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

非技术摘要：实现零电阻的电传输（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发挥作用。因此，寻找更高的“高tc”超导体的工作正在进行中。最近的两个发现在这个领域引起了极大的兴奋。首先是在生长的FeSe单层薄膜中发现了增强的超导转变温度（Tc）。这些薄膜中的Tc比块状晶体提高了大约8到10倍。第二个令人兴奋的发现是在一种叫做铱酸盐的新材料中观察到潜在的超导性。在FeSe中观察到的显著高Tc和铱酸盐中潜在的超导性不仅重新激发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解决的问题。本项目利用光谱成像扫描隧道显微镜（SI-STM）研究掺杂FeSe薄膜和铱酸盐单晶的超导现象和机制，该技术已成为获得材料结构和电子特性纳米级信息的有力技术。该项目的目的是研究超薄FeSe薄膜中超导转变温度（Tc）提高的机制，并探索铱酸盐化合物的超导性。了解高超导转变温度的机制可能为在其他材料中实现更高的Tcs指明道路。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方面的培训。PI的综合推广和教育活动使有才华的高中生接触到最前沿的研究。技术摘要：本项目利用光谱成像扫描隧道显微镜（SI-STM）研究掺杂FeSe薄膜和铱酸盐单晶的超导现象和机制，该技术已成为获得材料结构、电子和玻色子性质纳米信息的有力技术。该项目包括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的钾(K)，以及块体和表面掺杂的iridates。PI的实验室拥有使用高分辨率扫描隧道显微镜（STM）和光谱学（STS），傅里叶变换STS和自旋极化STS探测真实和动量空间中复杂材料的电子，声子和自旋激发的技术和专业知识。此外，PI实验室的定制分子束外延系统允许将薄膜样品传输到STM，而无需将薄膜暴露在空气中。这些工具对项目的成功至关重要。该项目的目标是多重的。该项目研究超薄FeSe薄膜中超导转变温度（Tc）提高的机制，并提供数据来区分不同的Tc增强机制。对于Iridates， SI-STM数据用于区分费米弧和费米袋。如果观察到弧形，研究铱酸盐系统可能会揭示这种弧形在铱酸盐和铜酸盐中的起源。了解FeSe中惊人的高超导转变温度的机制可能为在其他材料中实现更高的Tcs指明道路。如果证实了铱酸盐的d波超导性，将有助于阐明铜超导性中的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。这个项目的影响超出了对这些新材料的超导性的理解。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的材料和仪器方面的培训。PI以前的许多顾问都是女性。该项目的成功增加了女性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到前沿科学。

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.