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

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 j( q). For $J=1$, the photocurrent remains broadly positive, whereas higher-chirality sectors (J ≥ 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, j(q) ∝ ẑ, 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 $Δ$ 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.