  Excited electrons in the conduction band of germanium collect into four
energy minima, or valleys, in momentum space. These local minima have highly
anisotropic mass tensors which cause the electrons to travel in directions
which are oblique to an applied electric field at sub-Kelvin temperatures and
low electric fields, in contrast to the more isotropic behavior of the holes.
This experiment produces, for the first time, a full two-dimensional image of
the oblique electron and hole propagation and the quantum transitions of
electrons between valleys for electric fields oriented along the [0,0,1]
direction. Charge carriers are excited with a focused laser pulse on one face
of a germanium crystal and then drifted through the crystal by a uniform
electric field of strength between 0.5 and 6 V/cm. The pattern of charge
density arriving on the opposite face is used to reconstruct the trajectories
of the carriers. Measurements of the two-dimensional pattern of charge density
are compared in detail with Monte Carlo simulations developed for the Cryogenic
Dark Matter Search (CDMS) to model the transport of charge carriers in
high-purity germanium detectors.
