No Arabic abstract
We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (EF) and the Ti3+ component increased, while the Cl 2p intensity decreased. Ti 2p-3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.
Raman and combined trasmission and reflectivity mid infrared measurements have been carried out on monoclinic VO$_2$ at room temperature over the 0-19 GPa and 0-14 GPa pressure ranges, respectively. The pressure dependence obtained for both lattice dynamics and optical gap shows a remarkable stability of the system up to P*$sim$10 GPa. Evidence of subtle modifications of V ion arrangements within the monoclinic lattice together with the onset of a metallization process via band gap filling are observed for P$>$P*. Differently from ambient pressure, where the VO$_2$ metal phase is found only in conjunction with the rutile structure above 340 K, a new room temperature metallic phase coupled to a monoclinic structure appears accessible in the high pressure regime, thus opening to new important queries on the physics of VO$_2$.
Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ~ 60 GPa using multiple experimental techniques and ab -initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.
Magnetic spiral structures can exhibit ferroelectric moments as recently demonstrated in various multiferroic materials. In such cases the helicity of the magnetic spiral is directly correlated with the direction of the ferroelectric moment and measurement of the helicity of magnetic structures is of current interest. Soft x-ray resonant diffraction is particularly advantageous because it combines element selectivity with a large magnetic cross-section. We calculate the polarization dependence of the resonant magnetic x-ray cross-section (electric dipole transition) for the basal plane magnetic spiral in hexaferrite Ba0.8Sr1.2Zn2Fe12O22 and deduce its domain population using circular polarized incident radiation. We demonstrate there is a direct correlation between the diffracted radiation and the helicity of the magnetic spiral.
Unknown changes in the crystalline order of regular TiO$_2$ result in the formation of black titania, which has garnered significant interest as a photocatalytic material due to the accompanying electronic changes. Herein, we determine the nature of the lattice distortion caused by an oxygen vacancy that in turn results in the formation of mid-band gap states found in previous studies of black titania. We introduce an innovative technique using a state-of-the-art silicon drift detector, which can be used in conjunction with extended x-ray absorption fine structure (EXAFS) to measure bulk interatomic distances. We illustrate how the energy dispersive nature of such a detector can allow us an unimpeded signal, indefinitely in energy space, thereby sidestepping the hurdles of more conventional EXAFS, which is often impeded by other absorption edges.
We fabricate LaxSr2-x-yBayIrO4-delta thin films by pulsed laser deposition, in an effort to realize the effective carrier doping and metallization in the Sr2IrO4 system. We design ideal in-plane Ir-O-Ir frame structure by utilizing tensile substrate strain and Ba substitution, as well as control La doping and oxygen deficiency. This enables us to elucidate relation between the charge transport and the carrier density through systematic changes from original p-type spin-orbit Mott insulator to highly doped n-type metal.