Structural phase transformations and superconductivity in La2−xBaxCuO4

Abstract

${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ba}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ has been found to undergo the following sequence of transformations upon cooling: tetr.(I4/mmm) →ortho.(Bmab)→tetr.(P$4_2$/ncm), over a range of composition 0.05<x<0.20. The newly discovered low-temperature tetragonal phase can be thought of as a coherent superposition of the twin-related Bmab structures. The system can be modeled as an XY-spin system with temperature-dependent quartic anisotopy, v(T). Slight differences between orthorhombic and tetragonal structures appear to have large effects upon the superconductivity.