EVIDENCE FOR POLARONIC STATES IN METALLIC YBA2CU3O6.9 AND LA1.85SR0.15CUO4 FROM ULTRAFAST PHONON RAMAN-SPECTROSCOPY

There is now an abundance of experimental evidence for the existence o f polaronic states in insulating precursors of cuprate superconductors . We present new data using a non-equilibrium Raman technique studying the temperature dependence of photoinduced carrier relaxation which p oints to the existence of polaronic states in the metallic phase, i.e. in the normal state of superconducting YBa2Cu3O6.9 and La1.85Sr0.15Cu O4. From the symmetry of the phonons coupled to them, we deduce a pred ominantly A(g) symmetry for the Cu-O-2 plane electronic states in the CT gap. As a result, it is argued that both itinerant and localized (a nd polaronic) states are present at the same time in the normal phase of high-T-c cuprate superconductors and it is suggested that a number of apparently conflicting experimental results can be understood using this hypothesis.