Raman study of the anharmonicity in YBa$_2$Cu$_3$O$_x$

TL;DR

This Raman study investigates anharmonicity in YBa2Cu3O_x across different doping levels, revealing how phonon behavior varies with oxygen content and structural phases, and challenging some existing models of atomic potential.

Contribution

The paper provides the first systematic Raman analysis of anharmonicity in YBa2Cu3O_x, highlighting doping-dependent variations and their relation to structural phases.

Findings

Anharmonicity varies with oxygen doping and structural phase.

Almost no anharmonicity at optimal doping persists at 77K.

Anharmonicity is not enhanced by CuO2 buckling in overdoped samples.

Abstract

A systematic Raman study in the visible carried out on the YBa2Cu316,18Ox (x=6-7) compounds, with isotopic substitution of 18O for 16O, has detected a doping dependent deviation from harmonic behavior for the frequency shift of the in-phase mode, a smaller amount of anharmonicity for the apex mode, and almost no effect for the out-of-phase B1g-symmetry phonon. It appears that the amount of anharmonicity depends strongly on the oxygen concentration; it diminishes close to the tetragonal to orthorhombic structural phase transition and close to optimal doping, while it reaches its maximum value for the ortho-II and a tetragonal phase. The almost zero anharmonicity at optimal doping persists even at 77K. The data in the overdoped oxygen concentration, where a softening of the in-phase phonon frequency occurs, indicate that the anharmonicity is not enhanced by the sudden increase in the CuO2 buckling. The results fully agree with recent studies of the ortho-II phase but they do not comply with a static double-well potential of the apical oxygen atom at optimal doping.