  We study the level surfaces of quantum discord for a class of two-qubit
states with parallel nonzero Bloch vectors. The dynamic behavior of quantum
discord under decoherence is investigated. It is shown that a class of X states
has sudden transition between classical and quantum correlations under
decoherence. Our results include the ones in M. D. Lang and C. M. Caves [Phys.
Rev. Lett. 105, 150501 (2010)] as a special case and show new pictures and
structures of quantum discord.
