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 $1/\sqrt{3}(d_{xy}+d_{yz}+d_{zx})$ 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