9 - Electron–phonon interaction of non-equilibrium carriers in the photoinduced state of YBa₂Cu₃O_7−δ

Summary

Abstract

Using picosecond pulses we excite a large number of carriers in YBa₂Cu₃O_7−δ and drive the system through an insulator-to-metal transition. As we do this, we study the carrier interaction with the apical O and the 340 cm⁻¹ planar O buckling vibrations by counting the number of non-equilibrium phonons that the carriers generate as they relax towards equilibrium. Counting is done directly by photoexcited anti-Stokes Raman scattering (PEARS). Carrier transport is found to be thermally activated; presumably the carriers relax by hopping between localized states. The in-plane and chain activation energies are found to be very different, suggesting that different carriers are involved: the chain carriers form polarons, which are strongly coupled to apical O vibrations, while the planar holes, which have a smaller activation energy, are suggested to interact less strongly with the lattice, especially in the metallic state.

Introduction

One may ask a very simple question about charge carrier transport in a cuprate superconductor: are the carriers moving through the crystal as in an extended band, or are they hopping from site to site? Given that the structure of these materials is composed of two distinct parts – the CuO₂ planes plus charge reservoirs – one may perhaps also wonder whether the behaviour of carriers in the two parts is different. In order to answer such questions, we need a microscopic probe, which, it is to be hoped, might tell us something about both the type of interaction between the carriers and the symmetry and location of the interaction.