  Hydrogen intercalation in solids is common, complicated, and very difficult
to monitor. In a new approach to the problem, we have studied the profile of
hydrogen diffusion in single-crystal nanobeams and plates of VO2, exploiting
the fact that hydrogen doping in this material leads to visible darkening near
room temperature connected with the metal-insulator transition at 65 {\deg}C.
We observe hydrogen diffusion along the rutile c-axis but not perpendicular to
it, making this a highly one-dimensional diffusion system. We obtain an
activated diffusion coefficient, ~0.01 e^(-0.6 eV/k_B T) cm2sec-1, applicable
in metallic phase. In addition, we observe dramatic supercooling of the
hydrogen-induced metallic phase and spontaneous segregation of the hydrogen
into stripes implying that the diffusion process is highly nonlinear, even in
the absence of defects. Similar complications may occur in hydrogen motion in
other materials but are not revealed by conventional measurement techniques.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics