College Park, Maryland      June 6 - 10 , 2004

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MP37: Recent Neutron and X-ray Scattering Studies of the Electron-Doped Superconductor Nd_2-xCe_xCuO₄

P.K. Mang (Department of Applied Physics, Stanford University), M. Greven (Department of Applied Physics, Stanford University; Stanford Synchrotron Radiation Laboratory)

We have performed a series of studies on the electron-doped high temperature superconductor Nd_2-xCe_xCuO₄. Inelastic magnetic neutron scattering measurements of the evolution of the spin-spin correlation length with doping and temperature in non superconducting samples have revealed that magnetic correlations in the system are described at a quantitative level by the randomly diluted square-lattice quantum Heisenberg antiferromagnet (SLHAF) [1]. However the low temperature ordered moment evolves more rapidly than for the SLHAF, suggesting that quantum fluctuations manifest themselves differently for different observables. High energy resolution (~ 6 meV) inelastic x-ray scattering (IXS) has identified an anomalous softening of the highest energy (~ 70 meV) longitudinal optical phonon branch [2]. Similar features have been observed via neutron scattering in the hole-doped cuprate superconductors and a related "kink" structure was observered in the electronic dispersion by photoemission studies. These features have been interpreted as evidence for a strong electron-phonon coupling in the cuprate superconductors. We have established the existence of such a feature in NCCO, and determined that the feature is formed for concentrations x > 0.04. Using x-ray diffraction we have established that the oxygen-reduction procedure commonly employed to induce superconductivity in NCCO results in the decomposition of a small fraction (0.1 % to 1 %) of the sample to a cubic phase of Nd₂O₃ [3]. This secondary phase forms epitaxially with the copper-oxygen plane, but extends only a few lattice constants along the c-axis. A subset of structural diffraction peaks of the secondary phase overlap magnetic reflections associated with NCCO in reciprocial space, and at low temperatures the application of a magnetic field results in a dramatic polarization of the moment of the Neodymium atoms. These two effects have led others to mistakenly report the existence of a quantum phase transition from a superconducting to an antiferromagnetic phase.

[1]P.K. Mang et al., cond-mat/030793 (2003).
[2]M. d'Astuto et al., Phys. Rev. Lett. 88, 167002 (2002).
[3]P.K. Mang et al., Nature 426, 139 (2003).

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