172 research outputs found
Observations of the 8 December 1987 occultation of AG+40 deg 0783 by 324 Bamberga
The occultation of AG+40 deg 0783 by 324 Bamberga on 8 December 1987 was observed at 13 sites in the United States, Japan, and China. At four sites the event was observed photoelectrically; the other observations were visual. A least-squares fit of a circular limb profile to the data gives a diameter of 227.6 + or - 1.9 km. However, this solution is inconsistent with a negative visual observation near the northern edge of the ground track. The inconsistency cannot be removed by assuming an elliptical profile. The data suggest that Bamberga, despite its low-amplitude lightcurve, may depart significantly from a spherical or ellipsoidal shape. The asteroid also appears to be at least 10 percent smaller than indicated by infrared radiometry
The interplay between double exchange, super-exchange, and Lifshitz localization in doped manganites
Considering the disorder caused in manganites by the substitution of Mn by Fe
or Ga, we accomplish a systematic study of doped manganites begun in previous
papers. To this end, a disordered model is formulated and solved using the
Variational Mean Field technique. The subtle interplay between double exchange,
super-exchange, and disorder causes similar effects on the dependence of T_C on
the percentage of Mn substitution in the cases considered. Yet, in
LaCaMnGaO our results suggest a quantum
critical point (QCP) for , associated to the localization of
the electronic states of the conduction band. In the case of
LaCaMnFeO (with ) no such QCP is expected.Comment: 6 pages + 3 postscript figures. Largely extended discussio
Role of anisotropic impurity scattering in anisotropic superconductors
A theory of nonmagnetic impurities in an anisotropic superconductor including
the effect of anisotropic (momentum-dependent) impurity scattering is given. It
is shown that for a strongly anisotropic scattering the reduction of the
pair-breaking effect of the impurities is large. For a significant overlap
between the anisotropy functions of the scattering potential and that of the
pair potential and for a large amount of anisotropic scattering rate in
impurity potential the superconductivity becomes robust vis a vis impurity
concentration. The implications of our result for YBCO high-temperature
superconductor are discussed.Comment: 14 pages, RevTeX, 5 PostScript figures, to be published in Phys. Rev.
B (December 1, 1996
Perturbation study on the spin and charge susceptibilities of the two-dimensional Hubbard model
We investigate the spin and charge susceptibilities of the two-dimensional
Hubbard model based upon the perturbative calculation in the strength of
correlation . For comparable to a bare bandwidth, the charge
susceptibility decreases near the half-filling as hole-doping approaches zero.
This behavior suggesting the precursor of the Mott-Hubbard gap formation cannot
be obtained without the vertex corrections beyond the random phase
approximation. In the low-temperature region, the spin susceptibility deviates
from the Curie-Weiss-like law and finally turns to decrease with the decrease
of temperature. This spin-gap-like behavior is originating from the van Hove
singularity in the density of states.Comment: Revtex file + 11 figures, to appear in Phys. Rev.
Quasiclassical description of transport through superconducting contacts
We present a theoretical study of transport properties through
superconducting contacts based on a new formulation of boundary conditions that
mimics interfaces for the quasiclassical theory of superconductivity. These
boundary conditions are based on a description of an interface in terms of a
simple Hamiltonian. We show how this Hamiltonian description is incorporated
into quasiclassical theory via a T-matrix equation by integrating out
irrelevant energy scales right at the onset. The resulting boundary conditions
reproduce results obtained by conventional quasiclassical boundary conditions,
or by boundary conditions based on the scattering approach. This formalism is
well suited for the analysis of magnetically active interfaces as well as for
calculating time-dependent properties such as the current-voltage
characteristics or as current fluctuations in junctions with arbitrary
transmission and bias voltage. This approach is illustrated with the
calculation of Josephson currents through a variety of superconducting
junctions ranging from conventional to d-wave superconductors, and to the
analysis of supercurrent through a ferromagnetic nanoparticle. The calculation
of the current-voltage characteristics and of noise is applied to the case of a
contact between two d-wave superconductors. In particular, we discuss the use
of shot noise for the measurement of charge transferred in a multiple Andreev
reflection in d-wave superconductors
Hidden Order in the Cuprates
We propose that the enigmatic pseudogap phase of cuprate superconductors is
characterized by a hidden broken symmetry of d(x^2-y^2)-type. The transition to
this state is rounded by disorder, but in the limit that the disorder is made
sufficiently small, the pseudogap crossover should reveal itself to be such a
transition. The ordered state breaks time-reversal, translational, and
rotational symmetries, but it is invariant under the combination of any two. We
discuss these ideas in the context of ten specific experimental properties of
the cuprates, and make several predictions, including the existence of an
as-yet undetected metal-metal transition under the superconducting dome.Comment: 12 pages of RevTeX, 9 eps figure
Assessment of motor and process skills reflects brain-injured patients' ability to resume independent living better than neuropsychological tests
Critical temperature of an anisotropic superconductor containing both nonmagnetic and magnetic impurities
The combined effect of both nonmagnetic and magnetic impurities on the
superconducting transition temperature is studied theoretically within the BCS
model. An expression for the critical temperature as a function of potential
and spin-flip scattering rates is derived for a two-dimensional superconductor
with arbitrary in-plane anisotropy of the superconducting order parameter,
ranging from isotropic s-wave to d-wave (or any pairing state with nonzero
angular momentum) and including anisotropic s-wave and mixed (d+s)-wave as
particular cases. This expression generalizes the well-known Abrikosov-Gor'kov
formula for the critical temperature of impure superconductors. The effect of
defects and impurities in high temperature superconductors is discussed.Comment: 4 eps figure
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