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

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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.