arXiv subjects

I. Vekhter

Publications and source records attributed to I. Vekhter.

At least 55 records · Page 3Linked to original sources

arXiv2007

Thermal conductivity evidence for d_{x^2-y^2} pairing symmetry in the heavy-fermion CeIrIn5 superconductor

Quasi-two dimensional CeIrIn5 contains two distinct domes with different heavy fermion superconducting states in its phase diagram. Here we pinned down the superconducting gap structure of CeIrIn5 in the second dome, located away from the antiferromagnetic quantum critical point, by the thermal transport measurements in magnetic fields rotated relative to the crystal axes. Clear fourfold oscillation was observed when the field is rotated within the ab-plane, while no oscillation was observed within the bc-plane. In sharp contrast to previous reports, our results are most consistent with d_{x^2-y^2} symmetry, implying that two superconducting phases have the same gap symmetry which appears to be mediated by antiferromagnetic spin fluctuations.

cond-mat.supr-con↗

arXiv2007

Unconventional superconductors under rotating magnetic field I: density of states and specific heat

We develop a fully microscopic theory for the calculations of the angle-dependent properties of unconventional superconductors under a rotated magnetic field. We employ the quasiclassical Eilenberger equations, and use a variation of the Brandt-Pesch-Tewordt (BPT) method to obtain a closed form solution for the Green's function. The equations are solved self-consistently for quasi-two-dimensional $d_{x^2-y^2}$ ($d_{xy}$) superconductors with the field rotated in the basal plane. The solution is used to determine the density of states and the specific heat. We find that applying the field along the gap nodes may result in minima or maxima in the angle-dependent specific heat, depending on the location in the T-H plane. This variation is attributed to the scattering of the quasiparticles on vortices, which depends on both the field and the quasiparticle energy, and is beyond the reach of the semiclassical approximation. We investigate the anisotropy across the T-H phase diagram, and compare our results with the experiments on heavy fermion CeCoIn$_5$.

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arXiv2007

Unconventional superconductors under rotating magnetic field II: thermal transport

We present a microscopic approach to the calculations of thermal conductivity in unconventional superconductors for a wide range of temperatures and magnetic fields. Our work employs the non-equilibrium Keldysh formulation of the quasiclassical theory. We solve the transport equations using a variation of the Brandt-Pesch-Tewordt (BPT) method, that accounts for the quasiparticle scattering on vortices. We focus on the dependence of the thermal conductivity on the direction of the field with the respect to the nodes of the order parameter, and discuss it in the context of experiments aiming to determine the shape of the gap from such anisotropy measurements. We consider quasi-two dimensional Fermi surfaces with vertical line nodes and use our analysis to establish the location of gap nodes in heavy fermion CeCoIn$_5$ and organic superconductor $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$.

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arXiv2006

Interplane and intraplane heat transport in quasi two-dimensional nodal superconductors

We analyze the behavior of the thermal conductivity in quasi-two dimensional superconductors with line nodes. Motivated by measurements of the anisotropy between the interplane and intraplane thermal transport in CeIrIn_5 we show that a simple model of the open Fermi surface with vertical line nodes is insufficient to describe the data. We propose two possible extensions of the model taking into account a) additional modulation of the gap along the axial direction of the open Fermi surface; and b) dependence of the interplane tunneling on the direction of the in-plane momentum. We discuss the temperature dependence of the thermal conductivity anisotropy and its low T limit in these two models and compare the results with a model with a horizontal line of nodes (``hybrid gap''). We discuss possible relevance of each model for the symmetry of the order parameter in CeIrIn_5, and suggest further experiments aimed at clarifying the shape of the superconducting gap.

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arXiv2006

Nodal Structure of Unconventional Superconductors Probed by the Angle Resolved Thermal Transport Measurements

Over the past two decades, unconventional superconductivity with gap symmetry other than s-wave has been found in several classes of materials, including heavy fermion (HF), high-T_c, and organic superconductors. Unconventional superconductivity is characterized by anisotropic superconducting gap functions, which may have zeros (nodes) along certain directions in the Brillouin zone. The nodal structure is closely related to the pairing interaction, and it is widely believed that the presence of nodes is a signature of magnetic or some other exotic, rather than conventional phonon-mediated, pairing mechanism. Therefore experimental determination of the gap function is of fundamental importance. However, the detailed gap structure, especially the direction of the nodes, is an unresolved issue in most unconventional superconductors. Recently it has been demonstrated that the thermal conductivity and specific heat measurements under magnetic field rotated relative to the crystal axes are a powerful method for determining the shape of the gap and the nodal directions in the bulk. Here we review the theoretical underpinnings of the method and the results for the nodal structure of several unconventional superconductors, including borocarbide YNi$_2$B$_2$C, heavy fermions UPd$_2$Al$_3$, CeCoIn$_5$, and PrOs$_4$Sb$_{12}$, organic superconductor, $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$, and ruthenate Sr$_2$RuO$_4$, determined by angular variation of the thermal conductivity and heat capacity.

