979 research outputs found
Electron capture rates in a plasma
A new general expression is derived for nuclear electron capture rates within
dense plasmas. Its qualitative nature leads us to question some widely accepted
assumptions about how to calculate the effects of the plasma on the rates. A
perturbative evaluation, though not directly applicable to the strongly
interacting case, appears to bear out these suspicions.Comment: 9 page
Asymmetric Two-component Fermion Systems in Strong Coupling
We study the phase structure of a dilute two-component Fermi system with
attractive interactions as a function of the coupling and the polarization or
number difference between the two components. In weak coupling, a finite number
asymmetry results in phase separation. A mixed phase containing symmetric
superfluid matter and an asymmetric normal phase is favored. With increasing
coupling strength, we show that the stress on the superfluid phase to
accommodate a number asymmetry increases. Near the infinite-scattering length
limit, we calculate the single-particle excitation spectrum and the
ground-state energy at various polarizations. A picture of weakly-interacting
quasi-particles emerges for modest polarizations. In this regime near infinite
scattering length, and for modest polarizations, a homogeneous phase with a
finite population of excited quasi-particle states characterized by a gapless
spectrum should be favored over the phase separated state. These states may be
realized in cold atom experiments.Comment: 4 pages, 3 figur
Anomalous specific heat jump in a two-component ultracold Fermi gas
The thermodynamic functions of a Fermi gas with spin population imbalance are
studied in the temperature-asymmetry plane in the BCS limit. The low
temperature domain is characterized by anomalous enhancement of the entropy and
the specific heat above their values in the unpaired state, decrease of the gap
and eventual unpairing phase transition as the temperature is lowered. The
unpairing phase transition induces a second jump in the specific heat, which
can be measured in calorimetric experiments. While the superfluid is unstable
against a supercurrent carrying state, it may sustain a metastable state if
cooled adiabatically down from the stable high-temperature domain. In the
latter domain the temperature dependence of the gap and related functions is
analogous to the predictions of the BCS theory.Comment: 4 pages, 3 figures. v2 includes a discussion of instabilities; v3:
final version to appear in PR
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
Finite Temperature Phase Diagram of a Two-Component Fermi Gas with Density Imbalance
We investigated possible superfluid phases at finite temperature in a
two-component Fermi gas with density imbalance. In the frame of a general
four-fermion interaction theory, we solved in the BCS region the gap equations
for the pairing gap and pairing momentum under the restriction of fixed number
densities, and analyzed the stability of different phases by calculating the
superfluid density and number susceptibilities. The homogeneous superfluid is
stable only at high temperature and low number asymmetry, the inhomogeneous
LOFF survives at low temperature and high number asymmetry, and in between them
there exists another possible inhomogeneous phase, that of phase separation.
The critical temperatures and the orders of the phase transitions among the
superfluid phases and normal phase are calculated analytically and numerically.
The phase diagram we obtained in the temperature and number asymmetry plane is
quite different from the one in temperature and chemical potential difference
plane for a system with fixed chemical potentials.Comment: Final published versio
Constraining the nuclear equation of state at subsaturation densities
Only one third of the nucleons in Pb occupy the saturation density
area. Consequently nuclear observables related to average properties of nuclei,
such as masses or radii, constrain the equation of state (EOS) not at
saturation density but rather around the so-called crossing density, localised
close to the mean value of the density of nuclei: 0.11 fm.
This provides an explanation for the empirical fact that several EOS quantities
calculated with various functionals cross at a density significantly lower than
the saturation one. The third derivative M of the energy at the crossing
density is constrained by the giant monopole resonance (GMR) measurements in an
isotopic chain rather than the incompressibility at saturation density. The GMR
measurements provide M=1110 70 MeV (6% uncertainty), whose extrapolation
gives K=230 40 MeV (17% uncertainty).Comment: 4 pages, 4 figure
Theory of Diamagnetism in the Pseudogap Phase: Implications from the Self energy of Angle Resolved Photoemission
In this paper we apply the emerging- consensus understanding of the fermionic
self energy deduced from angle resolved photoemisssion spectroscopy (ARPES)
experiments to deduce the implications for orbital diamagnetism in the
underdoped cuprates. Many theories using many different starting points have
arrived at a broadened BCS-like form for the normal state self energy
associated with a d-wave excitation gap, as is compatible with ARPES data.
Establishing compatibility with the f-sum rules, we show how this self energy,
along with the constraint that there is no Meissner effect in the normal phase
are sufficient to deduce the orbital susceptibility. We conclude, moreover,
that diamagnetism is large for a d-wave pseudogap. Our results should apply
rather widely to many theories of the pseudogap, independent of the microscopic
details.Comment: 15 pages, 8 figure
Multi-scale fluctuations near a Kondo Breakdown Quantum Critical Point
We study the Kondo-Heisenberg model using a fermionic representation for the
localized spins. The mean-field phase diagram exhibits a zero temperature
quantum critical point separating a spin liquid phase where the f-conduction
hybridization vanishes, and a Kondo phase where it does not. Two solutions can
be stabilized in the Kondo phase, namely a uniform hybridization when the band
masses of the conduction electrons and the f spinons have the same sign, and a
modulated one when they have opposite sign. For the uniform case, we show that
above a very small Fermi liquid temperature scale (~1 mK), the critical
fluctuations associated with the vanishing hybridization have dynamical
exponent z=3, giving rise to a specific heat coefficient that diverges
logarithmically in temperature, as well as a conduction electron inverse
lifetime that has a T log T behavior. Because the f spinons do not carry
current, but act as an effective bath for the relaxation of the current carried
by the conduction electrons, the latter result also gives rise to a T log T
behavior in the resistivity. This behavior is consistent with observations in a
number of heavy fermion metals.Comment: 17 pages, 10 figure
Surface Energy in Cold Asymmetrical Fermion Superfluids
We derive the energy of the surface between the normal and superfluid
components of a mixed phase of a system composed of two particle species with
different densities. The surface energy is obtained by the integration of the
free energy density in the interface between the two phases. We show that the
mixed phase remains as the favored ground state over the gapless phase in weak
coupling. We find that the surface energy effects emerge only at strong
coupling.Comment: 12 pages, 2 figures, typos corrected, published versio
- …