Quantum-metric-induced plateau of nonlinear spin generation

The quantization of conductance is a fundamental concept in understanding topological phenomena in two-dimensional electronic systems. However, achieving quantization in the realm of nonlinear electric transport remains a significant challenge. In this study, we explore current-induced nonlinear spin generation using a quantum kinetic equation formulated within the framework of the nonequilibrium Green's function method. This approach provides a unified framework for analyzing both linear and nonlinear spin responses. By applying it to a tilted massive Dirac model, we reveal that the $z$ component of nonlinear spin generation exhibits plateaulike features within the band gap. Notably, this phenomenon is isotropic with respect to the applied field, independent of the tilt, and driven by the quantum metric rather than the Berry curvature. These results offer insights into quantization in nonlinear transport and highlight the pivotal role of quantum geometry.