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 . 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 (Hc2) of (BiSe)1.10NbSe2 exhibits a two-dimensional behavior with a cusp fitted by the Tinkham model . 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 Hc2, 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 Hc2 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 Hc2|| 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.
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Keywords: misfit layered compound, (BiSe)1.10NbSe2, two-dimensional superconductivity