Non-linear effects in hopping conduction of single-crystal La2CuO4+δ

Abstract

The unusual non-linear effects in hopping conduction of single-crystal La 2 CuO 4+δ with excess oxygen has been observed. The resistance is measured as a function of applied voltage U (voltage controlled regime) in the temperature range 5 K < ∼ T < ∼ 300 K and voltage range 10 −3 -25 V. At relatively high voltage (approximately at U > 0.1 V) the conduction of sample investigated corresponds well to variable-range hopping (VRH). Namely, in the range 0.1 V< U < ∼ 1 V the conductivity does not depend on U (Ohmic behavior) and the temperature dependence of resistance R(T ) follows closely the Mott's law of VRH [R ∝ exp(T 0 /T ) 1/4 ]. In the range of highest applied voltage the conduction has been non-Ohmic: the resistance decreases with increasing U . This non-linear effect is quite expected in the frame of VRH mechanism, since the applied electric field increases the hopping probability. A completely different and 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 unusual non-Ohmic behavior at low voltage range 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 in poor-conducting (dielectric) matrix. This allows to explain the observed unusual non-linear effects in the conduction of sample studied. The results obtained demonstrate that in some cases the measured transport properties of cuprate oxides can not be attributed to the intrinsic bulk properties. 72.20.Ht; 74.72Dn; 74.62Dh; Typeset using REVT E X 1

Magnetoresistive study of the antiferromagnetic–weak ferromagnetic transition in single-crystal La2CuO4+δ

“…For example, in Ref. [38], positive MR attributed to superconducting inclusions has been found at low temperature range (T < 10 K) in even more resistive La 2 CuO 4+δ with higher T N .…”

The AC Conductivity of Liquid‐Phase‐Sintered Silicon Carbide

“…Excitation voltage‐dependent behavior, as observed for these hopping conducting oxide layers, can be expected since the applied electric field raises the hopping probability. Voltage‐dependent effects in hopping conduction have been reported for other complex oxide systems, see for instance Belevtsev et al 16 …”