G. E. Tsydynzhapov

Destruction of localized electron pairs above the magnetic-field-driven superconductor-insulator transition in amorphous In-O films

We have investigated the field-induced superconductivity-destroying quantum transition in amorphous indium oxide films at low temperatures down to 30 mK. It has been found that, on the high-field side of the transition, the magnetoresistance reaches a maximum and the phase can be insulating as well as metallic. With further increasing magnetic field the film resistance drops and approaches in the high-field limit the resistance value at transition point so that at high fields the metallic phase occurs for both cases. We give a qualitative account of this behavior in terms of field-induced destruction of localized electron pairs.The field-induced superconductivity-destroying quantum transition in amorphous indium oxide films are investigated at low temperatures down to 30 mK. It is found that, on the high-field side of the transition, the magnetoresistance reaches a maximum and the phase can be insulating as well as metallic. With further increase of the magnetic field the resistance of the film drops and in the high-field limit approaches the resistance value at the transition point, so that at high fields the metallic phase occurs for both cases. We give a qualitative account of this behavior in terms of field-induced destruction of localized electron pairs.

Scaling Analysis of the Magnetic Field-Tuned Quantum Transition in Superconducting Amorphous In-O Films 1

We studied the magnetic field-tuned superconductor-insulator transition (SIT) in amorphous In-O films with different oxygen contents and, hence, different electron densities. Whereas the two-dimensional scaling behavior was confirmed for the states of the film near the zero-field SIT, the SIT scenario changed for the deeper states in the superconducting phase; in addition to the scaling function describing the conductivity of the fluctuation-induced Cooper pairs, the temperature-dependent contribution to the film resistance emerged. This contribution can originate from the conductivity of normal electrons.We have studied the magnetic-field-tuned superconductor-insulator quantum transition (SIT) in amorphous In-O films with different oxygen content and, hence, different electron density. While for states of the film near the zero-field SIT the two-dimensional scaling behaviour is confirmed, for deeper states in the superconducting phase the SIT scenario changes: in addition to the scaling function that describes the conductivity of fluctuation-induced Cooper pairs, there emerges a temperature-dependent contribution to the film resistance. This contribution can originate from the conductivity of normal electrons.

Observation of the parallel-magnetic-field-induced superconductor-insulator transition in thin amorphous InO films

We study the response of a thin superconducting amorphous InO film with variable oxygen content to a parallel magnetic field. A field-induced superconductor-insulator transition (SIT) is observed that is very similar to the one in normal magnetic fields. As the boson-vortex duality, which is the key-stone of the theory of the field-induced SIT, is obviously absent in the parallel configuration, we have to draw conclusion about the theory insufficiency.We study the response of a thin superconducting amorphous InO film with variable oxygen content to a magnetic field. A field-induced superconductor-insulator transition (SIT) is observed that is very similar to the one in normal magnetic fields. As the boson-vortex duality, which is the keystone of the theory of the field-induced SIT, is obviously absent in the parallel configuration, we have to draw a conclusion about the theory’s insufficiency.

Superconductor–insulator transition in amorphous In–O films

Abstract From experiments with amorphous InOx films, three conclusions are made: (i) The pattern of the superconductor–insulator transition (SIT) seen before for 2D electron gas in normal magnetic field can be reproduced under different experimental conditions, with field parallel to the film; (ii) The negative magnetoresistance (NMr) observed in high fields is due to breaking of localized pairs and transformation of bosons into fermions, the latter having higher mobility; hence, NMr is a confirmation of the conception of localized pairs.; (iii) Assumption of existence near the SIN of two complementary groups of electrons, paired, i.e. bosons with density nb, and unpaired fermions with density nf (2nb+nf=n, the total density), can explain the nonuniversality of the critical resistance Rc0 seen before and its temperature dependence which we found in films with comparatively high n.

