Ground state and finite temperature signatures of quantum phase transitions in the half-filled Hubbard model on a honeycomb lattice

Abstract

We investigate ground state and finite temperature properties of the half-filled Hubbard model on a honeycomb lattice using quantum monte carlo and series expansion techniques. Unlike the square lattice, for which magnetic order exists at T=0 for any non-zero $U$, the honeycomb lattice is known to have a semi-metal phase at small $U$ and an antiferromagnetic one at large $U$. We investigate the phase transition at T=0 by studying the magnetic structure$andcompressibilityusingquantummontecarlosimulationsandbycalculatingthesublatticemagnetization,uniformsusceptibility,spin-waveandsinglehole$U_c/t$intherange4-5.Finitetemperaturesignaturesofthisphasetransitionareseeninthebehaviorofthespecificheat,$C(T)$,whichchangesfromatwo-peakedstructurefor$U>U_c$toaone-peakedstructurefor$U < U_c$.Furthermore,the$U$dependenceofthelowtemperaturecoefficientof$C(T)$exhibitsananomalyat$U \approx U_c$.

11 pages, 19 figures