No abstract

Complementary to the recent experimental finding that the orbital magnetic moment is strongly quenched in small Fe clusters [M. Niemeyer, K. Hirsch, V. Zamudio-Bayer, A. Langenberg, M. Vogel, M. Kossick, C. Ebrecht, K. Egashira, A. Terasaki, T. Möller, B. v. Issendorff, and J. T. Lau, Phys. Rev. Lett. 108, 057201 (2012)], we provide the theoretical understanding of the spin and orbital moments as well as the electronic properties of neutral and cation Fen clusters (n = 2-20) by taking into account the effects of strong electronic correlation, spin-orbit coupling, and noncollinearity of inter-atomic magnetization. The generalized gradient approximation (GGA)+U method is used and its effluence on the magnetic moment is emphasized. We find that without inclusion of the Coulomb interaction U, the spin (orbital) moments have an average value between 2.69 and 3.50 μB/atom (0.04 and 0.08 μB/atom). With inclusion of U, the magnetic value is between 2.75 and 3.80 μB/atom (0.10 and 0.30 μB/atom), which provide an excellent agreement with the experimental measurements. Our results confirm that the spin moments are less quenched, while the orbital moments are strongly quenched in small Fe clusters. Both GGA and GGA+U functionals always yield collinear magnetic ground-state solutions for the fully relaxed Fe structures. Geometrical evolution, as a function of cluster size, illustrates that the icosahedral morphology competes with the hexagonal-antiprism morphology for large Fe clusters. In addition, the calculated trends of ionization potentials, electron affinities, fragment energies, and polarizabilities generally agree with respective experimental observations.

One of the fundamental problems relating to the properties of hydrogen is that of insulator-metal transition. Recent theoretical and experimental studies show that the metallization in liquid hydrogen could be a first-order phase transition and involve molecular to atomic transition. However, the location of the critical point is still an unresolved question. Earlier studies reported the critical point at a temperature of 1500–2000 K, but recent experimental observations on diamond-anvil cells show that the discontinuous transition still persists at temperatures well above 2000 K. We have carried out a detailed study on the liquid-liquid phase transition in dense hydrogen by uisng ab initio molecular dynamics simulations and found new evidence for the abrupt metallization between weakly dissociated and strongly dissociated fluid phases at temperatures as high as 3000 and 4000 K. Also, the predicted phase boundary is in excellent agreement with the recent experiments. Our results suggest that this first-order transition in liquid hydrogen likely ends in a critical point around 4000 K, which is significantly higher than the previous theoretical predictions.

SnS 2 nanosheets with three atom thickness have previously been synthesized and it has been shown that visible light absorption and hydrogen evolution through photocatalytic water splitting are restricted. In the present study, we have systematically investigated the electronic structures of anionic monodoped (N and P) and codoped (N-N, N-P, and P-P) SnS 2 nanosheets for the design of efficient water redox photocatalysts by adopting first principles calculations with the hybrid HSE06 functional. The resultsshow that the defect formation energies of both the anionic monodoped and all the codoped systems decrease monotonically with the decrease of the chemical potential of S. The P-P codoped SnS 2 nanosheets are not only more favorable than other codoped systems under an S-poor condition, but they also reduce the band gap without introducing unoccupied impurity states above the Fermi level.Interestingly, although the P-P(ii) codoped system gives a band gap reduction, this system is only suitable for oxygen production and not for hydrogen evolution, which indicates that it may serve as a Zscheme photocatalyst for water splitting. The P-P(i) codoped system may be a potential candidate for photocatalytic water splitting to generate hydrogen because of the appropriate band gap and band edge positions, which overcome the disadvantage that the pure SnS 2 nanosheet is not beneficial for hydrogen production. More importantly, the result of optical absorption spectral analysis shows that the P-P(i) codoped SnS 2 nanosheet absorbs a longer wavelength of the visible light spectrum as compared to the pristine SnS 2 nanosheet. The P-P(I) codoped system with a lower doping concentration also has an absorption shift towards the visible light region.