The quasiperiodic structure of high-pressure bismuth gives rise to unusually strong electron-phonon coupling.

The iron-based intermetallic YFe2Ge2 stands out among transition metal compounds for its high Sommerfeld coefficient of the order of 100 mJ/(molK 2 ), which signals strong electronic correlations. A new generation of high quality samples of YFe2Ge2 show superconducting transition anomalies below 1.8 K in thermodynamic as well as transport measurements, establishing that superconductivity is intrinsic in this layered iron compound outside the known superconducting iron pnictide or chalcogenide families. The Fermi surface geometry of YFe2Ge2 resembles that of KFe2As2 in the high pressure collapsed tetragonal phase, in which superconductivity at temperatures as high as 10 K has recently been reported, suggesting an underlying connection between the two systems.

Layered transition metal dichalcogenides (TMDs) are commonly classified as quasi-two-dimensional materials, meaning that their electronic structure closely resembles that of an individual layer, which results in resistivity anisotropies reaching thousands. Here, we show that this rule does not hold for 1T-TaS 2-a compound with the richest phase diagram among TMDs. Although the onset of charge density wave order makes the in-plane conduction non-metallic, we reveal that the out-of-plane charge transport is metallic and the resistivity anisotropy is close to one. We support our findings with ab initio calculations predicting a pronounced quasi-onedimensional character of the electronic structure. Consequently, we interpret the highly debated metal-insulator transition in 1T-TaS 2 as a quasi-one-dimensional instability, contrary to the long-standing Mott localisation picture. In a broader context, these findings are relevant for the newly born field of van der Waals heterostructures, where tuning interlayer interactions (e.g., by twist, strain, intercalation, etc.) leads to new emergent phenomena.

Applying elastic deformation can tune a material's physical properties locally and reversibly. Spatially modulated lattice deformation can create a bandgap gradient, favoring photogenerated charge separation and collection in optoelectronic devices. These advantages are hindered by the maximum elastic strain that a material can withstand before breaking. Nanomaterials derived by exfoliating transition metal dichalcogenides (TMDs) are an ideal playground for elastic deformation, as they can sustain large elastic strains, up to a few percent. However, exfoliable TMDs with highly strain-tunable properties have proven challenging for researchers to identify. We investigated 1T-ZrS 2 and 1T-ZrSe 2 , exfoliable semiconductors with large bandgaps. Under compressive deformation, both TMDs dramatically change their physical properties. 1T-ZrSe 2 undergoes a reversible transformation into an exotic three-dimensional lattice, with a semiconductor-to-metal transition. In ZrS 2 , the irreversible transformation between two different layered structures is accompanied by a sudden 14% bandgap reduction. These results establish that Zr-based TMDs are an optimal strain-tunable platform for spatially textured bandgaps, with a strong potential for novel optoelectronic devices and light harvesting.

In the layered iron-based superconductor YFe2Ge2, a high Sommerfeld ratio of ∼ 100 mJ/mol K 2 and a T 3/2 temperature dependence of the electrical resistivity at low temperature T indicate strong electronic correlations and point towards an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality YFe2Ge2. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than 2kBTc/ . Fe deficiency on the Fe site is identified as the dominant source of disorder, which can be minimised by precipitating from a slightly iron-rich melt, following by annealing.The iron-based superconductor YFe 2 Ge 2 [1] exhibits strong electronic correlations: its Sommerfeld ratio is enhanced by an order of magnitude over density functional theory (DFT) estimates [2][3][4], the normal state resistivity ρ follows a non-Fermi liquid temperature dependence, photoemission spectroscopy has revealed renormalised energy bands [5], and it displays enhanced magnetic fluctuations [6,7]. Further interest in this material derives from theoretical proposals for the superconducting state, which include s ± [3] or triplet pair wavefunctions [4] and from its striking similarities to some of the iron pnictide superconductors [8,9]. Moreover, several structurally and electronically related materials have recently been examined, some of which were found to superconduct at low temperatures [10][11][12][13], including a new iron-based superconductor, LaFeSiH [14].