Direct Detection of Pure ac Spin Current by X-Ray Pump-Probe Measurements

We acknowledge helpful discussion with Arne Brataas. Financial support from National Science Foundation DMR-1504568 NRF through the Global Research Laboratory project of Korea is gratefully acknowledged. The Advanced Light Source is supported by the US Department of Energy under contract number DE-AC02-05CH11231. JD acknowledges fellowship support from the China Scholarship Council and National Science Foundation of China under Grant Nos. 51331006. LRS, PSK, and RJH acknowledge the support of the Engineering and Physical Sciences Research Council (EPSRC) through grants EP/J018767/1 and EP/I038470/1. GvdL acknowledges support of the EPSRC through grant EP/J018767/1.