447 research outputs found
Superconductivity in heavily boron-doped silicon carbide
The discoveries of superconductivity in heavily boron-doped diamond (C:B) in
2004 and silicon (Si:B) in 2006 renew the interest in the superconducting state
of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a
metallic phase from which upon further doping superconductivity can emerge.
Recently, we discovered superconductivity in a closely related system:
heavily-boron doped silicon carbide (SiC:B). The sample used for that study
consists of cubic and hexagonal SiC phase fractions and hence this lead to the
question which of them participates in the superconductivity. Here we focus on
a sample which mainly consists of hexagonal SiC without any indication for the
cubic modification by means of x-ray diffraction, resistivity, and ac
susceptibility.Comment: 9 pages, 5 figure
BCS-BEC crossover of collective excitations in two-band superfluids
We use the functional integral approach to study low energy collective
excitations in a continuum model of neutral two-band superfluids at T=0 for all
couplings with a separable pairing interaction. In the long wavelength and low
frequency limit, we recover Leggett's analytical results in weak coupling (BCS)
for s-wave pairing, and further obtain analytical results in strong coupling
(BEC) for both two and three dimensional systems. We also analyse numerically
the behavior of the out-of-phase {\it exciton} (finite frequency) mode and the
in-phase {\it phonon} (Goldstone) mode from weak to strong coupling limits,
including the crossover region. In principle, the evolution of Goldstone and
finite frequency modes from weak to strong coupling may be accessible
experimentally in the superfluid phase of neutral Fermi atomic gases, and could
serve as a test of the validity of the theoretical analysis and approximations
proposed here.Comment: 14 pages, 9 figures. Submitted to PR
Presence of 3d Quadrupole Moment in LaTiO3 Studied by 47,49Ti NMR
Ti NMR spectra of LaTiO3 are reexamined and the orbital state of this
compound is discussed. The NMR spectra of LaTiO3 taken at 1.5 K under zero
external field indicate a large nuclear quadrupole splitting. This splitting is
ascribed to the presence of the rather large quadrupole moment of 3d electrons
at Ti sites, suggesting that the orbital liquid model proposed for LaTiO3 is
inappropriate. The NMR spectra are well explained by the orbital ordering model
expressed approximately as originating from
a crystal field effect. It is also shown that most of the orbital moment is
quenched.Comment: 4 pages, 3 fugures; to appear in Phys. Rev. Let
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