Observation of Half-Quantum Flux in Unconventional Superconductor $\beta$-Bi$_2$Pd

We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting $\beta$-Bi$_2$Pd thin films. The half-quantum fluxoid manifests itself as a $\pi$ phase shift in the quantum oscillation of the critical temperature. This result verifies unconventional superconductivity of $\beta$-Bi$_2$Pd, in accord with the expectation of a topological superconductor. We also discuss the strong indication that $\beta$-Bi$_2$Pd is a spin-triplet superconductor.