Title: Interfacial Engineering of Quantum Materials

Shuolong Yang

Cornell University

Abstract: Quantum materials are fascinating platforms where macroscopic quantum phenomena occur. As a prominent example, superconductors conduct electricity with no dissipation at low temperatures, holding great promise to address global energy challenges. It is of tremendous scientific and technological interest to enhance superconducting transition temperatures. In this talk I will elucidate the principles of interfacial engineering and co-operative interactions through studies of bulk superconductors, and demonstrate how I implement these concepts in fabricating an iron selenide/strontium titanate interfacial superconductor. In the first example, I map out the electronic band structure and quantify electron-phonon interactions of calcium-intercalated graphite. I discover a key phonon mode, enabled by the interface between calcium superlattice and graphene sheets, which facilitates superconductivity in this compound. In the second example, I combine two time-domain experiments to study an iron selenide superconductor. By precisely determining the atomic displacements and electronic energy shifts, I discover a co-operative interplay between electron-phonon and electron-electron interactions, which is crucial for unconventional superconductivity. Following the acquired insight, I fabricate ultrathin iron selenide on strontium titanate substrates and investigate the interfacial superconductivity. Characterized by in situ transport and in situ photoemission spectroscopy, the transition temperature of the interfacial layer is one order of magnitude higher than the counterpart for bulk iron selenide. This high transition temperature persists even when additional iron selenide layers are deposited. This study sets the foundation for manipulating superconductivity and other quantum phenomena via interfacial engineering.