Finite-q photon-drag shift current in two-dimensional massive chiral Dirac fermions

Abstract

We investigate the photon-drag shift current in an isotropic single-valley two-dimensional massive chiral Dirac model with chirality index $J=1,2,3$ by directly evaluating the full finite-$q$ non-vertical response beyond the perturbative small-$q$ regime. Our central result is that chirality qualitatively reorganizes the sign topology of the finite-$q$ photocurrent $\mathbf{ j}(\mathbf{ q})$. For $J=1$, the photocurrent remains broadly positive, whereas higher-chirality sectors ($J \ge 2$) generically develop internal zero-current contours and sign reversals within the kinematically allowed region. We further show that the photocurrent is symmetry-constrained to be purely transverse, $\mathbf{j}(\mathbf{q}) \propto \hat{\mathbf{z}}\times\mathbf{q}$, and vanishes in the strictly vertical-transition limit $q=0$ in centrosymmetric systems. Pauli blocking further shapes the response by selecting the active portion of the resonance contour, while its extinction at large $\Delta$ or $q$ follows from a simple kinematic cutoff. These results establish the isotropic massive chiral Dirac problem as a symmetry-controlled benchmark for chirality-dependent finite-$q$ shift currents.