Raman scattering versus infrared conductivity:  Evidence for one-dimensional conduction inLa2−xSrx

Experimental uncertainty in the far-infrared reflectivity of uniaxial superconductors

“…[1] On the other hand, the observation of resonances, i. e., peaks at finite frequencies in the far-infrared (FIR) lead to charges partially localized because of disorder, or phase separation, or ordering. [2,3,4,5,6,7,8,9] Such resonances, that have been reported most often for La 2−x Sr x CuO 4−y (LSCO), may lead the above models to unphysical predictions, like a change of sign in the carrier effective mass across the peak frequency. [10] In an attempt to explain such a complicated experimental landscape, it has been argued that most of the extra-Drude peaks observed in the σ ab 1 (ω) of LSCO are spurious [11,12] and caused by some leakage of the insulating c-axis response into that of the conducting ab plane.…”

In-plane optical features of the underdoped La2CuO4 based compounds: theoretical multiband analysis

“…Second, we illustrate the dependence of the low-frequency optical conductivity on the symmetry and magnitude of AF pseudogap. Finally, the validity of the model (5)− (8) in the lightly doped regime is tested on comparing the experimental spectra measured at ω < 3 eV to the ones which correspond to the model parameters independent of doping. The characteristic energy scales used in the numerical calculations, ∆ pd = 0.66 eV and t pd = 0.73 eV, are deduced from the estimate of the intracell hybridization t pd /∆ pd ≈ 1.1 [41] obtained on the basis of the intracell charge distributions measured in the La 2−x Sr x CuO 4 compounds by the EFG probes, and by assuming that the charge-transfer energy is roughly equal to 1.75 eV for the doping level δ = 0.2.…”

“…Both appear at nearly the same energy, exhibit similar doping dependence and possibly have the same physical origin. [4,5,9,10] It is interesting also to note that in the materials where the MIR peak appears at a relatively small energy, comparable with the typical damping energy Σ ≡ /τ [7,8], there is an essential difference between the predictions of the usual one-band optical models and measurements. It is explicitly shown that the development of the measured MIR structure with temperature can be fitted well with the orbital Kondo model of Emery and Kivelson [19] (representing the regime where ω ir peak ≈ Σ) rather than with the mentioned one-band models (underdamped regime: ω ir peak is proportional to characteristic correlation energy scales, such as the Hubbard interaction U [11,12] or the AF exchange energy J [13]).…”

“…A striking difference between these two groups of spectra occurs for the nearly half filled conduction band in all cases where there is a perturbation peaked at (usually commensurate) wave vector Q close to the nesting vector of the Fermi surface 2k F . The AF fluctuations at small dopings [4,9] and the charge-density-wave (CDW) fluctuations at the 1/8 doping [8] are typical examples. There is also a surprising similarity between the mid-infrared (MIR) peak in the optical conductivity, ω ir peak , and the magnetic peak in the B 1g Raman spectra.…”

“…Anomalous low-frequency structures related to the superconducting (SC) pseudogap, the antiferromagnetic (AF) pseudogap, the two-magnon excitations or the density waves are observed in the latter crystals, usually in one or two (of four) "optical" channels (optical conductivity and A 1g , B 1g and B 2g Raman symmetries). [3,4,5,6,7,8,9,10] Any theory of the low-frequency excitations in the underdoped cuprates should explain anomalies in both optical conductivity and Raman scattering spectra, taking the normal-state coherence effects properly into account.…”