47 research outputs found
Spin-size disorder model for granular superconductors with charging effects
A quantum pseudo-spin model with random spin sizes is introduced to study the
effects of charging-energy disorder on the superconducting transition in
granular superconducting materials. Charging-energy effects result from the
small electrical capacitance of the grains when the Coulomb charging energy is
comparable to the Josephson coupling energy. In the pseudo-spin model,
randomness in the spin size is argued to arise from the inhomogeneous
grain-size distribution. For a particular bimodal spin-size distribution, the
model describes percolating granular superconductors. A mean-field theory is
developed to obtain the phase diagram as a function of temperature, average
charging energy and disorder.Comment: 4 pages, 2 figure
Charging Effects and Quantum Crossover in Granular Superconductors
The effects of the charging energy in the superconducting transition of
granular materials or Josephson junction arrays is investigated using a
pseudospin one model. Within a mean-field renormalization-group approach, we
obtain the phase diagram as a function of temperature and charging energy. In
contrast to early treatments, we find no sign of a reentrant transition in
agreement with more recent studies. A crossover line is identified in the
non-superconducting side of the phase diagram and along which we expect to
observe anomalies in the transport and thermodynamic properties. We also study
a charge ordering phase, which can appear for large nearest neighbor Coulomb
interaction, and show that it leads to first-order transitions at low
temperatures. We argue that, in the presence of charge ordering, a non
monotonic behavior with decreasing temperature is possible with a maximum in
the resistance just before entering the superconducting phase.Comment: 15 pages plus 4 fig. appended, Revtex, INPE/LAS-00
Parity Effects in Stacked Nanoscopic Quantum Rings
The ground state and the dielectric response of stacked quantum rings are
investigated in the presence of an applied magnetic field along the ring axis.
For odd number of rings and an electric field perpendicular to the axis, a
linear Stark effect occurs at distinct values of the magnetic field. At those
fields energy levels cross in the absence of electric field. For even values of
a quadratic Stark effect is expected in all cases, but the induced electric
polarization is discontinuous at those special magnetic fields. Experimental
consequences for related nanostructures are discussed.Comment: typos corrected, to appear Phys. Rev. B (Rapid Communication) 15 Au
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
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
Doping Controlled Superconductor-Insulator Transition in Bi2Sr2-xLaxCaCu2O8+delta
We show that the doping-controlled superconductor-insulator transition (SIT)
in a high critical temperature cuprate system (Bi2Sr2-xLaxCaCu2O8+delta)
exhibits a fundamentally different behavior than is expected from conventional
SIT. At the critical doping, the sheet resistance seems to diverge in the zero
temperature limit. Above the critical doping, the transport is universally
scaled by a two-component conductance model. Below, it continuously evolves
from weakly to strongly insulating behavior. The two-component conductance
model suggests that a collective electronic phase separation mechanism may be
responsible for this unconventional SIT behavior.Comment: 21 pages, 5 figures, abstract changed. Introduction and conclusion
expanded. Slight changes in the main text. Accepted to PR
Spectral Flow, Magnus Force and Mutual Friction via the Geometric Optics Limit of Andreev Reflection
The notion of spectral flow has given new insight into the motion of vortices
in superfluids and superconductors. For a BCS superconductor the spectrum of
low energy vortex core states is largely determined by the geometric optics
limit of Andreev reflection. We use this to follow the evolution of the states
when a stationary vortex is immersed in a transport supercurrent. If the core
spectrum were continuous, spectral flow would convert the momentum flowing into
the core via the Magnus effect into unbound quasiparticles --- thus allowing
the vortex to remain stationary without a pinning potential or other sink for
the inflowing momentum. The discrete nature of the states, however, leads to
Bloch oscillations which thwart the spectral flow. The momentum can escape only
via relaxation processes. Taking these into account permits a physically
transparent derivation of the mutual friction coefficients.Comment: Plain TeX, 19 pages, 5 encapsulated postscript figure
Granular Electronic Systems
A granular metal is an array of metallic nano-particles imbedded into an
insulating matrix. Tuning the intergranular coupling strength a granular system
can be transformed into either a good metal or an insulator and, in case of
superconducting particles, experience superconductor-insulator transition. The
ease of adjusting electronic properties of granular metals makes them most
suitable for fundamental studies of disordered solids and assures them a
fundamental role for nanotechnological applications. This Review discusses
recent important theoretical advances in the study of granular metals,
emphasizing on the interplay of disorder, quantum effects, fluctuations and
effects of confinement in formation of electronic transport and thermodynamic
properties of granular materials.Comment: 51 pages, 23 figures, submitted to Reviews of Modern Physic
Microscopic approach to current-driven domain wall dynamics
This review describes in detail the essential techniques used in microscopic
theories on spintronics. We have investigated the domain wall dynamics induced
by electric current based on the - exchange model. The domain wall is
treated as rigid and planar and is described by two collective coordinates: the
position and angle of wall magnetization. The effect of conduction electrons on
the domain wall dynamics is calculated in the case of slowly varying spin
structure (close to the adiabatic limit) by use of a gauge transformation. The
spin-transfer torque and force on the wall are expressed by Feynman diagrams
and calculated systematically using non-equilibrium Green's functions, treating
electrons fully quantum mechanically. The wall dynamics is discussed based on
two coupled equations of motion derived for two collective coordinates. The
force is related to electron transport properties, resistivity, and the Hall
effect. Effect of conduction electron spin relaxation on the torque and wall
dynamics is also studied.Comment: manucript accepted to Phys. Re
Evaluation of serum chondroitin sulfate and hyaluronan: biomarkers for osteoarthritis in canine hip dysplasia
Hip dysplasia (HD) is one of the most important bone and joint diseases in dogs. Making the radiographic diagnosis is sometime possible when the disease has markedly progressed. Chondroitin sulfate (CS) and hyaluronan (HA) are the most important cartilage biomolecules that are elevated in the serum taken from dogs with osteoarthritis. The serum CS and HA can be detected by an ELISA technique, with using monoclonal antibodies against CS epitope 3B3 and WF6 and the HA chain as the primary antibodies. The aim of this study was to compare the levels of serum CS (both epitopes) and HA in non-HD and HD dogs. All 123 dogs were categorized into 2 groups. The non-HD group was composed of 98 healthy dogs, while the HD group was comprised of 25 HD dogs. Blood samples were collected for analyzing the serum CS and HA levels with using the ELISA technique. The results showed that the average serum level of the CS epitope WF6 in the HD group (2,594 ± 3,036.10 ng/ml) was significantly higher than that in the non-HD group (465 ± 208.97 ng/ml) (p < 0.01) while the epitope 3B3 in the HD group (105 ± 100.05 ng/ml) was significantly lower than that in the non-HD group (136 ± 142.03 ng/ml) (p < 0.05). The amount of serum HA in the HD group (134.74 ± 59.71 ng/ml) was lower than that in the non HD group (245.45 ± 97.84 ng/ml) (p < 0.05). The results indicate that the serum CS and HA levels might be used as biomarkers for osteoarthritis in HD dogs