1,789 research outputs found
Breathing Modes and Hidden Symmetry of Trapped Atoms in 2D
Atoms confined in a harmonic potential show universal oscillations in 2D. We
point out the connection of these ''breathing'' modes to the presence of a
hidden symmetry. The underlying symmetry SO(2,1), i.e. the two dimensional
Lorentz group, allows pulsating solutions to be constructed for the interacting
quantum system and for the corresponding nonlinear Gross-Pitaevskii equation.
We point out how this symmetry can be used as a probe for recently proposed
experiments of trapped atoms in 2D.Comment: 4 pages, small changes in title and text, references adde
Nonequilibrium Transport through a Kondo Dot: Decoherence Effects
We investigate the effects of voltage induced spin-relaxation in a quantum
dot in the Kondo regime. Using nonequilibrium perturbation theory, we determine
the joint effect of self-energy and vertex corrections to the conduction
electron T-matrix in the limit of transport voltage much larger than
temperature. The logarithmic divergences, developing near the different
chemical potentials of the leads, are found to be cut off by spin-relaxation
rates, implying that the nonequilibrium Kondo-problem remains at weak coupling
as long as voltage is much larger than the Kondo temperature.Comment: 16 pages, 4 figure
Wilson chains are not thermal reservoirs
Wilson chains, based on a logarithmic discretization of a continuous
spectrum, are widely used to model an electronic (or bosonic) bath for Kondo
spins and other quantum impurities within the numerical renormalization group
method and other numerical approaches. In this short note we point out that
Wilson chains can not serve as thermal reservoirs as their temperature changes
by a number of order Delta E when a finite amount of energy Delta E is added.
This proves that for a large class of non-equilibrium problems they cannot be
used to predict the long-time behavior.Comment: 2 page
Transport in Almost Integrable Models: Perturbed Heisenberg Chains
The heat conductivity kappa(T) of integrable models, like the one-dimensional
spin-1/2 nearest-neighbor Heisenberg model, is infinite even at finite
temperatures as a consequence of the conservation laws associated with
integrability. Small perturbations lead to finite but large transport
coefficients which we calculate perturbatively using exact diagonalization and
moment expansions. We show that there are two different classes of
perturbations. While an interchain coupling of strength J_perp leads to
kappa(T) propto 1/J_perp^2 as expected from simple golden-rule arguments, we
obtain a much larger kappa(T) propto 1/J'^4 for a weak next-nearest neighbor
interaction J'. This can be explained by a new approximate conservation law of
the J-J' Heisenberg chain.Comment: 4 pages, several minor modifications, title change
Giant mass and anomalous mobility of particles in fermionic systems
We calculate the mobility of a heavy particle coupled to a Fermi sea within a
non-perturbative approach valid at all temperatures. The interplay of particle
recoil and of strong coupling effects, leading to the orthogonality catastrophe
for an infinitely heavy particle, is carefully taken into account. We find two
novel types of strong coupling effects: a new low energy scale and
a giant mass renormalization in the case of either near-resonant scattering or
a large transport cross section . The mobility is shown to obey two
different power laws below and above . For ,
where is the Fermi wave length, an exponentially large effective
mass suppresses the mobility.Comment: 4 pages, 4 figure
Sign change of the Grueneisen parameter and magnetocaloric effect near quantum critical points
We consider the Grueneisen parameter and the magnetocaloric effect near a
pressure and magnetic field controlled quantum critical point, respectively.
Generically, the Grueneisen parameter (and the thermal expansion) displays a
characteristic sign change close to the quantum-critical point signaling an
accumulation of entropy. If the quantum critical point is the endpoint of a
line of finite temperature phase transitions, T_c \propto (p_c-p)^Psi, then we
obtain for p<p_c, (1) a characteristic increase \Gamma \sim T^{-1/(\nu z)} of
the Grueneisen parameter Gamma for T>T_c, (2) a sign change in the Ginzburg
regime of the classical transition, (3) possibly a peak at T_c, (4) a second
increase Gamma \sim -T^{-1/(nu z)} below T_c for systems above the upper
critical dimension and (5) a saturation of Gamma \propto 1/(p_c-p). We argue
that due to the characteristic divergencies and sign changes the thermal
expansion, the Grueneisen parameter and magnetocaloric effect are excellent
tools to detect and identify putative quantum critical points.Comment: 10 pages, 7 figures; final version, only minor change
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