The composite picture of the charge carriers in La_2 xSr_xCuO₄(0.063  x  0.11) superconductors

Abstract

Through far-infrared studies of La 2−x Sr x CuO 4 single crystals for x = 0.063, 0.07, 0.09, and 0.11, we found that only ∼0.2% of the total holes participated in the nearly dissipationless normal state charge transport and superconductivity. We have also observed characteristic collective modes at ω ∼ 18 and 22 cm −1 due to the bound carriers in an electronic lattice (EL) state, and the free carriers are massively screened by the EL. Our findings lead us to propose a composite picture of the charge system where the free carriers are coupled to and riding on the EL. This unique composite system of charge carriers may provide further insights into the understanding of the cuprate physics.

Anomalies at magic charge densities in under-doped La2−xSrxCuO4 superconductor crystals prepared by floating-zone method

“…Nernst signal measurements indicated that, in the underdoping regime, the superconducting condensation is established simultaneously with the coherent motion of Cooper pairs along the c-axis, while in the normal state the Nernst effect is of a strict 2D nature [18]. We find that all of the above results are consistent with the recently proposed composite charge model [15] with an intrinsically coherent c-axis charge transport [19].…”

“…These anomalies can be understood in light of our recently proposed composite charge model [15] of intrinsic electronic superconducting phases [8]. Recent high-resolution ARPES experiments on our La 2Kx Sr x CuO 4 crystals also revealed complicated charge dynamics in the cuprate.…”

“…Based on detailed far-infrared studies and especially the observation of characteristic collective modes at uw18 and 22 cm K1 , the ab-plane charge dynamics in under-doped SD-La214 can be understood through a composite charge model that only a very small fraction of the total holes contributes to the free charge transport and superconductivity while the rest of the holes are in an ordered bound state forming 2D electronic (charge) lattices. In this composite charge picture, the free-carriers innately coexist with the underlying 2D charge lattice that is formed at w200 K, which is much higher than T C , in the CuO 2 planes [15]. Other interesting results have also been obtained by applying various probes to our crystals.…”

In-plane and out-of-plane plasma resonances in optimally doped La1.84Sr0.16CuO4

“…This is in contrast to the common notion of the insulating behavior along the c-axis, which was conjectured based on the absence of the caxis plasma edge in the normal state far-IR reflectivity and by the T-dependence of the c-axis resistivity. Since the fraction of the free electrons becomes less than 1 % in underdoped S-LSCO [22] that increases to ~ 2.8 % at optimal doping as found in this work, the c-axis screened plasma frequency is smaller than Γ c , making the c-axis plasma oscillation appear over-damped. In fact the c-axis plasma edge was observed in the normal state c-axis reflectivity for x = 0.07 but not for x ≥ 0.08 even though the c-axis transport is metallic [32].…”

Implications of charge ordering in highTccuprate superconductors in far-infrared spectroscopy

“…The displayed data in Figure 2 are chosen for T well below T c to insure that the free electron contribution to the ab-plane σ 1 has been transferred to a delta function centered at ω = 0 in the superconducting state. The doping-dependent ab-plane far-IR σ 1 of LSCO reported by Padilla et al 20 and by Dumm et al 15 are displayed in the left column of Figure 2 and those obtained by Kim et al 16,19 in the right column. These two studies are chosen because they exhibit an internal consistency upon doping.…”