Starting from an effective two-dimensional dynamic interaction that includes screening of holes as carriers by charge density fluctuations and by optical phonons, we investigate the nature of a d-wave pairing mechanism leading to superconductivity in layered La-based cuprates. We consider the La–Sr–CuO system as an ionic solid containing layers of holes as carriers with a single CuO2 layer in a unit cell, where the localized spins form an antiferromagnetic (AF) order. The electron–phonon interaction matrix element in the case of an ordinary unit cell without the local AF order yields s-wave superconductivity. While for the unit cell with AF order, the wave-vector dependence of the intralayer effective interaction potential shows the sign reversal to create d-wave pairing due to localized antiferromagnetic spin order for the screened phonons. Following the strong coupling theory, the superconducting transition temperature, the isotope exponent, coherence length and magnetic penetration depth are also estimated. The implications of the intralayer pairing model and its analysis are discussed.
We have evolved an effective two-dimensional dynamic interaction, which embodies the screening of electrons by optical phonons and by plasmons to confer the attributes of the pairing mechanism leading to superconductivity in layered electron-doped cuprates. The electron-doped Nd-Ce-CuO cuprate behaves as an ionic solid containing isolated CuO2 layers of electrons as carriers, and a model dielectric function is set up that fulfils the appropriate sum rules on the electronic and ionic polarizabilities. Following strong coupling theory, the superconducting transition temperature of optimally-doped Nd-Ce-CuO is estimated as 28 K and the energy gap ratio is larger than the Bardeen-Cooper-Schrieffer value. The isotope effect exponent, coherence length and magnetic penetration depth are also estimated. Ziman's formula of resistivity is employed for analysis and a comparison is made with the temperature-dependent resistivity of a single crystal. The estimated phonon contribution together with the residual resistivity is lower than the reported data. The subtracted data infers a clear quadratic temperature dependence from Tc to near to room temperature. The implications of the model and its analysis are discussed.
Observed frequency dependent optical conductivity σ (ω) of electron-doped cuprate Nd 1.85 Ce 0.15 CuO 4−δ (δ ≈ 0.02, T c ≈ 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 CuO 2 planes and show similar results on optical conductivity in the mid-infrared as well as infrared frequency regions as those revealed from experiments.
Using a three square well model, we investigate the possibility that in low energy plasmons there could be an additional boson responsible for the attractive force binding the Cooper pairs that lead to superconductivity in layered electron-doped cuprates. The three square well model is, in principle, characterized by three coupling strengths, the electron-phonon (λ ph ), the electron-plasmon (λ pl ) and Coulomb screening parameter (µ * ), that estimate the transition temperature (T c ) and oxygen isotope effect coefficient (α). Starting with the three square well model within the framework of the Eliashberg theory, the energy gap kernels allow us to visualize the relative interplay of the Coulomb, electron-phonon and electron-plasmon interactions and we correlate the T c with these three coupling strengths. For a set of parameters (λ ph ≈ 1.0, λ pl ≈ 0.7 and µ * ≈ 0.18), a T c of 28 K is estimated for optimally doped Nd 1.85 Ce 0.15 CuO 4 . The present approach also explains the reported sizeable oxygen isotope effect in electron-doped cuprates. We suggest that the extended attractive force in the proposed three square well scheme consistently explains the superconductivity in electron-doped cuprates.
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