Observation of half-quantum flux in the unconventional superconductor β-Bi 2 Pd

Unconventional oscillations At sufficiently low temperatures, superconductors expel an applied magnetic field. However, if the topology of the superconductor is nontrivial—for example, if there is a hole in the sample—there can be a nonzero magnetic flux inside the hole. This flux can only take certain discrete values, and the superconducting critical temperature has maxima at the corresponding values of the magnetic field. Li et al. studied these so-called Little-Parks oscillations in superconducting rings made out of polycrystalline thin films of β-Bi 2 Pd. They found that the phase of the oscillations was shifted by π compared with oscillations observed in most superconductors, as predicted for certain unconventional pairing symmetries. Science , this issue p. 238