540 research outputs found
Exotic ground states and impurities in multiband superconductors
We consider the effect of isotropic impurity scattering on the exotic
superconducting states that arise from the usual BCS mechanism in substances of
cubic and hexagonal symmetry where the Fermi surface contains inequivalent but
degenerate pockets (e.g. around several points of high symmetry). As examples
we look at CeCo, CeRu, and LaB; all of which have such Fermi
surface topologies and the former exhibits unconventional superconducting
behavior. We find that while these non s-wave states are suppressed by
non-magnetic impurities, the suppression is much weaker than would be expected
for unconventional superconductors with isotropic non-magnetic impurity
scattering.Comment: 4 pages, no figure
Effective field theories and spin-wave excitations in helical magnets
We consider two classes of helical magnets. The first one has magnetic
ordering close to antiferromagnet and the second one has magnetic ordering
close to ferromagnet. The first case is relevant to cuprate superconductors and
the second case is realized in FeSrO and FeCaO. We derive the effective
field theories for these cases and calculate corresponding excitation spectra.
We demonstrate that the "hourglass" spin-wave dispersion observed
experimentally in cuprates is a fingerprint of the "antiferromagnetic spin
spiral state". We also show that quantum fluctuations are important for the
"ferromagnetic spin spiral", they influence qualitative features of the
spin-wave dispersion.Comment: 14 pages, 11 figure
Exact results for one-dimensional disordered bosons with strong repulsion
We study one-dimensional incommensurate bosons with strong repulsive
interactions and weak disorder. In analogy to the clean Tonks-Girardeau gas, a
Bose-Fermi mapping expresses this problem in terms of disordered free fermions.
Thereby many known results apply, in particular for the density-density
correlations, the distribution function of the local density of states, and the
complete spectral statistics. We also analyze the bosonic momentum
distribution, and comment on the experimental observability of these
predictions in ultracold atomic gases.Comment: 5 pages, 2 figures, published versio
Motion of vortices in type II superconductors
The methods of formal asymptotics are used to examine the behaviour of a system of curvilinear vortices in a type II superconductor as the thickness of the vortex cores tends to zero. The vortices then appear as singularities in the field equation and are analagous to line vortices in inviscid hydrodynamics. A local analysis near each vortex core gives an equation of motion governing the evolution of these singularities
Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets
We discuss the Josephson effect in strongly spin-polarized ferromagnets where
triplet correlations are induced by means of spin-active interface scattering,
extending our earlier work [Phys. Rev. Lett. 102, 227005 (2009)] by including
impurity scattering in the ferromagnetic bulk and the inverse proximity effect
in a fully self-consistent way. Our quasiclassical approach accounts for the
differences of Fermi momenta and Fermi velocities between the two spin bands of
the ferromagnet, and thereby overcomes an important short-coming of previous
work within the framework of Usadel theory. We show that non-magnetic disorder
in conjunction with spin-dependent Fermi velocities may induce a reversal of
the spin-current as a function of temperature.Comment: 12 pages, 9 figure
Peculiarities in Low Temperature Properties of Doped Manganites A1-xBxMnO3
The phase diagram and low temperature properties of the doped manganites
A1-xBxMnO3 are discussed for the concentrations x < 0.4. The transition from
insulating antiferromagnetic to metallic ferromagnetic state at x_cr = 0.16 is
treated by means of percolation theory. The unifying description of insulating
and metallic states is presented. The undoped manganite is a band insulator
consisting of ferromagnetic layers, which are coupled antiferromagnetically
along the c direction with a low Neel temperature. The metallic phase can be
described by the two-band Fermi liquid picture. The behavior of conductivity,
spin wave excitations, etc. is analyzed and the comparison with experimental
data is carried out.Comment: To appear in EPJ, 35 pages, 5 figure
Ginzburg-Landau Like Theory for High Temperature Superconductivity in the Cuprates: Emergent d-wave Order
High temperature superconductivity in the cuprates remains one of the most
widely investigated, constantly surprising, and poorly understood phenomena in
physics. Here, we describe briefly a new phenomenological theory inspired by
the celebrated description of superconductivity due to Ginzburg and Landau and
believed to describe its essence. This posits a free energy functional for the
superconductor in terms of a complex order parameter characterizing it. We
propose, for superconducting cuprates, a similar functional of the complex, in
plane, nearest neighbor spin singlet bond (or Cooper) pair amplitude psi_ij. A
crucial part of it is a (short range) positive interaction between nearest
neighbor bond pairs, of strength J'. Such an interaction leads to nonzero long
wavelength phase stiffness or superconductive long range order, with the
observed d-wave symmetry, below a temperature T_c\simzJ' where z is the number
of nearest neighbours; it is thus an emergent, collective consequence. Using
the functional, we calculate a large range of properties, e.g. the pseudogap
transition temperature T* as a function of hole doping x, the transition curve
T_c(x), the superfluid stiffness rho_s(x,T), the specific heat (without and
with a magnetic field) due to the fluctuating pair degrees of freedom, and the
zero temperature vortex structure. We find remarkable agreement with
experiment. We also calculate the self energy of electrons hopping on the
square cuprate lattice and coupled to electrons of nearly opposite momenta via
inevitable long wavelength Cooper pair fluctuations formed of these electrons.
The ensuing results for electron spectral density are successfully compared
with recent ARPES experiments, and comprehensively explain strange features
such as temperature dependent Fermi arcs above T_c and the 'bending' of the
superconducting gap below T_c .Comment: 22 pages, 14 figures, to appear in Int J Mod Phys
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