  We introduce a two-dimensional walk model in which a random walker can only
move on the first quarter of a two-dimensional plane. We calculate the
partition function of this walk model using a transfer matrix method and show
that the model undergoes a phase-transition. Surprisingly the partition
function of this two-dimensional walk model is exactly equal to that of a
driven-diffusive system defined on a discrete lattice with periodic boundary
conditions in which a phase transition occurs from a high-density to a
low-density phase. The driven-diffusive system can be mapped to a zero-range
process where the particles can accumulate in a single lattice site in the
low-density phase. This is very reminiscent of real-space Bose-Einstein
condensation.
