Date of Award

August 2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Michael Weinert

Committee Members

Lian Li, Daniel Agterberg, Peter Schwander, Jorg Woehl

Keywords

Bicollinear antiferromagnetic ordering, Iron-based superconductor, Molecular beam epitaxy, Pairing symmetry, Scanning tunneling microscopy, Size effect

Abstract

The search for high temperature superconductivity has been a prominent topic in the field of condensed matter physics ever since the discovery of this novel phenomenon more than 100 years ago. In addition to the search for new materials, interfacial superconductivity has shown great potential as demonstrated recently in monolayer FeSe grown on SrTiO3 (STO) (001) substrate, where superconducting transition temperature (Tc) has been enhanced by more than an order of magnitude compared to the bulk value. The uniqueness of this approach is the direct placement of the superconducting layer on a secondary substrate, which facilitates the independent control of interfacial interactions by methods such as electrical doping and optical gating. In addition, due to low dimensional nature of single layer film, quantum size effect is also expected to modify the superconductivity that allows for further tailoring. However, much is still unknown in this single layer FeSe/STO system. In particular, the substrate doping from the STO leads to distinct Fermi surface and band structure for the FeSe, giving rise to the pairing symmetry that is different from most of the iron pnictide superconductors. Furthermore, the interplay between magnetism and superconductivity at the single layer limit in iron chalcogenides also remains largely unexplored.

In this dissertation, I report on scanning tunneling microscopy/spectroscopy (STM/S) studies of single layer FeSe films grown on STO substrates using molecular beam epitaxy (MBE), focusing on the aspects discussed above. By mapping the spatially resolved superconducting gaps near the edges of single layer FeSe as a function of the edge orientations, I obtain evidence for sign-changing d wave pairing symmetry in single layer FeSe/STO. By further synthesizing well-defined rectangular nanoribbons with precisely controlled width, I establish the lowest length limit for superconducting single layer FeSe nanoribbons. To investigate the interplay between magnetism and superconductivity, I prepared single layer FeTe1-xSex films with different Se concentrations. A one-dimensional superconducting channel is observed on the edge of magnetically ordered single layer FeTe1-xSex films with Se concentration below 10%. This result suggests that the edge of the FeTe1-xSex (x < 0.1) film may help to destabilize the antiferromagnetic ordering and induce superconductivity. To identify the role of the interface, I prepared STO substrates with coexisting TiO2 and SrO terminations for the FeSe growth, and found that the superconducting gap of single layer FeSe grown on SrO is ~ 30% smaller than that on TiO2, confirming the critical role of the TiO2 termination in enhancing superconductivity. Those findings contribute significantly to the understanding of enhanced superconductivity in the FeSe/STO system, providing crucial insights into the design of interfacial control of superconductivity in the iron chalcogenide/oxide system.

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