Condensed-Matter Physics & Materials Science Seminar

We learned a long time ago how mechanical properties of alloys can depend on very subtle structural effects. Homogeneity most often only exists at high temperature. Upon cooling, a variety of structural effects occur depending on the cooling speed. Quenching is frequently the only way to preserve the high temperature homogeneity down to room temperature, however even in this case, with ageing some constituent atoms tend to segregate and form “pseudo-clusters” like Guinier-Preston zones1,2, with drastic and negative effects on the mechanical properties of the concerned materials. Most high Tc superconductors, in particular the well known cuprates (and the like…) have non stoichiometric chemical composition, as in the case of alloys, with for instance a high level of oxygen dopants. Such doping atoms are supposed to be located in the overall periodic ordered structure at interstitial sites. So far, in spite of over twenty years of extensive experimental and theoretical studies, since the original discovery of high Tc superconductivity by J. G. Bednorz and K. A. Müller3, little, or almost nothing is known about these interstitial atoms4, albeit their dominant role in the variation of the superconducting transition temperature upon the different doping level. This paper reports new structural features of interstitial oxygen atoms in HgBa2CuO4+d, near to optimal doping, deduced from X-Ray diffuse scattering. Experiments carried out at the ESRF synchrotron have revealed the existence of very long (over 90 unit cells) chains along both tetragonal axes. These chains are constituted of interstitial oxygen atoms dressed with correlated transverse displacements of mercury and apical oxygen atoms. Together with very faint secondary scattering which could be attributed to charge density waves these results suggest a completely new vision of this material that could also be relevant for the other cuprate families.