Thermoelectric Origin of Giant Nonreciprocal Charge Transport in NbAs Nanobelts

Abstract

Recently, nonreciprocal charge transport has emerged as an important experimental tool for detecting symmetry breaking in noncentrosymmetric crystals. Despite extensive studies on systems with intrinsic symmetry breaking from crystal structures or interfaces, nonreciprocal transport induced by extrinsic mechanisms, such as the asymmetric design of device geometry, remains largely unexplored. Here, we show that a giant nonreciprocal resistance arises extrinsically from the thermoelectric effect due to nonuniform current heating in mesoscopic devices. Taking $\mathrm{Nb}\mathrm{As}$ nanobelts as an example, we observe giant second-harmonic resistances induced by the Seebeck and Nernst effects, with a nonreciprocal coefficient value up to 5.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{A}$, which is higher than that of most noncentrosymmetric crystals. The symmetry of second-harmonic resistance in magnetic fields can be further controlled by designing the electrode geometry and is decoupled from that of the first-harmonic resistance or the crystal symmetry. We show that such thermoelectric related nonreciprocal resistances also exist in macroscopic samples up to hundreds of microns. Our results not only introduce a simple yet effective approach to probe the thermoelectric effect, but also reveal a previously underestimated mechanism that strongly modifies the second-order transport coefficients.