Shinji Kitao

Silicate melt has been proposed to exist near the base of the mantle and has been invoked to explain seismic and electrical conductivity anomalies observed near the core‐mantle boundary; however, its presence and stability under lowermost‐mantle conditions remain uncertain. Here we report high‐pressure electrical conductivity measurements of Fe ²⁺ ‐ and Fe ³⁺ ‐bearing pyroxene glasses (Fe ³⁺ /ΣFe ≈ 0.5), used as analogs of silicate melts, up to megabar pressures at room temperature. At lower pressures, conductivity increases with pressure and iron content, consistent with electron‐hole hopping between Fe ²⁺ and Fe ³⁺ . Above 77–85 GPa, all samples show a marked conductivity decrease, suggesting a pressure‐induced spin transition of Fe ³⁺ . Previous studies have suggested that lower‐mantle silicate melts may be enriched in Fe ³⁺ , and iron spin‐state changes may modify iron partitioning between silicate melts and coexisting crystalline phases. Therefore, the Fe ³⁺ spin transition inferred here may affect the evolution of deep‐mantle melts through pressure‐dependent changes in iron partitioning behavior.

Recent studies have shown that the lifetime of the isomeric ^{229}Th doped in crystals can be shortened by x-ray or laser irradiation, a phenomenon referred to as isomer quenching. We investigate the temperature dependence of x-ray-induced quenching in ^{229}Th:CaF_{2} and identify a correlation with the afterglow of the host crystal. These results suggest a mechanism in which x-ray-induced electrons migrate through the lattice and are captured at Th sites, enabling isomer deexcitation via internal conversion through electron-nucleus coupling. This mechanism links nuclear decay to charge carrier dynamics in the host crystal, providing a new interface between nuclear and solid-state physics. The findings offer a pathway to optimize the performance of solid-state nuclear clocks.

Electronic nematic states, characterized by broken rotational symmetry, are prevalent in correlated materials. In most iron-based superconductors, the nematic anisotropy aligns with the Fe–Fe direction of the iron square lattice. However, recent investigations propose a unique form of nematicity oriented along the diagonal Fe–As direction in heavily hole-doped AFe2As2 (A=Rb or Cs). Yet, the transport studies focusing on the fluctuations of such nematicity yield conflicting outcomes regarding the presence and orientation of the nematic fluctuations. Here, we report high-resolution heat capacity measurements conducted under in-plane field rotation in RbFe2As2. While no discernible anomaly associated with the nematic transition is found in the temperature dependence of specific heat, the field-angle dependence near the superconducting transition (at ∼2.8 K) reveals clear 2-fold oscillations within the plane, providing thermodynamic evidence for the presence of diagonal nematicity. Moreover, we find that Mössbauer spectroscopy sensitively probes the nematic transition at ∼50 K with no evidence of static magnetism. These findings imply that the diagonal nematicity in RbFe2As2 has a distinct mechanism involving charge degrees of freedom, exhibiting unusual thermodynamic properties of the transition.

We conducted in situ high‐pressure acoustic‐wave velocity measurements of Fe²⁺‐bearing MgSiO₃ glass up to 158 GPa by Brillouin scattering spectroscopy to clarify the effect of iron on the elasticity and structural evolution of silicate melts in the lower mantle. The change in trend of the V_S profile, likely induced by the structural transition of Si‐O coordination number from 6 to 6+ proposed in previous studies of silicate glasses, was confirmed to be located at ∼106 GPa. Given the iron contents of partial melts derived from a pyrolitic or chondritic mantle, the transition pressure would be at around 84–97 GPa, which is well within the lowermost‐mantle pressure regime. Our data show the substitution of 12 mol% Fe in MgSiO₃ glass decreases the V_S by ∼5.5%. This implies that iron affects the buoyancy relations between melts/crystals and the melts at the lowermost mantle will have the higher coordination number than 6.