Analysis of mid-infrared optical conductivity in electron-doped cuprates

Abstract

Observed frequency dependent optical conductivity σ(ω) of electron-doped cuprate Nd1.85Ce0.15CuO4−δ (δ ≈ 0.02, Tc ≈ 25 K) superconductors has been theoretically analysed. Starting from an effective two-dimensional (2D) interaction potential for superlattice of electron-doped cuprates treated as a layered electron gas, the spectral function is developed. Calculations of σ(ω) have been made within the two component scheme: one is the coherent Drude carriers responsible for superconductivity and the other is incoherent motion of carriers from one site to the other that leads to a pairing between Drude carriers. The approach accounts for the anomalies observed (frequency dependence of optical conductivity) in the optical measurements for the normal state. Estimating the effective mass from specific heat measurement and ε∞ from band structure calculations for the low-energy charge density waves, the model has only one free parameter, the relaxation rate. The frequency dependent relaxation rates are expressed in terms of memory functions, and the coherent Drude carriers from the effective interaction potential lead to a sharp peak at zero frequency and a long tail at higher frequencies, i.e. in the infrared region, while the hopping of carriers from one site to the other (incoherent motion of doped carriers) yields a peak value in the optical conductivity centred at mid-infrared region. We find that both the Drude and hopping carriers in the superlattice of electron-doped cuprates will contribute to the optical process of conduction in the CuO2 planes and show similar results on optical conductivity in the mid-infrared as well as infrared frequency regions as those revealed from experiments.