  Since 1979, many new classes of superconductors have been discovered,
including heavy-fermion compounds, organic conductors, high-Tc cuprates, and
Sr2RuO4. Most of these superconductors are unconventional and/or nodal.
Therefore it is of central importance to determine the symmetry of the order
parameter in each of these superconductors. In particular, the
angular-controlled thermal conductivity in the vortex state provides a unique
means of investigating the nodal structure of the superconducting energy gap
when high-quality single crystals in the extremely clean limit are available.
Using this method, Izawa et al have recently succeeded in identifying the
energy gap symmetry of superconductivity in Sr2RuO4, CeCoIn5,
kappa-(ET)2Cu(NCS)2, YNi2B2C, and PrOs4Sb12.
