  Noncentrosymmetric quantum materials can convert AC input current into DC
transverse current through the nonlinear Hall effect at zero magnetic field. We
analyze the AC-DC power conversion efficiency of such ``Hall rectifier'' and
suggest its application in wireless charging and energy harvesting. Our key
observation is that the development of Hall voltage results in a change of
longitudinal resistance, resulting in a violation of Ohm's law due to the
nonlinear Hall effect. This feedback mechanism balances the input power and the
output power and hence is crucial to understanding the power transfer from
source to load. We derive a general expression for the power conversion
efficiency in terms of material parameters, external load resistance, and input
power. As the Hall current is perpendicular to the electric field and does not
generate Joule heating by itself, we obtain high power conversion efficiency
when the Hall angle (which increases with the input power) is large and the
load resistance is optimized. Promising materials for high-efficiency Hall
rectifiers are also discussed.
