  In the presence of an external magnetic field Cooper pairs in
noncentrosymmetric superconductors can acquire finite momentum. Recent theory
predicts that such finite-momentum pairing can lead to an asymmetric critical
current, where a dissipationless supercurrent can flow along one direction but
not the opposite. However, to date this has not been observed. Here we report
the discovery of a giant Josephson diode effect (JDE) in Josephson junctions
formed from a type II Dirac semimetal, NiTe2. A distinguishing feature is that
the asymmetry in the critical current depends sensitively on the magnitude and
direction of an applied magnetic field and achieves its maximum value of ~60%
when the magnetic field is perpendicular to the current and is of the order of
just 10 mT. Moreover the asymmetry changes sign several times with increasing
field. These characteristic features are accounted for in a theoretical model
based on finite-momentum Cooper pairing derived from spin-helical topological
surface states, in an otherwise centrosymmetric system. The finite pairing
momentum is further established, and its value determined, from the evolution
of the interference pattern under an in-plane magnetic field. The observed
giant magnitude of the asymmetry in critical current and the clear exposition
of its underlying mechanism paves the way to building novel superconducting
computing devices using the Josephson diode effect.
