The c-axis fluctuation conductivity in layered superconductors in a strong electric field under a magnetic field

The effect of a high electric field on the c-axis fluctuation conductivity in layered superconductors near the superconducting transition is investigated by the time-dependent Ginzburg-Landau equation. The c-axis fluctuation conductivity is calculated in self-consistent Gaussian approximation for an arbitrarily strong electric field and a magnetic field perpendicular to the layers. Our results include all Landau levels and have refined analytical form. The results in linear response are in good agreement with the experimental data in a wide region around T _c in high T _c superconductor. We also show that high electric fields can be effectively used to suppress the c-axis fluctuation conductivity in high-temperature superconductors.     

DC electrical conductivity of vanadium-borate glass in weak and strong electric fields

The results of an investigation of the temperature and field dependences of the electrical conductivity of vanadium-borate glass with the composition 32.56% V₂O₅, 46.18% B₂O₃, and 21.26% CaO (mole %) are discussed. The measurements were carried out on metal-glass-metal sandwich structures in the temperature interval 200–500°K with electric field intensities up to 10⁸ V/m. It is shown that the experimental data for T 350°K can be described within the framework of the percolation conductivity theory with a variable jump length, which permits explaining the value of the critical field and the temperature dependence of the slope of the current-voltage characteristics (CVC). It is proposed that for T > 350°K the essential role in forming the CVC is played by small-radius polarons (SMP) localized in the vicinity of positively charged defect centers (bound SRP).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 90–94, May, 1982.     

Radiation-induced conductivity of alkali halide crystals in strong electric fields under X-ray and photoexcitation

A study is undertaken into radiation-induced conductivity of alkali halide crystals under X-ray excitation and sequential excitation with X-ray and laser pulses within the absorption band of F-and F ⁻-centers. The basic conduction parameters (concentration and lifetime of carriers upon X-ray and photoexcitation) are estimated. The possible processes responsible for the nonlinearity of the current-voltage characteristics are discussed. It is shown that an increase in the conductivity in strong electric fields may be due to a decrease in the spatial localization of electrons in the conduction band of the insulator.     

Effects of strong electric fields in a polyacetylene chain

In this work, we study the effects related to the creation of electron/hole pairs via application of an external electric field that acts on a pristine trans-polyacetylene molecular chain at zero-temperature. This phenomenon is termed Schwinger–Landau–Zener (SLZ) effect and arises when a physical system, which can even be the vacuum, is under the action of a strong, static and spatially homogeneous electric field. Initially, we investigate how the electrical conductivity of the polyacetylene changes with the applied field, by considering the carriers production as well as the variation of the interband gap according to certain ab initio models. Next, we analyse the competition between the SLZ effect and another one associated with the incidence of an uniform electric field on one-dimensional crystals – the Bloch oscillations. We evaluate the conditions in which these latter can be destroyed by the particles created through the same field that induces them, and verify the possibility of occurrence of the Bloch oscillations inside the trans-polyacetylene with frequencies equal to or higher than the terahertz scale.     

Upper critical fields of ferromagnet/superconductor layered structures

The temperature dependence of the upper critical fields, both perpendicular H_c2 and parallel H_c2 to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconducting order parameter for determining the phase diagram (H, T) is obtained by solving exactly the linearized Usadel equations in the multimode method taking into account the material parameter values. For the bilayers system, the influence of the boundary resistivity on the critical fields, and the dimensional crossover behavior of H_c2(T) are studied in details. For the multilayered structure, the effects of the π-phase state on both the superconducting transition temperature T_c and the upper critical fields (H_c2, and H_c2) are also considered. The nonmonotonic T_c behaviors are predicted. The interplay between 0- and π-phases leading to the strong oscillations of T_c as well as the temperature dependence of the zero temperature critical fields on the ferromagnetic layer thickness are investigated theoretically.     

On the fluctuation enhanced conductivity of a superconductor: The correct evaluation of the Maki graph

According to Maki, a particular diagram—the Maki graph—gives a contribution to the fluctuation enhanced conductivity of a superconductor which is infinite in the case of a thin film. It is shown that this result is spurious and that it is due to a breakdown of the standard Green function impurity technique. A new method is developed which is strictly based on the Boltzmann equation. It is shown that the temperature dependent contribution of the Maki graph to the conductivity is negligibly small in a dirty metal.     

