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

Abstract

We present an infrared transmission Pr 3ϩ crystal-field study of as-grown, reduced, and oxygenated Pr 2Ϫx Ce x CuO 4Ϯ␦ single crystals and thin films. Excitations from the ground-state multiplet 3 H 4 to the 3 H 5 , 3 H 6 , 3 F 2 , and 3 F 3 excited multiplets are observed in all samples. In addition to the Pr 3ϩ regular sites, which remain unperturbed following the cerium doping or the oxygen content modifications, Pr 3ϩ 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 Pr 1.85 Ce 0.15 CuO 4 following reduction. This observation questions the role attributed to the apical oxygen removal in triggering the superconductivity.

Microscopic annealing process and its impact on superconductivity in T′-structure electron-doped copper oxides

“…In this latter scenario, the presence of a small amount of randomly doped apical oxygen in the as-grown materials induces localization of doped electrons and thus prohibits superconductivity. The role of the oxygen reduction process would then be to remove the excess apical oxygen and increase the mobility of doped electrons to allow metallic behavior and superconductivity 8,10,11,23 .Although the reduction process is widely believed to involve removal of the apical oxygen, such notion has recently been challenged by Raman, infrared transmission, and ultrasound studies of electron-doped materials 16,17,22 . Instead of removing apical oxygen, these experiments on NCCO and PCCO suggest that the reduction process removes the oxygen in the CuO 2 plane [O(1), see Fig.…”

“…), electron-doping alone is insufficient, and annealing the as-grown sample in a low oxygen environment to remove a tiny amount of oxygen is necessary to induce superconductivity 2,3 . Previous work [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] suggests that oxygen reduction may influence mobile carrier concentrations 7 , decrease disorder/impurity scattering 8,10,11,23 , or suppress the long-range AF order 16,17,22 . However, the microscopic process of oxygen reduction, its effect on the large electron-hole phase diagram asymmetry and mechanism of superconductivity 2,3 are still unknown.…”

“…The observed increase in charge carrier mobility would then appear to be the result of eliminating the Cu vacancies and repairing the CuO 2 planes of the T' structure. Of course, the creation of oxygen defects in the CuO 2 plane as a byproduct of the oxygen reduction process (Table III) may also be necessary to suppress the AF order and increase the electron mobility 16,17,22 , therefore facilitating superconductivity in the electron-doped cuprates.Differences in the phase diagrams 2,3 and the electronic properties 24,25 of the electron and hole-doped cuprate superconductors have been obscured by the "materials issue" surrounding the annealing process in the electron-doped samples. The microscopic oxygen-reduction process presented here removes much of the mystery and will need to be taken into account in order to resolve any intrinsic differences between electron and hole-doped superconductivity.…”

“…5%) entirely suppress AF order and superconductivity appears over a wide range of hole concentrations (from 6% to 30%). In the case of T' structured electron-doped superconductors, doping alone by substituting the trivalent ions R 3+ in R 2 CuO 4 with tetravalent Ce 4+ is insufficient to induce superconductivity and the as-grown materials such as Nd 2-x Ce x CuO 4±δ (NCCO), Pr 2-x Ce x CuO 4±δ (PCCO), and Pr 0.88 LaCe 0.12 CuO 4 (PLCCO) are semiconducting with static long-range AF order [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] .In these materials, superconductivity (maximum T c ~25 K) is achieved only when the samples within a relatively narrow dopant range (0.1≤x≤0.18) are annealed in a reducing atmosphere . The reduction process suppresses the AF order most severely at higher Ce-doping levels above x≥0.1 (ref.…”

“…31) have recently determined the Fermi surface topology and the superconducting gap function in electron-doped materials. However, the quality of the sample surfaces studied has not been conclusively established, and one must now consider the possible effect of an exposed NSC R 2 O 3 surface in future ARPES and scanning tunneling microscopy experiments.Furthermore, it should also be possible to grow higher-quality single-crystal T' structured copper oxides by adding excess Cu in the starting materials to eliminate Cu deficiencies and subsequent R 2 O 3 contamination.Because of the availability of single crystals of the T' structured copper oxides, the vast majority of transport, structural, magnetic, and electronic-property measurements of electron-doped superconductors have been obtained from these materials [9][10][11][12][13][14][15][16][17][18][19][20][21][22]25,[27][28][29][30][31] . For other electron-doped high-T c superconductors with different crystal structure types, such as the infinite-layer Sr 1-x La x CuO 2 family 32-34 , the microscopic process of inducing superconductivity is still unclear 33,34 .…”