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

TL;DR

This paper studies the photon-drag shift current in 2D massive chiral Dirac fermions, revealing how chirality influences the sign and topology of the photocurrent beyond small momentum approximations.

Contribution

It provides a full finite-$q$ response analysis for different chirality indices, showing how chirality reorganizes photocurrent sign topology and symmetry constraints.

Findings

Photocurrent sign and topology depend on chirality index J.

Photocurrent is purely transverse and vanishes at q=0 in centrosymmetric systems.

Pauli blocking and kinematic cutoff shape the active resonance regions.

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.