Bipolaron theory of high-T[sub(c)] oxides

Our assumption is that all electrons are bound in small singlet or triplet inter site bipolarons stabilised by the lattice and spin dis-

tortion. Because the undoped plane has a half-filled CuSd? band there is no space for bipolarons to move if they are intersite. Their Brillouin zone is half of the original electron one and completely filled with hard-core bosons. Hole pairs, which appear with doping, have enough space to move and they are responsible for the low-energy charge excitations of the CuO-2 plane. Above Tc a material such as YBCO contains a non-degenerate gas of these hole bipolarons in singlet, or in triplet states. Triplets are separated from singlets by the spin gap J and have a slightly lower mass due to the lower binding energy, Fig.7.1,2.