2026/07/22 by Johannes S. Hofmann, Patrick Ledwith, Ashvin Vishwanath +2
#cond-mat.str-el #cond-mat.mes-hall
We study the phase diagram of twisted bilayer graphene at charge neutrality as a function of twist angle θ, uniaxial heterostrain ε, and temperature T using sign-problem-free quantum Monte Carlo simulations. At T=0 and zero strain, we find a continuous transition from a Dirac semimetal to a gapped Kramers inter-valley coherent (KIVC) phase as θ decreases toward the magic angle. With finite strain, the KIVC phase undergoes a further continuous transition at smaller θ into an anisotropic semimetal with gapless excitations near the center of the moiré Brillouin zone. In the KIVC regime, the entropy rises sharply with temperature and plateaus at 15 K \lesssim T \lesssim 40 K near the value expected from a Mott-like regime of localized electrons with nearly uncorrelated spin, valley, and orbital degrees of freedom, despite the topological obstruction preventing a localized tight-binding description of the active bands. The spectral function evolves continuously with θ: at low T, a gap opens at the K points and the minimal gap shifts to Γ as θ decreases; at intermediate T, the spectral function smoothly interpolates between a Dirac semimetal spectrum with coherent K-point quasiparticles and a spectrum with gapless Γ-centered quasiparticles near the magic angle. The later can be a anisotropic or a Mott semimetal and we discuss how to distiguish them in experiment.