86,702 research outputs found
Spin-wave excitation spectra and spectral weights in square lattice antiferromagnets
Using a recently developed method for calculating series expansions of the
excitation spectra of quantum lattice models, we obtain the spin-wave spectra
for square lattice, Heisenberg-Ising antiferromagnets. The calculated
spin-wave spectrum for the Heisenberg model is close to but noticeably
different from a uniformly renormalized classical (large-) spectrum with the
renormalization for the spin-wave velocity of approximately . The
relative weights of the single-magnon and multi-magnon contributions to neutron
scattering spectra are obtained for wavevectors throughout the Brillouin zone.Comment: Two postscript figures, 4 two-column page
Further Series Studies of the Spin-1/2 Heisenberg Antiferromagnet at T=0: Magnon Dispersion and Structure Factors
We have extended our previous series studies of quantum antiferromagnets at
zero temperature by computing the one-magnon dispersion curves and various
structure factors for the linear chain, square and simple cubic lattices. Many
of these results are new; others are a substantial extension of previous work.
These results are directly comparable with neutron scattering experiments and
we make such comparisons where possible.Comment: 15 pages, 12 figures, revised versio
A characterization of the central shell-focusing singularity in spherical gravitational collapse
We give a characterization of the central shell-focusing curvature
singularity that can form in the spherical gravitational collapse of a bounded
matter distribution obeying the dominant energy condition. This
characterization is based on the limiting behaviour of the mass function in the
neighbourhood of the singularity. Depending on the rate of growth of the mass
as a function of the area radius R, the singularity may be either covered or
naked. The singularity is naked if this growth rate is slower than R, covered
if it is faster than R, and either naked or covered if the growth rate is same
as R.Comment: 12 pages, Latex, significantly revised version, including change of
title. Revised version to appear in Classical and Quantum Gravit
Type-I superconductivity in noncentrosymmetric superconductor AuBe
The noncentrosymmetric superconductor AuBe have been investigated using the
magnetization, resistivity, specific heat, and muon-spin relaxation/rotation
measurements. AuBe crystallizes in the cubic FeSi-type B20 structure with
superconducting transition temperature observed at = 3.2 0.1 K.
The low-temperature specific heat data, (T), indicate a weakly-coupled
fully gapped BCS superconductivity with an isotropic energy gap
2 = 3.76, which is close to the BCS value of 3.52.
Interestingly, type-I superconductivity is inferred from the SR
measurements, which is in contrast with the earlier reports of type-II
superconductivity in AuBe. The Ginzburg-Landau parameter is = 0.4
1/. The transverse-field SR data transformed in the maximum
entropy spectra depicting the internal magnetic field probability distribution,
P(H), also confirms the absence of the mixed state in AuBe. The thermodynamic
critical field, , calculated to be around 259 Oe. The zero-field SR
results indicate that time-reversal symmetry is preserved and supports a
spin-singlet pairing in the superconducting ground state.Comment: 9 pages, 9 figure
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