arXiv subjects

I. Vekhter

Publications and source records attributed to I. Vekhter.

At least 37 records · Page 2Linked to original sources

arXiv2012

Structure of magnetic order in Pauli limited unconventional superconductors

We analyse the magnetic structure of the antiferromagnetic order induced in Pauli limited d-wave superconductors by Zeeman coupling to magnetic field. We determine the phase diagram in the H-T plane, and find that the magnetic phase, which is stabilized at low temperatures and just below the upper critical field, can have two realizations depending primarily on the shape of the underlying Fermi surface. The double-Q magnetic ordering may persist over the entire coexistence range. Alternatively, there may exist a weak first order transition from a double-Q structure at lower fields to a single-Q modulation at higher fields. Together with the calculations of the NMR line-shape these results suggest the second scenario as a serious candidate for describing the superconducting state of CeCoIn5.

cond-mat.supr-con↗

arXiv2012

Kondo effect in the presence of spin-orbit coupling

We study the T=0 Kondo physics of a spin-1/2 impurity in a non-centrosymmetric metal with spin-orbit interaction. Within a simple variational approach we compute ground state properties of the system for an {\it arbitrary} form of spin-orbit coupling consistent with the crystal symmetry. This coupling produces an unscreened impurity magnetic moment and can lead to a significant change of the Kondo energy. We discuss implications of this finding both for dilute impurities and for heavy-fermion materials without inversion symmetry.

cond-mat.str-el↗

arXiv2011

Theory of quasiparticle vortex bound states in Fe-based superconductors: application to LiFeAs

Spectroscopy of vortex bound states can provide valuable information on the structure of the superconducting order parameter. Quasiparticle wavefunctions are expected to leak out in the directions of gap minima or nodes, if they exist, and scanning tunneling spectroscopy (STS) on these low-energy states should probe the momentum dependence of the gap. Anisotropy can also arise from band structure effects, however. We perform a quasiclassical calculation of the density of states of a single vortex in an anisotropic superconductor, and show that if the gap itself is not highly anisotropic, the Fermi surface anisotropy dominates, preventing direct observation of superconducting gap features. This serves as a cautionary message for the analysis of STS data on the vortex state on Fe-based superconductors, in particular LiFeAs, which we treat explicitly.

cond-mat.supr-con↗

arXiv2011

Antiferromagnetic Order in Pauli Limited Unconventional Superconductors

We develop a theory of the coexistence of superconductivity (SC) and antiferromagnetism (AFM) in CeCoIn5. We show that in Pauli-limited nodal superconductors the nesting of the quasi-particle pockets induced by Zeeman pair-breaking leads to incommensurate AFM with the moment normal to the field. We compute the phase diagram and find a first order transition to the normal state at low temperatures, absence of normal state AFM, and coexistence of SC and AFM at high fields, in agreement with experiments. We also predict the existence of a new double-Q magnetic phase.

cond-mat.supr-con↗

arXiv2011

Volovik effect in a highly anisotropic multiband superconductor: experiment and theory

We present measurements of the specific heat coefficient γ(= C/T) in the low temperature limit as a function of an applied magnetic field for the Fe-based superconductor BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$. We find both a linear regime at higher fields and a limiting square root $H$ behavior at very low fields. The crossover from a Volovik-like $\sqrt{H}$ to a linear field dependence can be understood from a multiband calculation in the quasiclassical approximation assuming gaps with different momentum dependence on the hole- and electron-like Fermi surface sheets.

cond-mat.supr-con↗

arXiv2011

Nodal gap structure of BaFe_2(As_{1-x}P_x)_2 from angle-resolved thermal conductivity in a magnetic field

The structure of the superconducting order parameter in the iron-pnictide superconductor BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ ($T_c=31$\,K) with line nodes is studied by the angle-resolved thermal conductivity measurements in a magnetic field rotated within the basal plane. We find that the thermal conductivity displays distinct fourfold oscillations with minima when the field is directed at $\pm45^\circ$ with respect to the tetragonal a-axis. We discuss possible gap structures that can account for the data, and conclude that the observed results are most consistent with the closed nodal loops located at the flat parts of the electron Fermi surface with high Fermi velocity.

