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

The temperature dependence of the complex 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 for light polarized in the a-b planes and normal to the planes along the poorly conducting c axis. Below T_c the superconducting plasma frequency ω_ps ~ 2400 cm^-1 is determined from the Ferrell-Glover-Tinkham sum rule. An estimate for the plasma frequency ω_pD ~ 7400 cm^-1 may be obtained by fitting the optical conductivity to a Drude-Lorentz model, indicating that that less than 11% of the free carriers collapse into the superconducting condensate, in agreement with recent studies [1,2]. However, satisfactory fits may only be obtained by including a low-frequency bound excitation (two-component model), indicating two electronic subsystems [3], suggesting that the material may be inhomogeneous [4]. Alternatively, the system may be homogeneous, and the free-carrier scattering rate strongly renormalized (generalized Drude model), as a strong kink at ~650 cm^-1 may indicate. Both possibilities are considered.



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

[2] S. V. Dordevic and C. C. Homes, Phys. Rev. B 105, 214514 (2022).

[3] C. M. N. Kumar et al., Phys. Rev. B 107, 144515 (2023).

[4] Y. Li et al., Phys. Rev. B 106, 224515 (2022).