(2:45~3:15pm, Tea, Coffee, and Cookie)

I shall describe in this talk the difficulties we face in constructing low energy theories for spin liquid states and shall explain our recent work on a Landau Fermi-type effective low energy theory for S=1/2 systems.

Some experimental predictions of our theory and its relation to slave-rotor type theories will be discussed. The limitation of the theory and some plausible directions of extensions of the theory will be discussed, especially in relation to S=1 systems.