93 research outputs found
Approaching finite-temperature phase diagrams of strongly correlated materials: a case study for V2O3
Examining phase stabilities and phase equilibria in strongly correlated
materials asks for a next level in the many-body extensions to the
local-density approximation (LDA) beyond mainly spectroscopic assessments. Here
we put the charge-self-consistent LDA+dynamical mean-field theory (DMFT)
methodology based on projected local orbitals for the LDA+DMFT interface and a
tailored pseudopotential framework into action in order to address such
thermodynamics of realistic strongly correlated systems. Namely a case study
for the electronic phase diagram of the well-known prototype Mott-phenomena
system VO at higher temperatures is presented. We are able to describe
the first-order metal-to-insulator transitions with negative pressure and
temperature from the self-consistent computation of the correlated total energy
in line with experimental findings.Comment: 12 pages, 15 figures, new data adde
Comparison between a diagrammatic theory for the BCS-BEC crossover and Quantum Monte Carlo results
Predictions for the chemical potential and the excitation gap recently
obtained by our diagrammatic theory for the BCS-BEC crossover in the superfluid
phase are compared with novel Quantum Monte Carlo results at zero temperature
now available in the literature. A remarkable agreement is found between the
results obtained by the two approachesComment: 3 pages, 2 figure
Superscaling in dilute Fermi gas and its relation to general properties of the nucleon momentum distribution in nuclei
The superscaling observed in inclusive electron scattering is described
within the dilute Fermi gas model with interaction between the particles. The
comparison with the relativistic Fermi gas (RFG) model without interaction
shows an improvement in the explanation of the scaling function in
the region , where the RFG result is . It is found
that the behavior of for depends on the particular
form of the general power-law asymptotics of the momentum distribution
at large . The best agreement with the empirical
scaling function is found for in agreement with the asymptotics
of in the coherent density fluctuation model where . Thus,
superscaling gives information about the asymptotics of and the NN
forces.Comment: 6 pages, 5 figures, accepted for publication in Physical Review
Superfluid equation of state of dilute composite bosons
We present an exact theory of the BEC-BCS crossover in the BEC regime, which
treats explicitely dimers as made of two fermions. We apply our framework, at
zero temperature, to the calculation of the equation of state. We find that,
when expanding the chemical potential in powers of the density n up to the
Lee-Huang-Yang order, proportional to n^3/2, the result is identical to the one
of elementary bosons in terms of the dimer-dimer scattering length a_M, the
composite nature of the dimers appearing only in the next order term
proportional to n^2 .Comment: 5 pages, 3 figure
Bogoliubov theory of Feshbach molecules in the BEC-BCS crossover
We present the Bogoliubov theory for the Bose-Einstein condensation of
Feshbach molecules in a balanced Fermi mixture. Because the Bogoliubov theory
includes (Gaussian) fluctuations, we can in this manner accurately incorporate
both the two-body and many-body aspects of the BEC-BCS crossover that occurs
near a Feshbach resonance. We apply the theory in particular to the very broad
Feshbach resonance in atomic Li-6 at a magnetic field of B_0 = 834 G and find
good agreement with experiments in that case. The BEC-BCS crossover for more
narrow Feshbach resonances is also discussed.Comment: 13 pages of RevTex and 12 Figures. Submitted for publication in
Physical review
General relations for quantum gases in two and three dimensions. Two-component fermions
We derive exact relations for spin-1/2 fermions with zero-range or
short-range interactions, in continuous space or on a lattice, in or in
, in any external potential. Some of them generalize known relations
between energy, momentum distribution , pair distribution function
, derivative of the energy with respect to the scattering length
. Expressions are found for the second order derivative of the energy with
respect to (or to in ). Also, it is found that the leading
energy corrections due to a finite interaction range, are proportional to the
effective range in (and to in ) with exprimable
model-independent coefficients, that give access to the subleading short
distance behavior of and to the subleading tail of .
