FAR-INFRARED SPHERE RESONANCE IN ISOLATED SUPERCONDUCTING PARTICLES

A strong electric-dipole resonance has been observed in the far-infrared response of small isolated superconducting particles of La2-xSrxCuO4-y, Nd1.85Ce0.15CuO4, and Bi4Ca3Sr3Cu4Oz, but not in YBa2Cu3O7-y. Because of the small-particle boundary conditions, the bulk conductivity function of the superconducting condensate translates from zero to finite frequency, so that sphere resonance studies of the oxide superconductors provide important information about a dc property, the London penetration depth. The far-infrared data show that the frequency and strength of this resonance decrease as the temperature approaches Tc from below, and no resonance is observed above Tc. The effect of a magnetic field on this feature is similar to that produced by raising the temperature. The La2-xSrxCuO4-y system has been studied in some detail. By increasing the Sr doping it has been possible to drive the resonance frequency above the gap. An analysis based on the Maxwell-Garnett formalism is used to explain the observed changes in the resonant features. This analysis shows that for La2-xSrxCuO4-y the superconducting sphere resonance is polarized along the c axis of the particles. Moreover, the observed behavior of the resonant feature fixes some in- equalities that the normal and superconducting carrier parameters must satisfy, namely, that the c-axis energy-gap frequency is larger than the sphere resonance frequency but both of these frequencies are smaller than the normal-state carrier scattering rate. Since the sphere resonance position and the London penetration depth (T) are closely related to the oscillator strength of the superconducting condensate, we can show that (0)=5.8 m along the c axis for x=0.15. The temperature dependence of (T) for this sample follows the prediction of the empirical two-fluid approximation, but the behavior of samples for other values of x cannot be explained by any limiting case of BCS theory. Possible mechanisms for the large background absorption and the resonance line broadening are also discussed. This absorption below the gap may stem from disorder-induced lattice absorption, an unavoidable consequence of nonstoichiometric materials. © 1990 The American Physical Society.