Search

MgB2 is the highest transition temperature s-wave superconductor in ambient pressure with Tc ∼39 K. It has a layered structure with two superconducting gaps, where the larger gap is confined in two-dimensional planes perpendicular to the c-axis (Xi 2008), (Giubileo et al. 2001), (Bouquet et al. 2001), (Chen et al. 2012), (Iavarone et al. 2002). Recently, it has been observed that MgB2 displays Dirac nodal lines along the ab-axes, making it a potential topological superconductor (Jin et al. 2019), (Zhou et al. 2019). For probing many of these unique properties, MgB2 films with ab-axes exposed on the film surface are desirable. Recently, MgB2 [102] oriented films with bidirectional tilted c-axis have been fabricated on M-plane sapphire substrate using a HPCVD technique (Rondomanski et al. 2024). In this paper, we report the effect of the bidirectional grain structure in the [102] films on the anisotropy of Hc2 with the current applied in two directions: parallel to the a-axis, I∥a, and perpendicular to I∥a noted as I⊥a, respectively. For I∥a, two Hc2 maxima were observed when the magnetic field is applied parallel to the ab-axes of either of the bidirectional grains, which is at the offset angles of ±34° from the field parallel to the surface direction. Only one Hc2 maximum was observed at the applied field parallel to the film surface for I⊥a. Notably, there is a local Hc2 minimum at the field parallel to the surface for I∥a. The observed phenomena can largely be explained by the bidirectional orientation of the tilted grains. These results demonstrate the importance of crystallographic orientation of thin films on the physical properties of MgB2.

Ultrathin 2D-GaNxcan be formed by Ga intercalation into epitaxial graphene (EG) on SiC followed by nitridation in ammonia. Defects in the graphene provide routes for intercalation, but the nature and role of the defects have remained elusive. Here we examine the influence of graphene layer thickness and chemical functionalization on Ga intercalation and 2D-GaNxformation using a combination of experimental and theoretical studies. Thin buffer layer regions of graphene near steps on SiC readily undergo oxygen functionalization when exposed to air or a He/O2plasma in contrast to thicker regions which are not chemically modified. Oxygen functionalization is found to inhibit Ga intercalation leading to accumulation of Ga droplets on the surface. In contrast, Ga readily intercalates between EG and SiC in the thicker graphene regions that do not contain oxygen. When NH3annealing is carried out immediately after Ga exposure, 2D-GaNxformation is observed only in the oxygen-functionalized regions, and Ga intercalated under thicker nonfunctionalized graphene does not convert to GaNx. Density functional theory calculations demonstrate that oxygen functionalization of graphene alters the binding energy of Ga and NH3species to the graphene surface. The presence of hydroxyl groups on graphene inhibits binding of Ga to the surface; however, it enhances the chemical reactivity of the graphene surface to NH3which, in turn, enhances Ga binding and facilitates the formation of 2D-GaNx. By modifying the EG process to produce oxygen-functionalized buffer layer graphene, uniformly intercalated 2D-GaNxis obtained across the entire substrate surface.