  We propose that the recently observed violation of the Wiedemann-Franz law in
the normal state of underdoped cuprates is caused by spin-charge separation and
dynamical chiral symmetry breaking in a (2+1)-dimensional system consisting of
massless Dirac fermions, charged bosons and a gauge field. While the d-wave
spinon gap vanishes at the Fermi points, the nodal fermions acquire a finite
mass due to strong gauge fluctuations. This mass provides a gap below which no
free fermions can be excited. This implies that there is not a residual linear
term for the thermal conductivity, in good agreement with experiments. Other
physical implications of the CSB are also discussed.
