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Das Buch wendet sich an Theoretiker wie auch Experimentatoren, die auf dem Gebiet der Festkfirperphysik arbeiten. E s kann aber auch Physik-Studenten der hoheren Studienjahre mit der Spezialisierungsrichtung FestkGrperphysik empfohlen werden.nT. JOHN

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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 theoretical description of the zero-field and fieldinduced quantum superconductor-insulator transitions (SIT) in a 2D superconductor is based on a concept of electron pairs which are delocalized on the superconducting side and localized on the insulating side of transition [1][2][3]. According to Refs. [1][2][3], the temperature dependence of the film resistance near the field-induced SIT is controlled by deviation δ = B − B c from the critical field B c and the most specific among perceptible features of SIT is fan-like set of resistance-vs-temperature curves R δ (T ). Such a set is expected to collapse onto a single curve as a function of scaling variable δ/T 1/y , where y is the critical index, see review [4]. Many of the SIT studies were performed on amorphous In 2 O x (x < 3) films whose conductivity is caused by oxygen deficiency compared to fully stoichiometric insulating compound In 2 O 3 : by changing the oxygen content one can cover the range from a superconductor to an insulator and thus realize the zero-field SIT. On the insulating side of this SIT, observation was reported of the activation behavior of the resistance R ∝ exp(T 0 /T ) p with p = 1 (Arrhenius law) and activation energy T 0 tending to zero as the phase boundary is approached [5]. It was found later that switching a magnetic field results in decreasing the resistance and weakening its temperature dependence from the Arrhenius law to the Mott law with exponent p = 1/4 [6]. This was explained in Ref.[6] by magnetic-fieldcaused suppression of the binding energy ∆ of localized electron pairs which manifests as a gap at the Fermi level.Field-induced SIT is realized on the superconducting side of zero-field SIT. It is indicated by fan-like structure of experimental curves R δ (T ) such that, in accordance with the scaling analysis, the expected collapse is indeed the case [7]. Above the field-induced SIT, the existence of two insulating phases was postulated based on results of Hall measurements [8]; however, temperature dependences of the resistance of these phases were not studied. Reversal of a zero-bias peak in the differential resistance at the critical field B c was observed and attributed to the granular structure of films [9].

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