1,490 research outputs found
Pauli blocking effects and Cooper triples in three-component Fermi gases
We investigate the effect of Pauli blocking on universal two- and three-body
states in the medium. Their corresponding energies are extracted from the poles
of two- and three-body in-medium scattering amplitudes. Compared to the vacuum,
the binding of dimer and trimer states is reduced by the medium effects. In
two-body scattering, the well-known physics of Cooper pairs is recovered. In
the three-body sector, we find a new class of positive energy poles which can
be interpreted as Cooper triples.Comment: 19 pages, 9 figures, discussion expanded, final versio
Light nuclei quasiparticle energy shift in hot and dense nuclear matter
Nuclei in dense matter are influenced by the medium. In the cluster mean
field approximation, an effective Schr\"odinger equation for the -particle
cluster is obtained accounting for the effects of the correlated medium such as
self-energy, Pauli blocking and Bose enhancement. Similar to the single-baryon
states (free neutrons and protons), the light elements (,
internal quantum state ) are treated as quasiparticles with energies
. These energies depend on the center of mass
momentum , as well as temperature and the total densities
of neutrons and protons, respectively. No equilibrium is considered so
that (or the corresponding chemical potentials ) are
fixed independently.
For the single nucleon quasiparticle energy shift, different approximate
expressions such as Skyrme or relativistic mean field approaches are well
known. Treating the -particle problem in appropriate approximations, results
for the cluster quasiparticle shifts are given. Properties of dense nuclear
matter at moderate temperatures in the subsaturation density region considered
here are influenced by the composition. This in turn is determined by the
cluster quasiparticle energies, in particular the formation of clusters at low
densities when the temperature decreases, and their dissolution due to Pauli
blocking as the density increases. Our finite-temperature Green function
approach covers different limiting cases: The low-density region where the
model of nuclear statistical equilibrium and virial expansions can be applied,
and the saturation density region where a mean field approach is possible
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
Origin of electron-hole asymmetry in the scanning tunneling spectrum of
We have developed a material specific theoretical framework for modelling
scanning tunneling spectroscopy (STS) of high temperature superconducting
materials in the normal as well as the superconducting state. Results for
(Bi2212) show clearly that the tunneling process
strongly modifies the STS spectrum from the local density of states (LDOS) of
the orbital of Cu. The dominant tunneling channel to the surface
Bi involves the orbitals of the four neighbouring Cu atoms. In
accord with experimental observations, the computed spectrum displays a
remarkable asymmetry between the processes of electron injection and
extraction, which arises from contributions of Cu and other orbitals
to the tunneling current.Comment: 5 pages, 4 figures, published in PR
Normal Modes of a Vortex in a Trapped Bose-Einstein Condensate
A hydrodynamic description is used to study the normal modes of a vortex in a
zero-temperature Bose-Einstein condensate. In the Thomas-Fermi (TF) limit, the
circulating superfluid velocity far from the vortex core provides a small
perturbation that splits the originally degenerate normal modes of a
vortex-free condensate. The relative frequency shifts are small in all cases
considered (they vanish for the lowest dipole mode with |m|=1), suggesting that
the vortex is stable. The Bogoliubov equations serve to verify the existence of
helical waves, similar to those of a vortex line in an unbounded weakly
interacting Bose gas. In the large-condensate (small-core) limit, the
condensate wave function reduces to that of a straight vortex in an unbounded
condensate; the corresponding Bogoliubov equations have no bound-state
solutions that are uniform along the symmetry axis and decay exponentially far
from the vortex core.Comment: 15 pages, REVTEX, 2 Postscript figures, to appear in Phys. Rev. A. We
have altered the material in Secs. 3B and 4 in connection with the normal
modes that have |m|=1. Our present treatment satisfies the condition that the
fundamental dipole mode of a condensate with (or without) a vortex should
have the bare frequency $\omega_\perp
Benchmarking GW against exact diagonalization for semi-empirical models
We calculate groundstate total energies and single-particle excitation
energies of seven pi conjugated molecules described with the semi-empirical
Pariser-Parr-Pople (PPP) model using self-consistent many-body perturbation
theory at the GW level and exact diagonalization. For the total energies GW
captures around 65% of the groundstate correlation energy. The lowest lying
excitations are overscreened by GW leading to an underestimation of electron
affinities and ionization potentials by approximately 0.15 eV corresponding to
2.5%. One-shot G_0W_0 calculations starting from Hartree-Fock reduce the
screening and improve the low-lying excitation energies. The effect of the GW
self-energy on the molecular excitation energies is shown to be similar to the
inclusion of final state relaxations in Hartree-Fock theory. We discuss the
break down of the GW approximation in systems with short range interactions
(Hubbard models) where correlation effects dominate over screening/relaxation
effects. Finally we illustrate the important role of the derivative
discontinuity of the true exchange-correlation functional by computing the
exact Kohn-Sham levels of benzene.Comment: 9 pages, 5 figures, accepted for publication in Phys. Rev.
