166 research outputs found
Phase and Charge reentrant phase transitions in two capacitively coupled Josephson arrays with ultra-small junction
We have studied the phase diagram of two capacitively coupled Josephson
junction arrays with charging energy, , and Josephson coupling energy,
. Our results are obtained using a path integral Quantum Monte Carlo
algorithm. The parameter that quantifies the quantum fluctuations in the i-th
array is defined by . Depending on
the value of , each independent array may be in the semiclassical or
in the quantum regime: We find that thermal fluctuations are important when
and the quantum fluctuations dominate when . We have extensively studied the interplay between vortex and charge
dominated individual array phases. The two arrays are coupled via the
capacitance at each site of the lattices. We find a {\it
reentrant transition} in , at low temperatures, when one of
the arrays is in the semiclassical limit (i.e. ) and the
quantum array has , for the values considered for
the interlayer capacitance. In addition, when , and
for all the inter-layer couplings considered above, a {\it novel} reentrant
phase transition occurs in the charge degrees of freedom, i.e. there is a
reentrant insulating-conducting transition at low temperatures. We obtain the
corresponding phase diagrams and found some features that resemble those seen
in experiments with 2D JJA.Comment: 25 Latex pages including 8 encapsulated poscript figures. Accepted
for publication in Phys. Rev B (Nov. 2004 Issue
Quantum effects in a superconducting glass model
We study disordered Josephson junctions arrays with long-range interaction
and charging effects. The model consists of two orthogonal sets of positionally
disordered parallel filaments (or wires) Josephson coupled at each crossing
and in the presence of a homogeneous and transverse magnetic field. The large
charging energy (resulting from small self-capacitance of the ultrathin wires)
introduces important quantum fluctuations of the superconducting phase within
each filament. Positional disorder and magnetic field frustration induce
spin-glass like ground state, characterized by not having long-range order of
the phases. The stability of this phase is destroyed for sufficiently large
charging energy. We have evaluated the temperature vs charging energy phase
diagram by extending the methods developed in the theory of infinite-range spin
glasses, in the limit of large magnetic field. The phase diagram in the
different temperature regimes is evaluated by using variety of methods, to wit:
semiclassical WKB and variational methods, Rayleigh-Schr\"{o}dinger
perturbation theory and pseudospin effective Hamiltonians. Possible
experimental consequences of these results are briefly discussed.Comment: 17 pages REVTEX. Two Postscript figures can be obtained from the
authors. To appear in PR
The mechanism of hole carrier generation and the nature of pseudogap- and 60K-phases in YBCO
In the framework of the model assuming the formation of NUC on the pairs of
Cu ions in CuO plane the mechanism of hole carrier generation is
considered and the interpretation of pseudogap and 60 K-phases in
. is offered. The calculated dependences of hole
concentration in on doping and temperature
are found to be in a perfect quantitative agreement with experimental data. As
follows from the model the pseudogap has superconducting nature and arises at
temperature in small clusters uniting a number of
NUC's due to large fluctuations of NUC occupation. Here and
are the superconducting transition temperatures of infinite and finite
clusters of NUC's, correspondingly. The calculated and
dependences are in accordance with experiment. The area between
and corresponds to the area of fluctuations
where small clusters fluctuate between superconducting and normal states owing
to fluctuations of NUC occupation. The results may serve as important arguments
in favor of the proposed model of HTSC.Comment: 12 pages, 7 figure
Inertial Mass of a Vortex in Cuprate Superconductors
We present here a calculation of the inertial mass of a moving vortex in
cuprate superconductors. This is a poorly known basic quantity of obvious
interest in vortex dynamics. The motion of a vortex causes a dipolar density
distortion and an associated electric field which is screened. The energy cost
of the density distortion as well as the related screened electric field
contribute to the vortex mass, which is small because of efficient screening.
