Anomalous metals -- failed superconductors

TL;DR

This paper investigates the anomalous metallic state in 2D superconducting systems near the quantum superconductor-metal transition, revealing a universal behavior where resistivity saturates at low temperatures, challenging existing theories.

Contribution

It introduces the concept of the anomalous metal regime, combining experimental observations with theoretical modeling, and highlights the need for new theoretical frameworks.

Findings

Resistivity drops then saturates at low temperatures in the anomalous metal

Giant positive magneto-resistance observed in the anomalous metal

Existing theories cannot fully explain the robustness of the phenomena

Abstract

The observation of metallic ground states in a variety of two-dimensional electronic systems poses a fundamental challenge for the theory of electron fluids. Here, we analyze evidence for the existence of a regime, which we call the "anomalous metal regime," in diverse 2D superconducting systems driven through a quantum superconductor to metal transition (QSMT) by tuning physical parameters such as the magnetic field, the gate voltage in the case of systems with a MOSFET geometry, or the degree of disorder. The principal phenomenological observation is that in the anomalous metal, as a function of decreasing temperature, the resistivity first drops as if the system were approaching a superconducting ground state, but then saturates at low temperatures to a value that can be orders of magnitude smaller than the Drude value. The anomalous metal also shows a giant positive magneto-resistance. Thus, it behaves as if it were a "failed superconductor." This behavior is observed in a broad range of parameters. We moreover exhibit, by theoretical solution of a model of superconducting grains embedded in a metallic matrix, that as a matter of principle such anomalous metallic behavior can occur in the neighborhood of a QSMT. However, we also argue that the robustness and ubiquitous nature of the observed phenomena are difficult to reconcile with any existing theoretical treatment, and speculate about the character of a more fundamental theoretical framework.