Direct observation of the quasiparticle relaxation in YBa2Cu3O7-delta

The real and imaginary parts of the optical conductivity (delta(1) + i delt a(2)) in the superconducting state of YBa2Cu3O7 (- delta) thin films have b een obtained by submillimeter spectroscopy for frequencies from 3 to 35 cm( -1). The quasiparticle relaxation can be directly observed in this frequenc y range. A Drude-type analysis allows to extract the temperature dependence of the scattering rate; The collapse of the relaxation rates provides clea r experimental evidence that the spin fluctuations become gapped below T-C. (C) 1999 Elsevier Science B.V. All rights reserved.