Superheating and melting mechanisms of YBa2Cu3O7−x surfaces

Abstract

Recently, REBa2Cu3O7−x films were observed to possess excellent superheating performances, among which YBa2Cu3O7−x films deposited on LaAlO3 substrates could be superheated beyond the equilibrium melting point by 100 K. Although previous investigations have shown that the low surface energy of this multilayered structure is responsible for the superheating nature, micro-mechanism remains unclear. Here, based on molecular dynamics simulations, the superheating origin of YBa2Cu3O7−x was studied at the atomic scale. We found that the (001) free surface possesses strong superheating capability and exhibits a layer-by-layer melting mode with a flat melting front. Remarkably, the superheating phenomenon is attributed to the high stability of Ba–O layers, which serve as obstacles in suppressing the melting propagation. In contrast, with an absence of such significant barriers, the (100) surface displays a wavy melting front and a continuous melting process. Particularly, in that case, the highly mobile oxygen atoms facilitate the penetration of the melting liquid phase into the bulk, which further deteriorate the film thermal stability. Moreover, we proposed a modified thermodynamic model to describe the concrete melting process. The atomic simulations provide detailed evidence for understanding the microscopic origin of the melting of YBa2Cu3O7−x films and hint the way to enhance the thermal stability of functional oxides with multilayer structures.