Regensburg 2004 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 9: Supraleitung: Eigenschaften, elektronische Struktur, Ordnungsparameter
TT 9.5: Vortrag
Dienstag, 9. März 2004, 10:45–11:00, H20
The bulge in the basal CuO2 area of cuprate superconductors — •Jürgen Röhler — Universität zu Köln, D-50937 Köln, Germany
Hole doping of the superconducting cuprates not only shrinks their basal area B (square of the basal lattice parameter) – to be expected if electrons are removed from the antibonding Cu3dx2+y2O2pxy states –, but creates also a strong bulge around optimum doping, nopt≃ 0.16. The bulge starts growing in the lightly underdoped, and collapses in the overdoped regime near critical doping, ncrit ≃ 0.19 [1]. We show that the observed expansion of the basal lattice around nopt and its collapse at ncrit results from the Aufbau of the strongly correlated many electron state in the CuO2 sheets with “self-protecting” singlets (SPS) [2]. In them not only the Cu sites are protected against occupation with more than one hole, but also the oxygen cages of the Zhang-Rice (ZR) singlets. This additional non-double occupant constraint favours the formation of non magnetic (S=0), bond centered molecular pairs. The paired SPS exchange two oxygen holes across antiferromagnetic chains of 4 Cu spins, thereby flipping them, and create a delocalized singlet of oxygen holes. Most closest packing of paired SPS occurs at 1/6=0.166≃ nopt in a tweed-like mesoscopic pattern. At higher hole densities, n ≥ 0.2 ≃ ncrit, the non-double-occupant constraint for the oxygen cage collapses. We show that the exchange of two oxygen holes in a paired SPS is connected with the local half-breathing (LO) mode of planar oxygen vibrations found up to nopt, and that it expands the basal CuO2 area as observed.
[1] J. Röhler, Physica C (2003), cond-mat/0304628.
[2] J. Röhler, J. Supercond. (2003), cond-mat/0307310.