Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr₂RuO₄

Sr₂RuO₄ is expected to be a spin-triplet chiral p-wave superconductor, where the chirality means that the ground state is two-fold degenerate, with two different directions of the Cooper-pair orbital angular momentum. We fabricated Sr₂RuO₄ microrings and performed resistance (R) and critical-current (I_c) measurements in an axial magnetic field. In some such rings, Little-Parks magnetoresistance oscillations are observed close to the transition temperature T_c as recently reported [1]. In other rings, however, we find that I_c oscillates with a period corresponding to the fluxoid quantization down to temperatures far below T_c. This behavior resembles that of a superconducting quantum interference device (SQUID) and suggests that a pair of weak links of an intrinsic origin is spontaneously formed in the arms of the ring. Such weak links are most naturally attributable to domain walls separating domains with different chirality, which is also the outcome of order parameter calculations. We believe this to be strong new evidence for the chiral superconducting state in Sr₂RuO₄

[1] Y. Yasui, K. Lahabi, M. S. Anwar, Y. Nakamura, S. Yonezawa, T. Terashima, J. Aarts, and
Y. Maeno, Phys. Rev. B 96, 180507(R) (2017).