Superconducting ferecrystals: turbostratically disordered atomic-scale layered (PbSe)1.14(NbSe2)n thin films

Misfit Layer Compounds as Ultratunable Field Effect Transistors: From Charge Transfer Control to Emergent Superconductivity

Misfit layer compounds are heterostructures composed of rocksalt units stacked with few-layer transition metal dichalcogenides. They host Ising superconductivity, charge density waves, and good thermoelectricity. The design of misfits' emergent properties is, however, hindered by the lack of a global understanding of the electronic transfer among the constituents. Here, by performing first-principles calculations, we unveil the mechanism controlling the charge transfer and demonstrate that rocksalt units are always donor and dichalcogenides acceptors. We show that misfits behave as a periodic arrangement of ultratunable field effect transistors where a charging as large as ≈6 × 10¹⁴ e^- cm^-2 can be reached and controlled efficiently by the La-Pb alloying in the rocksalt. Finally, we identify a strategy to design emergent superconductivity and demonstrate its applicability in (LaSe)_1.27(SnSe₂)₂. Our work paves the way to the design synthesis of misfit compounds with tailored physical properties.

Tuning metal/superconductor to insulator/superconductor coupling via control of proximity enhancement between NbSe2monolayers

The interplay between charge transfer and electronic disorder in transition-metal dichalcogenide multilayers gives rise to superconductive coupling driven by proximity enhancement, tunneling and superconducting fluctuations, of a yet unwieldy variety. Artificial spacer layers introduced with atomic precision change the density of states by charge transfer. Here, we tune the superconductive coupling betweenNbSe2monolayers from proximity-enhanced to tunneling-dominated. We correlate normal and superconducting properties inSnSe1+δmNbSe21tailored multilayers with varying SnSe layer thickness (m=1-15). From high-field magnetotransport the critical fields yield Ginzburg-Landau coherence lengths with an increase of140%cross-plane (m=1-9), trending towards two-dimensional superconductivity form>9. We show cross-overs between three regimes: metallic with proximity-enhanced coupling (m=1-4), disordered-metallic with intermediate coupling (m=5-9) and insulating with Josephson tunneling (m>9). Our results demonstrate that stacking metal mono- and dichalcogenides allows to convert a metal/superconductor into an insulator/superconductor system, prospecting the control of two-dimensional superconductivity in embedded layers.

Substituent Effects in the Synthesis of Heterostructures

There have been a number of surprising reports of unexpected products when preparing heterostructures of Bi₂Se₃ with other 2D layers. These reports prompted us to explore the formation of metastable heterostructures containing Bi₂Se₃ using X-ray diffraction techniques to follow the reaction pathway. We discovered that the products formed depend on the electronic properties of the second constituent. Bi|Se layers deposited in a 2:3 ratio with enough atoms to make a single five-plane layer evolved to form thermodynamically stable Bi₂Se₃ as expected from the phase diagram. When the same Bi|Se layers were sequentially deposited with M|Se layers that form semiconductor layers (PbSe and 2H-MoSe₂), Bi₂Se₃-containing heterostructures formed. When the same Bi|Se layers were deposited with M|Se layers that form metallic layers (TiSe_2, VSe₂, and 1T-MoSe₂), BiSe-containing heterostructures formed. The amount of excess Se in the precursor controls whether [(Bi₂Se₃)_1+δ]₁[(MoSe₂)]₁ or [(BiSe)_1+γ]₁[(MoSe₂)]₁ forms. XPS data indicates that a mixture of both metallic 1T and semiconducting 2H-MoSe₂ is present in [(BiSe)_1+γ]₁[(MoSe₂)]₁, while only semiconducting 2H-MoSe₂ is present when layered with Bi₂Se₃. The electronic structure of adjacent layers impacts the formation of different structures from layers with similar local compositions. This provides an important additional parameter to consider when designing the synthesis of heterostructures, similar to substituent effects in molecular chemistry.

Superlattices based on van der Waals 2D materials

Two-dimensional (2D) materials exhibit a number of improved mechanical, optical, and electronic properties compared to their bulk counterparts. The absence of dangling bonds in the cleaved surfaces of these materials allows combining different 2D materials into van der Waals heterostructures to fabricate p-n junctions, photodetectors, and 2D-2D ohmic contacts that show unexpected performances. These intriguing results are regularly summarized in comprehensive reviews. A strategy to tailor their properties even further and to observe novel quantum phenomena consists in the fabrication of superlattices whose unit cell is formed either by two dissimilar 2D materials or by a 2D material subjected to a periodic perturbation, each component contributing with different characteristics. Furthermore, in a 2D material-based superlattice, the interlayer interaction between the layers mediated by van der Waals forces constitutes a key parameter to tune the global properties of the superlattice. The above-mentioned factors reflect the potential to devise countless combinations of van der Waals 2D material-based superlattices. In the present feature article, we explain in detail the state-of-the-art of 2D material-based superlattices and describe the different methods to fabricate them, classified as vertical stacking, intercalation with atoms or molecules, moiré patterning, strain engineering and lithographic design. We also aim to highlight some of the specific applications of each type of superlattices.