1,593 research outputs found
Probing the topological exciton condensate via Coulomb drag
The onset of exciton condensation in a topological insulator thin film was
recently predicted. We calculate the critical temperature for this transition,
taking into account screening effects. Furthermore, we show that the proximity
to this transition can be probed by measuring the Coulomb drag resistivity
between the surfaces of the thin film as a function of temperature. This
resistivity shows an upturn upon approaching the exciton-condensed state.Comment: 4 pages, 3 figure
Spin transport in a unitary Fermi gas close to the BCS transition
We consider spin transport in a two-component ultracold Fermi gas with
attractive interspecies interactions close to the BCS pairing transition. In
particular, we consider the spin-transport relaxation rate and the
spin-diffusion constant. Upon approaching the transition, the scattering
amplitude is enhanced by pairing fluctuations. However, as the system
approaches the transition, the spectral weight for excitations close to the
Fermi level is decreased by the formation of a pseudogap. To study the
consequence of these two competing effects, we determine the spin-transport
relaxation rate and the spin-diffusion constant using both a Boltzmann approach
and a diagrammatic approach. The former ignores pseudogap physics and finite
lifetime effects. In the latter, we incorporate the full pseudogap physics and
lifetime effects, but we ignore vertex corrections, so that we effectively
calculate single-particle relaxation rates instead of transport relaxation
rates. We find that there is qualitative agreement between these two approaches
although the results for the transport coefficients differ quantitatively.Comment: 9 pages, 10 figure
Interaction effects on dynamic correlations in non-condensed Bose gases
We consider dynamic, i.e., frequency-dependent, correlations in non-condensed
ultracold atomic Bose gases. In particular, we consider the single-particle
correlation function and its power spectrum. We compute this power spectrum for
a one-component Bose gas, and show how it depends on the interatomic
interactions that lead to a finite single-particle relaxation time. As another
example, we consider the power spectrum of spin-current fluctuations for a
two-component Bose gas and show how it is determined by the spin-transport
relaxation time.Comment: 9 pages, 3 figure
Vortex-lattice pinning in two-component Bose-Einstein condensates
We investigate the vortex-lattice structure for single- and two-component
Bose-Einstein condensates in the presence of an optical lattice, which acts as
a pinning potential for the vortices. The problem is considered in the
mean-field quantum-Hall regime, which is reached when the rotation frequency
of the condensate in a radially symmetric trap approaches the (radial)
trapping frequency and the interactions between the atoms are weak. We
determine the vortex-lattice phase diagram as a function of optical-lattice
strength and geometry. In the limit of strong pinning the vortices are always
pinned at the maxima of the optical-lattice potential, similar to the
slow-rotation case. At intermediate pinning strength, however, due to the
competition between interactions and pinning energy, a structure arises for the
two-component case where the vortices are pinned on lines of minimal potential
Mind the Gap:The Location of the Lower Edge of the Pair-instability Supernova Black Hole Mass Gap
Mind the Gap:The Location of the Lower Edge of the Pair-instability Supernova Black Hole Mass Gap
Gravitational-wave detections are now starting to probe the mass distribution
of stellar-mass black holes (BHs). Robust predictions from stellar models are
needed to interpret these. Theory predicts the existence of a gap in the BH
mass distribution because of pair-instability supernova. The maximum BH mass
below the gap is the result of pulsational mass loss. We evolve massive helium
stars through their late hydrodynamical phases of evolution using the
open-source MESA stellar evolution code. We find that the location of the lower
edge of the mass gap at 45 is remarkably robust against variations in
the metallicity (), the treatment of internal mixing
(), stellar wind mass loss (), making it
the most robust predictions for the final stages of massive star evolution. The
reason is that the onset of the instability is dictated by the near-final core
mass, which in turn sets the resulting BH mass. However, varying
reaction rate within its
uncertainties shifts the location of the gap between and
. We provide updated analytic fits for population synthesis
simulations. Our results imply that the detection of merging BHs can provide
constraints on nuclear astrophysics. Furthermore, the robustness against
metallicity suggests that there is a universal maximum for the location of the
lower edge of the gap, which is insensitive to the formation environment and
redshift for first-generation BHs. This is promising for the possibility to use
the location of the gap as a "standard siren" across the Universe.Comment: 17 pages, 5 figures, 1 Table, Accepted Ap
Sensitivity of the lower edge of the pair-instability black hole mass gap to the treatment of time-dependent convection
Solubility isotope effects in aqueous solutions of methane
The isotope effect on the Henry's law coefficients of methane in
aqueous solution (H/D and C-12/C-13 substitution) are interpreted using
the statistical mechanical theory of condensed phase isotope effects.
The missing spectroscopic data needed for the implementation of the
theory were obtained either experimentally (infrared measurements), by
computer simulation (molecular dynamics technique), or estimated using
the Wilson's GF matrix method. The order of magnitude and sign of both
solute isotope effects can be predicted by the theory. Even a crude
estimation based on data from previous vapor pressure isotope effect
studies of pure methane at low temperature can explain the inverse
effect found for the solubility of deuterated methane in water. (C)
2002 American Institute of Physics
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