Current fluctuations in a spin filter with paramagnetic impurities

We analyze the frequency dependence of shot noise in a spin filter consisting of a normal grain and ferromagnetic electrodes separated by tunnel barriers. The source of frequency-dependent noise is random spin-flip electron scattering that results from spin-orbit interaction and magnetic impurities. Though the latter mechanism does not contribute to the average current, it contributes to the noise and leads to its dispersion at frequencies of the order of the Korringa relaxation rate. Under nonequilibrium conditions, this rate is proportional to the applied bias $V$, but parametrically smaller than $eV/\hbar$.Comment: 6 pages, 2 figure

Frequency dependent third cumulant of current in diffusive conductors

We calculate the frequency dispersion of the third cumulant of current in diffusive-metal contacts. The cumulant exhibits a dispersion at the inverse time of diffusion across the contact, which is typically much smaller than the inverse $RC$ time. This dispersion is much more pronounced in the case of strong electron-electron scattering than in the case of purely elastic scattering because of a different symmetry of the relevant second-order correlation functions.Comment: 8 pages, 4 figure

Statistics of fluctuations for two types of crossover: from ballistic to diffusive regime and from orthogonal to unitary ensemble

In our previous publication [Kogan et al, Phys. Rev. {\bf 48}, 9404 (1993)] we considered the issue of statistics of radiation diffusively propagating in a disordered medium. The consideration was in the framework of diagrammatic techniques and a new representation for the intensity distribution function in terms of connected diagrams only was proposed. Here we use similar approach to treat the issue of statistics in the regime of the crossover between ballistic and diffusive transport. We find that even small contribution from coherent component decreases by one half the intensity distribution function for small values of intensity and also produces oscillations of the distribution function. We also apply this method to study statistics of fluctuations of wave functions of chaotic electrons in a quantum dot in an arbitrary magnetic field, by calculating the single state local density in the regime of the crossover between the orthogonal and unitary ensemble.Comment: Revtex, 3 pages + 2 ps.figures in uuencoded file, a version which clarifies and unites the results of two previous submission

Equilibrium basal-plane magnetization of superconductive YNi(2)B(2)C - the influence of non-local electrodynamics

For a single crystal of YNi(2)B(2)C superconductor, the equilibrium magnetization M in the square basal plane has been studied experimentally as a function of temperature and magnetic field. While the magnetization M(H) deviates from conventional London predictions, a recent extension of London theory (to include effects of non-local electrodynamics) describes the experiments accurately. The resulting superconductive parameters are well behaved. These results are compared with corresponding findings for the case with M perpendicular to the basal plane.Comment: 7 pages, 5 Postscript Figures, 2 table

Nature of 45 degree vortex lattice reorientation in tetragonal superconductors

The transformation of the vortex lattice in a tetragonal superconductor which consists of its 45 degree reorientation relative to the crystal axes is studied using the nonlocal London model. It is shown that the reorientation occurs as two successive second order (continuous) phase transitions. The transition magnetic fields are calculated for a range of parameters relevant for borocarbide superconductors in which the reorientation has been observed

Dual Response Models for the Fractional Quantum Hall Effect

It is shown that the Jain mapping between states of integer and fractional quantum Hall systems can be described dynamically as a perturbative renormalization of an effective Chern-Simons field theory. The effects of mirror duality symmetries of toroidally compactified string theory on this system are studied and it is shown that, when the gauge group is compact, the mirror map has the same effect as the Jain map. The extrinsic ingredients of the Jain construction appear naturally as topologically non-trivial field configurations of the compact gauge theory giving a dynamical origin for the Jain hierarchy of fractional quantum Hall states.Comment: 8 pages LaTe

Phenomenological noise model for superconducting qubits: two-state fluctuators and 1=f noise

We present a general phenomenological model for superconducting qubits subject to noise produced by two-state fluctuators whose couplings to the qubit are all roughly the same. In flux qubit experiments where the working point can be varied, it is possible to extract both the form of the noise spectrum and the number of fluctuators. We find that the noise has a broad spectrum consistent with 1=f noise and that the number of fluctuators with slow switching rates is surprisingly small: less than 100. If the fluctuators are interpreted as unpaired surface spins, then the size of their magnetic moments is surprisingly large.Comment: 7 pages, 2 figures, 1 tabl

The puzzle of 90 degree reorientation in the vortex lattice of borocarbide superconductors

We explain 90 degree reorientation in the vortex lattice of borocarbide superconductors on the basis of a phenomenological extension of the nonlocal London model that takes full account of the symmetry of the system. We propose microscopic mechanisms that could generate the correction terms and point out the important role of the superconducting gap anisotropy.Comment: 4 pages, 2 eps figure

Mass Spectrum in SQCD and Problems with the Seiberg Duality. Another Scenario

N=1 SQCD with SU(N_c) colors and N_F flavors of light quarks is considered within the dynamical scenario which assumes that quarks can be in two different phases only. These are: a) either the HQ (heavy quark) phase where they are confined, b) or they are higgsed, at the appropriate values of parameters of the Lagrangian. The mass spectra of this (direct) theory and its Seiberg's dual are obtained and compared, for quarks of equal or unequal masses. It is shown that in all cases when there is the additional small parameter at hand (it is 0<(3N_c-N_F)/N_F << 1 for the direct theory, or its analog 0<(2N_F-3N_c)/N_F << 1 for the dual one), the mass spectra of the direct and dual theories are parametrically different. A number of other regimes are also considered.Comment: 30 pages, purely technical improvements for readers convenienc

Temperature Dependence of the Flux Line Lattice Transition into Square Symmetry in Superconducting LuNi2_2B2_2C

We have investigated the temperature dependence of the H || c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi_2B_2C (T_c = 16.6 K). At temperatures below 10 K the transition onset field, H_2(T), is only weakly temperature dependent. Above 10 K, H_2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H_2(T) merging with the upper critical field, and suggests that just below the H_c2(T)-curve the flux line lattice might be hexagonal.Comment: 4 pages, 3 figure