1,066 research outputs found
High-temperature expansion of the viscosity in interacting quantum gases
We compute the frequency-dependent shear and bulk viscosity spectral
functions of an interacting Fermi gas in a quantum virial expansion up to
second quadratic order in the fugacity parameter , which is
small at high temperatures. Calculations are carried out using a diagrammatic
finite-temperature field-theoretic framework, in which the analytic
continuation from Matsubara to real frequencies is carried out in closed
analytic form. Besides a possible zero-frequency Drude peak, our results for
the spectral functions show a broad continuous spectrum at all frequencies with
an additional bound-state contribution for frequencies larger than the
dimer-breaking energy. Our results are consistent with various sum rules and
universal high-frequency tails. In the low-frequency limit, the shear viscosity
spectral function is recast as a collision integral, which reproduces known
results for the static shear viscosity from kinetic theory. Our findings for
the static bulk viscosity of a Fermi gas near unitarity, however, show a
nonanalytic dependence on the scattering length, at variance with kinetic
theory.Comment: 16 pages, 8 figure
Mesoscopic pairing without superconductivity
We discuss pairing signatures in mesoscopic nanowires with variable
attractive pairing interaction. Depending on wire length, density, and
interaction strength, these systems realize a simultaneous bulk-to-mesoscopic
and BCS-BEC crossover, which we describe in terms of the parity parameter that
quantifies the odd-even energy difference and generalizes the bulk Cooper pair
binding energy to mesoscopic systems. We show that the parity parameter can be
extracted from recent measurements of conductance oscillations in SrTiO
nanowires by G. Cheng et al. [Nature 521, 196 (2015)], where it marks the
critical magnetic field that separates pair and single-particle currents. Our
results place the experiment in the fluctuation-dominated mesoscopic regime on
the BCS side of the crossover.Comment: 6 pages, 4 figure
Current response, structure factor and hydrodynamic quantities of a two- and three-dimensional Fermi gas from the operator-product expansion
We apply the operator-product expansion to determine the asymptotic form of
the current response of a Fermi gas in two and three dimensions. The
leading-order term away from the one-particle peak is proportional to a
quantity known as the contact, the coefficient of which is determined exactly.
We also calculate the dynamic structure factor and the high-frequency tails of
the spectral viscosities as a function of the scattering length. Our results
are used to derive certain sum rules for the viscosities.Comment: 14 pages, 10 figure
Dimensional reduction in quantum field theories at finite temperature and density
In this work we present two correspondences between the massless Gross-Neveu
model with one or two coupling constants in 1+1 dimensions and nonrelativistic
field theories in 3+1 dimensions. It is shown that on a mean-field level the
massless Gross-Neveu model can be mapped onto BCS theory provided that
translational invariance of the condensate is assumed. The generalized massless
Gross-Neveu model with two coupling constants is mapped onto a quasi
one-dimensional extended Hubbard model used in the description of spin-Peierls
systems. It is shown that the particle hole symmetry of the Hubbard model
implies self-consistency of the condensate. The dimensional reduction allows an
identification of the phase diagrams of the models.Comment: 10 pages, 6 figures; v2: corrected typos and references, version
accepted for publication in PR
Quantum oscillations in Dirac magnetoplasmons
The plasmon frequency in standard electron gases with a parabolic
single-particle dispersion is a purely classical quantity that is not sensitive
to electron interactions or the equation of state. We demonstrate that this
canonical result no longer holds for plasmons in three-dimensional semimetals,
which can thus be used to probe many-body effects in these systems. In
particular, we show that the plasmon frequency in an external magnetic field
displays quantum oscillations, which is not the case for the electron gas.
Using the random phase approximation, results are presented for the
magnetoplasmon dispersion and the loss function in Dirac semimetals. We include
a full discussion of the loss function in a magnetic field as a function of the
direction of propagation with respect to the magnetic field direction and
discuss the transition from large magnetic fields to the low-field limit.Comment: 10 pages, 5 figure
Pairing effects in the non-degenerate limit of the two-dimensional Fermi gas
The spectral function of a spin-balanced two-dimensional Fermi gas with
short-range interactions is calculated by means of a quantum cluster expansion.
Good qualitative agreement is found with a recent experiment by Feld
[Nature (London) , 75 (2011)]. The effects of
pairing are clearly visible in the density of states, which displays a
suppression of spectral weight due to the formation of a two-body bound state.