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arXiv2006

Superconducting gap nodes in PrOs_4Sb_12

We examine the superconducting gap nodes in crystals with tetrahedral (T_h) symmetry. The (0,0,1) phase of the three dimensional order parameter in the triplet channel has nodes in the [001] directions. Following a second order phase transition to the state (0,i|eta_2|,|eta_1|), each node lifts away from the Fermi surface and splits into two deep dips. We discuss this scenario in the context of multiple superconducting phases in PrOs4Sb12.

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arXiv2006

Anisotropy of heat capacity in Pauli limited unconventional superconductors

Diamagnetically coupled magnetic field can be used as a probe of nodal positions in unconventional superconductors. The heat capacity depends on the angle $ϕ_0$ between the magnetic field and the nodal directions. We show that the anisotropy $C(ϕ_0)$ persists even in systems with strong paramagnetic coupling to the electrons's spins.

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arXiv2006

Nodal structure of quasi-2D superconductors probed by magnetic field

We consider a quasi two-dimensional superconductor with line nodes in an in-plane magnetic field, and compute the dependence of the specific heat, $C$, and the in-plane heat conductivity, $κ$, on the angle between the field and the nodal direction in the vortex state. We use a variation of the microscopic Brandt-Pesch-Tewordt method that accounts for the scattering of quasiparticles off vortices, and analyze the signature of the nodes in $C$ and $κ$. At low to moderate fields the specific heat anisotropy changes sign with increasing temperature. Comparison with measurements of $C$ and $κ$ in CeCoIn$_5$ resolves the contradiction between the two in favor of the $d_{x^2-y^2}$ gap.

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arXiv2004

Impurity-induced states in conventional and unconventional superconductors

We review recent developments in our understanding of how impurities influence the electronic states in the bulk of superconductors. Our focus is on the quasi-localized states in the vicinity of impurity sites in conventional and unconventional superconductors and our goal is to provide a unified framework for their description. The non-magnetic impurity resonances in unconventional superconductors are directly related to the Yu-Shiba-Rusinov states around magnetic impurities in conventional s-wave systems. We review the physics behind these states, including quantum phase transition between screened and unscreened impurity, and emphasize recent work on d-wave superconductors. The bound states are most spectacularly seen in scanning tunneling spectroscopy measurements on high-$T_c$ cuprates, which we describe in detail. We also discuss very recent progress on the states coupled to impurity sites which have their own dynamics, and impurity resonances in the presence of an order competing with superconductivity. Last part of the review is devoted to influence of local deviations of the impurity concentration from its average value on the density of states in s-wave superconductors. We review how these fluctuations affect the density of states and show that s-wave superconductors are, strictly speaking, gapless in the presence of an arbitrarily small concentration of magnetic impurities.

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arXiv2004

Evidence for the Coexistence of Anisotropic Superconducting Gap and Nonlocal Effects in the Non-magnetic Superconductor LuNi2B2C

A study of the dependence of the heat capacity Cp(alpha) on field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, Cp(alpha) exhibits a fourfold variation as the field H < Hc2 is rotated in the [001] plane, with minima along <100> (alpha = 0). A slightly disordered sample, however, develops anomalous secondary minima along <110> for H > 1 T, leading to an 8-fold pattern. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and non-local effects.

cond-mat.supr-con↗

arXiv2004

Non-fermi liquid behavior in itinerant antiferromagnets

We consider a two dimensional itinerant antiferromagnet near a quantum critical point. We show that, contrary to conventional wisdom, fermionic excitations in the ordered state are not the usual Fermi liquid quasiparticles. Instead, down to very low frequencies, the fermionic self energy varies as $ω^{2/3}$. This non-Fermi liquid behavior originates in the coupling of fermions to the longitudinal spin susceptibility $χ_{\parallel}(q, Ω)$ in which the order-induced ``gap'' in the spectrum at $q=0$ dissolves into the Landau damping term at $v_F q >Ω$. The transverse spin fluctuations obey $z=1$ scaling characteristic of spin waves, but remain overdamped in a finite range near the critical point.

cond-mat.str-el↗

arXiv2003

Tail states in clean superconductors with magnetic impurities

We analyse the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit. By using the method of optimal fluctuation and treating the order parameter self-consistently we show that the density of states is finite everywhere in the superconducting gap, and that it varies as $\ln N(E)\propto -|E-Δ_0|^{(7-d)/4}$ near the mean field gap edge $Δ_0$ in a d-dimensional superconductor. In contrast to most studied cases the optimal fluctuation is strongly anisotropic.

