  We propose a nonequilibrium variational polaron transformation, based on an
ansatz for nonequilibrium steady state (NESS) with an effective temperature, to
study quantum heat transport at the nanoscale. By combining the variational
polaron transformed master equation with the full counting statistics, we have
extended the applicability of the polaron-based framework to study
nonequilibrium process beyond the super-Ohmic bath models. Previously, the
polaron-based framework for quantum heat transport reduces exactly to the
non-interacting blip approximation (NIBA) formalism for Ohmic bath models due
to the issue of the infrared divergence associated with the full polaron
transformation. The nonequilibrium variational method allows us to
appropriately treat the infrared divergence in the low-frequency bath modes and
explicitly include cross-bath correlation effects. These improvements provide
more accurate calculation of heat current than the NIBA formalism for Ohmic
bath models. We illustrate the aforementioned improvements with the
nonequilibrium spin-boson model in this work and quantitatively demonstrate the
cross-bath correlation, current turnover, and rectification effects in quantum
heat transfer.
