▎ 摘　　要

We present results from lattice simulations of a monolayer graphene model at nonzero temperature. At low temperatures for sufficiently strong coupling the model develops an excitonic condensate of particle-hole pairs corresponding to an insulating phase. The Berezinskii-Kosterlitz-Thouless phase transition temperature is associated with the value of the coupling where the critical exponent delta governing the response of the order parameter at criticality to an external source has a value close to 15. The critical coupling on a lattice with temporal extent N-t = 32 [T = 1/(N(t)a(t)) where a(t) is the temporal lattice spacing] and spatial extent N-s = 64 is very close to infinite coupling. The value of the transition temperature normalized with the zero-temperature fermion mass gap Delta(0) is given by T-BKT/Delta(0) = 0.055(2). This value provides an upper bound on the transition temperature, because simulations closer to the continuum limit where the full U(4) symmetry is restored may result in an even lower (v)alue. In addition, we measured the helicity modulus gamma and the fermion thermal mass Delta(T)(T), the latter providing evidence for a pseudogap phase with Delta T > 0 extendi(n)g to arbitrarily high T. Analysis of the dispersion relation suggests that the Fermi velocity is not sensitive to thermal effects.