True superconductivity in a two-dimensional superconducting-insulating system

Abstract

We present results on disordered amorphous films which are expected to undergo a field-tuned Superconductor-Insulator Transition. Based on low-field data and I-V characteristics, we find evidence of a low temperature Metal-to-Superconductor transition. This transition is characterized by hysteretic magnetoresistance and discontinuities in the I-V curves. The metallic phase just above the transition is different from the "Fermi Metal" before superconductivity sets in.PACS numbers: 74.20.M, 73.40.Hm The two-dimensional (2D) Superconductor-Insulator (SI) Transition has attracted widespread attention in recent years [1]. Initially, theory argued for the existence of a magnetic field tuned, continuous phase transition from a superconducting to an insulating state, with a quantum critical point and corresponding quantum critical scaling about a critical field H SI [2][3][4]. Experiments seemed to confirm this scenario, although there was some concern that the apparent critical exponents resembled those expected from classical percolation [5] and that the critical resistance at the transition was not the quantum of resistance for Cooper pairs, h/4e2 [6][7][8]. However, recent experiments [9][10][11] have challenged the general existence of such a transition, demonstrating that the apparent transition is only a crossover to a new metallic state at low temperatures. Similar results have been obtained on quantum Hall liquid-to-insulator transitions and on arrays of Josephson junctions; both are presumed to be in the same universality class as the 2D SI transition [12][13][14]. The question of whether a true superconducting phase is possible within this new experimental situation was an unresolved issue.In this paper, we present results on magnetoresistance and current-voltage measurements carried out on 2D films of MoGe below H SI . Observations of a dramatic drop in resistance at a critical magnetic field H SM << H SI , along with corresponding hysteresis near that field and instabilities in I-V curves, point to the existence of a low-field quantum phase transition, possibly a first order one, to a true superconducting state. The metallic phase above the transition is not a normal Fermi Liquid, at least in any simple sense, because its resistance is orders of magnitudes smaller than the normal state ("Fermi") resistance, it has very large magnetoresistance, and it exhibits non-linear transport unlike a conventional metal. The superconducting state can be characterized by vortices (far separated due to the low field) that are pinned on impurities to provide the true zero resistance.Samples for which we present data in this paper are 30Ȧ and 40Ȧ M o 43 Ge 57 thin films, sandwiched between insulating layers of amorphous Ge on SiN substrates. The 30Ȧ and 40Ȧ samples have sheet resistances at 4.2K of R N ∼1300 Ω/✷ and R N ∼800 Ω/✷, respectively; T C 's of 0.5K and 1K; H C2 's of 1.4T and 1.9T. Previous studies have determined the films to be amorphous and homogeneous on all relevant length scales. We patterned the films in...

Concepts in High Temperature Superconductivity

“…What is the nature of this intermediate state? This is a widely observed phenomenon in systems which are A mysterious ground state expected to be undergoing a superconductor to insulator transition [272,273]. The physics of this anomalous metallic state is not understood at all, even in systems, such as the present one, where the microscopic physics is believed to be understood.…”

Singular or non-Fermi liquids

“…Interesting new developments in this problem with references to substantial earlier work may be found in [176,177,247]. The problem of transitions in Josephson arrays [247] is a variant of such problems.…”

Bose metal: Gauge-field fluctuations and scaling for field-tuned quantum phase transitions

“…Hence, it is not inconceivable that, in this very low field regime, at T = 0, quantum fluctuations and disorder can conspire to produce a pinned vortex state without any free dislocations, and consequently true (Cooper pair) superconductivity. In other words, there is a possibility that H c0 > 0 [5]. 2.…”