511 research outputs found
Magnetism of Superconducting UPt3
The phase diagram of superconducting in pressure-temperature
plane, together with the neutron scattering data is studied within a two
component superconducting order parameter scenario. In order to give a
qualitative explanation to the experimental data a set of two linearly
independent antiferromagnetic moments which emerge appropriately at the
temperature \mbox{} and \mbox{} and
couple to superconductivity is proposed. Several constraints on the fourth
order coefficients in the Ginzburg-Landau free energy are obtained.Comment: 17 pages, figures available on request to
[email protected]
c-axis Josephson Tunneling in Twinned YBCO Crystals
Josephson tunneling between YBCO and Pb with the current flowing along the
c-axis of the YBCO is persumed to come from an s-wave component of the
superconductivity of the YBCO. Experiments on multi-twin samples are not
entirely consistent with this hypothesis. The sign change of the s-wave order
parameter across the N_T twin boundaries should give cancelations, resulting in
a small tunneling current. The actual current is larger than this.
We present a theory of this unexpectedly large current based upon a surface
effect: disorder-induced supression of the d-wave component at the (001)
surface leads to s-wave coherence across the twin boundaries and a non-random
tunneling current. We solve the case of an ordered array of d+s and d-s twins,
and estimate that the twin size at which s-wave surface coherence occurs is
consistent with typical sizes observed in experiments. In this picture, there
is a phase difference of between different surfaces of the material. We
propose a corner junction experiment to test this picture.Comment: 5 pages, 4 eps figure
Phase diagram of UPt in the model
The phase diagram of the unconventional superconductor UPt is explained
under the long-standing hypothesis that the pair wavefunction belongs to the
representation of the point group. The main objection to this theory
has been that it disagrees with the experimental phase diagram when a field is
applied along the c-axis. By a careful analysis of the free energy this
objection is shown to be incorrect. This singlet theory also explains the
unusual anisotropy in the upper critical field curves, often thought to
indicate a triplet pair function.Comment: 11 pages, Revtex, 2 figures (uuencoded, gzip'ed Postscript
Theory of Optical Orientation in n-Type Semiconductors
Time resolved measurements of magnetization in n-GaAs have revealed a rich
array of spin decoherence processes, and have shown that fairly long lifetimes
(\sim 100 ns) can be achieved under certain circumstances. In time-resolved
Faraday rotation and time-resolved Kerr rotation the evolution of the
magnetization can be followed as a function of temperature, applied field,
doping level and excitation level. We present a theory for the spin relaxation
in n-GaAs based on a set of rate equations for two interacting thermalized
subsystems of spins: localized states on donor sites and itinerant states in
the conduction band. The conduction band spins relax by scattering from defects
or phonons through the D'yakonov-Perel' mechanism, while the localized spins
relax by interacting with phonons (when in an applied field) or through the
Dzyaloshinskii-Moriya interaction. In this model, numerous features of the
data, including puzzling temperature and doping dependences of the relaxation
time, find an explanation.Comment: 4 pages, 2 figures; revised version has a more complete discussion of
the Elliott-Yafet and spin-phonon decay mechanism
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