Superconductivity and Structural Phase Transition in La2-xBaxCuO4 with La Partially Replaced by Th

Abstract

In La2−x Ba x CuO4, striking suppression of superconductivity occurs in a narrow range of x around 0.125 accompanied by a structural phase transition from a midtemperature orthorhombic phase to a low temperature tetragonal phase below about 60 K. With partial replacement of La3+ by Th4+(La2−x−y Th y Ba x CuO4), we found that the anomalous low-temperature behavior is most prominent when x(Ba)=0.125+y(Th), instead of x=0.125. This result indicates that a particular value for the density of hole per Cu, p=x−y= 0.125, is of essential importance which leads to the low-temperature anomalies. The low-temperature structural phase transition of these oxides, therefore, is driven by conduction electron system. Because the volume fraction of the low-temperature tetragonal phase tends to saturate below T c, we suggest that the structural phase transition is suppressed when the electronic system becomes superconducting.