Within the tensor network framework, the (positive) thermal density operator can be approximated by a double layer of infinite Projected Entangled Pair Operator (iPEPO) tonic shower cap coupled via ancilla degrees of freedom.To investigate the thermal properties of the spin-1/2 Heisenberg model on the square lattice, we introduce a family of fully spin-$SU(2)$ and lattice-$C_{4v}$ symmetric on-site tensors (of bond dimensions $D=4$ or $D=7$) and a plaquette-based Trotter-Suzuki decomposition of the imaginary-time evolution operator.A variational optimization is performed on the plaquettes, using a full (for $D=4$) or simple (for $D=7$) environment obtained from the single-site Corner Transfer Matrix Renormalization Group fixed point.The method is benchmarked by a comparison to quantum Monte Carlo in the thermodynamic limit.
Although the iPEPO spin correlation length starts to deviate from the exact exponential growth for inverse-temperature $eta gtrsim 2$, the behavior of various observables chiggate.com turns out to be quite accurate once plotted w.r.t the inverse correlation length.We also find that a direct $T=0$ variational energy optimization provides results in full agreement with the $eta
ightarrowinfty$ limit of finite-temperature data, hence validating the imaginary-time evolution procedure.
Extension of the method to frustrated models is described and preliminary results are shown.