  We investigate nematic quantum phase transitions in two different Dirac
fermion models. The models feature twofold and fourfold, respectively, lattice
rotational symmetries that are spontaneously broken in the ordered phase. Using
negative-sign-free quantum Monte Carlo simulations and an $\epsilon$-expansion
renormalization group analysis, we show that both models exhibit continuous
phase transitions. In contrast to generic Gross-Neveu dynamical mass
generation, the quantum critical regime is characterized by large velocity
anisotropies, with fixed-point values being approached very slowly. Hence both
experimental and numerical investigations will not be representative of the
infrared fixed point, but of a crossover regime characterized by drifting
exponents.
