Magnetoresistance oscillations and the half-flux-quantum state in doubly connected superconducting cylinders of Sr2RuO4

Open Access
- Author:
- Cai, Xinxin
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 14, 2016
- Committee Members:
- Ying Liu, Dissertation Advisor/Co-Advisor
Ying Liu, Committee Chair/Co-Chair
Jainenedra Jain, Committee Member
Kin Fai Mak, Committee Member
Suzanne Mohney, Outside Member - Keywords:
- strontium ruthenate
superconductivity
half quantum vortices
transport propertices
magnetoresistance oscillations - Abstract:
- Sr2RuO4, the only layered perovskite known to become superconducting without the presence of Cu, was predicted to be an odd-parity, spin-triplet superconductor shortly after its superconductivity was discovered in 1994. Sr2RuO4 was found to feature exotic vortex physics including half-flux quanta trapped in doubly connected samples and the formation of vortex lattices at low fields. In this dissertation, I present low-temperature magnetoresistance oscillation measurements on micron-sized, doubly connected cylinders of Sr2RuO4 to explore the free energy modulation of the half-flux quantum state, inspired by the Little-Parks experiment, following a similar path taken in the original experiments for the establishment of fluoxid quantization. We fabricated transport devices of micron-sized superconducting cylinders of Sr2RuO4 on mechanically exfoliated single crystals. We carried out magnetoresistance oscillation measurements over a wide range of temperatures and magnetic fields for various samples. Magnetoresistance oscillations with an unexpected large amplitude were observed, suggesting a vortex-crossing origin of the magnetoresistance oscillations rather than a conventional Little-Parks effect. In thin-wall cylinders of Sr2RuO4, a large number of pronounced quantum oscillations with a conventional period of the full-flux-quantum were found. For cylinders with a thick wall, two distinct periods of oscillations were found in high- and low-field regimes, respectively, providing insight into the unconventional vortex physics in Sr2RuO4. No evidence for half-flux-quantum resistance oscillations were identified in any sample measured without the presence of an in-plane field. We demonstrated the tunability of the free energy of the superconducting Sr2RuO4 cylinders using various parameters, including an in-plane magnetic field, the measurement current, and structural factors. Distinct dips on magnetoresistance peaks were found, which we argue to be related to the emergence of the half-flux-quantum state, but only in cylinders possessing structural constrictions, providing insights into the stability of this complex topological object. This experiment not only supports the existence of half quantum vortices in this system, but also provides additional confirmation of the pairing mechanism of Sr2RuO4.