Nonreciprocal Spin Current Generation in Surface-Oxidized Copper Films

We experimentally demonstrate the nonreciprocal generation of spin current (${J}_{s}$) in a surface-oxidized copper film. The efficiency of conversion is at least 320 times larger than the inverse conversion. This nonreciprocity is due to a novel type of ${J}_{s}$ generation, which relies on the transfer of angular momentum from the velocity field of free electrons. A gradient in the electrical mobility in the film produces vorticity in the in-plane drift velocity of the free electrons. The inverse process can hardly occur when ${J}_{s}$ is collinear with the gradient in the electrical mobility.