arXiv subjects

Alexei E. Koshelev

Publications and source records attributed to Alexei E. Koshelev.

At least 19 recordsLinked to original sources

arXiv2026

Theory of frozen flux in a narrow uniform superconducting strip after cooling in a small magnetic field

We analyze residual frozen flux in a long narrow superconducting strip cooled through its transition temperature $T_{c}$ in a small perpendicular magnetic field. This problem is relevant for the issue of trapped magnetic flux in superconducting electronic devices. During cooling, the low-temperature vortex configuration is formed at temperatures very close to $T_{c}$, where the flux density is determined by dynamic balance between the thermally-activated exits and entries of vortices over the geometrical energy barrier formed by the interaction with the strip edges and the Meissner screening current. In the field range between the minimum flux-expulsion field and the penetration field, the equilibrium flux density is finite due to thermal activation and rapidly decreases with decreasing temperature. During cooling, however, the escape rate decreases exponentially, and the vortex density falls out of equilibrium at a field-dependent freezing temperature $T_{\mathrm{fr}}$. We derive and solve the dynamic-balance equation for this process, which yields definite quantitative results for $T_{\mathrm{fr}}$ and the frozen vortex density. The relative freezing temperature $1\!-\!T_{\mathrm{fr}}/T_{c}$ exceeds the fluctuation width of the transition by a large logarithmic factor, rapidly increases when the magnetic field approaches the minimum flux-expulsion field, and logarithmically increases with decreasing cooling rate. The resulting frozen flux density has a very strong magnetic-field dependence which can be used to define the effective flux-expulsion magnetic field.

cond-mat.supr-con↗

arXiv2025

Origin of the Unusual Temperature Dependence of the Upper Critical Field of Kagome Superconductor CsV3Sb5: Multiple Bands or van Hove Singularities?

Van Hove singularities (vHs) located close to the Fermi level in Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have profound influence on their electronic and transport characteristics. Specifically, magneto-transport and susceptibility measurements on CsV3Sb5 reveal an anomalous temperature dependence of the upper critical field H_c2 (T), characterized by a pronounced upward curvature for both in-plane and c-axis magnetic fields, with zero-temperature H_c2 values of ~6.0 T and ~1.2 T, respectively. Our theoretical analysis, using a newly developed single-band model incorporating vHs and gap anisotropy, suggests that the observed upper critical field behavior is predominantly driven by the anisotropy of the Fermi velocity originating from vHs, instead of multi-band effects or gap anisotropy. Increased electron scattering introduced by proton irradiation defects smears out the vHs, reduces anisotropy, and recovers the conventional H_c2 (T) behavior, corroborating our proposed model.

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arXiv2024

Anisotropic Paramagnetic Peak Effect in Reversible Magnetization of Crystalline Miassite Superconductor $\text{Rh}_{17}\text{S}_{15}$

We report an unusual anisotropic paramagnetic peak effect observed in reversible magnetization of a single crystalline nodal superconductor $\text{Rh}_{17}\text{S}_{15}$. Both temperature- and field-dependent magnetization measurements reveal a distinct novel vortex state above approximately 1 T. This peak effect is most pronounced when the magnetic field, $H$, is applied parallel to the $\left[111\right]$ direction, whereas it diminishes for $H\parallel\left[110\right]$. Intriguingly, for $H\parallel\left[100\right]$, instead of a peak, we observe a step-like decrease in $M(T)$, with the step amplitude increasing in larger applied magnetic fields. This behavior is opposite to the expectations of conventional Meissner expulsion. The magnitude of the peak effect, expressed in terms of dimensionless volume susceptibility, is on the order of $Δχ=10^{-5}$ (with full diamagnetic screening corresponding to $χ=-1$). The observed anisotropic paramagnetic vortex response is unusual considering the cubic symmetry of $\text{Rh}_{17}\text{S}_{15}$. We propose that in this distinct vortex phase, a small but finite attractive interaction between vortices below $H_{c2}$ may be responsible for this unusual phenomenon. Furthermore, the vortices seem to prefer aligning along the $\left[111\right]$ direction, rotating toward it when the magnetic field is applied in other directions. Our findings add another item to the list of unusual properties of $\text{Rh}_{17}\text{S}_{15}$ that attracted recent attention as the first unconventional superconductor that has a mineral analog, miassite, found in nature.

