Study of chromium chalcogenide

已知硫系铬的物理性质取决于其组成的比例。Cr_xY （Y=S, Se, Te）表示相变点处的相变。在过渡点观察到CrS和CrSe的金属-半导体相变。物理性质与费米能级E_F附近的电子结构有关。报道了CrS、CrSe和Cr_xTe的电子能带结构。在这项研究中，我们给出了这些系列的硫系铬的电子结构。从Cr位空位和硫化物种类对电子结构的影响的角度，比较和讨论了电子结构的态密度。并与带结构计算结果进行了比较。我们在Cr 3p-3d激发区通过共振逆光发射测量得到了Cr 3d的导带和未占据的部分DOSs。结果表明，CrS （x=1）、Cr_xSe和Cr_xTe的Cr三维pDOS与Cr位空位几乎无关，与价带空位相似。随着硫由S变为Se，再变为Te， Cr三维交换分裂能增大。这种趋势与计算得到的交换分裂能相似。但是，实测的交换分裂能比计算的要大。在CrS中观察到金属-半导体转变。因此，我们利用高分辨率和低温光电发射光谱仪在低温条件下测量了CrS在EF附近的电子结构。随着温度的降低，EF附近的光发射光谱强度逐渐降低。根据测量温度观察到结构变化。在CrS中，对电子结构的x依赖性现在也计划测量RIPES。为了研究三维过渡金属离子对电子结构的Cr取代效应，生长了（CrxVi_x）_3Te_4样品，并准备好测量DOSs。

It is known that the physical properties of chromium chalcogenides depend on the ratio of composition. Cr_xY (Y=S, Se, Te) show the phase transition at transition point. The metal-semiconductor phase transition is observed for CrS and CrSe at transition point. Physical properties are related with electronic structures near Fermi level E_F. The electronic band-structures of CrS, CrSe and Cr_xTe are reported. In this study, we present the electronic structures in these series of chromium chalcogenides. The density of states (DOSs) are compared and discussed from the view points of the effect of the vacancies at Cr sites and chalcogen species on electronic structures. These DOSs are also compared with the result of the band-structure calculation.We obtained the conduction-band and the unoccupied Cr 3d partial DOSs by resonant inverse photoemission measurements at the Cr 3p-3d excitation region. It can be concluded that the Cr 3d pDOS of CrS (x=1), Cr_xSe and Cr_xTe are almost independent on the vacancies at Cr sites, as similar as those of the valence-band. The Cr 3d exchange splitting energy becomes lager as chalcogen changes from S to Se and further to Te. This tendency is similar to the calculated exchange splitting energies. However, the measured exchange splitting energies are larger than those of calculation.The metal-semiconductor transition is observed in CrS. Therefore, we measured the electronic structure of CrS near EF at low temperatures, with high-resolution and low-temperature photoemission spectrometers. The intensity of photoemission spectra near EF becomes lower as the temperature decreases. A structural change was observed according to the measurement temperature.In CrS, x-dependence on the electronic structures are now also planning to measure the RIPES. In order to study Cr substitutional effect with 3d transition metal ions on the electronic structures, samples of (CrxVi_x)_3Te_4 have been grown, and now are ready to measure DOSs.