  The shift current responses of two-dimensional systems in the gapless limit
are investigated. As the energy gap becomes smaller, the interband transition
probability becomes larger at the band edges. We found the divergence of the
shift current response in a manner proportional to the inverse of the gap size
when the system becomes a two-dimensional Weyl semimetal in the gapless limit.
This behavior is different from that for electric polarization, which has a
jump across the gapless case. It means that the quantitative relationship
between electric polarizations and shift currents is broken in this gapless
limit.
