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
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 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
H_c
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
.