M. A. Belogolovskii

Interface resistive switching effects in bulk manganites

A physical mechanism driving the resistance switching in heterocontacts, formed by a metal counterelectrode and electrically conducting bulk perovskite manganites, is discussed. The nature of the inelastic, charge-hopping transport inside insulating and strongly inhomogeneous metal/manganite interfaces is studied theoretically. Comparison with measured current-voltage characteristics for a La0.67Ca0.33MnO3/Ag heterostructure in a high-resistance state reveals the presence of one or more charge traps along a conduction path within the interface. In a low-resistance state the main charge-transferring events are direct tunneling ones. The analysis of electrical noise measurements for a La0.82Ca0.18MnO3 single crystal in three different charge-transport regimes shows scattering centers with a broad, flat spectrum of excitation states, independent of manganite electrical and/or magnetic characteristics. All of these results are consistent with an oxygen-drift model for a bistable resistance state in perovskites.

Anomalous Inner-Gap Structure in Transport Characteristics of Superconducting Junctions with Degraded Interfaces

Quantitative description of charge transport across tunneling and break-junction devices with novel superconductors encounters some problems not present or not as severe for traditional superconducting materials. In this work, we explain unexpected features in related transport characteristics as an effect of a degraded nanoscaled sheath at the superconductor surface. A model capturing the main aspects of the ballistic charge transport across hybrid superconducting structures with normally conducting nanometer-thick interlayers is proposed. The calculations are based on a scattering formalism taking into account Andreev electron-into-hole (and inverse) reflections at normal metal-superconductor interfaces as well as transmission and backscattering events in insulating barriers between the electrodes. Current-voltage characteristics of such devices exhibit a rich diversity of anomalous (from the viewpoint of the standard theory) features, in particular shift of differential-conductance maxima at gap voltages to lower positions and appearance of well-defined dips instead expected coherence peaks. We compare our results with related experimental data.

Background conductance of HTSC tunneling junctions

It is shown that some principles of the tunneling spectroscopy of conventional metals cease to be true in the case of the tunneling spectroscopy of metal oxides.

Evidence for Strong Electron-Magnon Coupling in Gadolinium

Tunneling spectroscopy measurements on normal state Gd–Gd oxide–Al junctions have been performed. Small deviations from the overall parabolic dependence of conductance on voltage were revealed and analyzed using a theoretical approach, taking into account interactions of tunneling electrons with elementary excitations in the oxide layer and metal electrodes. These processes have been studied by calculating the even and odd conductances that characterize the symmetrical (emission of oxide phonons) and asymmetrical (self-energy effects in electrodes) processes, respectively. It has been found that in gadolinium, one of the simplest magnetic metals known, an interaction of electrons with magnetic excitations is as important as the electron-phonon contribution. This result explains the difference between the calculated electron-phonon coupling parameters and the experimentally derived electron mass enhancement factors in magnetic rare-earth metals.

Charge transport across a mesoscopic superconductor–normal metal junction: coherence and decoherence effects

We present a simple scattering approach to the charge transport across a realistic superconductor–normal injector interface of a finite transmittance that is modeled by a double-barrier mesoscopic junction. For a d-wave pairing symmetry, our calculations combine a fully quantum-mechanical scattering formalism with a self-consistent estimation of Andreev reflection coefficients within the quasi-classical Eilenberger equation scheme for a free specular superconducting surface. Numerical simulations confirm experimental criteria of Cucolo for the unconventional superconducting origin of conductance anomalies in high-temperature oxides. A discussion of dephasing effects caused by inelastic scattering processes in the interlayer and their impact on the conductance spectra is given.

ANOMALOUS TEMPERATURE EFFECT ON POINT-CONTACT Ag–LaBa2Cu3O7-x CONDUCTANCE SPECTRA

The observation of an anomalous temperature behavior of the differential conductance versus voltage curves in contacts formed by an Ag tip and a bulk ceramic LaBa2Cu3O7-x with Tc around 92 K is reported. For a wide range of temperatures from Tc to helium-liquid ones, we have found a crossover from curves typical for a pure conducting normal metal–superconductor interface up to Giaever tunneling characteristics with gap features shifted to high biases. We take into account the existence of a degraded interlayer with suppressed superconducting parameters between a normal injector and a superconducting bulk and interpret qualitatively the data in terms of mesoscopic proximity effects. We argue that as the temperature is decreased, (i) the electron localization in a disordered region is enhanced, and (ii) in the interlayer, inelastic scattering processes become more effective. The latter was considered as a result of the inelastic scattering rate changes for charge carriers interacting with magnetic excitations in the near-interface region of high-Tc compound junctions.

Sharp anomalies in the scanning tunneling microscope I–V characteristics at room temperature

A fine structure in the current-voltage characteristics of oxidized Zn, Cd, and Au measured by means of the room-temperature high stability STM technique is reported. The normalized second voltage derivatives obtained by numerical differentiation clearly reveal sharp anomalies near the voltages corresponding to the energies of oxide phonons. By analyzing the results, it is concluded that the physics of the effect involves the electron tunneling transfer via a series of quantum dots and the abrupt rearrangement of a dot local atomic structure accompanied by phonon emission.

POINT-CONTACT SPECTRA OF THE NONSUPERCONDUCTING COPPER-OXIDE CERAMICS: OBSERVATION OF MULTIPLE RESISTANCE PEAKS

Strong differential resistance peaks have been observed in the point-contact characteristics of normal state copper-oxide ceramics. A possible interpretation of the spikes that have been found previously for a variety of high-Tc superconducting polycrystalline samples is discussed.

Superluminal solutions to the Klein–Gordon equation and a causality problem

We present a new axially symmetric monochromatic free-space solution to the Klein-Gordon equation propagating with a superluminal group velocity and show that it gives rise to an imaginary part of the causal propagator outside the light cone. We address the question about causality of the spacelike paths and argue that the signal with a well-defined wavefront formed by the superluminal modes would propagate in vacuum with the light speed.

Electron-boson interaction in nonsuperconducting magnetic metals

Gadolinium-and chromium-based normal metal-insulator-metal tunnel junctions were studied. Spectral functions of the Gd and Cr electron-boson interaction were reconstructed, from which it follows that the magnetic subsystem plays a noticeable role in the electron transport in magnetic metals.