Richard S. Thompson

The superconducting transition temperatures of disordered Nb, W, and Mo films☆

Abstract Large changes in T c are observed in films of Nb, W and Mo evaporated under ultrahigh vacuum conditions onto cryogenic substrates. An explanation of these temperature changes in terms of the smearing of N (0) due to the small mean free path is discussed.

Weak Localization and Electron−Electron Interactions in Indium-Doped ZnO Nanowires

Single crystal ZnO nanowires doped with indium are synthesized via the laser-assisted chemical vapor deposition method. The conductivity of the nanowires is measured at low temperatures in magnetic fields with directions both perpendicular and parallel to the wire axes. A quantitative fit of our data is obtained, consistent with the theory of a quasi-one-dimensional metallic system with quantum corrections due to weak localization and electron-electron interactions. The anisotropy of the magneto-conductivity agrees with theory. The two quantum corrections are of approximately equal magnitude with respective temperature dependences of T(-1/3)and T(-1/2). The alternative model of quasi-two-dimensional surface conductivity is excluded by the absence of oscillations in the magneto-conductivity in parallel magnetic fields.

Estrogen and hippocampal synaptic plasticity

During the past several years, there has been increasing interest in the effects of estrogen on neural function. This enthusiasm is driven, in part, by the results of early clinical studies suggesting that estrogen therapy given after menopause may prevent, or at least delay, the onset of Alzheimers disease in older women. However, later clinical trials of women with probable Alzheimers disease had contrary results. Much of the current research related to estrogen and brain function is focused in two directions. One involves clinical studies that examine the potential of estrogen in protecting against cognitive decline during normal aging and against Alzheimers disease (neuroprotection). The other direction, which is the primary focus of this review, involves laboratory studies that examine the mechanisms by which estrogen can modify the structure of nerve cells and alter the way neurons communicate with other cells in the brain (neuroplasticity). In this review, we examine recent evidence from experimental and clinical research on the rapid effects of estrogen on several mechanisms that involve synaptic plasticity in the nervous system,including hippocampal excitability, long-term potentiation and depression related to sex and aging differences, cellular neuroprotection and probable molecular mechanisms of the action of estrogen in brain tissue.

Polysynaptic potentiation in the lateral septum following stimulation of the fimbria in anesthetized rats

In anesthetized rats, electrical stimulation of fimbria fibers evoked, in the ipsilateral lateral septum (LS), a field potential consisting of two negative components: an initial negativity (N2-3 complex wave) of high amplitude at 6.7 ms (+/- 0.8 ms; peak latency) and a slow negative wave (N4 wave) of small amplitude at 14.4 ms (+/- 2.4 ms). The N2-3 complex wave represents the monosynaptic activation of LS neurons while the N4 wave corresponds to polysynaptic activation of neurons in the mediolateral part of the LS. In this study, we investigated the effects of high-frequency stimulation of fimbria fibers on LS field potentials and compared them with those observed in the CA3 area. Tetanic stimulation of the fimbria did not change the characteristics of the N2-3 wave but induced a long-lasting increase in amplitude and slope of the N4 wave. A positive correlation was found between the magnitude of CA3 LTP and lateral septal polysynaptic potentiation of the N4 component. These results indicate that patterns of stimulation delivered to the same input fibers (fimbria fibers) produce similar changes in a polysynaptic input to the LS and in a monosynaptic input to the CA3 and emphasize the complexity of signal processing in serial networks.

Numerical calculation of the fidelity for the Kondo and the Friedel-Anderson impurities

The fidelities of the Kondo and the Friedel-Anderson (FA) impurities are calculated numerically. The ground states of both systems are calculated with the FAIR (Friedel artificially inserted resonance) theory. The ground state in the interacting systems is compared with a nullstate in which the interaction is zero. The different multi-electron states are expressed in terms of Wilson states. The use of N Wilson states simulates the use of a large effective number Neff of states. A plot of ln(F) versus N ∝ ln(Neff) reveals whether one has an Anderson orthogonality catastrophe at zero energy. The results are at first glance surprising. The ln(F) – ln(Neff) plot for the Kondo impurity diverges for large Neff. On the other hand, the corresponding plot for the symmetric FA impurity saturates for large Neff when the level spacing at the Fermi level is of the order of the singlet-triplet excitation energy. The behavior of the fidelity allows one to determine the phase shift of the electron states in this regime.

Fluctuation contribution to flux-flow conductivity

Abstract We find the leading corrections to the flux-flow conductivity due to fluctuations of the order parameter for magnetic field and temperature below the superconducting phase transition. These corrections are of the same order of magnitude, but opposite in sign, to those found above the transition.

The resistive transition in weakly coupled superconductors

Abstract Films of weakly connected particles have narrower transitions above the half resistance point than uniform films of the same resistance. In contrast, the region of finite resistivity below the halfway point remains very broad.

Fluctuation-induced pair breaking in high Tc superconductors

Abstract We show that pair breaking induced by superconducting fluctuations may play an important role in the anomalous fluctuation conductivity above the transition temperature T c .

Continuous-distribution puddle model for conduction in trilayer graphene

Abstract The temperature dependence of the resistance in trilayer graphene is observed under different applied gate voltages. At small gate voltages the resistance decreases with increasing temperature due to the increase in carrier concentration resulting from thermal excitation of electron-hole pairs, characteristic of a semimetal. At large gate voltages excitation of electron-hole pairs is suppressed, and the resistance increases with increasing temperature because of the enhanced electron-phonon scattering, characteristic of a metal. We find that the simple model with overlapping conduction and valence bands, each with quadratic dispersion relations, is unsatisfactory. Instead, we conclude that impurities in the substrate that create local puddles of higher electron or hole densities are responsible for the residual conductivity at low temperatures. The best fit is obtained using a continuous distribution of puddles. From the fit the average of the electron and hole effective masses can be determined.

Infrared Hall angle in the d-density-wave state: A comparison of theory and experiment

Infrared Hall measurements in the pseudogap phase of the high-T-c cuprates are addressed within the framework of the ordered d-density-wave state. The zero-temperature Hall frequency omega(H) is computed as a function of the hole-doping x. Our results are consistent with recent experiments in absolute units. We also discuss the signature of the quantum critical point in the Hall frequency at a critical doping inside the superconducting dome, which can be tested in future experiments.