Intrinsic nonlinear valley Nernst effect

Abstract: We investigate the intrinsic nonlinear valley Nernst effect, which induces a transverse valley current via a second-order thermoelectric response to a longitudinal temperature gradient. The effect arises from the Berry connection polarizability dipole of valley electrons and is permissible in both inversion-symmetric and inversion-asymmetric materials. We demonstrate that the response tensor is connected to the intrinsic nonlinear valley Hall conductivity through a generalized Mott relation, with the two being directly proportional at low temperatures, scaled by the Lorenz number. We elucidate the symmetry constraints governing this effect and develop a theory for its nonlocal measurement, revealing a nonlocal second-harmonic signal with a distinct $\rho^2$ scaling. This signal comprises two scaling terms, with their ratio corresponding to the square of the thermopower normalized by the Lorenz number. Key characteristics are demonstrated using a tilted Dirac model and first-principles calculations on bilayer WTe$_2$. Possible extrinsic contributions and alternative experimental detection methods, e.g., by valley pumping and by nonreciprocal directional dichroism, are discussed. These findings underscore the significance of band quantum geometry on electron dynamics and establish a theoretical foundation for nonlinear valley caloritronics.