Observation of an energy gap in the far infrared spectra of iron-doped YBa 2Cu 3O x superconductors

Abstract

We have measured the far infrared reflectance of superconducting YBa 2(Cu 1−cFe c) 3O x ceramics with an iron concentration c in the range of O < c < 0.06 at temperatures between 10 and 300 K. For c larger than 0.01, the energy gap structure shifts into a frequency range, where the reflectance is no longer dominated by infrared active phonon modes, thus allowing an accurate determination of the gap value. Although the decrease of the transition temperature T c is only moderate for such small iron concentrations, we observe a strong reduction of the energy gap Δ with increasing c, i.e. a decrease in the ratio 2Δ k BT c . The results are qualitatively similar to the predictions of the Abrikosov-Gorkov theory for conventional BCS-superconductors with paramagnetic impurities. We cannot, however, conclude that paramagnetic pair breaking dominates the gap supression, since samples without iron doping also show this behaviour when the oxygen content is reduced (x≤6.85).