Growth and Low Temperature Spectroscopy of Layered Quantum Materials

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This proposal takes an integrated approach to topics in quantum materials, by combining synthesis of materials with two forms of low temperature spectroscopy: Microwave Spectroscopy and Scanning Tunneling Microscopy (STM). The materials focus is layered compounds, because of the intrinsic interest in their diverse properties, their suitability for both of these spectroscopic measurements, and their promise for applications.Both known families of high temperature superconductors, the cuprates and the iron-based materials, are layered compounds. A trend in superconductivity is to scrutinize both families to gain clues to the mechanism driving Tc so high. Much of the debate revolves around the role of spin fluctuations versus phonons and evidence for the importance of spin fluctuations can be sought in both types of spectroscopy used here. The materials that we will grow focus on clean, stoichiometric compounds such as YBa2Cu3O6+x, and the much wider array of clean materials being found amongst the iron-based compounds, such as LiFeAs and FeSe. Our use of vapour transport growth for FeSe also opens up the possibility of growing many materials in the family of transition metal chalcogenides, a field that is taking off due to their ability to be grown or exfoliated in monolayers like graphene.A project using all of these techniques combines microwave surface impedance and STM to study superconducting order parameters. Microwave measurements of the London penetration depth determine the magnitude and anisotropy of a superconducting gap. STM measurements offer a rich source of information, from local gap spectroscopy, study of bound states at individual defects, and the use of quasiparticle interference (QPI) to detect sign changes in the superconducting gap. Another target for this combination is an integrated study of the charge transport by quasiparticles in these materials. It has been discovered that FeSe, like YBa2Cu3O6+x has quasiparticle excitations in the superconducting state that develop very long mean free paths. Adding QPI measurements to this project will allow us to use QPI to identify the nature of the native defects, their scattering parameters, and look for a consistent description of both electronic transport measurements and local STM measurements of scattering.The development of new STM techniques, together with in situ growth of films will open these studies up to manipulating surface electronic properties. Members of these families that have polar surfaces can be tuned with adatoms or molecules, enabling STM to study a phase diagram as a function of doping, all on one sample. In situ growth will also be used to create hybrid structures, such as superconductors on topological materials, which provide a platform for Majorana fermions and other exotic electronic phenomena presently being predicted by theorists in the field of topological materials