Critical behaviour of bulk phonons in YBa2Cu3O7-δas observed by infrared absorption spectroscopy

The frequencies and the absorption cross sections of infrared active phonons show anomalies related to the transition between the normal and superconducting phase in YBa 2 Cu 3 O 7 . The excess intensity of the absorption line and the frequency shift of the mode near 585 cm -1 are proportional to each other. Correlating these parameters with the thermodynamic order parameter using hard model spectroscopy leads to an order parameter of the superconducting phase which follows formally mean field theory of the BCS or Landau-Ginzburg type.