221 research outputs found
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 and organic
superconductor -(BEDT-TTF)Cu(NCS).Comment: 17 pages, 13 figure
Angle-dependent magnetothermal conductivity in d-wave superconductors
We analyse the behavior of the thermal conductivity, , 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 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.Comment: 2 pages, submitted to proceedings of M2S-HTSC-VI (Houston
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 () 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.Comment: 18 pages, 10 figure
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
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
. This non-Fermi liquid behavior originates in the coupling of
fermions to the longitudinal spin susceptibility
in which the order-induced ``gap'' in the spectrum at dissolves into the
Landau damping term at . The transverse spin fluctuations obey
scaling characteristic of spin waves, but remain overdamped in a finite
range near the critical point.Comment: 5p., 3fig
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
Importance of van der Waals interactions for ab initio studies of topological insulators
We investigate the lattice and electronic structures of the bulk and surface
of the prototypical layered topological insulators BiSe and
BiTe using ab initio density functional methods, and systematically
compare the results of different methods of including van der Waals (vdW)
interactions. We show that the methods utilizing semi-empirical energy
corrections yield accurate descriptions of these materials, with the most
precise results obtained by properly accounting for the long-range tail of the
vdW interactions. The bulk lattice constants, distances between quintuple
layers and the Dirac velocity of the topological surface states (TSS) are all
in excellent agreement with experiment. In BiTe, hexagonal warping of
the energy dispersion leads to complex spin textures of the TSS at moderate
energies, while in BiSe these states remain almost perfectly helical
away from the Dirac point, showing appreciable signs of hexagonal warping at
much higher energies, above the minimum of the bulk conduction band. Our
results establish a framework for unified and systematic self-consistent first
principles calculations of topological insulators in bulk, slab and interface
geometries, and provides the necessary first step towards ab initio modeling of
topological heterostructures.Comment: 26 pages, 7 figures. This is the Accepted Manuscript version of an
article accepted for publication in Journal of Physics: Condensed Matter. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The Version of Record is
available online at https://dx.doi.org/10.1088/1361-648X/abbdb
Quasiparticle transport in the vortex state of d-wave superconductors
We calculate the magnetic field and temperature dependence of the Raman response, superfluid density, and the NMR relaxation rate in the vortex state of a d-wave superconductor arising from the Doppler energy shift of extended quasiparticle states. Our results are valid both at low temperatures, where we observe scaling with variable (Formula presented) and obtain an explicit form of the scaling functions, and beyond this region. We derive a universal frequency-dependent scaling relation for the Raman intensity, and discuss the breakdown of the single relaxation rate approach to the NMR response. © 1999 The American Physical Society
Anisotropic thermodynamics of (formula presented)-wave superconductors in the vortex state
We show that the density of states and the thermodynamic properties of a two-dimensional (Formula presented)-wave superconductor in the vortex state with applied magnetic field H in the plane depend on the angle between H and the order-parameter nodes. Within a semiclassical treatment of the extended quasiparticle states, we obtain fourfold oscillations of the specific heat, measurement of which provides a simple probe of gap symmetry. The frequency dependence of the density of states and the temperature dependence of thermodynamic properties obey different power laws for field in the nodal and antinodal direction. The fourfold pattern is changed to twofold when orthorhombicity is considered. © 1999 The American Physical Society
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
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