Charge puddles complicate nonlinear transport in large Berry phase materials

Nonlinear transport in narrow band-gap materials has recently attracted significant attention, being commonly recognized as a useful tool to study a variety of complex physical effects related to Berry phase. Here we demonstrate that a mundane, commonly overlooked mechanism stemming from tunneling between charge puddles, can drive substantial nonlinearities in these narrow band gap materials. As a test bed, we use an epitaxial topological HgTe layer, underscoring that even good crystalline quality materials suffer from this issue. Indeed, we show that signatures of electrical transport through a network of charge puddles are not limited to the planar magnetoresistance, but also dominate transport. Our findings highlight the need for utmost caution when interpreting nonlinear electrical transport phenomena, as harmonic distortion of the electrical signal, caused by charge puddles, is unavoidable in any realistic narrow-gap system, complicating the identification of other proposed effects related to the Berry phase.