  Spin-charge conversion via spin-orbit interaction is one of the core concepts
in the current spintronics research. The efficiency of the interconversion
between charge and spin current is estimated based on Berry curvature of Bloch
wavefunction in the linear-response regime. Beyond the linear regime, nonlinear
spin-charge conversion in the higher-order electric field terms has recently
been demonstrated in noncentrosymmetric materials with nontrivial spin texture
in the momentum space. Here we report the observation of the nonlinear
charge-spin conversion in a nominally centrosymmetric oxide material, SrIrO3,
by breaking inversion symmetry at the interface. A large second-order
magnetoelectric coefficient is observed at room temperature because of the
antisymmetric spin-orbit interaction at the interface of Dirac semimetallic
bands, which is subject to the symmetry constraint of the substrates. Our study
suggests that nonlinear spin-charge conversion can be induced in many materials
with strong spin-orbit interaction at the interface by breaking the local
inversion symmetry to give rise to spin splitting in otherwise spin degenerate
systems.
