Resonating valence bonds

In this paper the authors briefly review the recent efforts to understand the novel resonating valence bond (RVB) state for the S=1/2 antiferromagnetic (AFM) Heisenberg model. The authors propose a pair hopping mechanism which generates a new form of AFM superexchange, and argue that the RVB wavefunction is successful because it takes full advantage of this pair hopping. Finally the authors argue that the high Tc copper oxides represent a new fixed point of mixed valence systems, in which the low energy charge transfer process is governed by pair hopping between copper and oxygen orbitals