Misfit layered compounds, denoted by (MX)_1+δ(TX₂)_n (M = Sn, Pb, Sb, Bi, rare earth metals, T = Ti, V, Nb, Ta, Cr, X = S, Se ; n = 1, 2, 3), are built of alternately double layers MX with quasi NaCl-type structure and TX₂ with slightly distorted trigonal prism [1]. The two sublattices have different spatial symmetry and periodicity from each other, resulting in incommensurability in the in-plane with a misalignment of lattice constants in the a-axis direction. Misfit layered compounds have attracted attention not only for their physical properties such as superconductivity and charge-density wave, but also as materials that can spontaneously realize van der Waals heterostructures.

In this study, we focus on the misfit layered superconductor (BiSe)_1.10NbSe₂, which exhibits quasi-two-dimensional behavior. Previously, we have shown that out-of-plane angular dependence of the upper critical field (H_c2) of (BiSe)_1.10NbSe₂ exhibits a two-dimensional behavior with a cusp fitted by the Tinkham model [2]. On the surface of this material, stripe structures appear along the b-axis direction, i.e., the in-plane alignment direction of the crystal. These stripe structures are unique to many of misfit layered compounds. To investigate the relationship between such a non-uniform crystal structure and the two-dimensional behavior of H_c2, we formed a micro-bridge using FIB fabrication to measure uniform regions without stripe structures within a single crystal shown in Fig.1. As a result, we found that the behavior of H_c2 is still two-dimensional even in sufficiently uniform crystals as shown in Fig.2, indicating that (BiSe)_1.10NbSe₂ exhibits an essentially two-dimensional superconducting state. We also investigated the possibility that H_c2^|| is enhanced by 3D-2D dimensional crossover or surface superconductivity.

Fig. 1 (BiSe)_1.10NbSe₂ single crystal micro-bridge formed by FIB processing.
Fig. 2 Out-of-plane angular dependence of upper critical field in (BiSe)_1.10NbSe₂ micro-bridge.

[1] G. A. Wiegers, Prog. Solid State Chem. 24, 1 (1996).
[2] S. Matsuzawa, P. Pyon, and T. Tamegai, J. Phys.: Conf. Ser. 2323, 012008 (2022).

Keywords: misfit layered compound, (BiSe)1.10NbSe2, two-dimensional superconductivity