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.Comment: Proceedings of SCES-0

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

On the use of the ratio of small to large separations in asteroseismic model fitting

Context. The use of ratios of small to large separations as a diagnostic of stellar interiors. Aims. To demonstrate that model fitting by comparing observed and model separation ratios at the same n values is in error, and to present a correct procedure. Methods. Theoretical analysis using phase shifts and numerical models. Results. We show that the separation ratios of stellar models with the same interior structure, but different outer layers, are not the same when compared at the same n values, but are the same when evaluated at the same frequencies by interpolation. The separation ratios trace the phase shift differences as a function of frequency not of n. We give examples from model fitting where the ratios at the same n values agree within the error estimates, but do not agree when evaluated at the same frequencies and the models do not have the same interior structure. The correct procedure is to compare observed ratios with those of models interpolated to the observed frequencies.Comment: 7 pages, 14 figures, 3 table

Heat transport in nonuniform superconductors

We calculate electronic energy transport in inhomogeneous superconductors using a fully self-consistent non-equilibrium quasiclassical Keldysh approach. We develop a general theory and apply it a superconductor with an order parameter that forms domain walls, of the type encountered in Fulde-Ferrell-Larkin-Ovchinnikov state. The heat transport in the presence of a domain wall is inherently anisotropic and non-local. Bound states in the nonuniform region play a crucial role and control heat transport in several ways: (i) they modify the spectrum of quasiparticle states and result in Andreev reflection processes, and (ii) they hybridize with impurity band and produce local transport environment with properties very different from those in uniform superconductor. As a result of this interplay, heat transport becomes highly sensitive to temperature, magnetic field and disorder. For strongly scattering impurities we find that the transport across domain walls at low temperatures is considerably more efficient than in the uniform superconducting state.Comment: 14 pages, 13 figure

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$_5$.Comment: 18 pages, 10 figure

Domain Walls in Superfluid 3He-B

We consider domain walls between regions of superfluid 3He-B in which one component of the order parameter has the opposite sign in the two regions far from one another. We report calculations of the order parameter profile and the free energy for two types of domain wall, and discuss how these structures are relevant to superfluid 3He confined between two surfaces.Comment: 6 pages with 3 figures. Conference proceedings of QSF 2004, Trento, Ital