980 research outputs found
One-dimensional Josephson arrays as superlattices for single Cooper pairs
We investigate uniform one-dimensional arrays of small Josephson junctions
(, ) with a realistic Coulomb interaction (here is the screening length
in units of the lattice constant of the array). At low energies this system can
be described in terms of interacting Bose particles (extra single Cooper pairs)
on the lattice. With increasing concentration of extra Cooper pairs, a
crossover from the Bose gas phase to the Wigner crystal phase and then to the
superlattice regime occurs. The phase diagram in the superlattice regime
consists of commensurable insulating phases with ( is integer)
separated by superconducting regions where the current is carried by
excitations with {\em fractional} electric charge . The Josephson
current through a ring-shaped array pierced by magnetic flux is calculated for
all of the phases.Comment: 4 pages (LATEX), 2 figure
Bose-Einstein condensation in inhomogeneous Josephson arrays
We show that spatial Bose-Einstein condensation of non-interacting bosons
occurs in dimension d < 2 over discrete structures with inhomogeneous topology
and with no need of external confining potentials. Josephson junction arrays
provide a physical realization of this mechanism. The topological origin of the
phenomenon may open the way to the engineering of quantum devices based on
Bose-Einstein condensation. The comb array, which embodies all the relevant
features of this effect, is studied in detail.Comment: 4 pages, 5 figure
Quantum Heisenberg Antiferromagnet: Improved Spin-Wave Theories Versus Exact-Diagonalization Data
We reconsider the results cocerning the extreme-quantum
square-lattice Heisenberg antiferromagnet with frustrating diagonal couplings
( model) drawn from a comparison with exact-diagonalization data. A
combined approach using also some intrinsic features of the self-consistent
spin-wave theory leads to the conclusion that the theory strongly overestimates
the stabilizing role of quantum flutcuations in respect to the N\'{e}el phase
in the extreme-quantum case . On the other hand, the analysis implies
that the N\'{e}el phase remains stable at least up to the limit which is pretty larger than some previous estimates. In addition, it is
argued that the spin-wave ansatz predicts the existence of a finite range
( in the linear spin-wave theory) where the Marshall-Peierls
sigh rule survives the frustrations.Comment: 13 pages, LaTex, 7 figures on reques
Superconductor-insulator transition driven by local dephasing
We consider a system where localized bound electron pairs form an array of
"Andreev"-like scattering centers and are coupled to a fermionic subsystem of
uncorrelated electrons. By means of a path-integral approach, which describes
the bound electron pairs within a coherent pseudospin representation, we derive
and analyze the effective action for the collective phase modes which arise
from the coupling between the two subsystems once the fermionic degrees of
freedom are integrated out. This effective action has features of a quantum
phase model in the presence of a Berry phase term and exhibits a coupling to a
field which describes at the same time the fluctuations of density of the bound
pairs and those of the amplitude of the fermion pairs. Due to the competition
between the local and the hopping induced non-local phase dynamics it is
possible, by tuning the exchange coupling or the density of the bound pairs, to
trigger a transition from a phase ordered superconducting to a phase disordered
insulating state. We discuss the different mechanisms which control this
occurrence and the eventual destruction of phase coherence both in the weak and
strong coupling limit.Comment: 16 pages, 5 figures, submitted to PRB (05-Feb04
On the Coexistence of Diagonal and off-Diagonal Long-Range Order, a Monte Carlo Study
The zero temperature properties of interacting 2 dimensional lattice bosons
are investigated. We present Monte Carlo data for soft-core bosons that
demonstrate the existence of a phase in which crystalline long-range order and
off-diagonal long-range order (superfluidity) coexist. We comment on the
difference between hard and soft-core bosons and compare our data to mean-field
results that predict a larger coexistence region. Furthermore, we determine the
critical exponents for the various phase transitions.Comment: 7 pages and 8 figures appended in postscript, KA-TFP-93-0
Nonlinear Transport and Current Fluctuation in an AB Ring with a Quantum Dot
Nonequilibrium steady states are explicitly constructed for a noninteracting
electron model of an Aharonov-Bohm (AB) ring with a quantum dot (QD) with the
aid of asymptotic fields. The Fano line shapes and AB oscillations are shown to
strongly depend on the bias voltage. Current fluctuations are studied as well.Comment: 4pages, 6figure
Effects of dephasing on shot-noise in an electronic Mach-Zehnder interferometer
We present a theoretical study of the influence of dephasing on shot noise in
an electronic Mach-Zehnder interferometer. In contrast to phenomenological
approaches, we employ a microscopic model where dephasing is induced by the
fluctuations of a classical potential. This enables us to treat the influence
of the environment's fluctuation spectrum on the shot noise. We compare against
the results obtained from a simple classical model of incoherent transport, as
well as those derived from the phenomenological dephasing terminal approach,
arguing that the latter runs into a problem when applied to shot noise
calculations for interferometer geometries. From our model, we find two
different limiting regimes: If the fluctuations are slow as compared to the
time-scales set by voltage and temperature, the usual partition noise
expression T(1-T) is averaged over the fluctuating phase difference. For the
case of ``fast'' fluctuations, it is replaced by a more complicated expression
involving an average over transmission amplitudes. The full current noise also
contains other contributions, and we provide a general formula, as well as
explicit expressions and plots for specific examples.Comment: 18 pages, 8 figures. A brief version is contained in cond-mat/030650
Kondo-resonance, Coulomb blockade, and Andreev transport through a quantum dot
We study resonant tunneling through an interacting quantum dot coupled to
normal metallic and superconducting leads. We show that large Coulomb
interaction gives rise to novel effects in Andreev transport. Adopting an exact
relation for the Green's function, we find that at zero temperature, the linear
response conductance is enhanced due to Kondo-Andreev resonance in the Kondo
limit, while it is suppressed in the empty site limit. In the Coulomb blockaded
region, on the other hand, the conductance is reduced more than the
corresponding conductance with normal leads because large charging energy
suppresses Andreev reflection.Comment: 3 pages Revtex, 4 Postscript figures, accepted for publication in
Phys. Rev.
Quantum-Phase Transitions of Interacting Bosons and the Supersolid Phase
We investigate the properties of strongly interacting bosons in two
dimensions at zero temperature using mean-field theory, a variational Ansatz
for the ground state wave function, and Monte Carlo methods. With on-site and
short-range interactions a rich phase diagram is obtained. Apart from the
homogeneous superfluid and Mott-insulating phases, inhomogeneous charge-density
wave phases appear, that are stabilized by the finite-range interaction.
Furthermore, our analysis demonstrates the existence of a supersolid phase, in
which both long-range order (related to the charge-density wave) and
off-diagonal long-range order coexist. We also obtain the critical exponents
for the various phase transitions.Comment: RevTex, 20 pages, 10 PostScript figures include
New quantum phases in a one-dimensional Josephson array
We examine the phase diagram of an ordered one-dimensional Josephson array of
small grains. The average grain charge in such a system can be tuned by means
of gate voltage. At small grain-to-grain conductance, this system is strongly
correlated because of the charge discreteness constraint (Coulomb blockade). At
the gate voltages in the vicinity of the charge degeneracy points, we find new
phases equivalent to a commensurate charge density wave and to a repulsive
Luttinger liquid. The existence of these phases can be probed through a special
dependence of the Josephson current on the gate voltage.Comment: 4 pages, including 1 eps figur
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