a-b plane optical conductivity in YBa_2Cu_3O_{7-delta} above and below T*

Abstract

Analysis of the a-b plane optical conductivity ${\sigma }_{ab}$ for both twinned and untwinned YBa${}_2$Cu${}_3$O${}_{7-\delta }$ as a function of temperature and doping shows that below a well defined temperature T*, a dip in the spectrum systematically appears separating the infrared charge excitation spectrum into two components with distinct energy scales. The change from monotonic behaviour in ${\sigma }_{ab}$ is found to be concurrent with the onset of phonon anomalies in Raman and infrared spectra below T*. The optical data are suggested to be evidence for the appearance of an inhomogeneous distribution of carriers rather than the opening of a simple gap for charge excitations below T*, an interpretation wich is consistent with recent angle-resoved photoemission and electronic Raman spectra. We find that the behaviour below T* and the absence of any anomalies at Tc can be interpreted assuming a Bose-Einstein condensation of preformed pairs.

6 pages, 4 figures

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