Bragg Spectroscopy of Cold Atomic Fermi Gases
We propose a Bragg spectroscopy experiment to measure the onset of superfluid
pairing in ultracold trapped Fermi gases. In particular, we study two component
Fermi gases in the weak coupling BCS and BEC limits as well as in the strong
coupling unitarity limit. The low temperature Bragg spectrum exhibits a gap
directly related to the pair-breaking energy. Furthermore, the Bragg spectrum
has a large maximum just below the critical temperature when the gas is
superfluid in the BCS limit. In the unitarity regime, we show how the pseudogap
in the normal phase leads to a significant suppression of the low frequency
Bragg spectrum.Comment: 8 pages, 9 figures. Typos corrected. Reference update
Kinetic Theory for Electron Dynamics Near a Positive Ion
A theoretical description of time correlation functions for electron
properties in the presence of a positive ion of charge number Z is given. The
simplest case of an electron gas distorted by a single ion is considered. A
semi-classical representation with a regularized electron - ion potential is
used to obtain a linear kinetic theory that is asymptotically exact at short
times. This Markovian approximation includes all initial (equilibrium) electron
- electron and electron - ion correlations through renormalized pair
potentials. The kinetic theory is solved in terms of single particle
trajectories of the electron - ion potential and a dielectric function for the
inhomogeneous electron gas. The results are illustrated by a calculation of the
autocorrelation function for the electron field at the ion. The dependence on
charge number Z is shown to be dominated by the bound states of the effective
electron - ion potential. On this basis, a very simple practical representation
of the trajectories is proposed and shown to be accurate over a wide range
including strong electron - ion coupling. This simple representation is then
used for a brief analysis of the dielectric function for the inhomogeneous
electron gas.Comment: 30 pages, 5 figures, submitted to Journal of Statistical Mechanics:
Theory and Experimen
Connecting scaling with short-range correlations
We reexamine several issues related to the physics of scaling in electron
scattering from nuclei. A basic model is presented in which an assumed form for
the momentum distribution having both long- and short-range contributions is
incorporated in the single-particle Green function. From this one can obtain
saturation of nuclear matter for an NN interaction with medium-range attraction
and short-range repulsion, and can obtain the density-density polarization
propagator and hence the electromagnetic response and scaling function. For the
latter, the shape of the scaling function and how it approaches scaling as a
function of momentum transfer are both explored.Comment: 24 pages, 15 figures. A reference has been corrected and update
The effect of a velocity barrier on the ballistic transport of Dirac fermions
We propose a novel way to manipulate the transport properties of massless
Dirac fermions by using velocity barriers, defining the region in which the
Fermi velocity, , has a value that differs from the one in the
surrounding background. The idea is based on the fact that when waves travel
accross different media, there are boundary conditions that must be satisfied,
giving rise to Snell's-like laws. We find that the transmission through a
velocity barrier is highly anisotropic, and that perfect transmission always
occurs at normal incidence. When in the barrier is larger that the
velocity outside the barrier, we find that a critical transmission angle
exists, a Brewster-like angle for massless Dirac electrons.Comment: 4.3 pages, 5 figure
- …