cond-mat.supr-con↗

arXiv2011

Effect of annealing on the specific heat of Ba(Fe1-xCox)2As2

We report on the effect of annealing on the temperature and field dependencies of the low temperature specific heat of the electron-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for under-(x = 0.045), optimal- (x = 0.08) and over-doped (x = 0.105 and 0.14) regimes. We observed that annealing significantly improves some superconducting characteristics in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. It considerably increases $T_{c}$, decreases $γ_{0}$ in the superconducting state and suppresses the Schottky-like contribution at very low temperatures. The improved sample quality allows for a better identification of the superconducting gap structure of these materials. We examine the effects of doping and annealing within a self-consistent framework for an extended s-wave pairing scenario. At optimal doping our data indicates the sample is fully gapped, while for both under and overdoped samples significant low-energy excitations possibly consistent with a nodal structure remain. The difference of sample quality offers a natural explanation for the variation in low temperature power laws observed by many techniques.

cond-mat.supr-con↗

arXiv2010

Calorimetric Evidence for Nodes in the Overdoped Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$

We present low-temperature specific heat of the electron-doped Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$, which does not show any indication of an upturn down to 400 mK, the lowest measuring temperature. The lack of a Schottky-like feature at low temperatures or in magnetic fields up to 9 Tesla enables us to identify enhanced low-temperature quasiparticle excitations and to study anisotropy in the linear term of the specific heat. Our results can not be explained by a single or multiple isotropic superconducting gap, but are consistent with multi-gap superconductivity with nodes on at least one Fermi surface sheet.

cond-mat.supr-con↗

arXiv2010

Nodes vs. minima in the energy gap of iron-pnictides from field-induced anisotropy

We develop the formalism for computing the oscillations of the specific heat and thermal transport under rotated magnetic field in multiband superconductors with anisotropic gap and apply it to iron-pnictides. We show that these oscillations change sign at low temperatures and fields, which strongly influences the conclusions about the gap structure based on experiment. We find that recent measurements of the specific heat oscillations indicate that the iron-based superconductors possess an anisotropic gap with deep minima or nodes close to the line connecting electron and hole pockets. We make predictions for the behavior of the thermal conductivity that will help distinguish between these cases.

cond-mat.supr-con↗

arXiv2010

Structure and physical properties of the noncentrosymmetric superconductor Mo3Al2C

We have synthesized polycrystalline samples of the noncentrosymmetric superconductor Mo3Al2C by arc and RF melting, measured its transport, magnetic and thermodynamic properties, and computed its band structure. Experimental results indicate a bulk superconducting transition at Tc ~ 9.2 K, while the density of states at the Fermi surface is found to be dominated by Mo d-orbitals. Using the measured values for the lower critical field Hc1, upper critical field Hc2, and the specific heat C, we estimated the thermodynamic critical field Hc(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ(0). The specific heat jump at Tc, ΔC/γTc = 2.14, suggests that Mo3Al2C is moderately-to-strongly coupled, consistent with the fast opening of the gap, as evidenced by the rapid release of entropy below Tc from our electronic specific heat measurements. Above 2K the electronic specific heat exhibits the power law behavior, suggesting that synthesis of single crystals and measurements at lower temperature are needed to establish whether the gap is anisotropic. The estimated value of the upper critical field Hc2(0) is close to the calculated Pauli limit, therefore further studies are needed to determine whether the absence of an inversion center results in a significant admixture of the triplet component of the order parameter.

cond-mat.supr-con↗

arXiv2010

Fermi surface evolution through a heavy fermion superconductor-to-antiferromagnet transition: de Haas-van Alphen effect in Cd-substituted CeCoIn$_5$

We report the results of de-Haas-van-Alphen (dHvA) measurements in Cd doped CeCoIn$_5$ and LaCoIn$_5$. Cd doping is known to induce an antiferromagnetic order in the heavy fermion superconductor CeCoIn$_5$, whose effect can be reversed with applied pressure. We find a slight but systematic change of the dHvA frequencies with Cd doping in both compounds, reflecting the chemical potential shift due to the addition of holes. The frequencies and effective masses are close to those found in the nominally pure compounds with similar changes apparent in the Ce and La compounds with Cd substitution. We observe no abrupt changes to the Fermi surface in the high field paramagnetic state for $x \sim x_c$ corresponding to the onset of antiferromagnetic ordering at H=0 in CeCo(In$_{1-x}$Cd$_x$)$_5$. Our results rule out $f-$electron localization as the mechanism for the tuning of the ground state in CeCoIn$_5$ with Cd doping.

cond-mat.str-el↗

arXiv2010

Vortex state in d-wave superconductors with strong paramagnetism: transport and specific heat anisotropy