In addition, the momentum distribution and the radio-frequency spectrum are
derived, which are in excellent agreement with exact universal results. It is
demonstrated that in the limit of high temperature, the quasiparticle
excitations are well defined, allowing for a kinetic description of the gas.Comment: 11 pages, 9 figures, updated to status of published versio
Efimov correlations in strongly interacting Bose gases
We compute the virial coefficients, the contact parameters, and the momentum
distribution of a strongly interacting three-dimensional Bose gas by means of a
virial expansion up to third order in the fugacity, which takes into account
three-body correlations exactly. Our results characterize the nondegenerate
regime of the interacting Bose gas, where the thermal wavelength is smaller
than the interparticle spacing but the scattering length may be arbitrarily
large. We observe a rapid variation of the third virial coefficient as the
scattering length is tuned across the three-atom and the atom-dimer thresholds.
The momentum distribution at unitarity displays a universal high-momentum tail
with a log-periodic momentum dependence, which is a direct signature of Efimov
physics. We provide a quantitative description of the momentum distribution at
high momentum as measured by P. Makotyn et al. [Nat. Phys. 10, 116 (2014)], and
our calculations indicate that the lowest trimer state might not be occupied in
the experiment. Our results allow for a spectroscopy of Efimov states in the
unitary limit.Comment: 6 pages, 5 figure
Plasmon signature in Dirac-Weyl liquids
We consider theoretically as a function of temperature the plasmon mode
arising in three-dimensional Dirac liquids, i.e., systems with linear chiral
relativistic single-particle dispersion, within the random phase approximation.
We find that whereas no plasmon mode exists in the intrinsic (undoped) system
at zero temperature, there is a well-defined finite-temperature plasmon with
superlinear temperature dependence, rendering the plasmon dispersion widely
tunable with temperature. The plasmon dispersion contains a logarithmic
correction due to the ultraviolet-logarithmic renormalization of the electron
charge, manifesting a fundamental many-body interaction effect as in quantum
electrodynamics. The plasmon dispersion of the extrinsic (doped) system
displays a minimum at finite temperature before it crosses over to the
superlinear intrinsic behavior at higher temperature, implying that the
high-temperature plasmon is a universal feature of Dirac liquids irrespective
of doping. This striking characteristic temperature dependence of intrinsic
Dirac plasmons along with the logarithmic renormalization is a unique
manifestation of the three-dimensional relativistic Dirac nature of
quasiparticle excitations and serves as an experimentally observable signature
of three-dimensional Dirac materials.Comment: 5 pages, 3 figure
Surface plasmon polaritons in topological Weyl semimetals
We consider theoretically surface plasmon polaritons in Weyl semimetals.
These materials contain pairs of band touching points - Weyl nodes - with a
chiral topological charge, which induces an optical anisotropy and anomalous
transport through the chiral anomaly. We show that these effects, which are not
present in ordinary metals, have a direct fundamental manifestation in the
surface plasmon dispersion. The retarded Weyl surface plasmon dispersion
depends on the separation of the Weyl nodes in energy and momentum space. For
Weyl semimetals with broken time-reversal symmetry, the distance between the
nodes acts as an effective applied magnetic field in momentum space, and the
Weyl surface plasmon polariton dispersion is strikingly similar to
magnetoplasmons in ordinary metals. In particular, this implies the existence
of nonreciprocal surface modes. In addition, we obtain the nonretarded Weyl
magnetoplasmon modes, which acquire an additional longitudinal magnetic-field
dependence. These predicted surface plasmon results are observable
manifestations of the chiral anomaly in Weyl semimetals and might have
technological applications.Comment: 8 pages, 2 figure
Short-distance properties of Coulomb systems
We use the operator product expansion to derive exact results for the
momentum distribution and the static structure factor at high momentum for a
jellium model of electrons in both two and three dimensions. It is shown that
independent of the precise state of the Coulomb system and for arbitrary
temperatures, the asymptotic behavior is a power law in the momentum, whose
strength is determined by the contact value of the pair distribution function
. The power-law tails are quantum effects which vanish in the classical
limit . A leading order virial expansion shows that the classical
and the high-temperature limit do not agree.Comment: 13 pages, 4 figures, updated to status of published versio
- …