Correlation between structural and transport properties in epitaxial films of Nd2-xCexCuO4 +/-delta

We present here a study on the influence of the oxygen reduction process on the structural and transport properties of epitaxial thin films of the electron-doped cuprate Nd2-xCexCuO4 +/-delta. As is well known, the gradual removal from as-grown samples of a tiny percentage of excess oxygen ions leads to a drastic improvement of the metallic character of this system, which eventually becomes superconducting for suitable values of the cerium concentration, with a maximal critical temperature T-c similar or equal to 25 K. We find that the oxygen loss occurring in thermal treatments in the temperature range 500-850 degrees C leads to a reduction of the disorder hindering conductance processes, but is insufficient to make the system become superconducting. On the other hand, as soon as the annealing temperature is raised above 850 degrees C, superconductivity appears, and at the same time a systematic variation of the length of the unit cell along the c-axis direction is detected. This is a clear indication that the transition to the superconducting phase is always accompanied by a structural modification. A further salient feature characterizing samples annealed at high temperatures is the emergence of a linear contribution in the normal state resistivity, which superimposes to the quadratic one already present in samples which are oxygen reduced below 850 degrees C. This contribution is probably associated with the formation of hole-like carriers located at hole pockets developing at the Fermi energy along the nodal direction in the Brillouin zone. We conjecture that the evolution of the electronic states with oxygen removal for a given cerium concentration close to optimal doping, is similar to the one taking place in optimally annealed samples where cerium concentration is raised from the underdoped to the lightly overdoped regime value. (C) 2012 Elsevier B. V. All rights reserved.