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Cumulants of heat transfer across nonlinear quantum systems

We consider thermal conduction across a general nonlinear phononic junction. Based on two-time observation protocol and the field theoretical/algebraic method, the cumulants of the heat transferred in both transient and steady-state regimes are studied on an equal footing, and practical formulae for the calculation of the cumulant generating function of heat transfer are obtained. As an application, the developed general formalism is used to study anharmonic effects on fluctuation of steady-state heat transfer across a single-site junction with a quartic nonlinear on-site pinning potential. An explicit nonlinear modification to cumulant generating function exact up to the first order is given, in which Gallavotti-Cohen fluctuation symmetry is verified. Numerically a self-consistent procedure is introduced, which works well for strong nonlinearity.