Crossover from symplectic to orthogonal class in a two-dimensional honeycomb lattice.

We have calculated the weak-localization correction to the conductivity for disordered electrons in a two-dimensional honeycomb lattice and shown that it can be either positive or negative depending on the interaction range of impurity potentials. From symmetry considerations, the symplectic class turns out to be realized at nonzero temperatures and crossover to the orthogonal class is predicted with decreasing temperature.