Hopping conductivity of Ni-doped p-CdSb in strong magnetic fields

Magnetoresistance (MR) of oriented single crystals of the anisotropic semiconductor p-CdSb doped with 2 at% of Ni is investigated between T=1.5 and 300 K in transversal pulsed magnetic fields up to B=30 T. In fields B∼4-15 T at T below 4.2 K, the resistivity obeys the law ln ρ∼η[B?(B)]^1/2 with ?(B)=a(0)/a(B), where a is the carrier localization radius and parameter η depends on a(0), on the acceptor concentration N_A and on the direction of the magnetic field with respect to the crystallographic axes, but does not depend on T. Such behavior gives evidence for MR realized by hopping charge transfer over the nearest-neighbor sites in strong magnetic field. The analysis of the experimental data yields the values of η, agreeing with calculated ones within an error of 10%, taking into account the effects of the anisotropy of the acceptor states and of the explicit dependence of a(B) due to the increase in the activation energy of shallow acceptors in magnetic field and the sensitivity of the metal-insulator transition to B.     

Nonlinear interaction of strong laser fields in vacuum

Quantum electrodynamics (QED) predicts that electromagnetic fields interact with each other in vacuum. We study the possibility of revealing this interaction experimentally with intensities on the order of 10²⁴–10²⁶ W/cm², which may be available in the next generation of laser systems. In particular, we investigate high-order harmonic generation in vacuum via the collision of two ultrastrong counterpropagating laser pulses. The experimental feasibility of the related process of stimulated light-by-light scattering is also examined. Finally, the importance of including diffractive effects to describe the nonlinear interaction between an x-ray probe and a strong, focused optical standing wave is pointed out.     

Field-emission characteristics of layered systems in high electric fields

A study has been made of the field-emission characteristics of cerium and BaO films on W within a broad range of field-emission currents and fields. An anomalous broadening of the spectra and a deviation of Fowler-Nordheim characteristics from linearity have been revealed for high (above 10⁹ A/m²) field-emission current densities. The dependence of this anomalous behavior of field-emission characteristics on the work function and topography of the surface has been investigated. Possible reasons for the observed phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 732–735 (April 1999)     

Hall effect in three-dimensional layered structures in strong magnetic fields

A three-dimensional electron system with a corrugated-cylinder-type Fermi surface is considered. We show that the quantum Hall effect must be observed in such a system with a smooth random potential. Our results are in qualitative agreement with available experimental data.     

Electron-phonon interaction and current in strong electric fields

Using the Houston-wave approach we investigate the electron-phonon interaction in electric fieldsF which are sufficiently strong to change an electron's Bloch state drastically during the relaxation time τ, but still too weak to support standing electron waves between the band edges. We calculate τ for the low density, high temperature limit and find τ～F ^?1/2. The electric current in the field region of the “hot electrons” between its ohmic rise and its decrease proportional toF ^?1 is determined. Its increase withF ^+1/2 in the limit of relatively low fields agrees with Shockley's result.     

Energy transfer processes in non-ohmic hopping conductivity in a strong electric field

Energy transfer processes between jumping current carriers in non-ohmic activationless $\text{F}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$-hopping conductivity in a strong electric field are considered. An energy gained during the downward spontaneous hopping of one electron can be radiationlessly transferred to another electron enabling it to make an upward energy hopping transition. It is shown that the contribution from these two-electron four-site energy transfer processes into the total hopping conductivity may be comparable or even exceed the contribution from one-electron radiative tunnel transitions. Numerical estimates are based on an example of beta-rhombohedral crystalline boron which exhibits $\text{F}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$-field law.     

Luminescence of rock salt in very strong electric fields

The luminescence of thin single-crystal layers of NaCl in very strong electric fields was observed. Oscillograms of the brightness waves were obtained. The light flux and luminescence delay were measured as functions of the applied field strength amplitude. The reasons for the delay phenomenon and the drop in the electric strength of samples when electrodes with electron conductivity (metal or graphite) are used are discussed.The authors sincerely thank Prof. G. A. Vorob'ev for his guidance and P. E. Ramazanov for valuable observations.