Robust anomalous metallic states and vestiges of self-duality in two-dimensional granular In-InOx composites

Abstract

Many experiments investigating magnetic-field tuned superconductor-insulator transition (H-SIT) often exhibit low-temperature resistance saturation, which is interpreted as an anomalous metallic phase emerging from a ‘failed superconductor’, thus challenging conventional theory. Here we study a random granular array of indium islands grown on a gateable layer of indium-oxide. By tuning the intergrain couplings, we reveal a wide range of magnetic fields where resistance saturation is observed, under conditions of careful electromagnetic filtering and within a wide range of linear response. Exposure to external broadband noise or microwave radiation is shown to strengthen the tendency of superconductivity, where at low field a global superconducting phase is restored. Increasing magnetic field unveils an ‘avoided H-SIT’ that exhibits granularity-induced logarithmic divergence of the resistance/conductance above/below that transition, pointing to possible vestiges of the original emergent duality observed in a true H-SIT. We conclude that anomalous metallic phase is intimately associated with inherent inhomogeneities, exhibiting robust behavior at attainable temperatures for strongly granular two-dimensional systems.

Superconductor–insulator transition in two-dimensional indium–indium-oxide composite

“…These studies reveal several common features, such as well-pronounced peaks in the magnetoresistance (MR) of the insulating phase, the existence of a critical magnetic field (HC), which is manifested in a single crossing point of the MR isotherms, and observation of scaling behavior which is related to universal critical exponents. Recently, the theoretically predicted "failed superconductor" state [12] was reported in experimental studies of InOx and In-InOx 2D systems by observation of low temperature saturation of sub-Drude values of resistivity [13,11]. Although theoretically the saturation of resistivity at low temperatures was also predicted in 3D [12] there was no experimental evidence for that.…”

“…The transition observed using this method is called magneticfield-tuned SIT (HSIT) [5][6][7][8][9]. Magnetic-field-tuned phase transitions in 2D films of InOx were reported in numerous studies [5,6,[9][10][11]. These studies reveal several common features, such as well-pronounced peaks in the magnetoresistance (MR) of the insulating phase, the existence of a critical magnetic field (HC), which is manifested in a single crossing point of the MR isotherms, and observation of scaling behavior which is related to universal critical exponents.…”

Anomalous metals: from "failed superconductor" to "failed insulator"

“…The robustness of the anomalous metallic phase in the In/InO x system [8] further enforces these expanded observations, while strongly contending interpretations solely based on non-equilibrium effects and response to external effects of the electronic system [20] (see also "Methods" section). At the same time, the temperatureindependent term associated with MQT depends delicately on the details of the distribution of grains and junctions, which may be controlled by external effects leading to different saturation value of the resistance, emphasizing its non-universal value in the anomalous metal regime.…”

“…In this letter we propose a unified phenomenology for the emergence of anomalous metallic states from an avoided SIT in disordered superconducting films. Focusing on the magnetic-field tuned SIT (H-SIT) in strongly granular system of In-on-InO x composites, we show that vestiges of the self-duality observed in a true H-SIT [14,15] are still pronounced in the AMS on both sides of the avoided-H-SIT [8], including the resistance satura-tion. Bearing in mind that from its nature, the critical point of any SIT yields an anomalous metal, we propose that the broadening of the metallic phase on both sides of the H-SIT has the same origin modulo the dual relation between phase of the order parameter (or vortices) and charge (both cooper-pairs and single electrons).…”

“…An increasing number of recent experiments on disordered superconducting films, initially searching for a superconductor-to-insulator transition (SIT), end up instead with an "avoided" transition, where scaling about a putative zero-temperature quantum critical point fails in the limit T → 0, giving way to a phase characterized by a resistance that levels off to a constant value independent of temperature [1][2][3][4][5][6][7][8]. This "anomalous metallic state" (AMS) seems to exhibit electronic properties that cannot be understood on the basis of the standard paradigms for transport in disordered two-dimensional (2d) metals (for a recent review see [9]).…”