1,897 research outputs found
On the chiral anomaly in non-Riemannian spacetimes
The translational Chern-Simons type three-form coframe torsion on a
Riemann-Cartan spacetime is related (by differentiation) to the Nieh-Yan
four-form. Following Chandia and Zanelli, two spaces with non-trivial
translational Chern-Simons forms are discussed. We then demonstrate, firstly
within the classical Einstein-Cartan-Dirac theory and secondly in the quantum
heat kernel approach to the Dirac operator, how the Nieh-Yan form surfaces in
both contexts, in contrast to what has been assumed previously.Comment: 18 pages, RevTe
Effective rate equations for the over-damped motion in fluctuating potentials
We discuss physical and mathematical aspects of the over-damped motion of a
Brownian particle in fluctuating potentials. It is shown that such a system can
be described quantitatively by fluctuating rates if the potential fluctuations
are slow compared to relaxation within the minima of the potential, and if the
position of the minima does not fluctuate. Effective rates can be calculated;
they describe the long-time dynamics of the system. Furthermore, we show the
existence of a stationary solution of the Fokker-Planck equation that describes
the motion within the fluctuating potential under some general conditions. We
also show that a stationary solution of the rate equations with fluctuating
rates exists.Comment: 18 pages, 2 figures, standard LaTeX2
Temperature in One-Dimensional Bosonic Mott insulators
The Mott insulating phase of a one-dimensional bosonic gas trapped in optical
lattices is described by a Bose-Hubbard model. A continuous unitary
transformation is used to map this model onto an effective model conserving the
number of elementary excitations. We obtain quantitative results for the
kinetics and for the spectral weights of the low-energy excitations for a broad
range of parameters in the insulating phase. By these results, recent Bragg
spectroscopy experiments are explained. Evidence for a significant temperature
of the order of the microscopic energy scales is found.Comment: 8 pages, 7 figure
Quantum Oscillations in the Underdoped Cuprate YBa2Cu4O8
We report the observation of quantum oscillations in the underdoped cuprate
superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed
magnetic fields up to 85T. There is a clear signal, periodic in inverse field,
with frequency 660+/-15T and possible evidence for the presence of two
components of slightly different frequency. The quasiparticle mass is
m*=3.0+/-0.3m_e. In conjunction with the results of Doiron-Leyraud et al. for
YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a
general feature of underdoped copper oxide planes and provide information about
the doping dependence of the Fermi surface.Comment: Contains revisions addressing referees' comments including a
different Fig 1b. 4 pages, 4 figure
Molecular Rayleigh Scattering Diagnostic for Measurement of High Frequency Temperature Fluctuations
A novel technique for measurement of high frequency temperature fluctuations in unseeded gas flows using molecular Rayleigh scattering is investigated. The spectrum of laser light scattered from molecules in a gas flow is resolved using a Fabry-Perot interferometer. The width of the spectral peak is broadened by thermal motion of the molecules and hence is related to gas temperature. The interference fringe pattern containing spectral information is divided into four concentric regions using a series of mirrors angled with respect to one another. Light from each of these regions is directed towards photomultiplier tubes and sampled at 10 kHz using photon counting electronics. Monitoring the relative change in intensity within each region allows measurement of gas temperature. Independently monitoring the total scattered intensity provides a measure of gas density. This technique also has the potential to simultaneously measure a single component of flow velocity by monitoring the spectral peak location. Measurements of gas temperature and density are demonstrated using a low speed heated air jet surrounded by an unheated air co-flow. Mean values of temperature and density are shown for radial scans across the jet flow at a fixed axial distance from the jet exit plane. Power spectra of temperature and density fluctuations at several locations in the jet are also shown. The instantaneous measurements have fairly high uncertainty; however, long data records provide highly accurate statistically quantities, which include power spectra. Mean temperatures are compared with thermocouple measurements as well as the temperatures derived from independent density measurements. The accuracy for mean temperature measurements was +/- 7 K
Supersymmetric quantum cosmology for Bianchi class A models
The canonical theory of (N=1) supergravity, with a matrix representation for
the gravitino covector-spinor, is applied to the Bianchi class A spatially
homogeneous cosmologies. The full Lorentz constraint and its implications for
the wave function of the universe are analyzed in detail. We found that in this
model no physical states other than the trivial "rest frame" type occur.Comment: 10 pages, Revte
Calculating critical temperatures of superconductivity from a renormalized Hamiltonian
It is shown that one can obtain quantitatively accurate values for the
superconducting critical temperature within a Hamiltonian framework. This is
possible if one uses a renormalized Hamiltonian that contains an attractive
electron-electron interaction and renormalized single particle energies. It can
be obtained by similarity renormalization or using flow equations for
Hamiltonians. We calculate the critical temperature as a function of the
coupling using the standard BCS-theory. For small coupling we rederive the
McMillan formula for Tc. We compare our results with Eliashberg theory and with
experimental data from various materials. The theoretical results agree with
the experimental data within 10%. Renormalization theory of Hamiltonians
provides a promising way to investigate electron-phonon interactions in
strongly correlated systems.Comment: 6 pages, LaTeX, using EuroPhys.sty, one eps figure included, accepted
for publication in Europhys. Let
Anisotropy Reversal of the Upper Critical Field at Low Temperatures and Spin-Locked Superconductivity in K2Cr3As3
We report the first measurements of the anisotropic upper critical field
for KCrAs single crystals up to 60 T and K. Our results show that the upper critical field parallel to the Cr
chains, , exhibits a paramagnetically-limited behavior,
whereas the shape of the curve (perpendicular to the Cr
chains) has no evidence of paramagnetic effects. As a result, the curves
and cross at K, so that
the anisotropy parameter
increases from near to at 0.6 K. This behavior of is inconsistent with triplet
superconductivity but suggests a form of singlet superconductivity with the
electron spins locked onto the direction of Cr chains
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