  Dynamical structural defects exist naturally in a wide variety of solids.
They fluctuate temporally, and hence can deteriorate the performance of many
electronic devices. Thus far, the entities of such dynamic objects have been
identified to be individual atoms. On the other hand, it is a long-standing
question whether a nanocrystalline grain constituted of a large number of atoms
can switch, as a whole, reversibly like a dynamical atomic defect (i.e., a
two-level system). This is an emergent issue considering the current
development of nanodevices with ultralow electrical noise, qubits with long
quantum coherence time, and nanoelectromechanical system (NEMS) sensors with
ultrahigh resolution. Here we demonstrate experimental observations of dynamic
nanocrystalline grains which repeatedly switch between two or more metastable
coordinate states. We study temporal resistance fluctuations in thin ruthenium
dioxide (RuO2) metal nanowires and extract microscopic parameters including
relaxation time scales, mobile grain sizes, and the bonding strengths of
nanograin boundaries. Such material parameters are not obtainable by other
experimental approaches. When combined with previous in-situ high-resolution
transmission electron microscopy (HRTEM), our electrical method can be used to
infer rich information about the structural dynamics of a wide variety of
nanodevices and new 2D materials.

Mesoscale and Nanoscale Physics