Speaker
Description
Nonequilibrium dynamical mean-field theory (DMFT) is a powerful framework for studying the real-time dynamics of strongly correlated electron systems. In this approach, an interacting lattice model such as the Hubbard model is mapped onto a quantum impurity problem, but solving this impurity problem accurately remains the central numerical bottleneck. Continuous-time quantum Monte Carlo (CT-QMC) impurity solvers systematically sum high-order Feynman diagrams and have been highly successful in equilibrium. In nonequilibrium problems, however, they suffer from the dynamical sign problem, which has severely restricted numerically accurate simulations to limited settings.
To overcome this limitation, we implement a deterministic impurity solver based on the weak-coupling expansion and the tensor-train decomposition with tensor cross interpolation (TCI) [1,2]. The method approximates the high-dimensional integrands of Feynman diagrams in a tensor-train format and evaluates the integrals without stochastic sampling [3]. This makes the method suitable for regimes where Monte Carlo sampling is strongly affected by sign or phase cancellations.
In this talk, I will explain the formulation of our tensor-train impurity solver and discuss its application to nonequilibrium DMFT calculations for interaction quenches in the Hubbard model. I will particularly focus on thermalization dynamics away from half filling, where particle-hole symmetry is broken and previous CT-QMC-based studies have been strongly hindered by the sign problem. Using the tensor-train-based nonequilibrium DMFT approach, we investigate how the fast thermalization and dynamical transition known at half filling [4] are affected by doping.
References:
[1] S. Matsuura, H. Shinaoka, P. Werner, and N. Tsuji, Phys. Rev. B 111, 155150 (2025).
[2] S. Matsuura, H. Shinaoka, P. Werner, and N. Tsuji, arXiv:2607.00702 (2026).
[3] Y. Núñez Fernández et al., Phys. Rev. X 12, 041018 (2022).
[4] M. Eckstein, M. Kollar, and P. Werner, Phys. Rev. Lett. 103, 056403 (2009).