110 research outputs found
Mean-field phase diagram for Bose-Hubbard Hamiltonians with random hopping
The zero-temperature phase diagram for ultracold Bosons in a random 1D
potential is obtained through a site-decoupling mean-field scheme performed
over a Bose-Hubbard (BH) Hamiltonian whose hopping term is considered as a
random variable. As for the model with random on-site potential, the presence
of disorder leads to the appearance of a Bose-glass phase. The different phases
-i.e. Mott insulator, superfluid, Bose-glass- are characterized in terms of
condensate fraction and superfluid fraction. Furthermore, the boundary of the
Mott lobes are related to an off-diagonal Anderson model featuring the same
disorder distribution as the original BH Hamiltonian.Comment: 7 pages, 6 figures. Submitted to Laser Physic
Gutzwiller approach to the Bose-Hubbard model with random local impurities
Recently it has been suggested that fermions whose hopping amplitude is
quenched to extremely low values provide a convenient source of local disorder
for lattice bosonic systems realized in current experiment on ultracold atoms.
Here we investigate the phase diagram of such systems, which provide the
experimental realization of a Bose-Hubbard model whose local potentials are
randomly extracted from a binary distribution. Adopting a site-dependent
Gutzwiller description of the state of the system, we address one- and
two-dimensional lattices and obtain results agreeing with previous findings, as
far as the compressibility of the system is concerned. We discuss the expected
peaks in the experimental excitation spectrum of the system, related to the
incompressible phases, and the superfluid character of the {\it partially
compressible phases} characterizing the phase diagram of systems with binary
disorder. In our investigation we make use of several analytical results whose
derivation is described in the appendices, and whose validity is not limited to
the system under concern.Comment: 12 pages, 5 figures. Some adjustments made to the manuscript and to
figures. A few relevant observations added throughout the manuscript.
Bibliography made more compact (collapsed some items
Expansion dynamics in the one-dimensional Fermi-Hubbard model
Expansion dynamics of interacting fermions in a lattice are simulated within
the one-dimensional (1D) Hubbard model, using the essentially exact
time-evolving block decimation (TEBD) method. In particular, the expansion of
an initial band-insulator state is considered. We analyze the simulation
results based on the dynamics of a two-site two-particle system, the so-called
Hubbard dimer. Our findings describe essential features of a recent experiment
on the expansion of a Fermi gas in a two-dimensional lattice. We show that the
Hubbard-dimer dynamics, combined with a two-fluid model for the paired and
non-paired components of the gas, gives an efficient description of the full
dynamics. This should be useful for describing dynamical phenomena of strongly
interacting Fermions in a lattice in general.Comment: Fig. 9 changed, text + supplementary material revise
Spin-asymmetric Josephson effect
The Josephson effect is a manifestation of the macroscopic phase coherence of
superconductors and superfluids. We propose that with ultracold Fermi gases one
can realise a spin-asymmetric Josephson effect in which the two spin components
of a Cooper pair are driven asymmetrically - corresponding to driving a
Josephson junction of two superconductors with different voltages V_\uparrow
and V_\downarrow for spin up and down electrons, respectively. We predict that
the spin up and down components oscillate at the same frequency but with
different amplitudes. Our results reveal that the standard description of the
Josephson effect in terms of bosonic pair tunnelling is insufficient. We
provide an intuitive interpretation of the Josephson effect as interference in
Rabi oscillations of pairs and single particles, the latter causing the
asymmetry.Comment: Article: 4 pages, 3 figures. Supplementary material: 12 pages, 7
figure
Glassy features of a Bose Glass
We study a two-dimensional Bose-Hubbard model at a zero temperature with
random local potentials in the presence of either uniform or binary disorder.
Many low-energy metastable configurations are found with virtually the same
energy as the ground state. These are characterized by the same blotchy pattern
of the, in principle, complex nonzero local order parameter as the ground
state. Yet, unlike the ground state, each island exhibits an overall random
independent phase. The different phases in different coherent islands could
provide a further explanation for the lack of coherence observed in experiments
on Bose glasses.Comment: 14 pages, 4 figures
The fidelity approach to the Hubbard model
We use the fidelity approach to quantum critical points to study the zero
temperature phase diagram of the one-dimensional Hubbard model. Using a variety
of analytical and numerical techniques, we analyze the fidelity metric in
various regions of the phase diagram, with particular care to the critical
points. Specifically we show that close to the Mott transition, taking place at
on-site repulsion U=0 and electron density n=1, the fidelity metric satisfies
an hyper-scaling form which we calculate. This implies that in general, as one
approaches the critical point U=0, n=1, the fidelity metric tends to a limit
which depends on the path of approach. At half filling, the fidelity metric is
expected to diverge as U^{-4} when U is sent to zero.Comment: 8 pages, 4 figures, added results on the hyper-scaling form of the
fidelity metri
Collision of one dimensional (1D) spin polarized Fermi gases in an optical lattice
In this work we analyze the dynamical behavior of the collision between two
clouds of fermionic atoms with opposite spin polarization. By means of the
time-evolving block decimation (TEBD) numerical method, we simulate the
collision of two one-dimensional clouds in a lattice. There is a symmetry in
the collision behaviour between the attractive and repulsive interactions. We
analyze the pair formation dynamics in the collision region, providing a
quantitative analysis of the pair formation mechanism in terms of a simple
two-site model
Generalized Hartree-Fock Theory for Interacting Fermions in Lattices: Numerical Methods
We present numerical methods to solve the Generalized Hartree-Fock theory for
fermionic systems in lattices, both in thermal equilibrium and out of
equilibrium. Specifically, we show how to determine the covariance matrix
corresponding to the Fermionic Gaussian state that optimally approximates the
quantum state of the fermions. The methods apply to relatively large systems,
since their complexity only scales quadratically with the number of lattice
sites. Moreover, they are specially suited to describe inhomogenous systems, as
those typically found in recent experiments with atoms in optical lattices, at
least in the weak interaction regime. As a benchmark, we have applied them to
the two-dimensional Hubbard model on a 10x10 lattice with and without an
external confinement.Comment: 16 pages, 22 figure
Microwave amplification with nanomechanical resonators
Sensitive measurement of electrical signals is at the heart of modern science
and technology. According to quantum mechanics, any detector or amplifier is
required to add a certain amount of noise to the signal, equaling at best the
energy of quantum fluctuations. The quantum limit of added noise has nearly
been reached with superconducting devices which take advantage of
nonlinearities in Josephson junctions. Here, we introduce a new paradigm of
amplification of microwave signals with the help of a mechanical oscillator. By
relying on the radiation pressure force on a nanomechanical resonator, we
provide an experimental demonstration and an analytical description of how the
injection of microwaves induces coherent stimulated emission and signal
amplification. This scheme, based on two linear oscillators, has the advantage
of being conceptually and practically simpler than the Josephson junction
devices, and, at the same time, has a high potential to reach quantum limited
operation. With a measured signal amplification of 25 decibels and the addition
of 20 quanta of noise, we anticipate near quantum-limited mechanical microwave
amplification is feasible in various applications involving integrated
electrical circuits.Comment: Main text + supplementary information. 14 pages, 3 figures (main
text), 18 pages, 6 figures (supplementary information
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