Direct evidence of a bulk nodal gap in the overdoped regime of Y0.9Ca0.1Ba2Cu3O7−δ thin films from THz spectroscopy

Abstract

We measured the terahertz (THz) complex conductivity of Ca-doped YBa2Cu3O7−δ thin films in the frequency range of 0.1 to 3 THz (3 to 100 cm−1) and in the temperature range of 20 to 300 K. The films were measured using time domain and frequency domain THz spectroscopy methods. We show evidences for the existence of an additional energy sub-gap in overdoped Y0.9Ca0.1Ba2Cu3O7−δ samples. The evidences for this gap appear as a sharp decrease in the real part of the optical conductivity, , and as a dip in the imaginary part of the optical conductivity multiplied by frequency, . These features are observed at a frequency equivalent to an energy of 1.2 meV. Our complex conductivity spectra are in agreement with the theoretical prediction assuming a mixed symmetry order parameter (Schürrer I . et al., Physica C, 303 (1998) 287). We suggest that these observations are direct evidence of a nodal gap obtained in a -wave superconductor when adding an imaginary s or dxy bulk component in the overdoped regime.