Title: Anomalous metals -- failed superconductors,
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.