# A Nonequilibrium Variational Polaron Theory to Study Quantum Heat Transport

## TL;DR

This paper introduces a nonequilibrium variational polaron approach to accurately analyze quantum heat transport at the nanoscale, overcoming limitations of previous models and capturing complex effects like cross-bath correlations.

## Contribution

It develops a variational polaron framework for nonequilibrium steady states that handles infrared divergence and cross-bath effects, extending beyond super-Ohmic bath models.

## Key findings

- More accurate heat current calculations than NIBA for Ohmic baths
- Demonstration of cross-bath correlation effects
- Observation of current turnover and rectification phenomena

## Abstract

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.