  Electron interactions in and between wires become increasingly complex and
important as circuits are scaled to nanometre sizes, or employ
reduced-dimensional conductors like carbon nanotubes, nanowires and gated high
mobility 2D electron systems. This is because the screening of the long-range
Coulomb potential of individual carriers is weakened in these systems, which
can lead to phenomenon such as Coulomb drag: a current in one wire induces a
voltage in a second wire through Coulomb interactions alone. Previous
experiments have observed electron drag in wires separated by a soft
electrostatic barrier $\gtrsim$ 80 nm. Here, we measure both positive and
negative drag between adjacent vertical quantum wires that are separated by
$\sim$ 15 nm and have independent contacts, which allows their electron
densities to be tuned independently. We map out the drag signal versus the
number of electron subbands occupied in each wire, and interpret the results in
terms of momentum-transfer and charge-fluctuation induced transport models. For
wires of significantly different subband occupancies, the positive drag effect
can be as large as 25%.
