  Stimulated by the recent experiment [F. Ando et al., Nature 584, 373 (2020)],
we propose an intrinsic mechanism to cause the superconducting diode effect
(SDE). SDE refers to the nonreciprocity of the critical current for the
metal-superconductor transition. Among various mechanisms for the critical
current, the depairing current is known to be intrinsic to each material and
has recently been observed in several superconducting systems. We clarify the
temperature scaling of the nonreciprocal depairing current near the critical
temperature and point out its significant enhancement at low temperatures. It
is also found that the nonreciprocal critical current shows sign reversals upon
increasing the magnetic field. These behaviors are understood by the
nonreciprocity of the Landau critical momentum and the change in the nature of
the helical superconductivity. The intrinsic SDE unveils the rich phase diagram
and functionalities of noncentrosymmetric superconductors.
