Superconductor-(Metal)-Insulator Transitions: Understanding the Emergence of Anomalous Metallic States

Stanford University, Stanford CA

Investigators

Abstract

Non-Technical Abstract: Conductivity of metals is determined by impurity scattering, and is given by a simple expression known as Drude Formula. As explained by Bardeen-Cooper-Schriefer in their celebrated "BCS" theory, at low temperatures electrons in such metals may also form an exotic state known as a condensate of bosonic Cooper pairs. However, despite the vast experimental data supporting the above "standard paradigm", anomalies in experimental data, particularly in the limit of two-dimensions (2D), have been accumulating. It became clear that there exists a new class of metallic states at low temperatures, where the electronic properties cannot be understood on the basis of existing theories. The project focuses on the investigations of novel metallic states that emerge at low temperature, primarily in proximity to superconducting states. The project includes training of graduate students, and a post-doctoral scholar. The project will also seek to continue this tradition by actively recruiting women graduate students and post-docs to the group, while also maintaining strong international collaborations. Technical Abstract: A metallic state is defined as a state in which the conductivity remains finite as the temperature tends to zero. There is an extraordinarily successful "Fermi liquid theory" of clean 3D metals with long mean free path and relatively weak interactions. In this theory fermionic excitations (quasiparticles) have a finite density of states at the Fermi level, while bosonic excitations (e.g. zero sound) have lesser role at low temperatures, since their contribution to the current is negligible due to their vanishing density of states. The low-T conductivity of relatively pure 3D metals is determined by impurity scattering, and is given by the Drude formula. Another well-established paradigm is the BCS theory of superconductivity, which is based on the idea that under some circumstances electron attraction can dominate the electron repulsion so that at low temperatures electrons form a condensate of bosonic Cooper pairs. It is this condensate that carries the supercurrent. As parameters controlling the electronic environment change, the system may exhibit a superconductor to metal transition, which at zero temperature is a quantum transition. The conventional theory of metals also predict that as the system approaches the BCS superconducting state from the metallic side, its properties in no way reflect the proximity of the superconducting phase, and the conductivity of the system is controlled by the fermionic excitations everywhere in the metallic phase. Despite the vast experimental data supporting the above "standard paradigm", especially on bulk superconductors, anomalies in experimental data, particularly in the limit of two dimensions, have been accumulating. It became clear that there exists a new class of metallic states in the zero-temperature limit, where the electronic properties cannot remotely be understood on the basis of Fermi liquid and Drude theories. This observation by itself is astonishing because it points to a new paradigm for the electronic properties of metals. Exploring the properties of such anomalous metallic states is the focus of this project. This will be done by fabricating a variety of thin-films superconductors, and study their transport properties. While in the past such studies focused solely on longitudinal conductivity measurements, this project will emphasize transverse transport, and will include thermal transport, which can further shed light on the nature of the metallic state. The application of electromagnetic perturbations such as magnetic and RF electric fields will be used to tune through these metallic states, or in some circumstances promote them. The project trains one or two graduate students, one post-doctoral scholar. The project will also seek to continue this tradition by actively recruiting women graduate students and post-docs to the group, while also maintaining strong international collaborations. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.