We present the results of an experimental study of superconducting, disordered, thin films of amorphous indium oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field (B). We find that the high-B insulator exhibits activated transport with a characteristic temperature, TI. TI has a maximum value (TpI) that is close to the superconducting transition temperature (Tc) at B=0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. Tp(I) and Tc display opposite dependences on the disorder strength.

The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid-solid phase transitions such as those involved in metal-insulator switching, ferroelectricity, and superconductivity. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution W x V 1Àx O 2 . Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal / insulator and insulator / metal phase transformations. Both transitions are depressed to lower critical temperatures upon the incorporation of substitutional tungsten dopants but the impact on the former transition seems far more prominent. In general, the depression in the critical temperatures upon tungsten doping far exceeds corresponding values for bulk W x V 1Àx O 2 of the same composition. Notably, the depression in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W 6+ doping in the W x V 1Àx O 2 nanobelts.

We present the results of an experimental study of the current-voltage characteristics in strong magnetic field (B) of disordered, superconducting, thin-films of amorphous Indium-Oxide. As the B strength is increased superconductivity degrades, until a critical field (B c ) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high B.

We report a comprehensive study of the complex ac conductivity of thin effectively two-dimensional amorphous superconducting InO x films at zero applied field. Below a temperature scale T c0 where the superconducting order parameter amplitude becomes well defined, there is a temperature where both the generalized superfluid stiffness acquires a frequency dependence and the dc magnetoresistance becomes linear in field. We associate this with a transition of the Kosterlitz-Thouless-Berezinskii ͑KTB͒ type. At our measurement frequencies the superfluid stiffness at T KTB is found to be larger than the universal value. Although this may be understood with a vortex dielectric constant of ⑀ v Ϸ 1.9 within the usual KTB theory, this is a relatively large value and indicates that such a system may be out of the domain of applicability of the low-fugacity ͑lowvortex-density͒ KTB treatment. This opens up the possibility that at least some of the discrepancy from a nonuniversal magnitude is intrinsic. Our finite-frequency measurements allow us access to a number of other phenomena concerning the charge dynamics in superconducting thin films, including an enhanced conductivity near the amplitude fluctuation temperature T c0 and a finite dissipation at low temperature which appears to be a universal aspect of highly disordered superconducting films.

We present the results from an experimental study of the magneto-transport of superconducting wires of amorphous Indium-Oxide, having widths in the range 40 -120 nm. We find that, below the superconducting transition temperature, the wires exhibit clear, reproducible, oscillations in their resistance as a function of magnetic field. The oscillations are reminiscent of those which underlie the operation of a superconducting quantum interference device.PACS numbers: 74.78. Na, 85.35.Ds, 73.21.Hb The central challenge in the study of thin superconducting wires is to understand how superconductivity is affected when approaching the one-dimensional (1D) limit. Earlier studies have predicted that intrinsic thermal [1, 2] and quantum [3,4,5] fluctuations play an increasingly important role in this limit, causing the wires to remain resistive much below the superconducting transition temperature, T c . Recent theories [6,7], incorporating the effect of electron-electron interactions, describe the suppression of T c when approaching the 1D limit, which was observed in experiments [8].In recent years new experimental techniques enabling the fabrication of superconducting wires with a diameter approaching the 1D limit were developed. The experiments that followed [9] focused primarily on whether the quantum resistance for a Cooper-pair, R Q = h/4e 2 ≈ 6.45 kΩ (h is Planck's constant and e is the charge of the electron) is the resistance scale that solely controls the existence of superconductivity. While some works [9,10] provided evidence that wires with a normal state resistance R N < R Q are superconducting and those with R N > R Q become insulating at low T , this point is still under debate [11].In this letter we report on an experimental study of the magnetic field (B) dependence of the resistance of superconducting wires whose dimensions are close to the 1D limit. We find that, while a strong B drives our wires into an insulating state, the magnetoresistance is dominated by reproducible periodic oscillations similar to those observed in superconducting quantum interference devices (SQUIDs) [12]. We also find that wires with R N >> R Q can exhibit superconductivity.In order to fabricate our 1D wires we utilized the method of Bezryadin et al. [9], in which a non-conducting nanotube, suspended across a narrow gap etched in a semiconductor substrate, is used as a template on which the superconductor is deposited. There are two experimental differences between our work and that of Ref.[9]. First, instead of carbon, our nanotubes were made of WS 2 [13]. Being a semiconductor with a bandgap of about 2 eV [14], WS 2 nanotubes are electrically insulating at low T and do not create a parallel conduction channel. We have verified that the nanotubes are insulating before depositing the superconducting material.Second, and more importantly, for the disordered superconductor we chose amorphous indium-oxide (a:InO). This choice was influenced by several of its properties. Since a:InO is known to form relatively uniform, superco...

We report a comprehensive study of the complex AC conductance of amorphous superconducting InOx thin films. We measure the explicit frequency dependency of the complex conductance and the phase stiffness over a range from 0.21 to 15 GHz at temperatures down to 350 mK using a novel broadband microwave Corbino spectrometer. The dynamic ac measurements are sensitive to the temporal correlations of the superconducting order parameter in the fluctuation range above Tc. Among other aspects, we explicitly demonstrate the critical slowing down of the characteristic fluctuation rate on the approach to the superconducting state and show that its behavior is consistent with vortex-like phase fluctuations and a phase-ordering scenario of the transition.

The lifetime of two dimensional electrons in GaAs quantum wells, placed in weak quantizing magnetic fields, is measured using a simple transport method in broad range of temperatures from 0.3 K to 20 K. The temperature variations of the electron lifetime are found to be in good agreement with conventional theory of electron-electron scattering in 2D systems.

The roster of materials exhibiting metal–insulator transitions with sharply discontinuous switching of electrical conductivity close to room temperature remains rather sparse, despite the fundamental interest in the electronic instabilities manifested in such materials and the plethora of potential technological applications ranging from frequency‐agile metamaterials to electrochromic coatings and Mott field‐effect transistors. Here, unprecedented, pronounced metal‐insulator transitions induced by application of a voltage are demonstrated for nanowires of a vanadium oxide bronze with intercalated divalent cations, β‐PbxV2O5 (x ≈ 0.33). The induction of the phase transition through application of an electric field at room temperature makes this system particularly attractive and viable for technological applications. A mechanistic basis for the phase transition is proposed based on charge disproportionation evidenced at room temperature in near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements, ab initio density functional theory calculations of the band structure, and electrical transport data, suggesting that transformation to the metallic state is induced by melting of specific charge localization and ordering motifs extant in these materials.