****Technical Abstract**** Studies of the phase diagram of two-dimensional superconductors has been intimately connected to finding new materials. With each materials system a different set of properties is highlighted. Over the years, NSF support enabled the optimization of a variety of materials for studies of thin superconducting films in general and the superconductor to insulator transition (SIT) in particular. These include amorphous-MoGe films that allowed the discovery of "metallic phases," amorphous-InOx films that allowed the studies of the regime of strong disorder, amorphous TaNx that allowed for the detailed study of the Hall effect near the transition, and amorphous- MgB2 films which in addition to the overlap in parameters with MoGe, TaNx, and InOx, also provide a new knob in the form of controlling spin-orbit interaction. This project explores the rich phase diagram near the superconductor-insulator transition with emphasis on the intervening metallic states. In particular we will concentrate on understanding of what is the true phase diagram of two-dimensional SIT in the presence of a metallic background (i.e. dissipation); what is the electronic microstructure of a 2D SIT system; can we infer the various length scales from experiments such as transport, Hall and Nernst effects, etc.; and, starting from artificial films made of orderly metallic dots on a metal films, what is the range of validity of this approach as we increase disorder. The proposed work is expected to shed new light on the nature of quantum phase transitions in reduced dimensions and impact the understanding of the material science of amorphous superconducting films.
Quantum phase transitions continue to attract intense theoretical and experimental interest. Such transitions -- where changing an external parameter in the Hamiltonian induces a transition from one quantum ground state to another, fundamentally different one -- have been invoked to understand experimental data in many electronic systems, as well as to analyze more challenging quantum phenomena such as the limitations of quantum computing. A paradigm for quantum phase transitions has been the superconductor to insulator transition in two-dimensional films. This transition is found to have a variety of realizations depending on intrinsic properties of the materials used. Controlling the strength of superconductivity, disorder, or dimensionality of the films will impact the nature of the transition, which is accessed through the variation of external parameters such as temperature and magnetic field. This award supports a project to explore a series of unique quantum states near the superconductor to insulator transition in amorphous films, particularly amorphous-MoGe, MgB2, InOx and TaNx. The uniqueness of this set of systems is the ability to control the strength of superconductivity and disorder over a very wide range of parameters, and and in particular allow access to the poorly understood ubiquitous metallic states that have been first identified by our group. The program involves graduate students in this area of great interest to physics and future applications.
Superconductivity is a fundamental collective phenomenon in condensed matter physics in which an instability of the electron system to the formation of a condensate of electron-pairs (called Cooper-pairs) result in a state that is characterized by zero resistance to the flow of electrical and heat currents, and by the repulsion of the magnetic field from the interior of the system. A precursor to the transition to the superconducting state is a state of superconducting fluctuations which is characterized by short-living Cooper pairs that alter the longitudinal and transverse (Hall) electrical resistances before both vanish at the superconducting transition. While the problem of superconducting fluctuations was first studied in the early 60th, it was never completely solved, especially in reduced dimensionality, and for the Hall coefficient. Studying fluctuation effects in the Hall conductivity has been an experimental challenge in systems with high carrier concentration and large longitudinal resistance. Our work, first measured the superconducting fluctuations contribution to the Hall effect, and then, in collaboration with theoreticians used a new theoretical approach to fit the data. We further studied the "ghost critical field," that is, the field scale for the supression of superconducting fluctuations, in disordered superconducting thin films. We observe an enhancement in the Hall effect with a maximum that tracks the upper critical field below Tc, disappears near Tc, and returns to higher fields above Tc. Simultaneous magnetoresistance measurements allow us to extract the ghost critical field for superconducting fluctuations, which is good agreement with theory.
