89 research outputs found
A large-N analysis of the local quantum critical point and the spin-liquid phase
We study analytically the Kondo lattice model with an additional
nearest-neighbor antiferromagnetic interaction in the framework of large-N
theory. We find that there is a local quantum critical point between two
phases, a normal Fermi-liquid and a spin-liquid in which the spins are
decoupled from the conduction electrons. The local spin susceptibility displays
a power-law divergence throughout the spin liquid phase. We check the
reliability of the large-N results by solving by quantum Monte Carlo simulation
the N=2 spin-liquid problem with no conduction electrons and find qualitative
agreement. We show that the spin-liquid phase is unstable at low temperatures,
suggestive of a first-order transition to an ordered phase.Comment: 4 pages and 1 figur
Continuous quantum phase transition in a Kondo lattice model
We study the magnetic quantum phase transition in an anisotropic Kondo
lattice model. The dynamical competition between the RKKY and Kondo
interactions is treated using an extended dynamic mean field theory (EDMFT)
appropriate for both the antiferromagnetic and paramagnetic phases. A quantum
Monte Carlo approach is used, which is able to reach very low temperatures, of
the order of 1% of the bare Kondo scale. We find that the finite-temperature
magnetic transition, which occurs for sufficiently large RKKY interactions, is
first order. The extrapolated zero-temperature magnetic transition, on the
other hand, is continuous and locally critical.Comment: 4 pages, 4 figures; updated, to appear in PR
Disorder effects in the quantum Heisenberg model: An Extended Dynamical mean-field theory analysis
We investigate a quantum Heisenberg model with both antiferromagnetic and
disordered nearest-neighbor couplings. We use an extended dynamical mean-field
approach, which reduces the lattice problem to a self-consistent local impurity
problem that we solve by using a quantum Monte Carlo algorithm. We consider
both two- and three-dimensional antiferromagnetic spin fluctuations and
systematically analyze the effect of disorder. We find that in three dimensions
for any small amount of disorder a spin-glass phase is realized. In two
dimensions, while clean systems display the properties of a highly correlated
spin-liquid (where the local spin susceptibility has a non-integer power-low
frequency and/or temperature dependence), in the present case this behavior is
more elusive unless disorder is very small. This is because the spin-glass
transition temperature leaves only an intermediate temperature regime where the
system can display the spin-liquid behavior, which turns out to be more
apparent in the static than in the dynamical susceptibility.Comment: 15 pages, 7 figure
Universal Distribution of Kondo Temperatures in Dirty Metals
Kondo screening of diluted magnetic impurities in a disordered host is
studied analytically and numerically in one, two and three dimensions. It is
shown that in the T_K \to 0 limit the distribution of Kondo temperatures has a
universal form, P(T_K) \sim T_K^{-\alpha} that holds in the insulating phase
and persists in the metallic phase close to the metal insulator transition.
Moreover, the exponent \alpha depends only on the dimensionality. The most
important consequence of this result is that the T-dependence of thermodynamic
properties is smooth across the metal-insulator transition in three dimensional
systems.Comment: 4 pages, 3 figures; added referenc
Bose-Einstein condensate of kicked rotators with time-dependent interaction
A modification of the quantum kicked rotator is suggested with a
time-dependent delta-kicked interaction parameter which can be realized by a
pulsed turn-on of a Feshbach resonance. The mean kinetic energy increases
exponentially with time in contrast to a merely diffusive or linear growth for
the first few kicks for the quantum kicked rotator with a constant interaction
parameter. A recursive relation is derived in a self-consistent random phase
approximation which describes this superdiffusive growth of the kinetic energy
and is compared with numerical simulations. Unlike in the case of the quantum
rotator with constant interaction, a Lax pair is not found. In general the
delta-kicked interaction is found to lead to strong chaotic behaviour.Comment: 4 pages, 3 figure
Quantum transport through a deformable molecular transistor
The linear transport properties of a model molecular transistor with
electron-electron and electron-phonon interactions were investigated
analytically and numerically. The model takes into account phonon modulation of
the electronic energy levels and of the tunnelling barrier between the molecule
and the electrodes. When both effects are present they lead to asymmetries in
the dependence of the conductance on gate voltage. The Kondo effect is observed
in the presence of electron-phonon interactions. There are important
qualitative differences between the cases of weak and strong coupling. In the
first case the standard Kondo effect driven by spin fluctuations occurs. In the
second case, it is driven by charge fluctuations. The Fermi-liquid relation
between the spectral density of the molecule and its charge is altered by
electron-phonon interactions. Remarkably, the relation between the
zero-temperature conductance and the charge remains unchanged. Therefore, there
is perfect transmission in all regimes whenever the average number of electrons
in the molecule is an odd integer.Comment: 9 pages, 6 figure
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