  The motion of a vibrating object is determined by the way it is held. This
simple observation has long inspired string instrument makers to create new
sounds by devising elegant string clamping mechanisms, whereby the distance
between the clamping points is modulated as the string vibrates. At the
nanoscale, the simplest way to emulate this principle would be to controllably
make nanoresonators slide across their clamping points, which would effectively
modulate their vibrating length. Here, we report measurements of flexural
vibrations in nanomechanical resonators that reveal such a sliding motion.
Surprisingly, the resonant frequency of vibrations draws a loop as a tuning
gate voltage is cycled. This behavior indicates that sliding is accompanied by
a delayed frequency response of the resonators, making their dynamics richer
than that of resonators with fixed clamping points. Our work elucidates the
dynamics of nanomechanical resonators with unconventional boundary conditions,
and offers opportunities for studying friction at the nanoscale from resonant
frequency measurements.
