Pr3+crystal-field excitation study of apical oxygen and reduction processes inPr2−xCexCuO4±δ

Abstract

We present an infrared transmission ${\mathrm{Pr}}^{3+}$ crystal-field study of as-grown, reduced, and oxygenated ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$ single crystals and thin films. Excitations from the ground-state multiplet ${}^{3}{\mathrm{H}}_{4}$ to the ${}^{3}{\mathrm{H}}_{5},$ ${}^{3}{\mathrm{H}}_{6},$ ${}^{3}{\mathrm{F}}_{2},$ and ${}^{3}{\mathrm{F}}_{3}$ excited multiplets are observed in all samples. In addition to the ${\mathrm{Pr}}^{3+}$ regular sites, which remain unperturbed following the cerium doping or the oxygen content modifications, ${\mathrm{Pr}}^{3+}$ sites are detected. A precise set of crystal-field parameters, which reproduces the energy and the symmetry of the levels, is determined. The reduction process, which drives the electron-doped cuprates superconducting, is discussed in detail and scenarios for the reduction mechanism and induced vacancies are proposed. In contrast to the common belief, the apical oxygen, which is clearly detected in all samples, is not removed in ${\mathrm{Pr}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4}$ following reduction. This observation questions the role attributed to the apical oxygen removal in triggering the superconductivity.