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arXiv2024

In-plane anisotropic magnetoresistance in detwinned $BaFe_{2-x}Ni_{x}As_{2}$ ($x$ = 0, 0.6)

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including $BaFe_2As_2$, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic (AF) ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned $BaFe_2As_2$ and compare the results to the non-magnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, $BaFe_2As_2$ exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in $BaFe_2As_2$ that is not present in the Ni-doped material.

cond-mat.str-el↗

arXiv2023

Nematic superconductivity from selective orbital pairing in Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals

We use transport measurements to determine the in-plane anisotropy of the upper critical field Hc2 in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence (x) of the upper critical field anisotropy for fields along the inter-planar (c-axis) and in-plane field directions was found to increase in the overdoped regime. For underdoped samples, which exhibit a spin nematic phase below the tetragonal to orthorhombic structural transition temperature Ts , we find that Hc2 along the a-axis is considerably lower than that along the b-axis. The upper critical field anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with inelastic neutron scattering studies of spin excitations, and angle-resolved photoemission spectroscopy, we conclude that superconductivity in under-doped iron pnictides is orbital selective - with a dominant contribution from electrons with the dyz orbital character and being intimately associated with spin excitations.

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arXiv2020

Magnetoresistance scaling, disorder, `hot spots' and the origin of $T$-linear resistivity in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

The scaling of $H$-linear magnetoresistance in field and temperature was measured in under-doped (x = 0.19) and optimally-doped (x=0.31)~BaFe$_2$(As$_{1-x}$P$_x$)$_2$. We analyze the data based on an orbital model in the presence of strongly anisotropic quasiparticle spectra and scattering time due to antiferromagnetism. The magnetoresistance is dominated by the properties of small regions of the Fermi surface called `hot spots' where antiferromagnetic excitations induce a large quasiparticle scattering rate. Approximate temperature-magnetic field scaling relations are derived and shown to be consistent with the experimental data. We argue that these results link the origin of linear-in-temperature resistivity to hot spots arising from an antiferromagnetic critical point, and magnetoresistance measurements provide a route to quantify this link.

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arXiv2019

Nanocalorimetric Evidence for Nematic Superconductivity in the Doped Topological Insulator Sr$_{0.1}$Bi$_{2}$Se$_{3}$

Spontaneous rotational-symmetry breaking in the superconducting state of doped $\mathrm{Bi}_2\mathrm{Se}_3$ has attracted significant attention as an indicator for topological superconductivity. In this paper, high-resolution calorimetry of the single-crystal $\mathrm{Sr}_{0.1}\mathrm{Bi}_2\mathrm{Se}_3$ provides unequivocal evidence of a two-fold rotational symmetry in the superconducting gap by a \emph{bulk thermodynamic} probe, a fingerprint of nematic superconductivity. The extremely small specific heat anomaly resolved with our high-sensitivity technique is consistent with the material's low carrier concentration proving bulk superconductivity. The large basal-plane anisotropy of $H_{c2}$ is attributed to a nematic phase of a two-component topological gap structure $\vecη = (η_{1}, η_{2})$ and caused by a symmetry-breaking energy term $δ(|η_{1}|^{2} - |η_{2}|^{2}) T_{c}$. A quantitative analysis of our data excludes more conventional sources of this two-fold anisotropy and provides the first estimate for the symmetry-breaking strength $δ\approx 0.1$, a value that points to an onset transition of the second order parameter component below 2K.

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arXiv2019

Quantum FFLO state in clean layered superconductors

We investigate the influence of Landau quantization on the superconducting instability for a pure layered superconductor in the magnetic field directed perpendicular to the layers. We demonstrate that the quantization corrections to the Cooper-pairing kernel with finite Zeeman spin splitting promote the formation of the nonuniform state in which the order parameter is periodically modulated along the magnetic field, i.e., between the layers (Fulde-Ferrell-Larkin-Ovchinnikov [FFLO] state). The conventional uniform state experiences such a quantization-induced FFLO instability at low temperatures even in a common case of predominantly orbital suppression of superconductivity when the Zeeman spin splitting is expected to have a relatively weak effect. The maximum relative FFLO temperature is given by the ratio of the superconducting transition temperature and the Fermi energy. This maximum is realized when the ratio of the spin-spitting energy and the Landau-level separation is half-integer. These results imply that the FFLO states may exist not only in the Pauli-limited superconductors but also in very clean materials with small Zeeman spin-splitting energy. We expect that the described quantization-promoted FFLO instability is a general phenomenon, which may be found in materials with different electronic spectra and order-parameter symmetries.

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arXiv2019

Targeted evolution of pinning landscapes for large superconducting critical currents

The ability of type-II superconductors to carry large amounts of current at high magnetic fields is a key requirement for future design innovations in high-field magnets for accelerators and compact fusion reactors and largely depends on the vortex pinning landscape comprised of material defects. The complex interaction of vortices with defects that can be grown chemically, e.g., self-assembled nanoparticles and nanorods, or introduced by post-synthesis particle irradiation precludes a priori prediction of the critical current and can result in highly non-trivial effects on the critical current. Here, we borrow concepts from biological evolution to create a genetic algorithm evolving pinning landscapes to accommodate vortex pinning and determine the best possible configuration of inclusions for two different scenarios: an evolution process starting from a pristine system and one with pre-existing defects to demonstrate the potential for a post-processing approach to enhance critical currents. Furthermore, the presented approach is even more general and can be adapted to address various other targeted material optimization problems.

