Among the family of transition metal dichalcogenides, ReS 2 occupies a special position, which crystalizes in a unique distorted lowsymmetry structure at ambient conditions. The interlayer interaction in ReS 2 is rather weak, thus its bulk properties are similar to those of monolayer. However, how compression changes its structure and electronic properties is unknown so far. Here using ab initio crystal structure searching techniques, we explore the high-pressure phase transitions of ReS 2 extensively and predict two new high-pressure phases. The ambient pressure phase transforms to a "distorted-1T" structure at very low pressure and then to a tetragonal I4 1 /amd structure at around 90 GPa. The "distorted-1T" structure undergoes a semiconductor-metal transition at around 70 GPa with a band overlap mechanism. Electron-phonon calculations suggest that the I4 1 /amd structure is superconducting and has a critical superconducting temperature of about 2 K at 100 GPa. We further perform high-pressure electrical resistance measurements up to 102 GPa. Our experiments confirm the semiconductor-metal transition and the superconducting phase transition of ReS 2 under high pressure. These experimental results are in good agreement with our theoretical predictions.

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Atomically thin 2D materials have drawn great attention due to their many potential applications. We herein report two novel structures of 2D C4N identified by first-principles calculations in combination with a swarm structure search. These two structures (with symmetry of Pm and P2/m) are almost degenerate in energy (with only 4 meV/atom difference) and exhibit quite similar structural topologies, both consisting of alternative arrays of C–N hexagon and arrays of C–N pentagon–octagon–pentagon. The Pm structure is semiconducting with a direct band gap of 90 meV at HSE. In contrast, the P2/m structure is a zero-band-gap semimetal and possesses the distorted Dirac cone, showing the direction-dependent Fermi velocity and electronic properties. Thus the predicted C4N monolayers are promising for applications in nanoelectronics.

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In the framework of the ab initio random structure search method, we show that elemental Se and Te undergo pressure-induced structural transition from the bcc to fcc phase, in agreement with the theoretical results previously reported. By means of the pseudopotential plane-wave method based on density functional perturbation theory, the fcc structure for both elements is found to be another phonon-mediated superconducting phase of these materials. With a reasonable value for the Coulomb pseudopotential 𝜇 * = 0.12, the maximum superconducting transition temperature 𝑇c in the fcc phase of Se and Te is estimated to be about 5.7 K and 4.6 K, respectively. Furthermore, we show that in the entire fcc phase for Se and Te, the superconducting transition temperature decreases together with the increase in pressure, leading to the final suppression of the superconductivity. It is suggested that such behavior is mainly caused by the rapid increase in the mean-square phonon frequency ⟨𝜔 2 ⟩ with pressure. Finally, a very strong electron-phonon coupling value, for both Se and Te in the fcc phase, is found along the 𝐺-𝐾 high symmetry lines.