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Gedik et al have very recently demonstrated using a pump/probe femto-secondlaser technique that the c-axis lattice parameter of LaCuO4+d temporarilybecomes expanded by as much as 2.5% following pulsed laser optical excitationat 1.55 eV. Access to an out-of-equilibrium metastable excited state isobserved to develop on a time scale of 30 ps. Subsequently the latter statedecays displaying a still longer half-life of just over 300 ps. Observation ofthe temperature independence of this laser induced interstate transfer and ofthe linear dependence of the production of the metastable population upon theenergy delivered per unit area by the initiating light pulse (beyond a keythreshold fluence) have been interpreted by Gedik et al within the framework ofstandard p-to-d, O-to-Cu, charge transfer excitations. By contrast these samedata are reinterpreted here in terms of pumped local pairs, within a negative-Uscenario of cuprate HTSC behaviour long advocated by the current author. Thed8-to-d10 laser-induced augmentation in the negative-U state population(10CuIII2-) brings marked c-axis expansion by virtue of (i) the localelectrostatic charge imbalance, (ii) the increased antibonding nature of theelectron double-loading d10(p6) configuration created at pair-receptive CuIIIcoordination units, and (iii) the layered nature of the cuprate crystalstructure. The new observations are related through to Rohlers striking,standard crystallographic observations, to the stripe domain formation, and toprevious pump/probe experiments.

journal of physics. condensed matter

A negative-U interpretation of the femto-second laser pulse induced  crystallographic expansion of a cuprate HTSC material reported recently by  Gedik et al