Advances in single crystal growth and annealing treatment of electron-doped HTSC

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High quality electron-doped HTSC single crystals of Pr₂₋ₓCeₓCuO₄₊δ and Nd₂₋ₓCeₓCuO₄₊δ have been successfully grown by the container-free traveling solvent floating zone technique. The optimally doped Pr₂₋ₓCeₓCuO₄₊δ and Nd₂₋ₓCeₓCuO₄₊δ crystals have transition temperatures T c of 25K and 23.5K, respectively, with a transition width of less than 1K. We found a strong dependence of the optimal growth parameters on the Ce content x. We discuss the optimization of the post-growth annealing treatment of the samples, the doping extension of the superconducting dome for both compounds as well as the role of excess oxygen. The absolute oxygen content of the as-grown crystals is determined from thermogravimetric experiments and is found to be ≥ 4.0. This oxygen surplus is nearly completely removed by a post-growth annealing treatment. The reduction process is reversible as demonstrated by magnetization measurements. In as-grown samples the excess oxygen resides on the apical site O(3). This apical oxygen has nearly no doping effect, but rather influences the evolution of superconductivity by inducing additional disorder in the CuO₂ layers. The very high crystal quality of Nd₂₋ₓCeₓCuO₄₊δ is particularly manifest in magnetic quantum oscillations observed on several samples at different doping levels. They provide a unique opportunity of studying the Fermi surface and its dependence on the carrier concentration in the bulk of the crystals.