391 research outputs found
The Bose Metal: gauge field fluctuations and scaling for field tuned quantum phase transitions
In this paper, we extend our previous discussion of the Bose metal to the
field tuned case. We point out that the recent observation of the metallic
state as an intermediate phase between the superconductor and the insulator in
the field tuned experiments on MoGe films is in perfect consistency with the
Bose metal scenario. We establish a connection between general dissipation
models and gauge field fluctuations and apply this to a discussion of scaling
across the quantum phase boundaries of the Bose metallic state. Interestingly,
we find that the Bose metal scenario implies a possible {\em two} parameter
scaling for resistivity across the Bose metal-insulator transition, which is
remarkably consistent with the MoGe data. Scaling at the superconductor-metal
transition is also proposed, and a phenomenolgical model for the metallic state
is discussed. The effective action of the Bose metal state is described and its
low energy excitation spectrum is found to be .Comment: 15 pages, 1 figur
Functional Brain Imaging with Multi-Objective Multi-Modal Evolutionary Optimization
Functional brain imaging is a source of spatio-temporal data mining problems.
A new framework hybridizing multi-objective and multi-modal optimization is
proposed to formalize these data mining problems, and addressed through
Evolutionary Computation (EC). The merits of EC for spatio-temporal data mining
are demonstrated as the approach facilitates the modelling of the experts'
requirements, and flexibly accommodates their changing goals
Screening effects in superconductors
The partition function of the Hubbard model with local attraction and long
range Coulomb repulsion between electrons is written as a functional integral
with an action involving a pairing field and a local potential
. After integration over and over fluctuations in , the
final form of involves a Josephson coupling between the local phases of
and a "kinetic energy" term, representing the screened Coulomb
interaction between charge fluctuations. The competition between Josephson
coupling and charging energy allows to understand the relation between
and composition in high- materials, in particular superlattices, alloys
and bulk systems of low doping.Comment: 4 pages, revtex, no figures, submitted to Physica B (Proceedings of
SCES '96 International Conference, held in Zurich from 19th to 21st of
August
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
Anisotropy in the helicity modulus of a quantum 3D XY-model: application to YBCO
We present a variational study of the helicity moduli of an anisotropic
quantum three-dimensional (3D) XY-model of YBCO in superconducting state. It is
found that both the ab-plane and the c-axis helicity moduli, which are
proportional to the inverse square of the corresponding magnetic field
penetration depth, vary with temperature T as T to the fourth power in the zero
temperature limit. Moreover, the c-axis helicity modulus drops with temperature
much faster than the ab-plane helicity modulus because of the weaker Josephson
couplings along the c-axis compared to those along the ab-plane. These findings
are in disagreement with the experiments on high quality samples of YBCO.Comment: 9 pages, 1 figur
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
Monte Carlo study of the superfluid weight in doped antiferromagnets
The phase fluctuations of the condensate in doped antiferromagnets, described
by a t-t'-J model and a suitable 1/N expansion, provide a mechanism for a
Kosterlitz-Thouless (KT) type of transition to a superconducting state below
T_{c}. In this paper, we present a Monte Carlo study of the corresponding
superfluid weight D_{s}(T) in the classical (large-N) limit, as a function of
temperature and doping. Consistent with generic experimental trends, D_{s}(T)
exhibits a T-linear decrease at low temperatures, with the magnitude of the
slope D_{s}'(0) increasing upon doping. Finite-size scaling in the underdoped
regime predicts values for the dimensionless ratio A=k_{B}T_{c}/D_{s}(0) of
order unity, with A=0.4435(5) in the half-filled-band limit, thus confirming
D_{s}(0) as the fundamental energy scale determining T_{c}. Our Monte Carlo
results for D_{s}(T)/D_{s}(0) vs k_{B}T/D_{s}(0), at 10% hole doping, are found
to be in reasonable agreement with recent measurements on
La_{2-x}Sr_{x}CuO_{4}, with x=0.10, throughout the temperature range below the
theoretical KT transition temperature T_{c}.Comment: 9 pages, REVTEX file (4 Postscript figures). To appear in Phys. Rev.
Nature of the quantum phase transitions in the two-dimensional hardcore boson model
We use two Quantum Monte Carlo algorithms to map out the phase diagram of the
two-dimensional hardcore boson Hubbard model with near () and next near
() neighbor repulsion. At half filling we find three phases: Superfluid
(SF), checkerboard solid and striped solid depending on the relative values of
, and the kinetic energy. Doping away from half filling, the
checkerboard solid undergoes phase separation: The superfluid and solid phases
co-exist but not as a single thermodynamic phase. As a function of doping, the
transition from the checkerboard solid is therefore first order. In contrast,
doping the striped solid away from half filling instead produces a striped
supersolid phase: Co-existence of density order with superfluidity as a single
phase. One surprising result is that the entire line of transitions between the
SF and checkerboard solid phases at half filling appears to exhibit dynamical
O(3) symmetry restoration. The transitions appear to be in the same
universality class as the special Heisenberg point even though this symmetry is
explicitly broken by the interaction.Comment: 10 pages, 14 eps figures, include
Low energy collective modes, Ginzburg-Landau theory, and pseudogap behavior in superconductors with long-range pairing interactions
We study the superconducting instability in systems with long but finite
ranged, attractive, pairing interactions. We show that such long-ranged
superconductors exhibit a new class of fluctuations in which the internal
structure of the Cooper pair wave function is soft, and thus lead to
"pseudogap" behavior in which the actual transition temperature is greatly
depressed from its mean field value. These fluctuations are {\it not} phase
fluctuations of the standard superconducting order parameter, and lead to a
highly unusual Ginzburg-Landau description. We suggest that the crossover
between the BCS limit of a short-ranged attraction and our problem is of
interest in the context of superconductivity in the underdoped cuprates.Comment: 20 pages with one embedded ps figure. Minor revisions to the text and
references. Final version to appear in PRB on Nov. 1st, 200
On Effect of Equilibrium Fluctuations on Superfluid Density in Layered Superconductors
We calculate suppression of inter- and intralayer superconducting currents
due to equilibrium phase fluctuations and find that, in contrast to a recent
prediction, the effect of thermal fluctuations cannot account for linear
temperature dependence of the superfluid density in high-Tc superconductors at
low temperatures. Quantum fluctuations are found to dominate over thermal
fluctuations at low temperatures due to hardening of their spectrum caused by
the Josephson plasma resonance. Near Tc sizeable thermal fluctuations are found
to suppress the critical current in the stack direction stronger, than in the
direction along the layers. Fluctuations of quasiparticle branch imbalance make
the spectral density of voltage fluctuations at small frequencies non zero, in
contrast to what may be expected from a naive interpretation of Nyquist
formula.Comment: 5 pages, LaTeX, RevTeX, Submitted to PR
- âŠ