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arXiv2003

A proposal to determine the spectrum of pairing-glue in high-temperature superconductors

We propose a method for an analysis of the angle-resolved photoemission data in two-dimensional anisotropic superconductors which directly yields the spectral function of the bosons mediating Cooper pairing. The method includes a self-consistency check for the validity of the approximations made in the analysis. We explicitly describe the experimental data needed for implementing the proposed procedure.

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arXiv2003

Avoided Antiferromagnetic Order and Quantum Critical Point in CeCoIn$_5$

We measured specific heat and resistivity of heavy fermion CeCoIn5 between the superconducting critical field $H_{c2} = 5 T$ and 9 T, with field in the [001] direction, and at temperatures down to 50mK. At 5T the data show Non Fermi Liquid behavior down to the lowest temperatures. At field above 8T the data exhibit crossover from the Fermi liquid to a Non Fermi Liquid behavior. We analyzed the scaling properties of the specific heat, and compared both resistivity and the specific heat with the predictions of a spin-fluctuation theory. Our analysis leads us to suggest that the NFL behavior is due to incipient antiferromagnetism (AF) in CeCoIn5, with the quantum critical point in the vicinity of the $H_{c2}$. Below $H_{c2}$ the AF phase which competes with the paramagnetic ground state is superseded by the superconducting transition.

cond-mat.str-el↗

arXiv2002

Tail states in superconductors with weak magnetic impurities

We analyse the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit by using the method of optimal fluctuation. We show that the density of states varies as $\ln N(E)\propto -|E-Δ_0|^{(7-d)/4}$ near the mean field gap edge $Δ_0$ in a d-dimensional superconductor. The optimal fluctuation in d>1 is strongly anisotropic. We compare the density of states with that obtained in other recent approaches.

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arXiv2000

Thermodynamics of d-wave superconductors in a magnetic field

We investigate the thermodynamic properties of a two-dimensional d-wave superconductor in the vortex state using a semiclassical approach, and argue that such an approach is valid for the analysis of the experimental data on high-temperature superconductors. We develop a formalism where the spatial average of a physical quantity is written as an integral over the probability density of the Doppler shift, and evaluate this probability density for several model cases. The approach is then used to analyse the behavior of the specific heat and the NMR spin-lattice relaxation rate in a magnetic field. We compare our results with the experimental measurements, and explain the origin of the discrepancy between the results from different groups. We also address the observability of the recently predicted fourfold oscillations of the specific heat for the magnetic field parallel to the copper oxide planes. We consider both the orbital and the Zeeman effects, and conclude that at experimentally relevant temperatures Zeeman splitting does not appreciably reduce the anisotropy, although it does change the field dependence of the anisotropic specific heat. We predict a scaling law for the non-exponentially decaying NMR magnetization, and discuss different approaches to the effective relaxation rate.

cond-mat.supr-con↗

arXiv1999

Thermal conductivity in the vortex state of d-wave superconductors

We present the results of a microscopic calculation of the longitudinal thermal conductivity of quasiparticles, $κ_{xx}$, in a 2D d-wave superconductor in the vortex state. Our approach takes into account both impurity scattering and a contribution to the thermal transport lifetime due to the scattering of quasiparticles off of vortices. We compare the results with the experimental measurements on high-T$_c$ cuprates and organic superconductors.

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arXiv1999

Angle-dependent magnetothermal conductivity in d-wave superconductors

We analyse the behavior of the thermal conductivity, $κ(H)$, in the vortex state of a quasi-two-dimensional d-wave superconductor when both the heat current and the applied magnetic field are in the basal plane. At low temperature the effect of the field is accounted for in a semiclassical approximation, via a Doppler shift in the spectrum of the nodal quasiparticles. In that regime $κ(H)$ exhibits twofold oscillations as a function of the angle between the direction of the field in the plane and the direction of the heat current, in agreement with experiment.

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