NEAR-INFRARED ABSORPTION OF YBA2CU3O7-DELTA - EVIDENCE FOR BOSE-EINSTEIN CONDENSATION OF SMALL BIPOLARONS

A significant change with temperature in intensity of the optical absorption spectrum of YBa2Cu3O7-delta in the range 2000 to 10000 cm-1 at the superconducting transition is shown to be a manifestation of Bose-Einstein condensation of small bipolarons. The sum rule for bipolaronic infrared absorption is derived and applied for the explanation of this unusual effect on the basis of the polaron theory of high-T(c) superconductivity. The temperature dependence of the normal state absorption is controlled by the thermal excitation of singlet inter-site bipolarons into triplet states. As determined from optical absorption experiments singlet-triplet exchange energy has the same value as the spin gap discovered by Rossat Mignod et al. by neutron scattering.