Superconductivity and Proximity Effect in Two-Dimensional Films and Multilayers

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Non-Technical AbstractQuantum 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 the superconductor to insulator transition in amorphous MgB2 films and multilayers. The uniqueness of this system is the ability to control the strength of superconductivity over a very wide range, control the disorder, and fabricate bilayers and multilayers. However, more uniquely, due to the lightness of both magnesium and boron, spin orbit interaction is probably the lowest that can be achieved in any thin film superconductor, providing us with new opportunities to study its effect on the transition. The proposed work is expected to impact science in several areas including shedding new light on the nature of quantum phase transitions in reduced dimensions, better understanding of the material science of amorphous superconducting films, and the development of new methodologies using novel measurements techniques. The program involves graduate students in this area of great interest to physics and future applications.Technical AbstractStudies 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, and more recently amorphous- MgB2 films which in addition to the overlap in parameters with MoGe and InOx, also provide a new knob in the form of controlling spin-orbit interaction. A newly available, novel technique to fabricate multilayers of amorphous MgB2/MgO allows further exploration of dimensionality effects. This award supports a project to explore the rich physics this MgB2 model system offers. Experiments will be performed to study SIT in perpendicular magnetic field on MgB2 thin films bilayers of various types (e.g. proximity systems) and multilayers. Specific attention will be given to the effect of quantum melting near the SIT point. The manifestation of low spin-orbit interaction will be explored through measurements in parallel magnetic field searching for signatures of first order transition and irreversibility. In addition to transport measurements, mesoscopic effects near the transition will be explored using novel techniques including scanning tunneling potentiometry and Kerr microscopy. The proposed work is expected shed new light on the nature of quantum phase transitions in reduced dimensions, as well as impact the understanding of the material science of amorphous superconducting films, and the development of new methodologies using novel measurements techniques. The program involves graduate students in this area of great interest to fundamental physics and future applications.