Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing from a charge-density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved controversial. Here, we study a prototypical example, 2H-NbSe2, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterized by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin–orbit interactions and local inversion symmetry breaking, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi-surface spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.
Clay minerals can adsorb large amounts of CO2 and are present in anthropogenic storage sites for CO2. Nanoscale functionalization of smectite clay minerals is essential for developing technologies for carbon sequestration based on these materials and for safe-guarding relevant long-term carbon storage sites. We investigate the adsorption mechanisms of CO2 in dried and hydrated synthetic Ni-exchanged fluorohectorite clayusing a combination of powder X-ray diffraction, Raman spectroscopy, and inelastic neutron scattering. Both dried and hydrated Ni-exchanged fluorohectorite show crystalline swelling and spectroscopic changes in response to CO2 exposure. These changes can be attributed to interactions with [Ni(OH)0.83(H2O)1.17]0.37 1.17+-interlayer species, and swelling occurs solely in the interlayers where this condensed species is present. The experimental conclusions are supported by density functional theory simulations. This work demonstrates a hitherto overlooked important mechanism, where a hydrogenous species present in the nanospace of a clay mineral creates sorption sites for CO2.
Due to the compact two-dimensional interlayer pore space and the high density of interlayer molecular adsorption sites, clay minerals are competitive adsorption materials for carbon dioxide capture. We demonstrate that with a decreasing interlayer surface charge in a clay mineral, the adsorption capacity for CO 2 increases, while the pressure threshold for adsorption and swelling in response to CO 2 decreases. Synthetic nickel-exchanged fluorohectorite was investigated with three different layer charges varying from 0.3 to 0.7 per formula unit of Si 4 O 10 F 2 . We associate the mechanism for the higher CO 2 adsorption with more accessible space and adsorption sites for CO 2 within the interlayers. The low onset pressure for the lower-charge clay is attributed to weaker cohesion due to the attractive electrostatic forces between the layers. The excess adsorption capacity of the clay is measured to be 8.6, 6.5, and 4.5 wt % for the lowest, intermediate, and highest layer charges, respectively. Upon release of CO 2 , the highest-layer charge clay retains significantly more CO 2 . This pressure hysteresis is related to the same cohesion mechanism, where CO 2 is first released from the edges of the particles thereby closing exit paths and trapping the molecules in the center of the clay particles.
Developing new technologies for carbon sequestration and long-term carbon storage is important. Clay minerals are interesting in this context as they are low-cost, naturally abundant, can adsorb considerable amounts of CO 2 , and are present in storage sites for anthropogenic carbon. Here, to better understand the intercalation mechanisms of CO 2 in dehydrated and hydrated synthetic Na-fluorohectorite clay, we have combined powder X-ray diffraction, inelastic and quasi-elastic neutron scattering, and density functional theory calculations. For dehydrated Nafluorohectorite, we observe no crystalline swelling or spectroscopic changes in response to CO 2 , whereas for the hydrated case, damping of the librational modes related to the intercalated water was clearly observed. These findings suggest the formation of a more disordered water coordination in the interlayer associated with highly confined water molecules. From the simulations, we conclude that intercalated water molecules decrease the layer−layer cohesion energy and create physical space for CO 2 intercalation. Furthermore, we confirm that interlayer confinement reduces the Na + hydration number when compared to that in bulk aqueous water, which may allow for proton transfer and hydroxide formation followed by CO 2 adsorption in the form of carbonates. The experimental results are discussed in the context of previous and present observations on, a similar smectite, Ni-fluorohectorite, for which it is established that CO 2 attaches to the edge of nickel hydroxide islands present in the interlayer.
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