First-Principles Study on the Structural and Magnetic Properties of Low-Index Cu$_2$O and CuO Surfaces

Copper oxides play a crucial role in a wide range of research areas, such as catalysis, photocatalysis, sensing, energy storage, biomedicine, and spintronics. However, further insights into the surface structure and the related magnetic properties of copper oxides are required to improve their performance. Here, we present a computational study on the structural and magnetic properties of low-index Cu$_2$O and CuO surfaces including their bulk oxides based on spin-polarized density functional theory (DFT) via DFT + U and the ab initio atomistic thermodynamics approach. We found that Cu$_2$O surfaces with an excess of oxygen atoms show surface ferromagnetism, while CuO surfaces with an excess of copper atoms exhibit surface atoms without a magnetic moment. By analyzing the density of states and the Bader charges of the surface atoms, we discuss the electronic properties of the copper oxide surfaces and the origin of the observed magnetism. Finally, we derive a correlation between the structure and the magnetic properties of copper oxide surfaces and suggest a possible explanation for the observed magnetism within a simplified model.