We analyse the combined effect of orbital and Pauli depairing on the superconducting state, and apply the results to the heavy fermion CeCoIn$_5$. We find that: a) standard extrapolation based on the slope of $H_{c2}(T)$ in the vicinity of the transition temperature to T=0 does not always give accurate values of the orbital upper critical field; b) critical value of the Maki parameter, $α$, that determines onset of the first order transition depends on the Fermi surface shape and the symmetry of the gap, and is $α^*\approx 3$ for CeCoIn$_5$; c) the anisotropy of the thermodynamic and transport coefficients in the low-temperature, low-field part of the phase diagram is essentially insensitive to the Zeeman field and can be used to determine the nodal directions in Pauli-limited superconductors. The latter result confirms the finding of the $d_{x^2-y^2}$ order parameter CeCoIn$_5$.

cond-mat.supr-con↗

arXiv2009

Direct measurements of the penetration depth in a superconducting film using magnetic force microscopy

We report the local measurements of the magnetic penetration depth $λ$ in a superconducting Nb film using magnetic force microscopy (MFM). We developed a method for quantitative extraction of the penetration depth from single-parameter simultaneous fits to the lateral and height profiles of the MFM signal, and demonstrate that the obtained value is in excellent agreement with that obtained from the bulk magnetization measurements.

cond-mat.mtrl-sci↗

arXiv2009

Theory of thermal conductivity in extended-$s$ state superconductors: application to ferropnictides

Within a two-band model for the recently discovered ferropnictide materials, we calculate the thermal conductivity assuming general superconducting states of $A_{1g}$ ("s-wave") symmetry, considering both currently popular isotropic "sign-changing" $s$ states and states with strong anisotropy, including those which manifest nodes or deep minima of the order parameter. We consider both intra- and interband disorder scattering effects, and show that in situations where a low-temperature linear-$T$ exists in the thermal conductivity, it is not always "universal" as in d-wave superconductors. We discuss the conditions under which such a term can disappear, as well as how it can be induced by a magnetic field. We compare our results to several recent experiments.

cond-mat.supr-con↗

arXiv2008

Inversion of specific heat oscillations with in-plane magnetic field angle in 2D d-wave superconductors

Experiments on several novel superconducting compounds have observed oscillations of the specific heat when an applied magnetic field is rotated with respect to the crystal axes. The results are commonly interpreted as arising from the nodes of an unconventional order parameter, but the identifications of nodal directions are sometimes controversial. Here we show with a semiclassical model calculation that when the magnetic field points in the direction of the nodes, either minima or maxima can occur in the specific heat depending on the the temperature $T$ and the magnetic field $H$. An inversion of the angular oscillations takes place with respect to those predicted earlier at low temperature by the nodal approximation. This result, together with the argument that the inversion takes place based on an approximation valid at moderate fields, indicates that the inversion of specific heat oscillations is an intrinsic feature of nodal superconductors.

cond-mat.supr-con↗

arXiv2008

Pauli-limited upper critical field in dirty d-wave superconductors

We calculate the Pauli-limited upper critical field and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) instability for {\it dirty} d-wave superconductors within the quasiclassical theory using the self-consistent $\hat{t}$-matrix approximation for impurities. We find that the phase diagram depends sensitively on the scattering rate and phase shift of nonmagnetic impurities. The transition into the superconducting state is always second order for weak (Born) scattering, while in the unitarity (strong) scattering limit a first-order transition into both uniform and spatially modulated superconducting states is stabilized. Contrary to general belief, we find that the FFLO phase is robust against disorder and survives impurity scattering equivalent to a $T_c$ suppression of roughly 40%. Our results bear on the search of FFLO states in heavy-fermion and layered organic superconductors.

cond-mat.supr-con↗

arXiv2008

Surface bound states and spin currents in non-centrosymmetric superconductors

We investigate the influence of spin-orbit coupling in a non-centrosymmetric superconductor on its ground state properties near a surface. We determine the spectrum of Andreev bound states due to surface-induced mixing of bands with opposite spin helicities for a Rashba-type spin-orbit coupling. We find a qualitative change of the Andreev spectrum when we account for the suppression of the order parameter near the surface, leading to clear signatures in the surface density of states. We also compute the spin current at the surface, which has spin polarization normal to that of the bulk current. The magnitude of the current at the surface is enhanced in the normal state compared to the bulk, and even further enlarged in the superconducting phase. The particle and hole coherence amplitudes show Faraday-like rotations of the spin along quasiparticle trajectories.

cond-mat.supr-con↗

arXiv2008

Searching for zeroes: unconventional superconductors in a magnetic field

We review the results of the microscopic approach to the calculation of the anisotropy in the specific heat in unconventional superconductors under rotated field. Treating vortex scattering on equal footing with the energy shift we find that the electronic specific heat may have minima or maxima when the field is aligned with the nodes, depending on the temperature and field range. We discuss the influence of the paramagnetic limiting and Fermi surface shape on the location of the inversion line.

cond-mat.supr-con↗