Using density functional theory plus Hubbard U calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O(3) perovskite, the major mineral phase in Earth's lower mantle, has high-spin ferric iron (S=5/2) at both dodecahedral (A) and octahedral (B) sites. With increasing pressure, the B-site iron undergoes a spin-state crossover to the low-spin state (S=1/2) between 40 and 70 GPa, while the A-site iron remains in the high-spin state. This B-site spin-state crossover is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent x-ray diffraction and Mössbauer spectroscopy measurements. The anomalous volume reduction leads to a significant softening in bulk modulus during the crossover, suggesting a possible source of seismic-velocity anomalies in the lower mantle.

No abstract

No abstract

Weyl semimetals are novel topological conductors that host Weyl fermions as emergent quasiparticles. In this paper, we propose a new type of Weyl semimetal state that breaks both time-reversal symmetry and inversion-symmetry in the RAlGe (R=Rare earth) family. Compared to previous predictions of magnetic Weyl semimetal candidates, the prediction of Weyl nodes in RAlGe are more robust and less dependent on the details of the magnetism, because the Weyl nodes are already generated by the inversion breaking and the ferromagnetism acts as a simple Zeeman coupling that shifts the Weyl nodes in k space. Moreover, RAlGe offers remarkable tunability, which covers all varieties of Weyl semimetals including type-I, type-II, inversion-breaking and time-reversal breaking, depending on a suitable choice of the rare earth elements. Further, the unique noncentrosymmetric and ferromagnetic Weyl semimetal state in RAlGe enables the generation of spin-currents.