Observation of a Bulk Nodal-Gap in Overdoped Y0.9Ca0.1Ba2Cu3O7-delta Thin Films

Complex conductivity of overdoped Y(Formula presented.)Ca(Formula presented.)Ba(Formula presented.)Cu(Formula presented.)O(Formula presented.) thin films was measured in Terahertz frequency using frequency and time domain methods. The films were measured in the frequency range of 3–100 cm(Formula presented.) and in the temperature range of 20–300 K. Results show a possible deviation from a pure (Formula presented.)-wave superconductor, indicated by the existence of an energy sub-gap in overdoped Y(Formula presented.)Ca(Formula presented.)Ba(Formula presented.)Cu(Formula presented.)O(Formula presented.) films. Evidence for this sub-gap appears as non-monotonic behavior of (Formula presented.) as a function of frequency followed by a sharp decrease at low frequencies, and a dip in the imaginary part of the optical conductivity multiplied by frequency, (Formula presented.). The mentioned features were observed at energy of about 1.2 meV in 10 % Ca-doped YBCO thin films. Our complex conductivity spectra are in agreement with the theoretical prediction obtained by using a mixed symmetry order parameter within the Born limit, shown by Schürrer et al. We suggest that these observations are direct evidence for a nodal gap obtained in a (Formula presented.)-wave superconductor and can be theoretically clarified by adding an imaginary component as (Formula presented.) or (Formula presented.) to the main (Formula presented.)-wave order parameter.