Growth and Low Temperature Spectroscopy of Layered Quantum Materials

该提案对量子材料的主题采取了综合方法，将材料合成与两种形式的低温光谱学相结合：微波光谱学和扫描隧道显微镜（STM）。材料的重点是层状化合物，因为它们的不同性质，它们对这两种光谱测量的适用性以及它们的应用前景的内在兴趣。两种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。超导性的一个趋势是仔细研究这两个家族，以获得驱动Tc如此之高的机制的线索。大部分的争论都围绕着自旋涨落与声子的作用，自旋涨落的重要性的证据可以在这里使用的两种类型的光谱学中找到。我们将开发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LiFeAs和FeSe。我们对FeSe的气相传输生长的使用也开辟了在过渡金属硫族化物家族中生长许多材料的可能性，这是一个由于它们能够在单层中生长或剥离而起飞的领域，如石墨烯。利用所有这些技术的一个项目结合微波表面阻抗和STM研究超导序参数。微波测量伦敦的穿透深度确定超导间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，在个别缺陷的束缚态的研究，并使用准粒子干涉（QPI）来检测超导间隙的符号变化。这种组合的另一个目标是在这些材料中的准粒子的电荷输运的综合研究。已经发现，FeSe，像YBa_2Cu_3O_（6 +x）一样，在超导状态下具有准粒子激发，其发展非常长的平均自由程。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，连同原位生长的薄膜将打开这些研究操纵表面的电子特性。这些具有极性表面的家族成员可以用吸附原子或分子进行调整，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，这为马约拉纳费米子和拓扑材料领域的理论家目前预测的其他奇异电子现象提供了平台 *

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***