Raman analysis of asymmetrical chains in YBa2Cu3O7−δ films

YBa2Cu3O7−δ/MgO(1 0 0) thin films with depletion of oxygen have been studied with Raman spectroscopy. The depletion changes the intensity of 233 and 596 cm−1 phonons induced from Cu(1)-O(1) asymmetrical chains. A simple model of a symmetry-broken chain row is introduced to explain this phenomenon. With depletion, the row is configured as asymmetrical [O(1)-Cu(1)-o(1)] chains, where O(1) and o(1) are full and deficient oxygen in the orthorhombic and tetragonal structures, respectively. The Raman peaks at 233 and 596 cm−1 increase with an increase in a population density of the asymmetrical chains in the row and decrease as the row has more symmetric sites of full or deficient oxygen. As a result, the intensity of the chain-induced modes reflects a degree of the opposite structure involved in the phase transition in a single-phase film, which is verified by both the relative intensity of the 232 cm−1 phonon and the full width at half maximum of a Cu(2) superposition band from the coexisting phases in the film. All the results support the asymmetrical-chain model.