Fractional Little-Parks effect observed in a topological superconductor

Abstract

In superconductors, the condensation of Cooper pairs gives rise to fluxoid quantization in discrete units of ${\Phi }_0=hc/2e$. The denominator of $2e$ is the signature of electron pairing, which is evidenced by a number of macroscopic quantum phenomena, such as the Little-Parks effect and the Josephson effect, where the critical temperature or the critical current oscillates in the period of ${\Phi }_0$. Here we report the observation of fractional Little-Parks effect in mesoscopic rings of epitaxial $\beta$-Bi${}_2$Pd, a topological superconductor. Besides ${\Phi }_0$, novel Little-Parks oscillation periodicities of $2{\Phi }_0$, $3{\Phi }_0$ and $4{\Phi }_0$ are also observed, implying quasiparticles with effective charges being a fraction of a Cooper pair. We show that the fractional Little-Parks effect may be closely related to the fractional Josephson effect, which is a key signature of chiral Majorana edge states.

30 pages total; 16 pages and 4 figures for the main text; 14 pages and 13 figures for the supplementary information