Vibrational anomalies in the superconducting compound La_1.85Ba_0.15CuO₄

Abstract

La_1.85Ba_0.15CuO₄ is the prototype member of a new class of unusual high temperature (T_c>24K) superconductors. These novel materials whose properties lie at the interface between metallic and insulating behaviour show a wide variety of physical phenomena. For example, the electrical resistivity of the parent compound La₂CuO₄ shows metallic behaviour down to approxi-mately 25 K where an abrupt upturn occurs towards a semiconduct-ing or insulating state. The barium-doped superconductor exhibits a linear fall in resistivity from room to liquid-nitrogen tem-peratures followed by a plateau to 35 K, at which point supercon-ducting behaviour begins. Several theoretical suggestions regard-ing the origin of this behaviour have been put forward, but the role played by dynamic or static lattice–instabilities is still an area of controversy (see refs 1–4). Evidence for the presence of a tetragonal to orthorhombic phase transition together with further subtle and anomalous structural instabilities in the barium-doped compound has been reported⁵. To elucidate the correlations between the lattice vibrational and electronic behaviour, we have undertaken neutron inelastic scattering experiments on the High Energy Transfer spectrometer at the ISIS facility of the Rutherford Appleton Laboratory, UK, using carefully prepared polycrystalline samples of both La_1.85Ba_0.15CuO₄ and La₂CuO₄. The experiments were performed over a range of temperatures between 12 K and 200 K. Here we report anomalous temperature dependence of modes centred at 10 meV and 20 meV respectively.