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 similar to 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-parallel to a, and perpendicular to I-parallel to a noted as I-perpendicular to a, respectively. For I-parallel to 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 degrees 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-perpendicular to a. Notably, there is a local H-c2 minimum at the field parallel to the surface for I-parallel to 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.