Field-induced Superconductor-insulator Transition Research Articles

Quantum Criticality inside the Anomalous Metallic State of a Disordered Superconducting Thin Film.

The field-induced superconductor-insulator transition (SIT) in two-dimensional (2D) systems is a famous example of a quantum phase transition. However, an emergence of an anomalous metallic state induced by field has been a long-standing problem in 2D superconductors. While theories predicted that the emergence is attributed to strong phase fluctuations of the superconducting order parameter due to quantum fluctuations, usual resistance measurements have not probed them directly. Here, using Nernst effect measurements, we uncover superconducting fluctuations in the vicinity of the field-induced metallic state in an amorphous Mo_{x}Ge_{1-x} thin film. The field range where the vortex Nernst signals are detectable remains nonzero toward zero temperature, and it locates inside the metallic state defined by the magnetoresistance, indicating that the metallic state results from quantum vortex liquid (QVL) with phase fluctuations due to quantum fluctuations. Slow decay of transport entropy of vortices in the QVL with decreasing temperature suggests that the metallic state originates from broadening of a quantum critical point in SIT.

Possible observation of the Berezinskii-Kosterlitz-Thouless transition in boron-doped diamond films

The occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated in heavily boron-doped nanocrystalline diamond films through a combination of current-voltage and resistance measurements. We observe transport features suggesting a robust BKT transition along with transport features related to vortex pinning in nanocrystalline diamond films with smaller grain size. The vortex core energy determined through analysis of the resistance temperature curves was found to be anti-correlated to the BKT transition temperatures. It is also observed that the higher BKT temperature is related to an increased vortex-antivortex binding energy derived from the activated transport regions. Further, the magnetic field induced superconductor insulator transition shows the possibility of the charge glass state. The consequences of granularity such as localization and vortex pinning can lead to tuneable BKT temperatures and strongly affects the field induced insulating state.

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

Magnetotransport of superconducting Nd2−x CexCuO4+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.

Temperature dependence of the Hall angle in disordered Y1−xPrxBa2Cu3O7−δ thin films

We have studied the behavior of carriers near the field-induced superconductor–insulator transition for different disordered Y1−xPrxBa2Cu3O7−δ thin films. The measurement of longitudinal and Hall resistivities has been carried out in the magnetic fields. For the lower disordered film the superconducting transition appeared even at 10 T, while, the insulating behavior was observed for the higher disordered film. Also the temperature dependence and values of Hall angle cotθH are related to the disorder. We discuss the behavior of carriers on the viewpoint of the electronic state.

Field-induced superconductor–insulator transition in layered organics

A partially deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br salt, which is a superconductor situated near the Mott insulator, has been investigated by resistivity measurements under magnetic fields. The superconducting transition of this salt is replaced by the first-order metal–insulator transition by magnetic field. This field-induced transition fades away when the sample cooling around 80 K is slowed down. The present findings lead us to bring to mind possible realization of the ideal electric switch controllable with the magnetic field.

Entropy of vortex cores near the superconductor-insulator transition in an underdoped cuprate.

We present a study of Nernst effect in underdoped La(2-x)Sr(x)CuO4 in magnetic fields as high as 28 T. At high fields, a sizable Nernst signal was found to persist in the presence of a field-induced nonmetallic resistivity. By simultaneously measuring resistivity and the Nernst coefficient, we extract the entropy of vortex cores in the vicinity of this field-induced superconductor-insulator transition. Moreover, the temperature dependence of the thermoelectric Hall angle provides strong constraints on the possible origins of the finite Nernst signal above T(c), as recently discovered by Xu et al. [Nature (London) 406, 486 (2000)].

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.

Indications of pair electron effect above field induced superconductor–insulator transition in amorphous ultra thin W films with Si/W/Si structure

We present the measurements of the superconductor–insulator transition (SIT) driven by both thickness and magnetic field for the Si/a–W/Si system. The thickness of the W-film was varied over a range from 1 to 5 nm. We found the universal quantum sheet resistance at SIT driven by thickness. Field induced SIT for two samples with 1.6 and 3 nm W thick was investigated in detail. In the insulator side, the evidence of Bose glass state for our homogeneous disordered two dimensional system is inferred by comparing SIT field B c with the upper critical field at 0 K H c2(0) estimated from measurements of d H c2/d T| T c and the field dependent magneto-resistance at various temperatures.

Field-induced superconductor-insulator transition and grain size in thin granular films of indium

Magnetic field, B, is used to tune two-dimensional granular films of indium with average grain sizes of 14 nm (Film G1), 22 nm (Film G2) and 28 nm (Film G3) through the superconductor-insulator (S-I) transition. For Film G1 the resistance, R( T, B), near the S-I transition obeys the scaling function which is similar to that observed in the amorphous film of indium with thickness about 3 nm, while for Films G2 and G3 deviations from the scaling behaviors are visible. The results are discussed in terms of a transformation of the system from a dirty superconductor to a Josephson network or inhomogeneities of the parameters in the Josephson network.