  We propose that an excitonic gap can be generated along nodal directions by
Coulomb interaction in the mixed state of d-wave cuprate superconductors. In a
superconductor, the Coulomb interaction usually can not generate any fermion
gap since its strength is weakened by superfluidity. It becomes stronger as
superfluid density is suppressed by external magnetic field, and is able to
generate a gap for initially gapless nodal quasiparticles beyond some critical
field $H_{c}$. By solving the gap equation, it is found that the nodal gap
increases with growing field $H$, which leads to a suppression of thermal
conductivity at zero temperature. This mechanism naturally produces the
field-induced thermal metal-insulator transition observed in transport
experiments.
