Growth and Low Temperature Spectroscopy of Layered Quantum Materials

本提案通过将两种低温光谱学形式：微波光谱学和扫描隧道显微镜（STM）结合起来，采用综合方法研究量子材料的主题。材料的重点是层状化合物，因为它们的各种特性，它们对这两种光谱测量的适用性以及它们的应用前景都有内在的兴趣。*******两个已知的高温超导体家族，铜酸盐和铁基材料，都是层状化合物。超导的一个趋势是仔细研究这两个家族，以获得驱动Tc如此高的机制的线索。许多争论围绕着自旋涨落与声子的作用，而自旋涨落重要性的证据可以在这里使用的两种光谱学中找到。我们将专注于清洁的化学计量化合物，如YBa2Cu3O6+x，以及在铁基化合物中发现的更广泛的清洁材料，如LiFeAs和FeSe。我们对FeSe的蒸汽传输生长也为过渡金属硫族化合物家族中的许多材料的生长开辟了可能性，由于它们能够在石墨烯等单层中生长或剥离，这一领域正在蓬勃发展。*******一个利用所有这些技术结合微波表面阻抗和STM来研究超导序参量的项目。对伦敦穿透深度的微波测量决定了超导隙的大小和各向异性。STM测量提供了丰富的信息来源，从局部间隙光谱，研究单个缺陷的束缚态，以及使用准粒子干涉（QPI）来检测超导间隙中的标志变化。这种结合的另一个目标是对准粒子在这些材料中的电荷输运进行综合研究。已经发现，FeSe，像YBa2Cu3O6+x一样，在超导态具有准粒子激发，产生很长的平均自由程。将QPI测量添加到这个项目将允许我们使用QPI来识别原生缺陷的性质，它们的散射参数，并寻找电子输运测量和散射的本地STM测量的一致描述。*******新的STM技术的发展，加上薄膜的原位生长，将打开这些研究，以操纵表面电子特性。这些具有极性表面的家族成员可以用原子或分子进行调谐，使STM能够在一个样品上研究作为掺杂函数的相图。原位生长还将用于创建混合结构，例如拓扑材料上的超导体，这为拓扑材料领域理论家目前预测的马约拉纳费米子和其他奇异电子现象提供了一个平台

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***