Superconductor-to-insulator Transition Research Articles

Superconductor-to-insulator transition and transport properties of underdoped YBa2Cu3O(y) crystals.

The carrier-concentration-driven superconductor-to-insulator (SI) transition as well as transport properties in underdoped YBa2Cu3O(y) twinned crystals is studied. The SI transition takes place at y approximately 6.3, carrier concentration n(SI)H approximately 3x10(20) cm(-3), anisotropy rho(c)/rho(ab) approximately 10(3), and the threshold resistivity rho(SI)ab approximately 0.8 mOmega cm which corresponds to a critical sheet resistance h/4e2 approximately 6.5 kOmega per CuO2 bilayer. The evolution of a carrier, nH infiniti y - 6.2, is clearly observed in the underdoped region. The resistivity and Hall coefficient abruptly acquire strong temperature dependence at y approximately 6.5 indicating a radical change in the electronic state.

Superconductor-to-insulator transition of λ-BETS 2Fe xGa 1 − xCl 4

The λ-type BETS alloys (BETS=bis(ethylenedithio)tetrathiafulvalene), λ-BETS 2Fe x Ga 1 − x Cl 4 exhibited an unprecedented superconductor-to-insulator (SC-I) transition around x = 0.43 at ambient pressure. The large susceptibility drop at T c and its recovery at t sc-i clearly show this SC-I transition reflects an intrinsic property of the crystals. The anisotropic field dependence of magnetization at 2 K indicated the insulating ground state to be antiferromagnetic. λ-BET 2Fe x Ga 1 − x Cl 4 undergoes a MI transition at x> x c1 (≈0.5), SC transition at x< x c2 (≈0.35) and SC-I transition at x c2< x< x c1. The SC-I transition was also observedin λ-BETS 2Fe x Ga 1 − x Cl 4 ( x ≈0.55) at 1 kbar, which exhibited a metal-insulator (MI) transition at ambient pressure. At high pressure, the SC phase was observed up to x≈0.9. The SC-to-M transition was also observed at high pressure in the Brcontaining system, λ-BETS 2Fe yGa 1 − y Br xCl 4 − x .

Tunneling measurements of fluctuation effects near the superconductor to insulator transition

We present measurements of the temperature dependence of the tunneling density of states near the Fermi energy, G 0 (T), and resistive transitions, R(T), of ultrathin PbBi films. Both R(T) and G 0 (T) broaden substantially near the superconductor to insulator transition (SIT). The broadening in R(T) is not affected by the proximity of a ground plane suggesting that long range Coulomb interactions are not important to the SIT. The results suggest that the transport properties of films near the SIT are influenced by fluctuations in both the phase and the amplitude of the order parameter. We discuss the data in terms of recent theories of the superconducting transition in low superfluid density systems.

Evidence for a quantum-vortex-liquid regime in ultrathin superconducting films.

We report the observation of activated dissipation in ultrathin superconducting films near the superconductor-to-insulator transition (SIT) in magnetic fields. The activation energy ${T}_{0}$ scales with the mean field transition temperature, film thickness, and logarithm of the applied field, in qualitative agreement with the activation of defects in a vortex solid. We find ${T}_{0}$ extrapolates to zero at ${H}_{0}$, a field well below the upper critical field implying the existence of a quantum-vortex-liquid regime. We discuss the implications of this result for the magnetic field tuned SIT in this system.

Electron tunneling studies of ultrathin films near the superconductor-to-insulator transition

Electron tunneling measurements on ultrathin quench-condensed films near the superconductor-to-insulator (SI) transition reveal that the superconducting state degrades with increasing normal state sheet resistance, R □, in a manner that depends strongly on film morphology. In homogeneously disordered films, the superconducting energy gap Δ o decreases continuously and appears to go to zero at the SI transition. In granular films the transport properties degrade while Δ o remains constant. Measurements in the normal state reveal disorder enhanced e - - e - interaction corrections to the density of states. These effects are strong and depend on morphology in a manner that is consistent with their playing an important role in driving the SI transition.