Zero-bias anomaly in cotunneling transport through quantum-dot spin valves

We predict a new zero-bias anomaly in the differential conductance through a quantum dot coupled to two ferromagnetic leads with antiparallel magnetization. The anomaly differs in origin and properties from other anomalies in transport through quantum dots, such as the Kondo effect. It occurs in Coulomb-blockade valleys with an unpaired dot electron. It is a consequence of the interplay of single- and double-barrier cotunneling processes and their effect on the spin accumulation in the dot. The anomaly becomes significantly modified when a magnetic field is applied.Comment: 4 pages, 3 figure

Metastable Voltage States of Coupled Josephson Junctions

We investigate a chain of capacitively coupled Josephson junctions in the regime where the charging energy dominates over the Josephson coupling, exploiting the analogy between this system and a multi-dimensional crystal. We find that the current-voltage characteristic of the current-driven chain has a staircase shape, beginning with an (insulating) non-zero voltage plateau at small currents. This behavior differs qualitatively from that of a single junction, which should show Bloch oscillations with vanishing dc voltage. The simplest system where this effect can be observed consists of three grains connected by two junctions. The theory explains the results of recent experiments on Josephson junction arrays.Comment: 5 pages, 4 figures include

The possible explanation of electric-field-doped C60 phenomenology in the framework of Eliashberg theory

In a recent paper (J.H. Schon, Ch. Kloc, R.C. Haddon and B. Batlogg, Nature 408 (2000) 549) a large increase in the superconducting critical temperature was observed in C60 doped with holes by application of a high electric field. We demonstrate that the measured Tc versus doping curves can be explained by solving the (four) s-wave Eliashberg equations in the case of a finite, non-half-filled energy band. In order to reproduce the experimental data, we assume a Coulomb pseudopotential depending on the filling in a very simple and plausible way. Reasonable values of the physical parameters involved are obtained. The application of the same approach to new experimental data (J.H. Schon, Ch. Kloc and B. Batlogg, Science 293 (2001) 2432) on electric field-doped, lattice-expanded C60 single crystals (Tc=117 K in the hole-doped case) gives equally good results and sets a theoretical limit to the linear increase of Tc at the increase of the lattice spacing.Comment: latex2e, 6 pages, 7 figures, 1 table, revised versio

Stable Fermion Bag Solitons in the Massive Gross-Neveu Model: Inverse Scattering Analysis

Formation of fermion bag solitons is an important paradigm in the theory of hadron structure. We study this phenomenon non-perturbatively in the 1+1 dimensional Massive Gross-Neveu model, in the large $N$ limit. We find, applying inverse scattering techniques, that the extremal static bag configurations are reflectionless, as in the massless Gross-Neveu model. This adds to existing results of variational calculations, which used reflectionless bag profiles as trial configurations. Only reflectionless trial configurations which support a single pair of charge-conjugate bound states of the associated Dirac equation were used in those calculations, whereas the results in the present paper hold for bag configurations which support an arbitrary number of such pairs. We compute the masses of these multi-bound state solitons, and prove that only bag configurations which bear a single pair of bound states are stable. Each one of these configurations gives rise to an O(2N) antisymmetric tensor multiplet of soliton states, as in the massless Gross-Neveu model.Comment: 10 pages, revtex, no figures; v2: typos corrected, references added; v3: version accepted for publication in the PRD. referencess added. Some minor clarifications added at the beginning of section

Full Counting Statistics for a Single-Electron Transistor, Non-equilibrium Effects at Intermediate Conductance

We evaluate the current distribution for a single-electron transistor with intermediate strength tunnel conductance. Using the Schwinger-Keldysh approach and the drone (Majorana) fermion representation we account for the renormalization of system parameters. Nonequilibrium effects induce a lifetime broadening of the charge-state levels, which suppress large current fluctuations.Comment: 4 pages, 1 figur

Low- and high-frequency noise from coherent two-level systems

Recent experiments indicate a connection between the low- and high-frequency noise affecting superconducting quantum systems. We explore the possibilities that both noises can be produced by one ensemble of microscopic modes, made up, e.g., by sufficiently coherent two-level systems (TLS). This implies a relation between the noise power in different frequency domains, which depends on the distribution of the parameters of the TLSs. We show that a distribution, natural for tunneling TLSs, with a log-uniform distribution in the tunnel splitting and linear distribution in the bias, accounts for experimental observations.Comment: minor corrections, references adde

Reading-out the state of a flux qubit by Josephson transmission line solitons

We describe the read-out process of the state of a Josephson flux qubit via solitons in Josephson transmission lines (JTL) as they are in use in the standard rapid single flux quantum (RSFQ) technology. We consider the situation where the information about the state of the qubit is stored in the time delay of the soliton. We analyze dissipative underdamped JTLs, take into account their jitter, and provide estimates of the measuring time and efficiency of the measurement for relevant experimental parameters.Comment: 13 pages, 12 figure

The role of law and ethics in developing business management as a profession

Currently, business management is far from being recognised as a profession. This paper suggests that a professional spirit should be developed which could function as a filter of commercial reasoning. Broadly, management will not be organised within the framework of a well-established profession unless formal knowledge, licensing, professional autonomy and professional codes of conduct are developed sufficiently. In developing business management as a profession, law may play a key role. Where the idea is that business management should be more professsionalised, managers must show that they are willing to adopt ethical values, while arriving at business decisions. The paper argues that ethics cannot survive without legal regulation, which, in turn, will not be supported by law unless lawyers can find alternative solutions to the large mechanisms of the official society, secured by the monopolised coercion of the nation state. From a micro perspective of law and business ethics, communities can be developed with their own conventions, rules and standards that are generated and sanctioned within the boundaries of the communities themselves

Gauged N=4 supergravities

We present the gauged N=4 (half-maximal) supergravities in four and five spacetime dimensions coupled to an arbitrary number of vector multiplets. The gaugings are parameterized by a set of appropriately constrained constant tensors, which transform covariantly under the global symmetry groups SL(2) x SO(6,n) and SO(1,1) x SO(5,n), respectively. In terms of these tensors the universal Lagrangian and the Killing Spinor equations are given. The known gaugings, in particular those originating from flux compactifications, are incorporated in the formulation, but also new classes of gaugings are found. Finally, we present the embedding chain of the five dimensional into the four dimensional into the three dimensional gaugings, thereby showing how the deformation parameters organize under the respectively larger duality groups.Comment: 36 pages, v2: references added, comments added, v3: published version, references added, typos corrected, v4: sign mistakes in footnote 4 and equation (2.13) correcte

Resonant Tunneling through Multi-Level and Double Quantum Dots

We study resonant tunneling through quantum-dot systems in the presence of strong Coulomb repulsion and coupling to the metallic leads. Motivated by recent experiments we concentrate on (i) a single dot with two energy levels and (ii) a double dot with one level in each dot. Each level is twofold spin-degenerate. Depending on the level spacing these systems are physical realizations of different Kondo-type models. Using a real-time diagrammatic formulation we evaluate the spectral density and the non-linear conductance. The latter shows a novel triple-peak resonant structure.Comment: 4 pages, ReVTeX, 4 Postscript figure