A Molecular Beam Epitaxy System for Studying Unconventional Superconductivity by Scanning Tunneling Microscopy

Abstract

An ultra-high vacuum molecular beam epitaxy (MBE) system was designed and built to enable growth of thin films for low-temperature scanning tunneling microscopy (STM). This is one of the first combinations of a fully functional MBE system with an STM at the low temperature, high magnetic field frontier. The system features six Knudsen cell and electron beam evaporators, allows sample growth from 77-1500 K, and enables rapid transfer to STM instruments via an ultra-high vacuum suitcase. The MBE system enables new experiments traditionally impossible in STM, and it has already proven successful with a wide variety of systems. First, bilayers of nickel and bismuth were grown on MgO to investigate the reported coexistence of superconductivity and ferromagnetic order. The resulting films had a T\(_c\) \(\sim\) 3.5 K in transport and showed evidence of magnetism. Torque magnetometry, transmission electron microscope cross section, and STM results, however, found that the nickel layer was amorphous and that inter-layer mixing at the interface to form the superconducting alloy NiBi\(_3\) was favored. NiBi\(_3\) nanocrystals were also grown and studied. Growth of bismuth (111) and tellurium-doped bismuth on silicon (111) was developed with the goal of accessing quantum hall physics with a doping-tuned Fermi level. This could enable STM study of a fractional quantum hall state in the lowest Landau level. Growth of niobium (110) on \(\alpha\)-sapphire was developed for potential use in ferromagnetic atom manipulation experiments to explore 1D topological superconductivity and Majorana fermions. Finally, the MBE was used to prepare high quality samples of monolayer iron selenide (001) on strontium titanate, an interface-engineered high temperature superconductor with \(T_c\) above 50 K. This project has proven most interesting for immediate study and became the main focus of this thesis. FeSe growth procedures were optimized in a variable temperature STM that will be used for mapping the superconducting gap as a function of temperature. Samples were also studied in a dilution refrigerator STM at 2 K and 250 mK, reproducing existing results and demonstrating quasiparticle interference mapping so far. This STM will allow access to unexplored low temperature and high magnetic field physics. Future proximitization of thin film bismuth or bismuth telluride with monolayer FeSe in an MBE-grown heterostructure is proposed as an avenue to achieve 2D topological superconductivity.