Tilted ab-Axes MgB₂ Films with High T_c and Anomalous Upper Critical Field Anisotropy

MgB₂ is the highest transition temperature s-wave superconductor in ambient pressure with T_c ∼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 H_c2 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.