As a preliminary, we present a discussion and calculation of the vortex mass
using a microscopically derivable phase-only action functional for the far
region which shows that the contribution from the far region is negligible, and
that most of it arises from the (small) core region of the vortex. A
calculation based on a phenomenological Ginzburg-Landau functional is performed
in the core region. Unfortunately such a calculation is unreliable, the reasons
for it are discussed. A credible calculation of the vortex mass thus requires a
fully microscopic, non-coarse grained theory. This is developed, and results
are presented for a s-wave BCS like gap, with parameters appropriate to the
cuprates. The mass, about 0.5 per layer, for magnetic field along the
axis, arises from deformation of quasiparticle states bound in the core, and
screening effects mentioned above. We discuss earlier results, possible
extensions to d-wave symmetry, and observability of effects dependent on the
inertial mass.Comment: 27 pages, Latex, 3 figures available on request, to appear in
Physical Review
Quantum critical point and scaling in a layered array of ultrasmall Josephson junctions
We have studied a quantum Hamiltonian that models an array of ultrasmall
Josephson junctions with short range Josephson couplings, , and charging
energies, , due to the small capacitance of the junctions. We derive a new
effective quantum spherical model for the array Hamiltonian. As an application
we start by approximating the capacitance matrix by its self-capacitive limit
and in the presence of an external uniform background of charges, . In
this limit we obtain the zero-temperature superconductor-insulator phase
diagram, , that improves upon previous theoretical
results that used a mean field theory approximation. Next we obtain a
closed-form expression for the conductivity of a square array, and derive a
universal scaling relation valid about the zero--temperature quantum critical
point. In the latter regime the energy scale is determined by temperature and
we establish universal scaling forms for the frequency dependence of the
conductivity.Comment: 18 pages, four Postscript figures, REVTEX style, Physical Review B
1999. We have added one important reference to this version of the pape
Superconducting to normal state phase boundary in arrays of ultrasmall Josephson junctions
We study the competition between Josephson and charging energies in
two-dimensional arrays of ultrasmall Josephson junctions, when the mutual
capacitance is dominant over the self-capacitance. Our calculations involve a
combination of an analytic WKB renormalization group approach plus
nonperturbative Quantum Monte Carlo simulations. We consider the zero
frustration case in detail and we are able to make a successful comparison
between our results and those obtained experimentally.Comment: 14 pages + 2 postscript figures, REVTEX. THU-9412
First Order Transition in the Ginzburg-Landau Model
The d-dimensional complex Ginzburg-Landau (GL) model is solved according to a
variational method by separating phase and amplitude. The GL transition becomes
first order for high superfluid density because of effects of phase
fluctuations. We discuss its origin with various arguments showing that, in
particular for d = 3, the validity of our approach lies precisely in the first
order domain.Comment: 4 pages including 2 figure
Resistance in Superconductors
In this pedagogical review, we discuss how electrical resistance can arise in
superconductors. Starting with the idea of the superconducting order parameter
as a condensate wave function, we introduce vortices as topological excitations
with quantized phase winding, and we show how phase slips occur when vortices
cross the sample. Superconductors exhibit non-zero electrical resistance under
circumstances where phase slips occur at a finite rate. For one-dimensional
superconductors or Josephson junctions, phase slips can occur at isolated
points in space-time. Phase slip rates may be controlled by thermal activation
over a free-energy barrier, or in some circumstances, at low temperatures, by
quantum tunneling through a barrier. We present an overview of several
phenomena involving vortices that have direct implications for the electrical
resistance of superconductors, including the Berezinskii-Kosterlitz-Thouless
transition for vortex-proliferation in thin films, and the effects of vortex
pinning in bulk type II superconductors on the non-linear resistivity of these
materials in an applied magnetic field. We discuss how quantum fluctuations can
cause phase slips and review the non-trivial role of dissipation on such
fluctuations. We present a basic picture of the superconductor-to-insulator
quantum phase transitions in films, wires, and Josephson junctions. We point
out related problems in superfluid helium films and systems of ultra-cold
trapped atoms. While our emphasis is on theoretical concepts, we also briefly
describe experimental results, and we underline some of the open questions.Comment: Chapter to appear in "Bardeen, Cooper and Schrieffer: 50 Years,"
edited by Leon N. Cooper and Dmitri Feldman, to be published by World
Scientific Pres
Three-dimensional Josephson-junction arrays in the quantum regime
We study the quantum phase transition properties of a three-dimensional
periodic array of Josephson junctions with charging energy that includes both
the self and mutual junction capacitances. We use the phase fluctuation algebra
between number and phase operators, given by the Euclidean group E_2, and we
effectively map the problem onto a solvable quantum generalization of the
spherical model. We obtain a phase diagram as a function of temperature,
Josephson coupling and charging energy. We also analyze the corresponding
fluctuation conductivity and its universal scaling form in the vicinity of the
zero-temperature quantum critical point.Comment: 9 pages, LATEX, three PostScript figures. Submitted to Phys. Rev.
Let
Mean Field Theory of Josephson Junction Arrays with Charge Frustration
Using the path integral approach, we provide an explicit derivation of the
equation for the phase boundary for quantum Josephson junction arrays with
offset charges and non-diagonal capacitance matrix. For the model with nearest
neighbor capacitance matrix and uniform offset charge , we determine,
in the low critical temperature expansion, the most relevant contributions to
the equation for the phase boundary. We explicitly construct the charge
distributions on the lattice corresponding to the lowest energies. We find a
reentrant behavior even with a short ranged interaction. A merit of the path
integral approach is that it allows to provide an elegant derivation of the
Ginzburg-Landau free energy for a general model with charge frustration and
non-diagonal capacitance matrix. The partition function factorizes as a product
of a topological term, depending only on a set of integers, and a
non-topological one, which is explicitly evaluated.Comment: LaTex, 24 pages, 8 figure
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