Suppression of 2D superconductivity by the magnetic field: Quantum corrections vs. The superconductor-insulator transition

Magnetotransport of superconducting Nd2−xCexCuO4+y(NdCeCuO) films is studied in the temperature interval 0.3–30 K. The microscopic theory of the quantum corrections to conductivity, both in the Cooper and in the diffusion channels, qualitatively describes the main features of the experiment, including the negative magnetoresistance in the high-field limit. Comparison with the model of the field-induced superconductor-insulator transition is included and a crossover between these two theoretical approaches is discussed.Magnetotransport of superconducting Nd_{2-x}Ce_xCuO_{4+y} (NdCeCuO) films is studied in the temperature interval 0.3-30 K. The microscopic theory of the quantum corrections to conductivity, both in the Cooper and in the diffusion channels, qualitatively describes the main features of the experiment including the negative magnetoresistance in the high field limit. Comparison with the model of the field-induced superconductor-insulator transition (SIT) is included and a crossover between these two theoretical approaches is discussed.

Resistive transition and upper critical field in underdoped YBa2Cu3O6+x single crystals

A superconducting transition in the temperature dependence of the ab-plane resistivity of underdoped YBa2Cu3O6+x crystals in the range Tc≲30 K has been investigated. Unlike the case of samples with the optimal level of doping, the transition width increased insignificantly with magnetic field, and in the range T≲13 K it decreased with increasing magnetic field. The transition point Tc(B) was determined by analyzing the fluctuation conductivity. The curves of Bc2(T) measured in the region T/Tc≳0.1 did not show a tendency to saturation and had a positive second derivative everywhere, including the immediate neighborhood of Tc. The only difference among the curves of Bc2(T) for different crystal states is the scales of Tand B, so they can be described in terms of a universal function, which fairly closely follows Alexandrov’s model of boson superconductivity.

Low-temperature resistivity of YBa2Cu3O6+x single crystals in the normal state

A scan of the superconductor-nonsuperconductor transformation in single crystals of YBa2Cu3O6+x (x≈0.37) is done in two alternative ways, namely, by applying a magnetic field and by reducing the hole concentration through oxygen rearrangement. The in-plane normal-state resistivity ρab obtained in the two cases is quite similar; its temperature dependence can be fitted by a logarithmic law in a temperature range of almost two decades. However, an alternative representation of the temperature dependence of σab=1/ ρ ab by a power law, typical for a 3D material near a metal-insulator transition, is also plausible. The vertical conductivity σc=1/ρc followed a power law, and neither σc(T), nor ρc(T) could be fitted by log T. It follows from the ρc measurements that the transformation at T=0 is split into two transitions: superconductor-normal-metal and normal-metal-insulator. In our samples, they are separated in oxygen content by Δx≈0.025.

Temperature dependence of the upper critical field as an indicator of boson effects in superconductivity in Nd2−xCexCuO4−y

The temperature dependence of the upper critical field Bc2 was determined from the shift of the resistive transition ΔT(B) in nearly optimally doped Nd2−xCexCuO4−y single crystals. Within the experimental accuracy, the weak-field data are described by the power function Bc2∝(ΔT)3/2. This result is compared with the data on heat capacity and analyzed in the context of possible manifestations of boson effects in superconductivity. The T dependence of Bc2 persists down to the lowest temperatures, but the numerical values of Bc2 below 1 K are different for different samples.

Linear low-temperature dependence of the surface impedance of a Ba0.6K0.4BiO3 single crystal

The temperature dependences of the real part Rs and the imaginary part Xs of the surface impedance Zs=Rs+iXs of the superconductor Ba0.6K0.4BiO3 (Tc≃30 K) are measured at a frequency of 9.4 GHz. Its temperature dependence Zs(T) and that of the complex conductivity σs(T) can be described on the basis of a two-fluid model under two assumptions: The density of superconducting carriers increases linearly, and the relaxation time increases as a power law (∝1/T5), with decreasing temperature T<Tc. This model also describes well the curves Zs(T) and σs (T) recently measured for YBa2Cu3O6.95 and Bi2Sr2CaCu2O8 single crystals.

Electron subsystem superheating as a cause of nonlinear current-voltage characteristics of amorphous InOx films

Current-voltage characteristics of amorphous InOx films in the region of the magnetic-field-induced superconductor-insulator transition were examined. The characteristics are shown to be nonlinear above the critical field because of the superheating of charge carriers in the lattice. There is some evidence that this mechanism is operative below the transition point as well. In this case, the heating fully masks the effect of an electric field in the vicinity of the superconductor-insulator transition.