Optical properties of severely overdoped La_1.75Sr_0.25CuO₄

The temperature dependence of the optical properties of the severely overdoped cuprate La_1.75Sr_0.25CuO₄ have been determined above and below the critical temperature T_c ~16 K over a wide frequency range for light polarized in the a-b plane. The real part of the optical conductivity can be reproduced reasonably well at low frequency using a simple Drude model, yielding an estimate for the plasma frequency w_pD ~ 9300 cm^-1. Below T_c the superconducting plasma frequency w_ps ~ 2700 cm^-1 is determined from the Ferrell-Glover-Tinkham sum rule, indicating that less than 8% of the free carriers collapse into the superconducting condensate, in agreement with a recent study [1]. However, there is unusual structure below ~0.1 eV that suggests that the free-carrier scattering rate is strongly renormalized with frequency. In addition, the nature of the magnetic susceptibility through the superconducting transition suggests that this compound may not be homogeneous.

[1] I. Bozovic et al., Nature 536, 309 (2016).