Investigation of half-quantized fluxoid states in Sr2RuO4 mesoscopic superconducting rings

Spin-triplet superconductors can support exotic objects, such as chiral edge currents and half-quantum vortices (HQVs) characterized by the nontrivial winding of the spin structure. In this dissertation, we present cantilever magnetometry measurements performed on mesoscopic samples of Sr2RuO4, a spin-triplet superconductor. Satisfying the total anti-symmetric property of the Cooper pair wave function, Sr2RuO4 is theoretically suggested to have angular momentum and form domain structure with order parameter that corresponds to . For micron-size samples, only a few number of domains would exist and signatures of domain walls and edge currents are expected to be measurable with current sensitivity. From the measurements of fluctuations of magnetic signal and the signatures of vortex entries, we found no evidence to support broken time-reversal symmetry (TRS) in these crystals. We argue that various scenarios exist to explain the negative result while still assuming the TRS breaking chiral order parameter. Also, micron-size annular-shaped Sr2RuO4 crystals were used to observe transitions between fluxoid states. Our observation of half-integer transitions is consistent with the existence of HQVs in a spin-triplet superconductor. Stability of the half states with an in-plane magnetic field is explained by the spin polarization in consequence of a differential phase winding of up and down spin components. These spin and charge dynamics can also be revealed in the current response to phase winding across a weak-link junction. The junctions were fabricated within ring geometry. The phase is varied by the external magnetic field and the current is calculated by measuring the magnetic moments of the ring. The current response shows second harmonics when the in-plane magnetic field is applied, and the data are successfully fitted when Gibbs free energy is expressed with additional spin degree of freedom. Our observations are consistent with spin-triplet pairing of the Sr2RuO4, while requiring more investigations to confirm order parameter in the crystal.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2012

Author dc:creator

• Jang, Joonho

Contributors dc:contributor

• Budakian, Raffi
• Van Harlingen, Dale J.
• Fradkin, Eduardo H.
• King, William P.

• half-quantum vortex
• triplet supercondcutivity
• chiral order parameter

Statement dc:rights

• Copyright 2012 Joonho Jang

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/31026

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/31026