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At least 19 recordsLinked to original sources
arXiv1995

Raman spectrum and charge fluctuations in the copper-oxide superconductors

The effect of the charge fluctuations on the electronic spectrum and the Raman spectrum of high temperature superconductors is examined within the slave boson approach. Instead of using the saddle point approximation for slave bosons, we confine ourselves to the non-crossing approximation (NCA) in summing the diagrams for the Green functions, thus obtaining the renormalized hole spectrum and its lifetime on equal footing. The electronic Raman spectrum is calculated, showing the characteristic featureless behaviour up to the frequency of the order of renormalized $Δ_{pd}$ parameter. The dependence on the polarization of the incident and the scattered light agrees with experiments.

cond-mat↗
arXiv1995

Quantum critical phenomena in ladders of Josephson junctions in a magnetic field

A model of a ladder of Josephson junctions in a magnetic field is considered. The topological features of the zero-temperature phase diagram, as a function of charging energy and small deviations from commensurability of the vortex lattice, are strongly dependent on the field. In addition to superconductor-insulator transitions, commensurate-incommensurate transitions and depinning by quantum fluctuations of the vortex-lattice are also possible. the critical behavior of the superconductor-insulator transition at f = 1/2 is found to depend on the ratio between interchain and intrachain Josephson couplings and is in the universality of the classical XY-Ising model.

cond-mat↗
arXiv1995

Heavy Fermion Superconductivity

A brief review is given on the current state of experimental and theoretical work in the field of heavy fermion superconductivity. Particular emphasis is placed on those experiments which demonstrate the presence of an unconventional order parameter, as well as the intricate interplay of magnetism and superconductivity in these metals. On the theoretical front, current phenomenological models of the superconductivity are reviewed along with potential pairing mechanisms.

cond-mat↗
arXiv1995

Phase transitions in a Kagome lattice of Josephson junctions

We investigate the nature of the phase transition in Josephson junctions arranged on a Kagome lattice. We find that an applied magnetic field corresponding to 1/2 flux quanta per elementary triangle results in a pi phase shift in the current phase relation for all bonds, resulting in an XY antiferromagnet. This corresponds to the order-from-disorder selected highly-degenerate coplanar state of the more extensively studied Heisenberg Kagome antiferromagnet. Using an histogram Monte-Carlo analysis, we observe a phase transition at 0.078 of the coupling energy. We find that the jump in the superfluid density at the transition temperature, determined from a finite-size scaling analysis of the magnetization fluctuations, retains its universal value within the Kosterlitz-Thouless scenario despite the well-known degeneracy of the low-temperature phase. This universal jump, combined with the dramatically suppressed transition temperature, point to a very strong renormalization of the spin stiffness just below the transition temperature. We also observe a sequence of ground states corresponding to flux quanta per elementary triangle of 1/2, 3/8, 1/4, 1/8, and 0 that, unlike ground states of other lattices, consist of currents of equal magnitude circulating around each elementary triangle in the structure. This sequence of structurally similar ground states will permit a detailed analysis of the dependence of transition temperature on frustration.

cond-mat↗
arXiv1995

Spectral Flow in Josephson Junctions and Effective Magnus Force

Momentum production during the phase slip process in SNS Josephson junction is discussed. It is caused by the spectral flow of bound states of fermions localized within the junction. This effectively reduces the Magnus force acting on vortices which provides an explanation for the experimental observation of the negligible Magnus force in 2D Josephson junction arrays. The flow of the fermionic levels is similar to that in sphalerons in particle physics, where it gives rise to the baryogenesis.

cond-mat↗
arXiv1996

Charge Transport in Superconductor - Normal Metal Proximity Structures

We develop a detailed analysis of electron transport in normal diffusive conductors in the presence of proximity induced superconducting correlation. We calculated the linear conductance of the system and the profile of the electric field. A rich structure of temperature dependences was found and explained. Our results are consistent with recent experimental findings.

cond-mat↗
arXiv1996

Coherent transport in a normal diffusive wire between two reservoirs

We develop a detailed analysis of electron transport in normal diffusive con- ductors in the presence of proximity induced superconductivity. A rich structure of temperature and energy dependencies for the system condcutcance, density of states and related quantities was found and explained. If the nor- mal conductor forms a loop its conductance changes h/2e-periodically with the magnetic flux inside the loop. The amplitude of these conductance oscillations shows a reentrant behavior and decays as 1/T at high T

cond-mat↗
arXiv1996

Comment on "Voltage-Current Characteristics of the Two-Dimensional Gauge Glass Model"

Li (Phys. Rev. Lett. {\bf 69},1819 (1992)) has computed the current-voltage characteristics (CVC) of a disordered two-dimensional (2D) Josephson-junction array and claimed that his data(nonlinear CVC for small temperatures), gives evidence for a finite-temperature phase transition in the 2D gauge-glass model in contradiction to the result of equilibrium simulations. In this Comment I show that the nonlinear CVC, found by Li, is merely a single-junction effect.

cond-mat↗
arXiv1996

Pairing Constraint on the Real Space Formalism of the Theory of Superconductivity

