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

Abstract

Gedik et al have very recently demonstrated using a pump/probe femto-second laser technique that the c-axis lattice parameter of LaCuO4+d temporarily becomes expanded by as much as 2.5% following pulsed laser optical excitation at 1.55 eV. Access to an out-of-equilibrium metastable excited state is observed to develop on a time scale of 30 ps. Subsequently the latter state decays displaying a still longer half-life of just over 300 ps. Observation of the temperature independence of this laser induced interstate transfer and of the linear dependence of the production of the metastable population upon the energy delivered per unit area by the initiating light pulse (beyond a key threshold fluence) have been interpreted by Gedik et al within the framework of standard p-to-d, O-to-Cu, charge transfer excitations. By contrast these same data are reinterpreted here in terms of pumped local pairs, within a negative-U scenario of cuprate HTSC behaviour long advocated by the current author. The d8-to-d10 laser-induced augmentation in the negative-U state population (10CuIII2-) brings marked c-axis expansion by virtue of (i) the local electrostatic charge imbalance, (ii) the increased antibonding nature of the electron double-loading d10(p6) configuration created at pair-receptive CuIII coordination units, and (iii) the layered nature of the cuprate crystal structure. The new observations are related through to Rohlers striking, standard crystallographic observations, to the stripe domain formation, and to previous pump/probe experiments.