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arXiv2018

Interplay between orbital-quantization effects and the Fulde-Ferrell-Larkin-Ovchinnikov instability in multiple-band layered superconductors

We explore superconducting instability for a clean two-band layered superconductor with deep and shallow bands in the magnetic field applied perpendicular to the layers. In the shallow band, the quasiclassical approximation is not applicable, and Landau quantization has to be accounted for exactly. The electronic spectrum of this band in the magnetic field is composed of the one-dimensional Landau-level minibands. With increasing magnetic field the system experiences series of Lifshitz transitions when the chemical potential enters and exits the minibands. These transitions profoundly influence the shape of the upper critical field at low temperatures. In addition, the Zeeman spin splitting may cause the nonuniform state with interlayer modulation of the superconducting order parameter (Fulde-Ferrell-Larkin-Ovchinnikov state). Typically, the quantization effects in the shallow band strongly promote the formation of this state. The uniform state remains favorable only in the exceptional resonance cases when the spin-splitting energy exactly matches the Landau-level spacing. Furthermore, for specific relations between electronic spectrum parameters, the alternating FFLO state may realize, in which the order parameter changes sign between the neighboring layers. For all above cases, the reentrant high-field superconducting states may emerge at low temperatures if the shallow band has significant contribution to the Cooper pairing.

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arXiv2018

Peak effect due to competing vortex ground states in superconductors with large inclusions

Superconductors can support large dissipation-free electrical currents only if vortex lines are effectively immobilized by material defects. Macroscopic critical currents depend on elemental interactions of vortices with individual pinning centers. Pinning mechanisms are nontrivial for large-size defects such as self-assembled nanoparticles. We investigate the problem of a vortex system interacting with an isolated defect using time-dependent Ginzburg-Landau simulations. In particular, we study the instability-limited depinning process and extract the dependence of the pin-breaking force on inclusion size and anisotropy for an \emph{isolated vortex line}. In the case of a \emph{vortex lattice} interacting with a large isolated defect, we find a series of first-order phase transitions at well-defined magnetic fields, when the number of vortex lines occupying the inclusion changes. The pin-breaking force has sharp local minima at those fields. As a consequence, in the case of isolated identical large-size defects, the field dependence of the critical current is composed of a series of peaks located in between the occupation-number transition points.

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arXiv2017

Strong Landau-quantization effects in high-magnetic-field superconductivity of a two-dimensional multiple-band metal near the Lifshitz transition

We investigate the onset of superconductivity in magnetic field for a clean two-dimensional multiple-band superconductor in the vicinity of the Lifshitz transition when one of the bands is very shallow. Due to small number of carriers in this band, the quasiclassical Werthamer-Helfand approximation breaks down and Landau quantization has to be taken into account. We found that the transition temperature TC2(H) has giant oscillations and is resonantly enhanced at the magnetic fields corresponding to full occupancy of the Landau levels in the shallow band. This enhancement is especially pronounced for the lowest Landau level. As a consequence, the reentrant superconducting regions in the temperature-field phase diagram emerge at low temperatures near the magnetic fields at which the chemical potential matches the Landau levels. The specific behavior depends on the relative strength of the intraband and interband pairing interactions and the reentrance is most pronounced in the purely interband coupling scenario. The reentrant behavior is suppressed by the Zeeman spin splitting in the shallow band, the separated regions disappear already for very small spin-splitting factors. On the other hand, the reentrance is restored in the resonance cases when the spin-splitting energy exactly matches the separation between the Landau levels. The predicted behavior may realize in the gate-tuned FeSe monolayer.

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arXiv2016

Surface Nematic Order in Iron Pnictides

Electronic nematicity plays important role in iron-based superconductors. These materials have layered structure and theoretical description of their magnetic and nematic transitions has been well established in two-dimensional approximation, i.e., when the layers can be treated independently. However, the interaction between iron layers mediated by electron tunneling may cause non-trivial three-dimensional behavior. Starting from the simplest model for orbital nematic in a single layer, we investigate the influence of interlayer tunneling on bulk nematic order and possible preemptive state where this order is only formed near the surface. We found that the interlayer tunneling suppresses the bulk nematicity which makes favorable formation of a surface nematic above the bulk transition temperature. The purely electronic tunneling Hamiltonian, however, favors alternating from layer-to-layer nematic order parameter in the bulk. The uniform bulk state typically observed experimentally may be stabilized by the coupling with the elastic lattice deformation. Depending on strength of this coupling, we found three regimes: (i) surface nematic and alternating bulk order, (ii) surface nematic and uniform bulk order, and (iii) uniform bulk order without the intermediate surface phase. The intermediate surface-nematic state may resolve the current controversy about the existence of the weak "meta-nematic transition" in the compound BaFe$_2$As$_{2-x}$P$_{x}$.

