ROUGH AND VICINAL SURFACES OF HE-4 CRYSTALS - MOBILITY MEASUREMENTS

By studying the transmission coefficient of ultrasonic waves perpendic ular to the solid-liquid interface, we have measured the mobility of v icinal surfaces of He-4 crystrals for different crystallographic orien tations and in the temperature range from 0.38 K to 1 K. The results s how an angular dependency of the vicinal mobility as predicted by theo ry. Further, in comparison with rough interfaces, a spectacular decrea se for the mobility of vicinal surfaces due to the decreasing number o f moving sites is in good agreement with preceeding results for the ph onon term, It has been observed for the first time for the roton term.