19 research outputs found
Local density approximation for a perturbative equation of state
The knowledge of a series expansion of the equation of state provides a deep
insight into the physical nature of a quantum system. Starting from a generic
``perturbative'' equation of state of a homogeneous ultracold gas we make
predictions for the properties of the gas in the presence of harmonic
confinement. The local density approximation is used to obtain the chemical
potential, total and release energies, Thomas-Fermi size and density profile of
a trapped system in three-, two-, and one- dimensional geometries. The
frequencies of the lowest breathing modes are calculated using scaling and
sum-rule approaches and could be used in an experiment as a high precision tool
for obtaining the expansion terms of the equation of state. The derived
formalism is applied to dilute Bose and Fermi gases in different dimensions and
to integrable one-dimensional models. Physical meaning of expansion terms in a
number of systems is discussed.Comment: 3 Figure
Quasi-one-dimensional anisotropic Heisenberg model in a transverse magnetic field
The phase diagram of weakly coupled chains in a transverse magnetic
field is studied using the mean-field approximation for the interchain coupling
and known exact results for an effective one-dimensional model. Results are
applied to the quasi-one-dimensional antiferromagnet and the
value of interchain interaction in this compound is estimated.Comment: 4 pages, 2 figure
Gap generation in the XXZ model in a transverse magnetic field
The ground state phase diagram of the 1D XXZ model in transverse magnetic
field is obtained. It consists of the gapped phases with different types of
long range order (LRO) and critical lines at which the gap and the LRO vanish.
Using scaling estimations and a mean-field approach as well as numerical
results we found critical indices of the gap and the LRO in the vicinity of all
critical lines.Comment: 4 pages, 1 figure, Late
Density of Neutral Solitons in Weakly Disordered Peierls Chains
We study the effects of weak off-diagonal disorder on Peierls systems with a
doubly degenerate ground state. We show that for these systems disorder in the
electron hopping amplitudes induces a finite density of solitons in the
minimal-energy lattice configuration of a single chain. These disorder-induced
dimerization kinks are neutral and have spin 1/2. Using a continuum model for
the Peierls chain and treating the lattice classically, we analytically
calculate the average free energy and density of kinks. We compare these
results to numerical calculations for a discrete model and discuss the
implications of the kinks for the optical and magnetic properties of the
conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 5 Postscript figures, to appear in Phys. Rev.
Superconductivity in an exactly solvable Hubbard model with bond-charge interaction
The Hubbard model with an additional bond-charge interaction is solved
exactly in one dimension for the case where is the hopping amplitude.
In this case the number of doubly occupied sites is conserved. In the sector
with no double occupations the model reduces to the Hubbard model.
In arbitrary dimensions the qualitative form of the phase diagram is obtained.
It is shown that for moderate Hubbard interactions the model has
superconducting ground states.Comment: Revtex, 14 pages, 1 figure (uuencoded compressed tar-file
Frustrated quantum Heisenberg ferrimagnetic chains
We study the ground-state properties of weakly frustrated Heisenberg
ferrimagnetic chains with nearest and next-nearest neighbor antiferromagnetic
exchange interactions and two types of alternating sublattice spins S_1 > S_2,
using 1/S spin-wave expansions, density-matrix renormalization group, and
exact- diagonalization techniques. It is argued that the zero-point spin
fluctuations completely destroy the classical commensurate- incommensurate
continuous transition. Instead, the long-range ferrimagnetic state disappears
through a discontinuous transition to a singlet state at a larger value of the
frustration parameter. In the ferrimagnetic phase we find a disorder point
marking the onset of incommensurate real-space short-range spin-spin
correlations.Comment: 16 pages (LaTex 2.09), 6 eps figure
Theory of ultracold Fermi gases
The physics of quantum degenerate Fermi gases in uniform as well as in
harmonically trapped configurations is reviewed from a theoretical perspective.
Emphasis is given to the effect of interactions which play a crucial role,
bringing the gas into a superfluid phase at low temperature. In these dilute
systems interactions are characterized by a single parameter, the s-wave
scattering length, whose value can be tuned using an external magnetic field
near a Feshbach resonance. The BCS limit of ordinary Fermi superfluidity, the
Bose-Einstein condensation (BEC) of dimers and the unitary limit of large
scattering length are important regimes exhibited by interacting Fermi gases.
In particular the BEC and the unitary regimes are characterized by a high value
of the superfluid critical temperature, of the order of the Fermi temperature.
Different physical properties are discussed, including the density profiles and
the energy of the ground-state configurations, the momentum distribution, the
fraction of condensed pairs, collective oscillations and pair breaking effects,
the expansion of the gas, the main thermodynamic properties, the behavior in
the presence of optical lattices and the signatures of superfluidity, such as
the existence of quantized vortices, the quenching of the moment of inertia and
the consequences of spin polarization. Various theoretical approaches are
considered, ranging from the mean-field description of the BCS-BEC crossover to
non-perturbative methods based on quantum Monte Carlo techniques. A major goal
of the review is to compare the theoretical predictions with the available
experimental results.Comment: Revised and abridged version accepted for publication in Rev. Mod.
Phys.: 63 pages, 36 figure