177 research outputs found
Competition between local and nonlocal dissipation effects in two-dimensional quantum Josephson junction arrays
We discuss the local and nonlocal dissipation effects on the existence of the
global phase coherence transitions in two dimensional Josephson-coupled
junctions. The quantum phase transitions are also examined for various lattice
geometries: square, triangular and honeycomb. The T=0 superconductor-insulator
phase transition is analyzed as a function of several control parameters which
include self-capacitance and junction capacitance and both local and nonlocal
dissipation effects. We found the critical value of the nonlocal dissipation
parameter \alpha_{1} depends on a geometry of the lattice. The critical value
of the normal state conductance seems to be difficult to obtain experimentally
if we take into consideration different damping mechanisms which are presented
in real physical systems.Comment: accepted to Physica C Ref. No.: PHYSC-D-06-00244R
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
Possible origin of 60-K plateau in the YBa2Cu3O(6+y) phase diagram
We study a model of YBa2Cu3O(6+y) to investigate the influence of oxygen
ordering and doping imbalance on the critical temperature Tc(y) and to
elucidate a possible origin of well-known feature of YBCO phase diagram: the
60-K plateau. Focusing on "phase only" description of the high-temperature
superconducting system in terms of collective variables we utilize a
three-dimensional semi microscopic XY model with two-component vectors that
involve phase variables and adjustable parameters representing microscopic
phase stiffnesses. The model captures characteristic energy scales present in
YBCO and allows for strong anisotropy within basal planes to simulate oxygen
ordering. Applying spherical closure relation we have solved the phase XY model
with the help of transfer matrix method and calculated Tc for chosen system
parameters. Furthermore, we investigate the influence of oxygen ordering and
doping imbalance on the shape of YBCO phase diagram. We find it unlikely that
oxygen ordering alone can be responsible for the existence of 60-K plateau.
Relying on experimental data unveiling that oxygen doping of YBCO may introduce
significant charge imbalance between CuO2 planes and other sites, we show that
simultaneously the former are underdoped, while the latter -- strongly
overdoped almost in the whole region of oxygen doping in which YBCO is
superconducting. As a result, while oxygen content is increased, this provides
two counter acting factors, which possibly lead to rise of 60K plateau.
Additionally, our result can provide an important contribution to understanding
of experimental data supporting existence of multicomponent superconductivity
in YBCO.Comment: 9 pages, 8 figures, submitted to PRB, see http://prb.aps.or
3D IC optimal layout design. A parallel and distributed topological approach
The task of 3D ICs layout design involves the assembly of millions of
components taking into account many different requirements and constraints such
as topological, wiring or manufacturability ones. It is a NP-hard problem that
requires new non-deterministic and heuristic algorithms. Considering the time
complexity, the commonly applied Fiduccia-Mattheyses partitioning algorithm is
superior to any other local search method. Nevertheless, it can often miss to
reach a quasi-optimal solution in 3D spaces. The presented approach uses an
original 3D layout graph partitioning heuristics implemented with use of the
extremal optimization method. The goal is to minimize the total wire-length in
the chip. In order to improve the time complexity a parallel and distributed
Java implementation is applied. Inside one Java Virtual Machine separate
optimization algorithms are executed by independent threads. The work may also
be shared among different machines by means of The Java Remote Method
Invocation system.Comment: 26 pages, 9 figure
Quantum rotor description of the Mott-insulator transition in the Bose-Hubbard model
We present the novel approach to the Bose-Hubbard model using the
quantum rotor description. The effective action formalism
allows us to formulate a problem in the phase only action and obtain an
analytical formulas for the critical lines. We show that the nontrivial
phase field configurations have an impact on the phase
diagrams. The topological character of the quantum field is governed by terms
of the integer charges - winding numbers. The comparison presented results to
recently obtained quantum Monte Carlo numerical calculations suggests that the
competition between quantum effects in strongly interacting boson systems is
correctly captured by our model.Comment: accepted to PR
Nexus between quantum criticality and the chemical potential pinning in high- cuprates
For strongly correlated electrons the relation between total number of charge
carriers and the chemical potential reveals for large Coulomb
energy the apparently paradoxical pinning of within the Mott gap, as
observed in high- cuprates. By unravelling consequences of the non-trivial
