Growth and Low Temperature Spectroscopy of Layered Quantum Materials

这项提议对量子材料的主题采取了一种综合的方法，将材料的合成与两种形式的低温光谱相结合：微波光谱和扫描隧道显微镜(STM)。材料的焦点是层状化合物，因为它们的不同性质引起了人们的内在兴趣，它们适用于这两种光谱测量，以及它们的应用前景。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。超导电性的一个趋势是仔细研究这两个家族，以获得推动T_c如此高的机制的线索。大部分争论都围绕着自旋波动对声子的作用，在这里使用的两种光谱学中都可以找到自旋波动重要性的证据。我们将种植的材料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LiLias和FeSe。我们对FeSe的蒸气传输生长也为生长过渡金属硫化物家族的许多材料打开了可能性，这一领域正在蓬勃发展，因为它们能够在石墨烯等单分子层中生长或剥离。*使用所有这些技术的项目结合了微波表面阻抗和扫描隧道显微镜来研究超导序参数。对伦敦穿透深度的微波测量决定了超导带隙的大小和各向异性。扫描隧道显微镜测量提供了丰富的信息来源，从局域能隙光谱，研究单个缺陷的束聚态，以及使用准粒子干涉(QPI)来检测超导带隙中的符号变化。这种结合的另一个目标是对这些材料中准粒子的电荷传输进行综合研究。人们已经发现，像YBa2Cu3O6+x一样，FeSe在超导态也有准粒子激发，产生很长的平均自由程。将QPI测量添加到这个项目中，将使我们能够使用QPI来识别本征缺陷的性质及其散射参数，并寻找对散射的电子传输测量和局部STM测量的一致描述。*新STM技术的发展，以及薄膜的原位生长，将使这些研究向操纵表面电子性质开放。具有极性表面的这些家族的成员可以用吸附原子或分子进行调节，使STM能够研究作为掺杂功能的相图，所有这些都在一个样品上。原位生长也将被用来创造混合结构，例如拓扑材料上的超导体，这为马约拉纳费米子和其他目前由拓扑材料领域的理论家预测的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***