Spin Transport Across Interfaces in Unconventional Superconductors

Nontechnical Description As conventional semiconductor technology faces increasing limits from heat management and device miniaturization, there is a need for and interest in finding new ways of transporting and processing information. One of the major thrusts is towards building devices from quantum materials where the currents are carried not by electric charge but by the magnetic moment of electrons, which is proportional to a quantum mechanical property of an electron called spin, and where such currents can travel without heat losses in the superconducting state. This project focuses on a class of quantum materials, called triplet superconductors, where the superconducting state itself can support spin current without the need for external magnetic fields or additional magnetic components. The principal investigator (PI) identifies superconductors where spin currents can be driven by heating and cooling, without a need for an applied voltage. When such superconductors are used in prototype devices, the superconducting behavior changes near contacts that inject or collect spin currents for device operation. The PI and his team analyze these changes and understand spin and charge transport in depth, including dependence on the type of the superconductor and the material of the leads, and the properties of the contacts and interfaces between them. The PI develops the methodology and provides quantitative estimates for the spin currents in such structures helping to guide the use of triplet superconductors as components for quantum information, metrology, and sensing applications. The project attracts, trains, and cultivates a highly qualified STEM workforce for quantum science and technology. It provides training opportunities for graduate and undergraduate students in technical skills, knowledge of high-performance computing, as well as soft skills targeted at communication, mentoring and collaboration. Through public lectures, the PI increases public scientific literacy, drives an understanding of the scientific enterprise as an economic engine for the national economy, and stimulates public engagement with science and technology. Technical Description: The broad goal of the project is to investigate new conceptual pathways towards deployment of quantum technology based on superconducting spin transport. The PI’s research reveals that several classes of triplet superconductors, including those with topological order, longitudinal and transverse spin currents can be induced solely by temperature gradients, eliminating the problem of leakage fields. The PI aims to identify types of superconducting order, including superconducting topological order, that support spin currents in the bulk. The PI and his research team work on the problem of coupled spin, charge, and heat transport at the interface between heavy metallic leads and triplet superconductors to focus on equilibrium and non-equilibrium spin response. Methodologically, the scattering matrix and the boundary conditions at the interface between the leads and triplet superconductors are derived, focusing on the mismatch in the spin-orbit coupling across interfaces and its consequences in the absence of and in combination with symmetry-breaking perturbations. The PI uses these boundary conditions within the quasi-classical Keldysh theory to self-consistently compute non-equilibrium spin and charge transport coefficients. The activity especially targets longitudinal and transverse response to thermal gradients, rather than direct charge or spin injection, and provides quantitative estimates of the spin currents, and the associated inverse spin Hall signal in prototype heterostructures. The developed methodology is transferable to general analysis of spin transport in the bulk and at interfaces of correlated and topological materials. The outcomes may contribute to US economic competitiveness and to maintaining national economic security through leadership in the field of superconducting spintronics. 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.