Pressure-induced insulator-metal transition in transition metal chalcogenides studied by nuclear resonant inelastic scattering

硫化铁，简称FeS，在地球科学和凝聚态物理中一直受到人们的关注。尽管压力-温度相图一直是矛盾的，但FeS在室温下经历了两次连续的一级相变，从3.5 GPa时的硫铁矿到MnP型结构，再到6.5 GPa时的单斜结构。在环境条件下，FeS是一种反铁磁性半导体，其中电子关联起着重要作用。压力下FeS的电阻率测量表明，半导体-金属和金属-半导体的转变分别发生在3.5 Gpa和6.5 Gpa，对应于室温下的结构相变。这些相变导致电子性质和结构性质的同时变化。因此，我们需要从凝聚态物理的角度研究FeS在各个相变和相变前后的声子性质。在本研究中，我们应用核共振非弹性x射线散射来提取加压下FeS的部分声子密度，然后计算总声子密度。发现FeS的声子态密度受到压力诱导相变的影响，并由这些声子态密度导出了热力学参数与压力的关系。在这些热力学参数中，力常数与压力的关系是由FeS中Fe~(2+)和Gt~(2+)离子的Hund规则决定的。对观测到的压缩系数和估算的压缩系数的比较表明，在金属状态下观测到的压缩系数中有很大的纯电子贡献。因此，金属状态下较大的观测压缩率是由纯电子贡献提供的，估计约为观测压缩率的40%。因此，这种纯电子贡献的不连续变化是3.5 GPa时相变的特征。

Iron sulfide, FeS, has been attracting attention in geosciences as well as condensed matter physics. Although the pressure vs.temperature phase diagram has been contradictory, FeS undergoes two successive first-order phase transitions at room temperature, from the troilite to an MnP-type structure at 3.5 GPa and then to a monoclinic structure at 6.5 GPa. At ambient conditions, FeS is an antiferromagnetic semiconductor, where an electron correlation plays an important role. Electrical resistivity measurements of FeS under pressure show that the semiconductor-metal and metal-semiconductor transitions occur at 3.5 and 6.5 GPa, respectively, corresponding to the structural phase transitions at room temperature. These phase transitions cause simultaneous changes in the electronic and structural properties. Therefore, we need to investigate the phonon properties of FeS in each phase and around these phase transitions from the condensed matter physical points of view.In this research, we have applied the nuclear resonant inelastic x-ray scattering to extract the partial phonon DOS of FeS under pressure and then calculated the total phonon DOS. It is found that the phonon DOS of FeS is modified by the pressure-induced phase transitions.We derive the pressure dependence of thermodynamic parameters from these phonon DOS. In these thermodynamic parameters, the pressure dependence of a force constant is determined by the Hund's rules of Fe^<2+> ion in FeS. A comparison of the observed and estimated compressibilities makes it clear that there is large pure electronic contribution in the observed compressibility in the metallic state. Consequently the larger observed compressibility in the metallic state is provided by a purely electronic contribution, which is estimated to be about 40% of the observed compressibility. Therefore, this discontinuous change of purely electronic contribution characterizes the phase transition at 3.5 GPa.