Strongly correlated electrons could display intriguing spontaneous broken
symmetries in the ground state. Understanding these symmetry breaking states is
fundamental to elucidate the various exotic quantum phases in condensed matter
physics. Here, we report an experimental observation of spontaneous rotational
symmetry breaking of the superconductivity at the interface of
YAlO3​/KTaO3​ (111) with a superconducting transition temperature of 1.86
K. Both the magnetoresistance and upper critical field in an in-plane field
manifest striking twofold symmetric oscillations deep inside the
superconducting state, whereas the anisotropy vanishes in the normal state,
demonstrating that it is an intrinsic property of the superconducting phase. We
attribute this behavior to the mixed-parity superconducting state, which is an
admixture of s-wave and p-wave pairing components induced by strong
spin-orbit coupling. Our work demonstrates an unconventional nature of the
pairing interaction in the KTaO3​ interface superconductor, and provides a
new platform to clarify a delicate interplay of electron correlation and
spin-orbit coupling.Comment: 7 pages, 4 figure