Absence of Polar Kerr Signal in the Superconducting Phase of Bi3Ni Single Crystal*

Superconductivity and ferromagnetism are two competing ground states in condensed matter systems. So far, the coexistence of superconductivity and ferromagnetism has only been discovered in a few systems. Bi₃Ni is an intermetallic superconductor with critical temperature around 4.06K, coexistence of superconductivity and ferromagnetism has been reported in Bi₃Ni nanostructures and polycrystals. Using ultrasensitive loop-less fiber-optic Sagnac interferometer, we studied Polar Kerr effect on Bi₃Ni single crystal. Crystal was cooled down to 3.5K in zero field and then warmed up to 5.7K, measurement showed no significant change in Kerr signal around critical temperature during the warm up. To find an upper boundary of Kerr rotation of Bi₃Ni, we used pure silicon as reference, the difference in Kerr rotation between Bi₃Ni and pure silicon was smaller than 50 nanoradians during the warm up process, suggesting the Kerr rotation caused by Bi₃Ni was smaller than 50 nanoradians. Sample cooled in magnetic field of 100 Oe showed similar result, indicating there is no ferromagnetic order and time reversal symmetry breaking in Bi₃Ni single crystal in its superconducting phase.