pdf, 9 MiB - Infoscience - EPFL
pdf, 9 MiB - Infoscience - EPFL
pdf, 9 MiB - Infoscience - EPFL
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18 CHAPTER 1. INTRODUCTION<br />
the CuO 2 planes. Since none of this is accompanied by any major structural<br />
change of the materials, it is quite natural to expect electronic correlations to<br />
play a significant role in these systems. The notion of overdoped or underdoped<br />
cuprates is frequently used to define the region of doping above, or below the<br />
so-called optimal doping concentration at which the superconducting transition<br />
temperature T c is highest. Obviously, holes and electrons doped into the antiferromagnetic<br />
insulators are very efficient in destroying the magnetic order. In<br />
La 2−x Sr x CuO 4 only 2% of holes are sufficient to achieve this. Therefore, the<br />
charge carriers seem to couple strongly to the spin systems. However, there is an<br />
apparent asymmetry between the electron and the hole-doped systems regarding<br />
the stability of antiferromagnetism as well as that of superconductivity, since<br />
the magnetic order is stable up to 30% in the electron doped side of the phase<br />
diagram.<br />
1.3 Theoretical approaches to superconductivity<br />
Regarding theory, despite the huge research efforts and tremendous scientific<br />
activity, the current understanding of both the normal state properties of the<br />
cuprates as well as the nature of the superconducting phase remains incomplete.<br />
Early on, it has been argued that many of the unusual properties of the<br />
cuprates are related to the electronic structure of the CuO 2 planes. It is widely<br />
believed that this structural unit supplies the carriers which form the superconducting<br />
condensate. At present time, it is commonly accepted that the relevant<br />
degrees of freedom in the perovskites are confined to the two dimension plans,<br />
though the role of the out-of-plane oxygens, the so called apical oxygens, remains<br />
unclear. Therefore, low dimensionality is one of the challenging aspects of the<br />
carrier dynamics in the cuprates which is directly related to their layered crystal<br />
structure.<br />
Another challenging aspect from the theoretical point of view is that the electron<br />
correlations constitute the key issue in the description of the elementary<br />
excitations in the cuprates. Ab initio LDA calculations (Local Density Approximations)<br />
corroborate the picture of strong local Coulomb correlations and give<br />
an onsite electronic repulsion of about 10eV in the d x2−y2 orbitals [4], which are<br />
located in the copper-oxide planes.<br />
More importantly, the Fermi level in La 2 CuO 4 is located such that at halffilling<br />
we get one hole per unit cell, which would exactly satisfy the valencies<br />
of La + 2 ,O− 2 ,andCu+ 2 . Therefore, any picture within a simple band structure<br />
theory would predict that at half-filling the cuprates have a metallic ground<br />
state and fail to reproduce the insulating behavior of undoped La 2 CuO 4 . The<br />
inconsistency of LDA is a direct consequence of an improper treatment of strong