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arXiv · 2608.09883

Anomalous temperature dependence of polaron mobility in a nonlinear double-well potential: unbiased X-propagator approach

Stefano Ragni·Osor S. Barišić·Naoto Nagaosa·Andrey S. Mishchenko
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Abstract

We develop an unbiased X-propagator method for calculating finite-temperature optical conductivity $\sigma(\omega)$ and dc mobility $\mu$ for arbitrary nonlinear electron-phonon interaction. We apply it to a polaron coupled to a double-well lattice potential, a minimal model for strongly anharmonic polar materials. At moderate coupling, the mobility exhibits three temperature regimes associated with confinement within one well, thermal competition with the barrier, and barrier-insensitive high-temperature dynamics. This sequence produces a concave temperature dependence of the mobility that is absent in conventional linear-coupling polaron models. At strong coupling, the mobility becomes nonmonotonic, and its temperature evolution is reflected in a characteristic redistribution of optical spectral weight. For parameters relevant to SrTiO$_3$, our results qualitatively reproduce both the anomalous concave mobility and the onset of violation of the Mott-Ioffe-Regel limit, thereby supporting nonlinear coupling to a soft anharmonic lattice mode as a microscopic mechanism for anomalous transport in dilute polar metals.

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Stefano Ragni, Osor S. Barišić, Naoto Nagaosa, Andrey S. Mishchenko. 2026-08-10. Anomalous temperature dependence of polaron mobility in a nonlinear double-well potential: unbiased X-propagator approach. https://arxiv.org/abs/2608.09883

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