Tilted ab-Axes MgB₂ Films With High Tc and Anomalous Upper Critical Field Anisotropy

MgB 2 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 MgB 2 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, MgB 2 films with ab -axes exposed on the film surface are desirable. Recently, MgB 2 [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 Hc 2 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 H c2 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 H c2 maximum was observed at the applied field parallel to the film surface for I⊥ a . Notably, there is a local H c2 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 MgB 2 .