A wafer-thin flake of bismuth telluride can act a little like a one-way street for electricity, even when the push comes from an alternating signal. But the direction of that “street” is not fixed. Moreover, if you warm the material up, the signal can flip. That temperature-triggered reversal sits at the center of a new study of the nonlinear Hall effect. This is an unusual quantum response that can turn alternating current into direct current without a magnetic field. The work was led by Professor Dongchen Qi at Queensland University of Technology’s School of Chemistry and Physics and Professor Xiao Renshaw Wang at Nanyang Technological University in Singapore. It traces the effect to a tug-of-war inside the material: tiny imperfections dominate at low temperatures. On the other hand, crystal vibrations take over closer to room temperature. Unlike a conventional rectifier that relies on diodes and other components, the nonlinear Hall effect can generate a DC output straight from an AC drive. That matters because many ambient energy sources, including wireless signals and other radio-frequency fields, …