Direct Measurement of Intrinsic Apical Oxygen Potential in Metallic and Insulating YBa2Cu3O7-σ

Using pulsed Raman spectroscopy, we directly examined the intrinsic apical O potential in insulating YBa2Cu3O6.25 and metallic YBa2Cu3O6.9. We selectively excited the apical O vibration mode with ultra short laser pulses to obtain a 'hot' apical O vibration mode in a 'cold' bulk phonon background. Measurements showed that the apical O phonon frequency shift and linewidth change are much smaller in the case of selective heating of the apical O vibration mode compared to bulk heating. The results agree well with theoretical predictions and indicate a strong influence of surrounding lattice ions on the apical O potential. (author)