The structure and superconductivity of ultrathin Ga films on GaN substrate: A first-principles calculations

Abstract

A free-standing ultrathin Ga film with hexagonal symmetry is unstable due to its incompatibility with the chemical aromatic rule. Recently, two or three monolayers Ga films have been grown on GaN substrate and exhibit superconductivity [Zhang et al., Phys. Rev. Lett. 114, 107003 (2015) and Xing et al. Science 350, 542 (2015)]. To elucidate the role of the GaN substrate, the properties of slab consisting of ultrathin Ga films and GaN substrate are calculated from first-principles. The results show that the stable interface structure is ultrathin Ga films deposited on N-terminated GaN substrate. Covalent and metallic bonding coexist in the deposited ultrathin Ga films and this peculiar feature contributes to their stability. For two monolayers Ga film deposited on GaN substrate, the obtained electron-phonon coupling λ is about 1.18 and the estimated superconducting temperature (5.1 K) is in excellent agreement with the experimental value, suggesting that it is a conventional BCS-type two-dimensional superconductor. For the deposited three monolayer Ga film, our calculations of the phonon structures show that it is dynamically unstable. While for one monolayer Ga film deposited on GaN substrate, due to the deposited metallic Ga layer with partial covalent character and meanwhile having the behavior of phonon softening, the obtained electron-phonon coupling λ (2.31) and the estimated superconducting transition temperature (9.4 K) are much larger than the case of the deposited two monolayer Ga film.