry. We find that beyond mean field approximation and for sufficiently strong frustration of interband interactions there appears an unusual metallic state precursory to a superconducting phase transition. In that state, the system is not superconducting. Nonetheless, it features a spontaneously broken $\groupZ{2}$ time reversal symmetry. By contrast, for weak frustration of interband coupling the energy of a domain wall between different $\groupZ{2}$ states is low and thus fluctuations restore broken time reversal symmetry in the superconducting state at low temperatures.

cond-mat.supr-con↗
arXiv2014

Phase transitions and anomalous normal state in superconductors with broken time reversal symmetry

Using Monte Carlo simulations, we explore the phase diagram and the phase transitions in $\groupUZ$ $n$-band superconductors with spontaneously broken time-reversal symmetry (also termed $s+is$ superconductors), focusing on the three-band case. In the limit of infinite penetration length, the system under consideration can, for a certain parameter regime, have a single first order phase transition from a $\groupUZ$ broken state to a normal state due to a nontrivial interplay between $\groupU$ vortices and $\groupZ$ domain walls. This regime may also apply to multicomponent superfluids. For other parameters, when the free energy of the domain walls is low, the system undergoes a restoration of broken $\groupZ$ time reversal symmetry at temperatures lower than the temperature of the superconducting phase transition.{We show that inclusion of fluctuations can strongly suppress the temperature of the $\groupZ$-transition when frustration is weak. The main result of our paper is that} for relatively short magnetic field penetration lengths, the system has a superconducting phase transition at a temperature lower than the temperature of the restoration of the broken $\groupZ$ symmetry. Thus, there appears a new phase which is $\groupU$-symmetric, but breaks $\groupZ$ time reversal symmetry, an anomalous dissipative (metallic) state.

cond-mat.supr-con↗
arXiv2014

Microscopic prediction of skyrmion lattice state in clean interface superconductors

When an in-plane field is applied to a clean interface superconductor, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like phase is stabilized. This phase has a $\mathrm{U}(1)\times\mathrm{U}(1)$ symmetry and, in principle, this symmetry allows for flux carrying topological excitations different from Abrikosov vortices (which are the simplest defects associated with $S^1 \to S^1$ maps). However, in practice, largely due to electromagnetic and other intercomponent interactions, such topological excitations are very rare in superconducting systems. Here we demonstrate that a realistic microscopic theory for interface superconductors, such as SrTiO$_3$/LaAlO$_3$, predicts an unconventional magnetic response where the flux-carrying objects are skyrmions, characterized by homotopy invariants of $S^2 \to S^2$ maps. Additionally, we show that this microscopic theory predicts that stable fractional vortices form near the boundary of these superconductors. It also predicts the appearance of type-1.5 superconductivity for some range of parameters. Central to these results is the assumption that the Rashba spin orbit coupling is much larger than the superconducting gap.

cond-mat.supr-con↗
arXiv2014

Honeycomb, square, and kagomé vortex lattices in superconducting systems with multi-scale inter-vortex interactions

The recent proposal of Romero-Isart {\em et al.}~\cite{romero-isart_superconducting_2013} to utilize the vortex lattice phases of superconducting materials to prepare a lattice for ultra-cold atoms-based quantum emulators, raises the need to create and control vortex lattices of different symmetries. Here we propose a mechanism by which honeycomb, hexagonal, square, and kagomé vortex lattices could be created in superconducting systems with multi-scale inter-vortex interaction. Multiple scales of the inter-vortex interaction can be created and controlled in layered systems made of different superconducting material or with differing interlayer spacing.

cond-mat.supr-con↗
arXiv2014

Topological defects in mixtures of superconducting condensates with different charges

We investigate the topological defects in phenomenological models describing mixtures of charged condensates with commensurate electric charges. Such situations are expected to appear for example in liquid metallic deuterium. This is modeled by a multicomponent Ginzburg-Landau theory where the condensates are coupled to the same gauge field by different coupling constants whose ratio is a rational number. We also briefly discuss the case where electric charges are incommensurate. Flux quantization and finiteness of the energy per unit length dictate that the different condensates have different winding and thus different number of (fractional) vortices. Competing attractive and repulsive interactions lead to molecule-like bound state between fractional vortices. Such bound states have finite energy and carry integer flux quanta. These can be characterized by $\mathbb{C}P^1$ topological invariant that motivates their denomination as skyrmions.

