A series of (PbSe)1+δ(VSe2) n heterostructures with extensive turbostratic disorder were synthesized with n = 1–3 through low temperature annealing of appropriately designed layered precursors. The crystal structures consist of alternating layers of CdI2 type structured VSe2 and distorted NaCl type structured PbSe. The n = 1 compound has a positive Hall coefficient and a charge density wave like transition at 100 K, during which the resistivity increases by a factor of 3.5 and the Hall coefficient increases by a factor of 8. The n = 2 and 3 compounds have negative Hall coefficients and significantly smaller changes in the slope of the resistivity and Hall coefficient as a function of temperature at similar temperatures. The distinctly different transport properties of the compound containing a monolayer of VSe2 compared to compounds with thicker VSe2 layers highlights the complexity of the electronic structure of these stacked systems. The differences cannot be simply explained by charge transfer between VSe2 and PbSe within a rigid band model. More sophisticated interactions between the constituent layers, electron–phonon interactions, and/or correlation between electrons need to be considered to explain the change in carrier type and the charge density wave (CDW) transition.

The [(BiSe) 11d ] 1 (VSe 2 ) 1 heterostructure was characterized structurally and electrically to determine the effects of interlayer interaction on the charge density wave (CDW) found in VSe 2 and compared to previously reported [(SnSe) 1.15 ] 1 (VSe 2 ) 1 . Out-of-plane x-ray diffraction scans contain reflections that can be indexed as 00l reflections of a BiSe-VSe 2 supercell. Structure refinement indicates that the VSe 2 layer is very similar structurally to that found in [(SnSe) 1.15 ] 1 (VSe 2 ) 1 . Scanning transmission electron microscopy images show a turbostratically disordered layer structure and the formation of anti-phase boundaries in the BiSe bilayer. The [(BiSe) 11d ] 1 (VSe 2 ) 1 heterostructure is metallic with a negative Hall coefficient, in contrast to the positive Hall coefficient found for [(SnSe) 1.15 ] 1 (VSe 2 ) 1 . The CDW found [(SnSe) 1.15 ] 1 (VSe 2 ) 1 is not present in [(BiSe) 11d ] 1 (VSe 2 ) 1 . This work illustrates the importance of inter constituent interactions in determining the transport properties of single layer films.

Ferecrystals are a new artificially layered material system, in which the individual layers are stacked with monolayer precision and are turbostratically disordered. Here, the superconducting coupling of the NbSe 2 layers in [(SnSe) 1+δ ] m [NbSe 2 ] 1 ferecrystals with m between 1 and 6 are investigated. The variation of m effectively increases the distance between the superconducting NbSe 2 monolayers. We find a systematic decrease of the transition temperature with an increasing number of SnSe layers per repeat unit. For m=9 a superconducting transition can no longer be observed at temperatures above 250 mK. In order to investigate the superconducting coupling between individual NbSe 2 layers, the cross-plane Ginzburg-Landau coherence lengths were determined. Electric transport measurements of the superconducting transition were performed in the presence of a magnetic field, oriented parallel and perpendicular to the layers, at temperatures closely below the transition temperature. A decoupling with increasing distance of the NbSe 2 layers is observed. However, ferecrystals with NbSe 2 layers separated by up to six layers of SnSe are still considered as three-dimensional superconductors.

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