We review the new theory of impure superconductors constructed by Kim and Overhauser, and further developed by Kim. It was shown that Gor'kov's self-consistency equation needs a pairing constraint derived from the Anomalous Green's function. Whereas earlier studies have applied a constraint only on the pair potential, we show that the kernel also should be fixed by the ground state. The Bogoliubov-de Gennes equations need a pairing constraint in order to obtain the proper vacuum state by the corresponding unitary transformation. The relation between the pair potential and the gap parameter is clarified. This new study opens up a reinvestigation of inhomogeneous superconductors. We will discuss (i) strong coupling theory of dirty superconductors, (ii) suppression of magnetic impurity effect by ordinary impurities, (iii) weak localization correction to the phonon-mediated interaction, and (iv) other inhomogeneous superconductors.

cond-mat↗
arXiv1996

Supercurrent Decay by Vortex Nucleation in the Presence of an Array of Pinning Centres: A Renormalization Group Approach

We study the phenomenon of decay of a supercurrent in a superconducting thin film due to the spontaneous homogeneous nucleation of quantized vortex-antivortex pairs in the absence of an external magnetic field and in the presence of a periodic pinning potential. We describe the vortex nucleation by means of a Schwinger-type path-integral calculation including the effects of quantum dissipation. The overdamped case is treated exactly, whilst the case in which a periodic array of pinning centres is present is dealt with by means of a renormalization group (RG) approach in frequency space. This yields an approximate but very appealing analytic result for the dependence of the vortex nucleation rate on the externally-driven supercurrent. In this formula the rate dependence displays oscillations which are connected to the pinning periodicity and correspond to the vortex nucleation length probing over the pinning sites as the current density is varied. Our RG treatment describes the vortex nucleation process in both the confined and the mobile phases of the dissipation-driven localization transition characterising the motion of dissipating quantum particles in a periodic potential.

cond-mat↗
arXiv1996

Vortices, flux tubes and other structures in the Ginzburg-Landau model: a possible fine structure of the mixed state?

We present new regular static isolated cylindrically symmetric solutions for the Ginzburg-Landau model which have finite Gibbs free energy. These configurations (which we call the {\it flux tube} and {\it type B} solutions) are energetically favorable in the interval of the external magnetic fields between the thermodynamic critical value $H_{c}$ and the upper critical field $H_{c_2}$ which indicates that they are important new elements of the mixed state describing a transition from vortices to the normal state.

hep-th↗
arXiv1995

Aharonov-Bohm Type Forces Between Magnetic Fluxons

Forces related to A-B phases between fluxons with $Φ=αΦ_0\ \ \ $ $α\ne integer$ are discussed. We find a $α^2\ln(r)$ type interaction screened on a scale $λ_s$. The forces exist only when the fluxons are actually immersed in the region with non vanishing charge density and are periodic in $α$. We briefly comment on the problem of observing such forces.

quant-ph↗
arXiv1994

Multiple Scattering Analysis of Cu-K EXAFS in Bi_2Sr_1.5Ca_1.5Cu_2O_8+d

We have analyzed the Cu K-EXAFS of Bi_2Sr_1.5Ca_1.5Cu_2O_8+d using a full multiple scattering analysis in a cluster with diameter d = 7.6 AA. The numerous quasi one-dimensional structural elements give rise to significant multiple scattering contributions in the EXAFS. We confirm the Sr/Ca ratio of the sample is 1:1, and one Ca atom is located close to a nominal Sr-site. At 40 K the dimpling angle in the $\rm CuO_2-plane is found to be < 3.5 degrees.

supr-con↗
arXiv1995

Verification of a New Non-Linear IV-exponent: Simulation of the 2D Coulomb Gas with Langevin Dynamics.

It has recently been suggested from scaling arguments that the non-linear IV-exponent a, for a two-dimensional superconductor is different from the exponent originally suggested by Ambegaokar et al. The relation between the new and the old exponent is a=a_AHNS-3. The new scaling behaviour is linked to the logarithmic vortex interaction and the long range time tail which this gives rise to. Consequently one may expect that the scaling behavior is generic for models which have these basic features. The simplest model of this type is the two-dimensional Coulomb gas model with Langevin dynamics. We here explicitly verify, through computer simulations, that the IV-characteristics of this model indeed scales according to the new scaling exponent a. Keywords: vortex, Coulomb gas, IV-exponent, Simulations, Langevin, 2D superconductor, thin films.

supr-con↗
arXiv1995

Synchronization in one-dimensional array of Josephson coupled thin layers

We obtain self-consistent macroscopic equations describing interlayer Josephson effect and intralayer disequilibrium in one-dimensional array of Josephson coupled layers. We show that ``nonequilibrium coupling'' can lead to effective spatial and time synchronization and formation of coherent dynamic resistive state (collective Josephson effect) in Nb-AlO-Nb stacked junctions and HTSC (intrinsic Josephson effect). We propose it to be the origin of collective switching phenomena observed in PbBiSrCaCuO.

supr-con↗
arXiv1996

Updating the theoretical analysis of the weak gravitational shielding experiment

The most recent data about the weak gravitational shielding produced recently through a levitating and rotating HTC superconducting disk show a very weak dependence of the shielding value ($\sim 1 \%$) on the height above the disk. We show that whilst this behaviour is incompatible with an intuitive vectorial picture of the shielding, it is consistently explained by our theoretical model. The expulsive force observed at the border of the shielded zone is due to energy conservation.

supr-con↗