cond-mat.supr-con↗
arXiv2014

Skyrmions induced by dissipationless drag in U(1)xU(1) superconductors

Rather generically, multicomponent superconductors and superfluids have intercomponent current-current interaction. We show that in superconductors with substantially strong intercomponent drag interaction, the topological defects which form in external field are characterized by a skyrmionic topological charge. We then demonstrate that they can be distinguished from ordinary vortex matter by a very characteristic magnetization process due to the dipolar nature of inter-skyrmion forces. The results provide an experimental signature to confirm or rule out the formation $p$-wave state with reduced spin stiffness in $p$-wave superconductors.

cond-mat.supr-con↗
arXiv2014

Rotational response of superconductors: magneto-rotational isomorphism and rotation-induced vortex lattice

The analysis of nonclassical rotational response of superfluids and superconductors was performed by Onsager (in 1949) \cite{Onsager} and London (in 1950) \cite{London} and crucially advanced by Feynman (in 1955) \cite{Feynman}. It was established that, in thermodynamic limit, neutral superfluids rotate by forming---without any threshold---a vortex lattice. In contrast, the rotation of superconductors at angular frequency ${\bf Ω}$---supported by uniform magnetic field ${\bf B}_L\propto {\bf Ω}$ due to surface currents---is of the rigid-body type (London Law). Here we show that, neglecting the centrifugal effects, the behavior of a rotating superconductor is identical to that of a superconductor placed in a uniform fictitious external magnetic filed $\tilde{\bf H}=- {\bf B}_L$. In particular, the isomorphism immediately implies the existence of two critical rotational frequencies in type-2 superconductors.

cond-mat.supr-con↗
arXiv2014

Classification of ground states and normal modes for phase-frustrated multicomponent superconductors

We classify ground states and normal modes for $n$-component superconductors with frustrated intercomponent Josephson couplings, focusing on $n = 4$. The results should be relevant not only to multiband superconductors, but also to Josephson-coupled multilayers and Josephson-junction arrays. It was recently discussed that three-component superconductors can break time-reversal symmetry as a consequence of phase frustration. We discuss how to classify frustrated superconductors with an arbitrary number of components. Although already for the four-component case there are a large number of different combinations of phase-locking and phase-antilocking Josephson couplings, we establish that there are a much smaller number of equivalence classes where properties of frustrated multicomponent superconductors can be mapped to each other. This classification is related to the graph-theoretical concept of Seidel switching. Numerically, we calculate ground states, normal modes, and characteristic length scales for the four-component case. We report conditions of appearance of new accidental continuous ground-state degeneracies.

cond-mat.supr-con↗
arXiv2014

Domain walls and their experimental signatures in s+is superconductors

Arguments were recently advanced that hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ exhibits $s+is$ state at certain doping. Spontaneous breaking of time reversal symmetry in $s+is$ state, dictates that it possess domain wall excitations. Here, we discuss what are the experimentally detectable signatures of domain walls in $s+is$ state. We find that in this state the domain walls can have dipole-like magnetic signature (in contrast to the uniform magnetic signature of domain walls $p+ip$ superconductors). We propose experiments where quench-induced domain walls can be stabilized by geometric barriers and be observed via their magnetic signature or their influence on the magnetization process, thereby providing an experimental tool to confirm $s+is$ state.

cond-mat.supr-con↗
arXiv2013

Unusual mechanism of vortex viscosity generated by mixed normal modes in superconductors with broken time reversal symmetry

We show that under certain conditions multiband superconductors with broken time-reversal symmetry have a new vortex viscosity-generating mechanism which is different from that in conventional superconductors. It appears due to the existence of mixed superfluid phase-density mode inside vortex core. This new contribution is dominant near the time reversal symmetry breaking phase transition. The results could be relevant for three band superconductor $Ba_{1-x}K_{x}Fe_2As_2$.

cond-mat.supr-con↗
arXiv2013

Hierarchical structure formation in layered superconducting systems with multi-scale inter-vortex interactions

We demonstrate formation of hierarchical structures in two-dimensional systems with multiple length scales in the inter-particle interaction. These include states such as clusters of clusters, concentric rings, clusters inside a ring, and stripes in a cluster. We propose to realize such systems in vortex matter (where a vortex is mapped onto a particle with multi-scale interactions) in layered superconducting systems with varying inter-layer thicknesses and different layer materials.

cond-mat.supr-con↗