  We investigate the current-phase relations of Al/InAs-quantum well planar
Josephson junctions fabricated using nanowire shadowing technique. Based on
several experiments, we conclude that the junctions exhibit an unusually large
second-order Josephson harmonic, the $\sin(2\varphi)$ term. First,
superconducting quantum interference devices (dc-SQUIDs) show half-periodic
oscillations, tunable by gate voltages as well as magnetic flux. Second,
Josephson junction devices exhibit kinks near half-flux quantum in supercurrent
diffraction patterns. Third, half-integer Shapiro steps are present in the
junctions. Similar phenomena are observed in Sn/InAs quantum well devices. We
perform data fitting to a numerical model with a two-component current phase
relation. Analysis including a loop inductance suggests that the sign of the
second harmonic term is negative. The microscopic origins of the observed
effect remain to be understood. We consider alternative explanations which can
account for some but not all of the evidence.
