  The Hall effect occurs only in systems with broken time-reversal symmetry,
such as solids under an external magnetic field in the ordinary Hall effect and
magnetic materials in the anomalous Hall effect (AHE). Here we show a new Hall
effect in a nonmagnetic material under zero magnetic field, in which the Hall
voltage depends quadratically on the longitudinal current. We observe the
effect (referred to as nonlinear AHE) in two-dimensional Td-WTe2, a semimetal
with broken inversion symmetry and only one mirror line in the crystal plane.
Our angle-resolved electrical measurements reveal that the Hall voltage changes
sign when the bias current reverses direction; it maximizes (vanishes) when the
bias current is perpendicular (parallel) to the mirror line. The observed
effect can be understood as an AHE induced by the bias current which generates
an out-of-plane magnetization. The temperature dependence of the Hall
conductivity further suggests that both intrinsic Berry curvature dipole and
extrinsic spin-dependent scatterings contribute to the observed nonlinear AHE.
Our results open the possibility of exploring the intrinsic Berry curvature
effect in nonlinear electrical transport in solids .
