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Tunneling spectroscopy on an atomically thin transition metal dichalcogenide superconductor

Author
Sohn, Egon
Published
[University Park, Pennsylvania] : Pennsylvania State University, 2020.
Physical Description
1 electronic document
Additional Creators
Chang, Cuizu
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Open Access.
Summary
Two dimensional (2D) materials have become very popular in the last decade. Among the many available 2D materials, 2H-NbSe2, which is one compound of the transition metal dichalcogenide (TMD) family, represents the 2D superconductors. The TMDs are unique in a sense that when the bulk is thinned down to a monolayer limit, inversion symmetry is broken. Together with the strong spin-orbit coupling, spin degeneracy is broken in each band and leads to electron spins pinned in out-of-plane direction, which are opposite in direction for opposite momentum. These are called Ising spins. Unlike the conventional superconductors which freely rotating spins pair, the Cooper pairs in 2D-NbSe2 are formed by Ising spins. This results in unconventional superconducting properties. We take advantage of the unique aspect of atomically thin superconductors which is the quenched orbital depairing effect under in-plane magnetic field and study the spin properties of the superconducting state. In the first part of this dissertation (Chapter 3), we study the superconducting properties of 2D-NbSe2 under large in-plane magnetic field. We characterize the magnetic field versus temperature phase diagram of one- to three-layer NbSe2 by electrical transport measurement and find an enhanced upper critical field, several times larger than the Pauli paramagnetic limit. By fitting the results to the pair breaking equation, we confirm large spin-orbit field in 2D-NbSe2. We fabricate planer tunneling junctions and measure differential conductance spectra under in-plane magnetic field. The superconducting gap is obtained by fitting the data with the Blonder-Tinkham-Klapwijk (BTK) model. We find that there is a second-order transition in contrast to first-order transition observed in conventional superconductors. The enhanced upper critical field and the second-order phase transition both suggests finite spin susceptibility in the superconducting state which originates from the mixed spin singlet-triplet Cooper pair wave function present in noncentrosymmetric superconductors. In the second part (Chapter 4), we study the in-plane magnetic field response of the two superconducting gaps of 2D-NbSe2 and show there is a possible field driven nodal phase transition in the [gamma]-pocket superconducting order parameter. This is achieved by fabricating high-quality tunneling devices in which the differential conductance spectrum reflects the density of states. We were able to fit the data with a two-gap Bardeen-Cooper-Schrieffer density of states and identify the two superconducting gaps in 2D-NbSe2. The distinct behavior of the two superconducting gaps are explained in terms of the different spin-orbit coupling strength of the K- and [gamma]-pocket, which is also consistent with earlier theoretical studies that suggest a magnetic field driven nodal superconducting phase. We also discuss limitations in our measurement that prevents us from discerning between a nodal and a fully gapped superconducting phase in high magnetic field. In the last part (Chapter 5), we construct tunneling devices with either a magnetic barrier or a magnetic material and observe a novel two-fold symmetry in the differential conductance spectra. We fit the measured spectra with a two-gap BTK model and find that the weight of the two gaps and the superconducting order parameter depends on in-plane magnetic field direction and field strength. Our observation is explained in terms of competing superconducting channels which stabilizes by small symmetry breaking fields such as strain or external magnetic field.
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Dissertation Note
Ph.D. Pennsylvania State University 2020.
Technical Details
The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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