  We report on a study of thermal Hall conductivity k_xy in the superconducting
state of CeCoIn_5. The scaling relation and the density of states of the
delocalized quasiparticles, both obtained from k_xy, are consistent with d-wave
superconducting symmetry. The onset of superconductivity is accompanied by a
steep increase in the thermal Hall angle, pointing to a striking enhancement in
the quasiparticle mean free path. This enhancement is drastically suppressed in
a very weak magnetic field. These results highlight that CeCoIn_5 is unique
among superconductors. A small Fermi energy, a large superconducting gap, a
short coherence length, and a long mean free path all indicate that CeCoIn_5 is
clearly in the superclean regime (E_F/Delta<<l/xi), in which peculiar vortex
state is expected.