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arXiv2015

Local spin-density-wave order inside vortex cores in multiband superconductors

Coexistence of antiferromagnetic order with superconductivity in many families of newly discovered iron-based superconductors has renewed interest to this old problem. Due to competition between the two types of order, one can expect appearance of the antiferromagnetism inside the cores of the vortices generated by the external magnetic field. The structure of a vortex in type II superconductors holds significant importance from the theoretical and the application points of view. Here we consider the internal vortex structure in a two-band s$_\pm$ superconductor near a spin-density-wave instability. We treat the problem in a completely self-consistent manner within the quasiclassical Eilenberger formalism. We study the structure of the s$_\pm$ superconducting order and magnetic field-induced spin-density-wave order near an isolated vortex. We examine the effect of this spin-density-wave state inside the vortex cores on the local density of states.

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arXiv2014

Complex state induced by impurities in multiband superconductors

We study the role of impurities in a two-band superconductor, and elucidate the nature of the recently predicted transition from s_{+-} state to s_{++} state induced by interband impurity scattering. Using a Ginzburg-Landau theory, derived from microscopic equations, we demonstrate that close to T_c this transition is necessarily a direct one, but deeper in the superconducting state an intermediate complex state appears. This state has a distinct order parameter, which breaks the time-reversal symmetry, and is separated from the s_{+-} and s_{++} states by phase transitions. Based on our results, we suggest a phase diagram for systems with weak repulsive interband pairing, and discuss its relevance to iron-based superconductors.

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arXiv2013

Linewidth of the electromagnetic radiation from Josephson junctions near cavity resonances

The powerful terahertz emission from intrinsic Josephson junctions in high-Tc cuprate superconductors has been detected recently. The synchronization of different junctions is enhanced by excitation of the geometrical cavity resonance. A key characteristic of the radiation is its linewidth. In this work, we study the intrinsic linewidth of the radiation near the internal cavity resonance. Surprisingly, this problem was never considered before, neither for a single Josephson junction nor for a stack of the intrinsic Josephson junctions realized in cuprate superconductors. The linewidth appears due to the slow phase diffusion, which is determined by the dissipation and amplitude of the noise. We found that both these parameters are resonantly enhanced when the cavity mode is excited but enhancement of the dissipation dominates leading to the net suppression of diffusion and dramatic narrowing of the linewidth. The line shape changes from Lorentzian to Gaussian when either the Josephson frequency is shifted away from the resonance or the temperature is increased.

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arXiv2013

Josephson vortex lattice in layered superconductors

Many superconducting materials are composed of weakly coupled conducting layers. Such a layered structure has a very strong influence on the properties of vortex matter in a magnetic field. This review focuses on the properties of the Josephson vortex lattice generated by the magnetic field applied in the layers direction. The theoretical description is based on the Lawrence-Doniach model in the London limit which takes into account only the phase degree of freedom of the superconducting order parameter. In spite of its simplicity, this model leads to an amazingly rich set of phenomena. We review in details the structure of an isolated vortex line as well as various properties of the vortex lattice, both in dilute and dense limits. In particular, we present an extensive consideration on the influence of the layered structure and thermal fluctuations on the selection of lattice configurations at different magnetic fields.

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arXiv2012

Anomalous proximity effects at the interface of s and s+- superconductors

We study proximity effects close to a boundary between s and s+- superconductors. Frustration, caused by interaction of the s-wave gap parameter with the opposite-sign gaps of s+- superconductor, leads to several anomalous features. In the case of strong frustration a nontrivial time-reversal-symmetry breaking (TRSB) state, with nonzero phase angles between all gap parameters, is possible. In a more typical state, the s-wave order parameter is aligned with one of the s+- gaps. The other (anti-aligned) gap induces negative feature in the s-wave density of states, which can serve as a fingerprint of s+- state. Another consequence of the frustration is an extended region in the parameter space in which s-wave superconductivity is suppressed, despite being in contact with nominally stronger superconductor. This negative proximity effect is always present for the TRSB state, but extends even into the aligned states. We study these effects within a simple microscopic model assuming dirty limit in all bands, which allows us to model the system in terms of minimum number of the most relevant parameters. The described anomalous features provide a route to establishing the possible s+- state in the iron-based superconductors

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