  Electronic heat transport in the normal state of a high-quality single
crystal of optimally-doped superconductor YBa2Cu3O6.95 was studied by
measurements of longitudinal and transverse transport coefficients. For the
temperature range from 100 to 300 K, the Hall-Lorenz number (Lxy) depends
weakly on temperature and is about two times larger than the Sommerfeld value
of the Lorenz number Lo = (pi^2)/3. Our results can be interpreted using a
Fermi liquid model when effects of the pseudogap that opens at the Fermi level
are included. However, we find that the bipolaron model can also explain both
the enhanced value and the weak temperature dependence of the Hall-Lorenz
number.
