The crystal structure, electronic structure, and transport properties of crystals with the nominal composition Nb 0.25 Bi 2 Se 3 are investigated. X-ray diffraction reveals that the as-grown crystals display phase segregation and contain major contributions of BiSe and the superconducting misfit layer compound (BiSe) 1.1 NbSe 2 . The inhomogeneous character of the samples is also reflected in the electronic structure and transport properties of different single crystals. Angle-resolved photoemission spectroscopy (ARPES) reveals an electronic structure that resembles poor-quality Bi 2 Se 3 with an ill-defined topological surface state. High-quality topological surface states are instead observed when using a highly focused beam size, i.e., nanoARPES. While the superconducting transition temperature is found to vary between 2.5 and 3.5 K, the majority of the bulk single crystals does not exhibit a zero-resistance state suggesting filamentary superconductivity in the materials. Susceptibility measurements of the system together with the temperature dependence of the coherence length extracted from the upper critical field are consistent with conventional BCS superconductivity of a type II superconductor.

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High-resolution diffraction data are essential to determine accurate charge density. We reported d-space resolution dependence of the charge densities in Ref. 1. To obtain high-resolution X-ray diffraction data, we have performed synchrotron diffraction experiments using 60 keV X-ray at PETRA-III [2], 50 keV X-ray at SPring-8 and multiple powder diffraction data method [1]. Recently, we have probed charge densities of iso-structural metal hexaborides as a pi-bonding system using ultra-high resolution d > 0.22 Å powder data [3]. Our present challenge is to evaluate the weak interaction in van der Waals-layered structures such as MoS 2 and TiS 2 . The layered transition metal dichalcogenides have been attractive due to their unique properties, e.g., as topological insulators, charge density wave system and materials for energy. The layered structure exhibits strong covalent intralayer bonding and weak van der Waals (vdW) interlayer interaction. Compared to strong intralayer bonding, weak interlayer interaction is difficult to be evaluated not only experimentally but also theoretically because vdW force is not described in ground states. The experimental evaluation of vdW interlayer interaction can be useful for the fundamental understanding of the vdW interaction and layered structure. In the present study, we evaluate and compare the weak vdW interlayer interaction in MoS 2 (space group: P6 3 /m m c) and TiS 2 (space group: P-3m 1) using experimental X-ray charge densities. The charge densities were modelled with the Hansen-Coppens multipole model using single-crystal diffraction data with a resolution of d > 0.3 Å measured at the BL02B1 of SPring-8. In the presentation, we will discuss the weak interlayer vdW interaction from experimental and theoretical charge densities.