1,730 research outputs found
An efficient method for the Quantum Monte Carlo evaluation of the static density-response function of a many-electron system
In a recent Letter we introduced Hellmann-Feynman operator sampling in
diffusion Monte Carlo calculations. Here we derive, by evaluating the second
derivative of the total energy, an efficient method for the calculation of the
static density-response function of a many-electron system. Our analysis of the
effect of the nodes suggests that correlation is described correctly and we
find that the effect of the nodes can be dealt with
Two-fluid model for a rotating trapped Fermi gas in the BCS phase
We investigate the dynamical properties of a superfluid gas of trapped
fermionic atoms in the BCS phase. As a simple example we consider the reaction
of the gas to a slow rotation of the trap. It is shown that the currents
generated by the rotation can be understood within a two-fluid model similar to
the one used in the theory of superconductors, but with a position dependent
ratio of normal and superfluid densities. The rather general result of this
paper is that already at very low temperatures, far below the critical one, an
important normal-fluid component appears in the outer regions of the gas. This
renders the experimental observation of superfluidity effects more difficult
and indicates that reliable theoretical predictions concerning other dynamical
properties, like the frequencies of collective modes, can only be made by
taking into account temperature effects.Comment: 6 pages, 4 figure
Kelvin mode of a vortex in a nonuniform Bose-Einstein condensate
In a uniform fluid, a quantized vortex line with circulation h/M can support
long-wavelength helical traveling waves proportional to e^{i(kz-\omega_k t)}
with the well-known Kelvin dispersion relation \omega_k \approx (\hbar k^2/2M)
\ln(1/|k|\xi), where \xi is the vortex-core radius. This result is extended to
include the effect of a nonuniform harmonic trap potential, using a quantum
generalization of the Biot-Savart law that determines the local velocity V of
each element of the vortex line. The normal-mode eigenfunctions form an
orthogonal Sturm-Liouville set. Although the line's curvature dominates the
dynamics, the transverse and axial trapping potential also affect the normal
modes of a straight vortex on the symmetry axis of an axisymmetric Thomas-Fermi
condensate. The leading effect of the nonuniform condensate density is to
increase the amplitude along the axis away from the trap center. Near the ends,
however, a boundary layer forms to satisfy the natural Sturm-Liouville boundary
conditions. For a given applied frequency, the next-order correction
renormalizes the local wavenumber k(z) upward near the trap center, and k(z)
then increases still more toward the ends.Comment: 9 pages, 1 figur
Neutrino Superfluidity
It is shown that Dirac-type neutrinos display BCS superfluidity for any
nonzero mass. The Cooper pairs are formed by attractive scalar Higgs boson
exchange between left- and right-handed neutrinos; in the standard SU(2)xU(1)
theory, right-handed neutrinos do not couple to any other boson. The value of
the gap, the critical temperature, and the Pippard coherence length are
calculated for arbitrary values of the neutrino mass and chemical potential.
Although such a superfluid could conceivably exist, detecting it would be a
major challenge.Comment: This is the version published in PR
Fermionic superfluidity with positive scattering length
Superfluidity in an ultracold Fermi gas is usually associated with either a
negative scattering length, or the presence of a two-body bound state. We show
that none of these ingredients is necessary to achieve superfluidity. Using a
narrow Feshbach resonance with strong repulsive background interactions, the
effective interactions can be repulsive for small energies and attractive for
energies around the Fermi energy, similar to the effective interactions between
electrons in a metallic superconductor. This can result in BCS-type
superfluidity while the scattering length is positive.Comment: 6 pages, 3 figures; v2: added references and details energy-dependent
interactio
Medical applications of nasa-developed science and technology quarterly progress report no. 2, 30 apr. - 31 jul. 1965
Medical applications of aerospace science and technolog
Quantum logic as superbraids of entangled qubit world lines
Presented is a topological representation of quantum logic that views
entangled qubit spacetime histories (or qubit world lines) as a generalized
braid, referred to as a superbraid. The crossing of world lines is purely
quantum in nature, most conveniently expressed analytically with
ladder-operator-based quantum gates. At a crossing, independent world lines can
become entangled. Complicated superbraids are systematically reduced by
recursively applying novel quantum skein relations. If the superbraid is closed
(e.g. representing quantum circuits with closed-loop feedback, quantum lattice
gas algorithms, loop or vacuum diagrams in quantum field theory), then one can
decompose the resulting superlink into an entangled superposition of classical
links. In turn, for each member link, one can compute a link invariant, e.g.
the Jones polynomial. Thus, a superlink possesses a unique link invariant
expressed as an entangled superposition of classical link invariants.Comment: 4 page
Correlation potentials for molecular bond dissociation within the self-consistent random phase approximation
Self-consistent correlation potentials for H and LiH for various
inter-atomic separations are obtained within the random phase approximation
(RPA) of density functional theory. The RPA correlation potential shows a peak
at the bond midpoint, which is an exact feature of the true correlation
potential, but lacks another exact feature: the step important to preserve
integer charge on the atomic fragments in the dissociation limit. An analysis
of the RPA energy functional in terms of fractional charge is given which
confirms these observations. We find that the RPA misses the derivative
discontinuity at odd integer particle numbers but explicitly eliminates the
fractional spin error in the exact-exchange functional. The latter finding
explains the accurate total energy in the dissociation limit.Comment: 9 pages, 10 figure
Flow equation approach to the linear response theory of superconductors
We apply the flow equation method for studying the current-current response
function of electron systems with the pairing instability. To illustrate the
specific scheme in which the flow equation procedure determines the
two-particle Green's functions we reproduce the standard response kernel of the
BCS superconductor. We next generalize this non-perturbative treatment
considering the pairing field fluctuations. Our study indicates that the
residual diamagnetic behavior detected above the transition temperature in the
cuprate superconductors can originate from the noncondensed preformed pairs.Comment: 12 pages, 4 figure
Competition between final-state and pairing-gap effects in the radio-frequency spectra of ultracold Fermi atoms
The radio-frequency spectra of ultracold Fermi atoms are calculated by
including final-state interactions affecting the excited level of the
transition, and compared with the experimental data. A competition is revealed
between pairing-gap effects which tend to push the oscillator strength toward
high frequencies away from threshold, and final-state effects which tend
instead to pull the oscillator strength toward threshold. As a result of this
competition, the position of the peak of the spectra cannot be simply related
to the value of the pairing gap, whose extraction thus requires support from
theoretical calculations.Comment: 4 pages, 3 figures, final version published in Phys. Rev. Let
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