This applies to lattice models for some magic dispersion relations, an example
of which is given. Corrections to exactly solvable two-body and three-body
problems are obtained. For the trapped unitary gas, the variation of the
finite- and finite energy corrections within each energy
ladder is obtained; it gives the frequency shift and the collapse time of the
breathing mode. For the bulk unitary gas, we compare to fixed-node Monte Carlo
data, and we estimate the experimental uncertainty on the Bertsch parameter due
to a finite .Comment: Augmented version: with respect to published version, subsection V.K
added (minorization of the contact by the order parameter). arXiv admin note:
text overlap with arXiv:1001.077
Dilute Fermi gas: kinetic and interaction energies
A dilute homogeneous 3D Fermi gas in the ground state is considered for the
case of a repulsive pairwise interaction. The low-density (dilution) expansions
for the kinetic and interaction energies of the system in question are
calculated up to the third order in the dilution parameter. Similar to the
recent results for a Bose gas, the calculated quantities turn out to depend on
a pairwise interaction through the two characteristic lengths: the former, ,
is the well-known s-wave scattering length, and the latter, , is related to
by , where stands for the fermion mass.
To take control of the results, calculations are fulfilled in two independent
ways. The first involves the Hellmann-Feynman theorem, taken in conjunction
with a helpful variational theorem for the scattering length. This way is used
to derive the kinetic and interaction energies from the familiar low-density
expansion of the total system energy first found by Huang and Yang. The second
way operates with the in-medium pair wave functions. It allows one to derive
the quantities of interest``from the scratch'', with no use of the total
energy. An important result of the present investigation is that the pairwise
interaction of fermions makes an essential contribution to their kinetic
energy. Moreover, there is a complicated and interesting interplay of these
quantities
Beyond the GW approximation: combining correlation channels
In many-body perturbation theory (MBPT) the self-energy \Sigma=iGW\Gamma
plays the key role since it contains all the many body effects of the system.
The exact self-energy is not known; as first approximation one can set the
vertex function \Gamma to unity which leads to the GW approximation. The latter
properly describes the high-density regime, where screening is important; in
the low-density regime, instead, other approximations are proposed, such as the
T matrix, which describes multiple scattering between two particles.
Here we combine the two approaches. Starting from the fundamental equations
of MBPT we show how one can derive the T-matrix approximation to the
self-energy in a common framework with GW. This allows us to elucidate several
aspects of this formulation, including the origin of, and link between, the
electron-hole and the particle-particle T matrix, the derivation of a screened
T matrix, and the conversion of the T matrix into a vertex correction. The
exactly solvable Hubbard molecule is used for illustration.Comment: 15 pages, 7 figure
Trapped Li : A high T_c superfluid ?
We consider the effect of the indirect interaction due to the exchange of
density fluctuations on the critical temperature of superfluid Li . We
obtain the strong coupling equation giving this critical temperature. This
equation is solved approximately by retaining the same set of diagrams as in
the paramagnon model. We show that, near the instability threshold, the
attractive interaction due to density fluctuations gives rise to a strong
increase in the critical temperature, providing a clear signature of the
existence of fluctuation induced interactions.Comment: 4 pages, revtex, 1 figur
Nonmonotonic Decay of Nonequilibrium Polariton Condensate in Direct-Gap Semiconductors
Time evolution of a nonequilibrium polariton condensate has been studied in
the framework of a microscopic approach. It has been shown that due to
polariton-polariton scattering a significant condensate depletion takes place
in a comparatively short time interval. The condensate decay occurs in the form
of multiple echo signals. Distribution-function dynamics of noncondensate
polaritons have been investigated. It has been shown that at the initial stage
of evolution the distribution function has the form of a bell. Then
oscillations arise in the contour of the distribution function, which further
transform into small chaotic ripples. The appearance of a short-wavelength wing
of the distribution function has been demonstrated. We have pointed out the
enhancement and then partial extinction of the sharp extra peak arising within
the time interval characterized by small values of polariton condensate density
and its relatively slow changes.Comment: 20 pages, LaTeX 2.09; in press in PR
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