In the past decade, there has emerged strong evidence to support spin-triplet supercon-ductivity in the layered-perovskite Sr2RuO4, whose ground state is thought to be analogous to the A-phase of 3He. It is believed that the spin and orbital degrees of freedom of the superconducting order parameter can give rise to states with remarkable properties, such as chiral domains and half-quantum vortices (HQV) that may obey non-Abelian statistics. With regards to the latter, recent theoretical work suggests that the HQV state could be made energetically favorable in me-soscopic samples [1]. In this talk, I will present a new method for ultrasensitive cantilever mag-netometry that allows us to probe the magnetic response of mesoscopic samples of Sr2RuO4. Us-ing this technique, we have detected the entry of individual vortices into micron-size rings of Sr2RuO4. Our most intriguing observation is the appearance of fractional fluxoid states that have half the magnetic moment of the full (integer) fluxoid. We find that the stability region of the fractional fluxoid state grows linearly with the magnitude of the in-plane magnetic field applied to the crystal. While the physical origin of the fractional state is yet unknown, I will present a recent theoretical proposal that predicts spontaneous spin polarization in the HQV state [2] which could explain the observed field dependence.
[1] S. B. Chung, H. Bluhm, and E. A. Kim, Phys. Rev. Lett. 99, 197002 (2007).
[2] V. Vakaryuk, and A. J. Leggett, Phys. Rev. Lett. 103, 057003 (2009).
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