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.Comment: 4+3 pages, published version with supplemen

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 $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$.Comment: 17 pages, 13 figure

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

We analyze 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 Hc2 (T) in the vicinity of the transition temperature does not always give accurate values of the orbital upper critical field; (b) critical value of the Maki parameter, α, which determines onset of the first order transition, depends on the Fermi surface shape and the symmetry of the gap and is α* ≈ 3 for CeCoIn5; and (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 x2 - y2 order parameter CeCoIn5. © 2010 The American Physical Society

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.Comment: 4 pages, 4 figure

Pauli-Limited Superconductivity with Classical Magnetic Fluctuations

We examine the effect of classical magnetic fluctuations on the phase diagram of paramagneticallylimited two-dimensional superconductors under a Zeeman magnetic field. We derive the free energy expansion in powers of the superconducting order parameter and analyze the character of the normalsuperconducting transition. While the transition is of the second order for all temperatures in the absence of magnetic fluctuations, we find that proximity to magnetism drives both the transition into the uniform state and that into the modulated (Fulde-Ferrell-Larkin-Ovchinnikov, FFLO) state to first order at intermediate temperatures. We compute the thermodynamic signatures of the normal-superconducting transition along the upper critical field.Comment: 16 pages, 9 figure

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 dirty d -wave superconductors within the quasiclassical theory using the self-consistent 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 Tc suppression of roughly 40%. Our results bear on the search of FFLO states in heavy-fermion and layered organic superconductors. © 2008 The American Physical Society

Role of the fermi-surface anisotropy in angle-dependent magnetic-field oscillations for identifying the energy-gap anisotropy of A yFe 2Se 2 superconductors

We present a numerical study of the field-angle resolved oscillations of the thermal conductivity and specific heat under a rotated magnetic field in the A yFe 2-xSe 2 [A=K, Rb, Cs, (Tl, K)] superconductors, using realistic two-band Fermi surface parametrization. Our key finding is that even for isotropic pairing on an anisotropic Fermi surface, the thermodynamic quantities exhibit substantial oscillatory behavior in the superconducting state, even much below the upper critical field. Furthermore, in multiband systems the competition of anisotropies between two Fermi surfaces can cause a double sign reversal of oscillations as a function of temperature, irrespective of gap anisotropy. Our findings put severe constraints on simple interpretations of field-angle resolved measurements widely used to identify the angular structure of the superconducting gap. © 2012 American Physical Society