topology of the charge gauge U(1) group and the associated ground state
degeneracy we found a close kinship between the pinning of and the
zero-temperature divergence of the charge compressibility , which marks a novel quantum criticality governed by
topological charges rather than Landau principle of the symmetry breaking.Comment: 4+ pages, 2 figures, typos corrected, version as publishe
SO(5) superconductor in a Zeeman magnetic field: Phase diagram and thermodynamic properties
In this paper we present calculations of the SO(5) quantum rotor theory of
high-T superconductivity in Zeeman magnetic field. We use the spherical
approach for five-component quantum rotors in three-dimensional lattice to
obtain formulas for critical lines, free energy, entropy and specific heat and
present temperature dependences of these quantities for different values of
magnetic field. Our results are in qualitative agreement with relevant
experiments on high-T cuprates.Comment: 4 pages, 2 figures, to appear in Phys. Rev. B, see http://prb.aps.or
Local dissipation effects in two-dimensional quantum Josephson junction arrays with magnetic field
We study the quantum phase transitions in two-dimensional arrays of
Josephson-couples junctions with short range Josephson couplings (given by the
Josephson energy) and the charging energy. We map the problem onto the solvable
quantum generalization of the spherical model that improves over the mean-field
theory method. The arrays are placed on the top of a two-dimensional electron
gas separated by an insulator. We include effects of the local dissipation in
the presence of an external magnetic flux f in square lattice for several
rational fluxes f=0,1/2,1/3,1/4 and 1/6. We also have examined the T=0
superconducting-insulator phase boundary as function of a dissipation alpha for
two different geometry of the lattice: square and triangular. We have found
critical value of the dissipation parameter independent on geometry of the
lattice and presence magnetic field.Comment: accepted to PR
A generalized spherical version of the Blume-Emery-Griffits model with ferromagnetic and antiferromagnetic interactions
We have investigated analitycally the phase diagram of a generalized
spherical version of the Blume-Emery-Griffiths model that includes
ferromagnetic or antiferromagnetic spin interactions as well as quadrupole
interactions in zero and nonzero magnetic field. We show that in three
dimensions and zero magnetic field a regular paramagnetic-ferromagnetic (PM-FM)
or a paramagnetic-antiferromagnetic (PM-AFM) phase transition occurs whenever
the magnetic spin interactions dominate over the quadrupole interactions.
However, when spin and quadrupole interactions are important, there appears a
reentrant FM-PM or AFM-PM phase transition at low temperatures, in addition to
the regular PM-FM or PM-AFM phase transitions. On the other hand, in a nonzero
homogeneous external magnetic field , we find no evidence of a transition to
the state with spontaneous magnetization for FM interactions in three
dimensions. Nonethelesss, for AFM interactions we do get a scenario similar to
that described above for zero external magnetic field, except that the critical
temperatures are now functions of . We also find two critical field values,
, at which the reentrance phenomenon dissapears and
(), above which the PM-AFM transition temperature
vanishes.Comment: 21 pages, 6 figs. Title changed, abstract and introduction as well as
section IV were rewritten relaxing the emphasis on spin S=1 and Figs. 5 an 6
were improved in presentation. However, all the results remain valid.
Accepted for publication in Phys. Rev.
Neel Order and Electron Spectral Functions in the Two-Dimensional Hubbard Model: a Spin-Charge Rotating Frame Approach
Using recently developed quantum SU(2)xU(1) rotor approach, that provides a
self-consistent treatment of the antiferromagnetic state we have performed
electronic spectral function calculations for the Hubbard model on the square
lattice. The collective variables for charge and spin are isolated in the form
of the space-time fluctuating U(1) phase field and rotating spin quantization
axis governed by the SU(2) symmetry, respectively. As a result interacting
electrons appear as composite objects consisting of bare fermions with attached
U(1) and SU(2) gauge fields. This allows us to write the fermion Green's
function in the space-time domain as the product CP^1 propagator resulting from
the SU(2) gauge fields, U(1) phase propagator and the pseudo-fermion
correlation function. As a result the problem of calculating the spectral line
shapes now becomes one of performing the convolution of spin, charge and
pseudo-fermion Green's functions. The collective spin and charge fluctuations
are governed by the effective actions that are derived from the Hubbard model
for any value of the Coulomb interaction. The emergence of a sharp peak in the
electron spectral function in the antiferromagnetic state indicates the decay
of the electron into separate spin and charge carrying particle excitations.Comment: 16 pages, 5 figures, submitted to Phys. Rev.
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