%AZhang, Yichao%AFlannigan, David%BJournal Name: Nano Letters; Journal Volume: 21; Journal Issue: 17 %D2021%I %JJournal Name: Nano Letters; Journal Volume: 21; Journal Issue: 17 %K %MOSTI ID: 10323986 %PMedium: X %TImaging Nanometer Phonon Softening at Crystal Surface Steps with 4D Ultrafast Electron Microscopy %X %0Journal Article