Superconducting proximity effect in topological insulator thin films and graphene
Restricted (Penn State Only)
- Author:
- Li, Cequn
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 20, 2024
- Committee Members:
- Chaoxing Liu, Major Field Member
Jun Zhu, Chair & Dissertation Advisor
Joshua Robinson, Outside Unit & Field Member
Cuizu Chang, Major Field Member
Mauricio Terrones, Program Head/Chair - Keywords:
- Topological insulator
graphene
topological superconductivity
Josephson junction
Superconductivity
Proximity effect
2D materials
tunneling spectroscopy
van der Waals heterostructures - Abstract:
- The superconducting proximity effect in topological materials is of great interest to the exploration of topological superconductivity and Majorana zero modes, which may support topological quantum computing. In topological insulator (TI)/superconductor (SC) heterostructures, the introduction of superconductivity to the Dirac surface states may lead to a topological superconductor. In this thesis, we studied the superconductivity and proximity-induced superconductivity in TI thin films and graphene by building tunnel junctions, vertical and lateral Josephson junctions. A new method to fabricate lithography-free, van der Waals-like interfaces between a 3D superconductor and graphene was developed. The first part of the thesis focuses on proximity-induced superconductivity in epitaxial (Bi,Sb)2Te3 (BST)/graphene (Gr)/2L-Ga heterostructures. We developed a clean, lithography-free van der Waals tunnel junction and performed transport tunneling spectroscopy measurements. We observed two superconducting gaps; One gap agrees with the SC gap of the 2L-Ga; The second gap is attributed to the proximity-induced superconductivity in the Dirac surface states of the BST film. The induced gap is approximately 0.2 meV, which is ~40% of the SC gap in 2L-Ga. In addition, we observed discrete tunneling conductance jumps corresponding to the addition of a single vortex under a magnetic field. Our work establishes a large-area, potentially scalable TI/SC platform for the studies of topological superconductivity. Further studies combining tunneling spectroscopy measurements and Josephson junction experiments are needed to explore topological phase transitions and Majorana zero modes. Experiments extended to magnetic TI/SC heterostructures are described in Chapter 3, where we investigated the interplay between superconductivity and ferromagnetism in Cr-doped Sb2Te3 (CST)/FeTe thin films. Neither CST nor FeTe superconducts on their own, but the combination is found to superconduct with a Tc of ~12 K. Below a Curie temperature of ~20 K, ferromagnetism develops in the CST film. We constructed NbSe2/CST/FeTe SC-ferromagnet-SC vertical junctions of different CST film thicknesses to investigate the possibility of 0-π transition. In some devices, the tunneling spectra reveal signatures of the superconducting gaps in NbSe2 (𝛥=1.1 meV) and the interfacial superconductivity (𝛥 ~2.4 meV). Our experiments provide a direct probe of the superconductivity at the interface of CST/FeTe. However, no supercurrent is observed in our vertical junctions. Continued efforts to improve the interface transparency are needed to explore the 0-π transition and the proximity-induced superconductivity in the surface states of CST/FeTe for chiral topological superconductivity. In the last chapter, we present our work in developing superconducting via contacts that allow us to make Josephson junctions using 3D superconductors but incorporate van der Waals transfer techniques. We describe the process to fabricate NbN/Pd via contacts and discuss some of the challenges. We first used this approach to construct clean, lithography-free NbN/Pd-graphene Josephson junctions. We observed a gate-tunable supercurrent that reaches ~110 nA and the signature of a small proximity-induced gap in graphene. However, the IcRn product is much reduced compared to the gap of NbN/Pd. One potential reason is the low interface transparency between NbN/Pd and graphene, which needs to be further optimized. Possible limitations to the interface quality are analyzed. Our approach may be generalized to other van der Waals materials and opens up opportunities for studying novel superconducting heterostructures.