311 research outputs found
Pairing state in multicomponent superconductors
We use the microscopic weak coupling theory to predict the pairing state in
superconductors of cubic, hexagonal, or tetragonal symmetry, where the order
parameter is multicomponent, i.e., transforms according to either a
2-dimensional or a 3-dimensional representation of the crystal point group. We
show that the superconducting phase usually breaks the time-reversal symmetry
for singlet multicomponent superconductors. The superconducting order parameter
for triplet superconductors in most cases turns out to be non-magnetic.Comment: 7 page
Phases of the 2D Hubbard model at low doping
We show that the planar spiral phase of the 2D Hubbard model at low doping,
x, is unstable towards a noncoplanar spin configuration. The novel equilibrium
state we found at low doping is incommensurate with the inverse pitch of the
spiral varying as x^(1/2), but nevertheless has a dominant peak in the
susceptibility at (\pi,\pi). Relevance to the NMR and neutron scattering
experiments in La_2-xSr_xCuO_4 is disccussed.Comment: 12 pages, emtex v.3.
Theory of Unconventional Spin Density Wave: A Possible Mechanism of the Micromagnetism in U-based Heavy Fermion Compounds
We propose a novel spin density wave (SDW) state as a possible mechanism of
the anomalous antiferromagnetism, so-called the micromagnetism, in URu_2Si_2
below 17.5[K]. In this new SDW, the electron-hole pair amplitude changes its
sign in the momentum space as in the case of the unconventional
superconductivity. It is shown that this state can be realized in an extended
Hubbard model within the mean field theory. We also examine some characteristic
properties of this SDW to compare with the experimental results. All these
properties well explain the unsolved problem of the micromagnetism.Comment: REVTeX v3.1, 4 pages, 5 figure
Impurity correlations in dilute Kondo alloys
The single impurity Kondo model is often used to describe metals with dilute
concentrations (n_i) of magnetic impurities. Here we examine how dilute the
impurities must be for this to be valid by developing a virial expansion in
impurity density. The O(n_i^2) term is determined from results on the
2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial
fixed point of the 2-impurity problem could produce novel singularities in the
heat capacity of dilute alloys at O(n_i^2).Comment: 6 pages, no figure
Correlation length in cuprates deduced from the impurity-induced magnetization
We report a new multi-nuclei based NMR method which allows us to image the
staggered polarization induced by nonmagnetic Li impurities in underdoped O6.6
and slightly overdoped O7 YBa2Cu3O6+y above T_C. The spatial extension of the
polarization xi_imp approximately follows a Curie law, increasing up to six
lattice constants at T=80K at O6.6 in the pseudogap regime. Near optimal
doping, the staggered magnetization has the same shape, with xi_imp reduced by
a factor 2. xi_imp is argued to reveal the intrinsic magnetic correlation
length of the pure system. It is found to display a smooth evolution through
the pseudogap regime.Comment: 8 latex pages + 8 figures, to appear in Physical Review B, this
resubmitted version is twice longer than the previous one : we detail here
our method to determine the impurity-induced magnetizatio
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
Superconductivity in an organic insulator at very high magnetic fields
We investigate by electrical transport the field-induced superconducting
state (FISC) in the organic conductor -(BETS)FeCl. Below 4 K,
antiferromagnetic-insulator, metallic, and eventually superconducting (FISC)
ground states are observed with increasing in-plane magnetic field. The FISC
state survives between 18 and 41 T, and can be interpreted in terms of the
Jaccarino-Peter effect, where the external magnetic field {\em compensates} the
exchange field of aligned Fe ions. We further argue that the Fe
moments are essential to stabilize the resulting singlet, two-dimensional
superconducting stateComment: 9 pages 3 figure
On superconducting and magnetic properties of iron-oxypnictides
Pairing symmetry in oxypnictides, a new family of multiband high-Tc
superconductors, is partially imposed by the positions of multiple Fermi
pockets, which itself can give rise to new order parameters, such as s+,-
states or the state of dx^2-y^2 symmetry. Other pairing states may appear on
small pockets for long range interactions, but they are expected to be
sensitive to defects. We identify the competing antiferromagnetic order with
the triplet exciton transition in the semi- metallic background and discuss
whether its coexistence with superconductivity explains the doping dependence
of Tc.Comment: Fig1b replace
Theory of anomalous magnetic interference pattern in mesoscopic SNS Josephson junctions
The magnetic interference pattern in mesoscopic SNS Josephson junctions is
sensitive to the scattering in the normal part of the system. In this paper we
investigate it, generalizing Ishii's formula for current-phase dependence to
the case of normal scattering at NS boundaries in an SNS junction of finite
width. The resulting flattening of the first diffraction peak is consistent
with experimental data for S-2DEG-S mesoscopic junctions.Comment: 6 pages, 5 figures. Phys. Rev. B 68, 144514 (2003
Magnetism, superconductivity, and pairing symmetry in Fe-based superconductors
We analyze antiferromagnetism and superconductivity in novel based
superconductors within the itinerant model of small electron and hole pockets
near and . We argue that the effective interactions in both
channels logarithmically flow towards the same values at low energies, {\it
i.e.}, antiferromagnetism and superconductivity must be treated on equal
footings. The magnetic instability comes first for equal sizes of the two
pockets, but looses to superconductivity upon doping. The superconducting gap
has no nodes, but changes sign between the two Fermi surfaces (extended s-wave
symmetry). We argue that the dependencies of the spin susceptibility and
NMR relaxation rate for such state are exponential only at very low , and
can be well fitted by power-laws over a wide range below .Comment: 12 pages, 9 figures. References added, the formula for the
susceptibility correcte
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