805 research outputs found
Induced interactions in dilute atomic gases and liquid helium mixtures
In dilute mixtures of two atomic gases, interactions between two minority
atoms acquire a contribution due to interaction with the majority component.
Using thermodynamic arguments, we derive expressions for this induced
interaction for both fermions and bosons for arbitrary strength of the
interaction between the two components. Implications of the work for the theory
of dilute solutions of He in liquid He are discussed.Comment: 7 pages, 1 figure, NORDITA-2012-3
One-Particle Anomalous Excitations of Gutzwiller Projected BCS Superconductors and Bogoliubov Quasi-Particle Characteristics
Low-lying one-particle anomalous excitations are studied for Gutzwiller
projected strongly correlated BCS states. It is found that the one-particle
anomalous excitations are highly coherent, and the numerically calculated
spectrum can be reproduced quantitatively by a renormalized BCS theory, thus
strongly indicating that the nature of low-lying excitations described by the
projected BCS states is essentially understood within a renormalized Bogoliubov
quasi-particle picture. This finding resembles a well known fact that a
Gutzwiller projected Fermi gas is a Fermi liquid. The present results are
consistent with numerically exact calculations of the two-dimensional t-J model
as well as recent photoemission experiments on high-T_{\rm C} cuprate
superconductors.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. B 74,
180504(R) (2006
Probing ultracold Fermi gases with light-induced gauge potentials
We theoretically investigate the response of a two component Fermi gas to
vector potentials which couple separately to the two spin components. Such
vector potentials may be implemented in ultracold atomic gases using optically
dressed states. Our study indicates that light-induced gauge potentials may be
used to probe the properies of the interacting ultracold Fermi gas, providing.
amongst other things, ways to measure the superfluid density and the strength
of pairing.Comment: 8 pages, 3 figure
Exchange cotunneling through quantum dots with spin-orbit coupling
We investigate the effects of spin-orbit interaction (SOI) on the exchange
cotunneling through a spinful Coulomb blockaded quantum dot. In the case of
zero magnetic field, Kondo effect is shown to take place via a Kramers doublet
and the SOI will merely affect the Kondo temperature. In contrast, we find that
the breaking of time-reversal symmetry in a finite field has a marked influence
on the effective Anderson, and Kondo models for a single level. The nonlinear
conductance can now be asymmetric in bias voltage and may depend strongly on
direction of the magnetic field. A measurement of the angle dependence of
finite-field cotunneling spectroscopy thus provides valuable information about
orbital, and spin degrees of freedom and their mutual coupling.Comment: 5 pages, 2 figure
Scaling properties of step bunches induced by sublimation and related mechanisms: A unified perspective
This work provides a ground for a quantitative interpretation of experiments
on step bunching during sublimation of crystals with a pronounced
Ehrlich-Schwoebel (ES) barrier in the regime of weak desorption. A strong step
bunching instability takes place when the kinetic length is larger than the
average distance between the steps on the vicinal surface. In the opposite
limit the instability is weak and step bunching can occur only when the
magnitude of step-step repulsion is small. The central result are power law
relations of the between the width, the height, and the minimum interstep
distance of a bunch. These relations are obtained from a continuum evolution
equation for the surface profile, which is derived from the discrete step
dynamical equations for. The analysis of the continuum equation reveals the
existence of two types of stationary bunch profiles with different scaling
properties. Through a mathematical equivalence on the level of the discrete
step equations as well as on the continuum level, our results carry over to the
problems of step bunching induced by growth with a strong inverse ES effect,
and by electromigration in the attachment/detachment limited regime. Thus our
work provides support for the existence of universality classes of step
bunching instabilities [A. Pimpinelli et al., Phys. Rev. Lett. 88, 206103
(2002)], but some aspects of the universality scenario need to be revised.Comment: 21 pages, 8 figure
Criteria of off-diagonal long-range order in Bose and Fermi systems based on the Lee-Yang cluster expansion method
The quantum-statistical cluster expansion method of Lee and Yang is extended
to investigate off-diagonal long-range order (ODLRO) in one- and
multi-component mixtures of bosons or fermions. Our formulation is applicable
to both a uniform system and a trapped system without local-density
approximation and allows systematic expansions of one- and multi-particle
reduced density matrices in terms of cluster functions which are defined for
the same system with Boltzmann statistics. Each term in this expansion can be
associated with a Lee-Yang graph. We elucidate a physical meaning of each
Lee-Yang graph; in particular, for a mixture of ultracold atoms and bound
dimers, an infinite sum of the ladder-type Lee-Yang 0-graphs is shown to lead
to Bose-Einstein condensation of dimers below the critical temperature. In the
case of Bose statistics, an infinite series of Lee-Yang 1-graphs is shown to
converge and gives the criteria of ODLRO at the one-particle level.
