Non-linear effects in hopping conduction of single-crystal La_{2}CuO_{4 + \delta}

The unusual non-linear effects in hopping conduction of single-crystal La_{2}CuO_{4 + \delta} with excess oxygen has been observed. The resistance is measured as a function of applied voltage U (10^{-3} V - 25 V) in the temperature range 5 K 0.1 V) the conduction of sample investigated corresponds well to Mott's variable-range hopping (VRH). An unusual conduction behavior is found, however, in low voltage range (approximately below 0.1 V), where the influence of electric field and (or) electron heating effect on VRH ought to be neglected. Here we have observed strong increase in resistance at increasing U at T < 20 K, whereas at T > 20 K the resistance decreases with increasing U. The magnetoresistance of the sample below 20 K has been positive at low voltage and negative at high voltage. The observed non-Ohmic behavior is attributable to inhomogeneity of the sample, and namely, to the enrichment of sample surface with oxygen during the course of the heat treatment of the sample in helium and air atmosphere before measurements. At low enough temperature (below 20 K) the surface layer with increased oxygen concentration is presumed to consist of disconnected superconducting regions (with T_{c} about 20 K) in poor-conducting matrix. The results obtained demonstrate that transport properties of cuprate oxides may be determined in essential degree by structural or stoichimetric inhomogeneities. This should be taken into account at evaluation of "quality" of high-temperature superconductors on the basis of transport properties measurements.Comment: 12 pages, REVTex, 11 Postscript figures, To be published in Fizika Nizkikh Temperatur (published by AIP as Low Temperature Physics

Anisotropic magnetoresistive and magnetic properties of La_{0.5}Sr_{0.5}CoO_{3-\delta} film

The magnetic and transport properties of La_{0.5}Sr_{0.5}CoO_{3-\delta} film grown on a LaAlO_3 substrate by pulsed-laser deposition are studied. The properties are found to be influenced by the magnetic anisotropy and inhomogeneity. Magnetoresistance anisotropy is determined by the shape anisotropy of the magnetization and the strain-induced magnetic anisotropy due to the film-substrate lattice interaction. Indications of the temperature-driven spin reorientation transition from an out-of plane orderded state at low temperatures to an in-plane ordered state at high temperatures as a result of competition between the mentioned sources of magnetic anisotropy are found.Comment: 5 pages, 8 figures, submitted to Fiz. Nizk. Temp, an extended version of short communication in cond-mat/020734

Spiral flames

AbstractWe describe computations of periodic and meandering spiral patterns in a reaction-diffusion model of flames

Filament tension and phase-locked drift of meandering scroll waves

Rotating scroll waves are self-organising patterns which are found in many oscillating or excitable systems. Here we show that quasi-periodic (meandering) scroll waves, which include the rotors that organise cardiac arrhythmias, exhibit filament tension when averaged over the meander cycle. With strong filament curvature or medium thickness gradients, however, scroll wave dynamics are governed by phase-locked drift instead of filament tension. Our results are validated in computational models of cycloidal meander and a cardiac tissue model with linear core.Comment: accepted for publication in Physical Review Letters (December 2017