536 research outputs found
Path integral formulation of the tunneling dynamics of a superfluid Fermi gas in an optical potential
To describe the tunneling dynamics of a stack of two-dimensional fermionic
superfluids in an optical potential, we derive an effective action functional
from a path integral treatment. This effective action leads, in the saddle
point approximation, to equations of motion for the density and the phase of
the superfluid Fermi gas in each layer. In the strong coupling limit (where
bosonic molecules are formed) these equations reduce to a discrete nonlinear
Schrodinger equation, where the molecular tunneling amplitude is reduced for
large binding energies. In the weak coupling (BCS) regime, we study the
evolution of the stacked superfluids and derive an approximate analytical
expression for the Josephson oscillation frequency in an external harmonic
potential. Both in the weak and intermediate coupling regimes the detection of
the Josephson oscillations described by our path integral treatment constitutes
experimental evidence for the fermionic superfluid regime.Comment: 13 pages + 2 figure
Optical Absorption Spectra of Bipolarons
The absorption of large bipolarons is investigated using the path-integral
method. The response of a bipolaron to an external electromagnetic field is
derived in the framework of the memory-function approach. The bipolaron optical
absorption spectrum consists of a series of relatively narrow peaks. The
peculiarities of the bipolaron optical absorption as a function of the
frequency of the electromagnetic field may be attributed to the transitions
involving relaxed excited states and scattering states of a bipolaron.Comment: 14 pages, 3 figures, E-mail addresses: [email protected],
[email protected]; to be published in Phys. Rev.
Ground state and optical conductivity of interacting polarons in a quantum dot
The ground-state energy, the addition energies and the optical absorption
spectra are derived for interacting polarons in parabolic quantum dots in three
and two dimensions. A path integral formalism for identical particles is used
in order to take into account the fermion statistics. The approach is applied
to both closed-shell and open-shell systems of interacting polarons. Using a
generalization of the Jensen-Feynman variational principle, the ground-state
energy of a confined N-polaron system is analyzed as a function of N and of the
electron-phonon coupling constant. As distinct from the few-electron systems
without the electron-phonon interaction, three types of spin polarization are
possible for the ground state of the few-polaron systems: (i) a spin-polarized
state, (ii) a state where the spin is determined by Hund's rule, (iii) a state
with the minimal possible spin. A transition from a state fulfilling Hund's
rule, to a spin-polarized state occurs when decreasing the electron density. In
the strong-coupling limit, the system of interacting polarons turns into a
state with the minimal possible spin. These transitions should be
experimentally observable in the optical absorption spectra of quantum dots.Comment: 33 pages, 9 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected],
accepted for Phys. Rev.
The vortex state in the BEC to BCS crossover: a path-integral description
We derive a path-integral description of the vortex state of a fermionic
superfluid in the crossover region between the molecular condensate (BEC)
regime and the Cooper pairing (BCS) regime. This path-integral formalism,
supplemented by a suitable choice for the saddle point value of the pairing
field in the presence of a vortex, offers a unified description that
encompasses both the BEC and BCS limits. The vortex core size is studied as a
function of the tunable interaction strength between the fermionic atoms. We
find that in the BEC regime, the core size is determined by the molecular
healing length, whereas in the BCS regime, the core size is proportional only
to the Fermi wave length. The observation of such quantized vortices in dilute
Fermi gases would provide an unambiguous proof of the realization of
superfluidity in these gases.Comment: 10 pages, 2 figure
Photoluminescence of tetrahedral quantum-dot quantum wells
Taking into account the tetrahedral shape of a quantum dot quantum well
(QDQW) when describing excitonic states, phonon modes and the exciton-phonon
interaction in the structure, we obtain within a non-adiabatic approach a
quantitative interpretation of the photoluminescence (PL) spectrum of a single
CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW
is bright, in contrast to the dark ground state for a spherical QDQW.Comment: 4 pages, 2 figure
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