We have studied the electronic transport properties of homogeneously disordered superconducting tantalum thin films in magnetic fields. The films exhibit three distinct transport regimes in the zero temperature limit which we identify as superconducting, metallic, and insulating phases. The metallic phase is unexpected. The transport characteristics of this metallic phase are found to be similar to those of MoGe films and high mobility dilute two-dimensional electrons or holes confined in semiconductor interface or transistor geometry.

We have studied the effect of perpendicular magnetic fields and temperatures on the nonlinear electronic transport in amorphous Ta superconducting thin films. The films exhibit a magnetic field induced metallic behavior intervening the superconductor-insulator transition in the zero temperature limit. We show that the nonlinear transport in the superconducting and metallic phase is of non-thermal origin and accompanies an extraordinarily long voltage response time.In recent years, the suppression of superconductivity in two-dimensions (2D) by means of increasing disorder (usually controlled by film thickness) or applying magnetic fields has been a focus of attention. Conventional treatments [1,2,3,4,5] of electronic transport predict that in 2D the suppression of the superconductivity leads to a direct superconductor-insulator transition (SIT) in the limit of zero temperature (T = 0). This traditional view has been challenged by the observation of magnetic field (B) induced metallic behavior in amorphous MoGe [6,7,8] and Ta thin films [9]. The unexpected metallic behavior, intervening the B-driven SIT, is characterized by a drop in resistance (ρ) followed by a saturation to a finite value as T → 0. The metallic resistance can be orders of magnitude smaller than the normal state resistance (ρ n ) implying that the metallic state exists as a separate phase rather than a point in the phase diagram. Despite many theoretical treatments [8,10,11,12,13,14,15,16,17], a consensus on the mechanism behind the metallic behavior is yet to be reached. Proposed origins of the metallic behavior include bosonic interactions in the nonsuperconducting phase [10,11], contribution of fermionic quasiparticles to the conduction [12,13], and quantum phase fluctuations [14,15].In a recent paper [9] on the magnetically induced metallic behavior in Ta films, we have reported the nonlinear voltage-current (I-V ) characteristics that can be used to identify each phase. The superconducting phase is unique in having both a hysteretic I-V and an "immeasurably" small voltage response to currents below an apparent critical current I c . The metallic phase can be identified by a differential resistance (dV /dI) that increases with increasing I, whereas the insulating phase is identified by a dV /dI that decreases with increasing I. The contrasting nonlinear I-V in the metallic and insulating phase are shown in Fig. 1(a).The main purpose of this Letter is to report that the origin of the nonlinear transport, particularly in the superconducting and metallic phase, is not a simple reflection of T -dependence of ρ via the unavoidable Joule heating. We describe the effect of B and T on the nonlinear TABLE I: List of sample parameters: nominal film thickness, mean field Tc at B = 0, normal state resistivity at 4.2 K, critical magnetic field at which the resistance reaches 90% of the high field saturation value, and correlation length calculated from ξ = Φ0/2πBc where Φ0 is the flux quantum.

Non-monotonic differential resistance (dV/dI) is observed in magnetically induced insulating films which exhibit apparent superconductor-metal-insulator transitions in the low temperature limit; at low bias currents the nonlinear transport is insulator-like while at high bias currents it is characteristic of metallic phase. The non-monotonic dV/dI may be evidence that the insulating state consists of metallic domains connected by point contacts (insulating gaps), implying that spatial inhomogeneities play a dominant role in determining the nature of the apparent metal-insulator transition.

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