Nanoscale Studies of Surface Doping Effects and Superconductivity in Fe-based Superconductors and Iridates

非技术摘要：实现零电阻的电输运（超导性）是凝聚态物理学家几十年来的梦想。然而，我们最好的“高温（高Tc）”超导体仍然需要低温才能起作用。因此，人们正在寻找更高的“高温”超导体。最近的两项发现在这一领域引起了极大的轰动。第一个是在FeSe单层薄膜中发现了增强的超导转变温度（Tc）。在这些薄膜中的Tc增强了约8或10倍的散装晶体相比。第二个令人兴奋的发现是在一种名为虹彩酸盐的新材料中观察到了潜在的超导性。在FeSe中观察到的显着的高Tc和在铱酸盐中观察到的潜在的超导性不仅重新引起了我们对寻找新的和更高Tc材料的兴趣，而且还产生了一系列新的和尚未回答的问题。本计画利用光谱成像扫描隧道显微镜（SI-STM）研究掺杂FeSe薄膜和铱酸盐单晶中超导电性的发生和机制，SI-STM已成为获得材料结构和电子性质的纳米级信息的强大技术。该项目的目标是研究的机制，其中超导转变温度（Tc）的提高，在铁硒薄膜和探索在铱酸盐化合物的超导电性。了解令人惊讶的高超导转变温度的机制可能会为在其他材料中实现更高的Tc指明方向。研究这些材料的学生正在接受当今研究前沿的材料和仪器的培训。PI的综合推广和教育活动暴露有才华的高中生的前沿研究。技术摘要：本项目调查的发生和机制的超导性掺杂FeSe薄膜和铱酸盐单晶与光谱成像扫描隧道显微镜（SI-STM），这已成为一个强大的技术，以获得纳米级信息的结构，电子和玻色子性质的材料。该项目特别关注表面掺杂对材料性能的影响，如FeSe单层和多层电子掺杂钾（K），以及体相和表面掺杂的铱酸盐。PI的实验室拥有的技术和专业知识，探测电子，声子和自旋激发的复杂材料在真实的-和动量-空间使用高分辨率扫描隧道显微镜（STM）和光谱（STS），傅立叶变换STS和自旋极化STS。此外，PI实验室中的定制分子束外延系统允许将薄膜样品运输到STM，而无需将薄膜暴露在空气中。这些工具对于项目的成功至关重要。该项目的目标是多方面的。该项目调查的机制，超导转变温度（Tc）是提高在FeSe薄膜，并提供数据，以区分不同的机制之间的Tc增强。对于虹膜，SI-STM数据被用来区分费米弧从费米口袋。如果观察到弧，研究虹彩系统可能会揭示虹彩和铜酸盐中这种弧的起源。了解FeSe中令人惊讶的高超导转变温度的机制可能会为在其他材料中实现更高的Tc指明方向。如果铱酸盐d波超导性得到证实，将有助于阐明铜酸盐超导性中许多悬而未决的问题，并为探索氧化物超导性提供一个新的平台。该项目的影响超出了对这些新材料超导性的理解。 使用SI-STS和MBE技术研究这些材料的学生正在接受当今研究前沿材料和仪器的培训。PI的许多前受害者都是女性。该项目的成功增强了妇女在物理学领域的研究经验。PI的综合推广和教育活动使有才华的高中生接触尖端科学。

Nontechnical abstract:Achieving electrical transport with zero resistance (superconductivity) has been a dream of condensed matter physicists for many decades. Our best `high-temperature (high-Tc)' superconductors still however require low temperatures to be functional. The search is therefore on to discover higher `high-Tc' superconductors. Two recent discoveries have created tremendous excitement in this field. The first is the discovery of enhanced superconducting transition temperatures (Tc) in monolayer films of FeSe grown. The Tc in these films is enhanced by a factor of approximately eight or ten compared to bulk crystals. The second exciting discovery is the observation of potential superconductivity in a new class of materials called iridates. The dramatically high Tcs observed in FeSe and potential superconductivity in the iridates have not only renewed our interest in the search for new and higher Tc materials but have also generated a series of new and as yet unanswered questions. This project investigates the occurrence and mechanisms of superconductivity in doped FeSe thin films and iridate single crystals with spectroscopic imaging scanning tunneling microscopy (SI-STM), which has emerged as a powerful technique to obtain nanoscale information on the structural and electronic properties of materials. The goals of the project are to investigate the mechanism by which superconducting transition temperature (Tc) is enhanced in ultrathin FeSe films and explore superconductivity in the iridate compounds. Understanding the mechanism of the surprisingly high superconducting transition temperatures may point the way to realizing higher Tcs in other materials. Students working on these materials with are being trained on materials and instruments at the forefront of today's research. The PI's integrated outreach and education activities exposes talented high school students to cutting edge research.Technical abstract:This project investigates the occurrence and mechanisms of superconductivity in doped FeSe thin films and iridate single crystals with spectroscopic imaging scanning tunneling microscopy (SI-STM), which has emerged as a powerful technique to obtain nanoscale information on the structural, electronic and bosonic properties of materials. The project includes a special focus on the effects of surface doping on the properties of materials such as FeSe monolayers and multilayers electron-doped with potassium (K), and bulk and surface doped iridates. The PI's laboratory has the technology and expertise to probe the electron, phonon, and spin excitations of complex materials in real- and momentum-space using high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS), Fourier transform STS, and spin-polarized STS. In addition, a custom molecular beam epitaxy system in the PI's lab allows transport of thin film samples to the STM without exposing the films to air. These instruments are critical in the success of the projects. The goals of the project are multifold. The project investigates the mechanism by which superconducting transition temperature (Tc) is enhanced in ultrathin FeSe films and provides data to distinguish between the different mechanisms of Tc enhancement. For the Iridates, SI-STM data is used to differentiate Fermi arcs from Fermi pockets. If arcs are observed, studying the iridate system might shed light on the origin of such arcs in both iridates and cuprates. Understanding the mechanism of the surprisingly high superconducting transition temperatures in FeSe may point the way to realizing higher Tcs in other materials. If iridate d-wave superconductivity were confirmed, it would help elucidate many of the outstanding questions in cuprate superconductivity and provide a new playground to explore oxide superconductivity in general. The project has impact beyond progress in understanding superconductivity in these new materials. Students working on these materials with SI-STS and MBE techniques are being trained on materials and instruments at the forefront of today's research. Many of PI's former advisees have been women. The success of the project enhances the research experience for women in physics. The PI's integrated outreach and education activities exposes talented high school students to cutting edge science.