1. P/Qs. Chmr. Solids Vol. 54. No. IO. pi. 1315-1322. 1993 0022-3697/93 $6.00 + 0.00 Printed zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in G re a t Brita in 0 1993 Pergamon PressLtd NON-EQUILIBRIUM ELECTRONS, HOLES AND PHONONS IN YBa2Cu307_x D. MIHAILOVIC and I. POBERAJ Joxef Stefan Institute, University of Ljubljana, Jamova 39, Ljubljana. Slovenia Abatrnct - We report an observation by picosecond Stokes/anti-Stokes Raman spectroscopy of high non-equilibrium O(4) apical oxygen and Cu-0 plane buckling phonon populations in insulating and metallic YBa2Cu307_x induced by high peak power pulsed laser excitation. Different excitation and probe geometries were used to selectively meaaun the non-equilibrium phonon populations of the two phonons. We attribute the effects to strong coupling of photoexcited carriers to the lattice. A photoinduced metal-insulator transition is observed in insulating YBa2Cu306.3 accompanied with a qualitative change in nonequilibtium behaviour for the two phonona and an increase in carrier lifetime upon increasing doping. A polaronic model is used to discuss the observations. Keywords: high T,. polarons. non-equilibrium phonons. picosecond, Roman zyxwvutsrqponmlkjihgfedcbaZYXW INTRODUCTION Since the discovery of a high T, superconducting materials by Bednorz and Mtlller (Bednorz and Mtlller, 1986) there has been an intense effort to explain the mechanism of a high Tc superconductivity. Now it is generally accepted that the most important role in high T, superconductivity is played by charge carriers which are confined to a planar structure of Cu and 0 atoms common to all high T, materials. Nevertheless it seems that the role of out plane structure must be taken into account if we want to fully understand the mechanism of high T, superconductivity. Almost trivially, the importance of the c-axis structure can be concluded from the fact that materials with identical plane structure like La&uO, and TlzBazCuOs have very different T,s (38K and 95K respectively) (D. Mihailovic, 1992). A well-known example of an out of plane structure that has a direct influence on charge carriers in planes are Cu-0 chains in YBa2Cu307_x. They provide states that act as a charge reservoir for electrons which are transferred from planes leaving behind holes. Through varying the oxygen concentration in the chains, different in-plane charge carrier concentrations can be obtained. The charge transfer between Cu-0 planes and chains proceeds through O(4) apical oxygen that serves as a bridge between the two substructures in YBa2Cu307_x. Similar out-of-plane charge reservoirs are found in most other high T, compounds. Apart from a static c-axis charge transfer, possible dynamic effects that are potentially even more important for superconductivity should also be considered. If we take YBa2Cu307_,as an example again, from the point of view of dynamical CT fluctuations, of particular interest is the 470 cm-r phonon that includes O(4) apical oxygen motion along the z direction. The 470 cm-l phonon modulates the distance between Cu-0 planes and O(4) apical oxygen and can thus change the efficiency of dynamic charge transfer between planes and chains. As a consequence, the in-plane charge carrier mobility could be directly modulated with the phonon frequency via the finite plane buckling. Recent measurements investigating the correlations between the apical oxygen phonon fkquency, T, and apical oxygen-to-Cu02 planes distances and the corresponding bond lengths in the 1212- structure compound (Ca, Y)Sr2(Tl, Pb)Cuz07 have shown that the dynamic charge transfer along the c-axis is directly coupled to superconducting charge fluctuations. (T. Mertelj et al, 1993) The presence of lattice polarons in HTSC materials has been suggested in many experiments (D. Mihailovic, 1991 and C. Taliani, 1988 and K. A. Mtlller, 1990), but many questions - such as: what is their behaviour on doping, their relevance in superconductivity (if any), the reason for their very long lifetimes, etc - have remained largely unanswered. As a 1315