Applications to a dilute Bose system of hard spheres are also made. In the case
of Fermi statistics, an infinite series of Lee-Yang 2-graphs is shown to
converge and gives the criteria of ODLRO at the two-particle level.
Applications to a two-component Fermi gas in the tightly bound limit are also
made.Comment: 21 pages, 10 figure
Strong vs. Weak Coupling Duality and Coupling Dependence of the Kondo Temperature in the Two-Channel Kondo Model
We perform numerical renormalization group (NRG) as well as analytical
calculations for the two-channel Kondo model to obtain the dependence of the
Kondo temperature on the dimensionless (bare) spin exchange coupling
over the complete parameter range from to . We show that there
exists a duality between the regimes of small and large coupling. It is unique
for the two-channel model and enables a mapping between the strong and the weak
coupling cases via the identification , implying an
exponential dependence of on and , respectively, in the two
regimes. This agrees quantitatively with our NRG calculations where we extract
over the complete parameter range and obtain a non-monotonous
dependence, strongly peaked at the 2CK fixed point coupling . These
results may be relevant for resolving the long-standing puzzle within the 2CK
interpretation of certain random defect systems, why no broad distribution of
is observed in those systems.Comment: 6 pages, 3 figures; PRB published version, shortened, nomenclature
clarifie
Kondo effect in a quantum dot coupled to ferromagnetic leads and side-coupled to a nonmagnetic reservoir
Equilibrium transport properties of a single-level quantum dot tunnel-coupled
to ferromagnetic leads and exchange-coupled to a side nonmagnetic reservoir are
analyzed theoretically in the Kondo regime. The equilibrium spectral functions
and conductance through the dot are calculated using the numerical
renormalization group (NRG) method. It is shown that in the antiparallel
magnetic configuration, the system undergoes a quantum phase transition with
increasing exchange coupling , where the conductance drops from its maximum
value to zero. In the parallel configuration, on the other hand, the
conductance is generally suppressed due to an effective spin splitting of the
dot level caused by the presence of ferromagnetic leads, irrespective of the
strength of exchange constant. However, for ranging from J=0 up to the
corresponding critical value, the Kondo effect and quantum critical behavior
can be restored by applying properly tuned compensating magnetic field.Comment: (8 pages, 8 figs) to appear in PR
Crossover temperature of Bose-Einstein condensation in an atomic Fermi gas
We show that in an atomic Fermi gas near a Feshbach resonance the crossover
between a Bose-Einstein condensate of diatomic molecules and a Bose-Einstein
condensate of Cooper pairs occurs at positive detuning, i.e., when the
molecular energy level lies in the two-atom continuum. We determine the
crossover temperature as a function of the applied magnetic field and find
excellent agreement with the experiment of Regal et al. [Phys. Rev. Lett. 92,
040403 (2004)] that has recently observed this crossover temperature.Comment: 4 pages, 2 figure
Spectroscopy of Superfluid Pairing in Atomic Fermi Gases
We study the dynamic structure factor for density and spin within the
crossover from BCS superfluidity of atomic fermions to the Bose-Einstein
condensation of molecules. Both structure factors are experimentally accessible
via Bragg spectroscopy, and allow for the identification of the pairing
mechanism: the spin structure factor allows for the determination of the two
particle gap, while the collective sound mode in the density structure reveals
the superfluid state.Comment: 4 pages, 3 figure
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