Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userPhotoinduced reduction of surface states in Fe:ZnO
347
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Transition metal doping is known to increase the photosensitivity to visible light for photocatalytically active ZnO. We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been shown to be an efficient photocatalyst. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material. Electronic states corresponding to low-spin Fe 2+ are observed and attributed to crystal field modification at the surface. These states can be important for the photocatalytic sensitivity to visible light due to their deep location in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is a crucial factor limiting the photocatalytic functionality of ZnO, where the most efficient doping concentration have been shown to be 2-4% for Fe doping. Using resonant photoemission spectroscopy we determine the location of Fe 3d states with sensitivity to the charge states of the Fe ion even for multi-valent and multi-coordinated Fe.
rate research
Read More
Thouless quantization of adiabatic particle transport permits to associate an integer topological charge with each atom of an electronically gapped material. If these charges are additive and independent of atomic positions, they provide a rigorous definition of atomic oxidation states and atoms can be identified as integer-charge carriers in ionic conductors. Whenever these conditions are met, charge transport is necessarily convective, i.e. it cannot occur without substantial ionic flow, a transport regime that we dub trivial. We show that the topological requirements that allow these conditions to be broken are the same that would determine a Thouless pump mechanism if the system were subject to a suitably defined time-periodic Hamiltonian. The occurrence of these requirements determines a non-trivial transport regime whereby charge can flow without any ionic convection, even in electronic insulators. These results are first demonstrated with a couple of simple molecular models that display a quantum pump mechanism upon introduction of a fictitious time dependence of the atomic positions along a closed loop in configuration space. We finally examine the impact of our findings on the transport properties of non-stoichiometric alkali-halide melts, where the same topological conditions that would induce a quantum pump mechanism along certain closed loops in configuration space also determine a non-trivial transport regime such that most of the total charge current results to be uncorrelated from the ionic ones.
By means of pump-probe time- and angle-resolved photoelectron spectroscopy, we provide evidence of a sizeable reduction of the Fermi velocity of out-of-equilibrium Dirac bands in the quasi-two-dimensional semimetal BaNiS$_2$. First-principle calculations indicate that this band renormalization is ascribed to a change in non-local electron correlations driven by a photo-induced enhancement of screening properties. This effect is accompanied by a slowing down of the Dirac fermions and by a non-rigid shift of the bands at the center of the Brillouin zone. This result suggests that other similar electronic structure renormalizations may be photoinduced in other materials in presence of strong non-local correlations.
We present an accurate study of the static-nucleus electronic energy band gap of solid molecular hydrogen at high pressure. The excitonic and quasiparticle gaps of the $C2/c$, $Pc$, $Pbcn$, and $P6_3/m$ structures at pressures of 250, 300, and 350~GPa are calculated using the fixed-node diffusion quantum Monte Carlo (DMC) method. The difference between the mean-field and many-body band gaps at the same density is found to be almost independent of system size and can therefore be applied as a scissor correction to the mean-field gap of an infinite system to obtain an estimate of the many-body gap in the thermodynamic limit. By comparing our static-nucleus DMC energy gaps with available experimental results, we demonstrate the important role played by nuclear quantum effects in the electronic structure of solid hydrogen. Our DMC results suggest that the metallization of high-pressure solid hydrogen occurs via a structural phase transition rather than band gap closure.
Electrides are an emerging class of materials with excess electrons localized in interstices and acting as anionic interstitial quasi-atoms (ISQs). The spatial ion-electron separation means that electrides can be treated physically as ionic crystals, and this unusual behavior leads to extraordinary physical and chemical phenomena. Here, a completely different effect in electrides is predicted. By recognizing the long-range Coulomb interactions between matrix atoms and ISQs that are unique in electrides, a nonanalytic correction to the forces exerted on matrix atoms is proposed. This correction gives rise to an LA-TA splitting in the acoustic branch of lattice phonons near the zone center, similar to the well-known LO-TO splitting in the phonon spectra of ionic compounds. The factors that govern this splitting are investigated, with isotropic fcc-Li and anisotropic hP4-Na as the typical examples. It is found that not all electrides can induce a detectable splitting, and criteria are given for this type of splitting. The present prediction unveils the rich phenomena in electrides and could lead to unprecedented applications.
By means of Raman spectroscopy of liquid microjets we have investigated the crystallization process of supercooled quantum liquid mixtures composed of parahydrogen (pH$_2$) diluted with small amounts of up to 5% of either neon or orthodeuterium (oD$_2$), and of oD$_2$ diluted with either Ne or pH$_2$. We show that the introduction of Ne impurities affects the crystallization kinetics in both the pH$_2$-Ne and oD$_2$-Ne mixtures in terms of a significant reduction of the crystal growth rate, similarly to what found in our previous work on supercooled pH$_2$-oD$_2$ liquid mixtures [M. Kuhnel et {it al.}, Phys. Rev. B textbf{89}, 180506(R) (2014)]. Our experimental results, in combination with path-integral simulations of the supercooled liquid mixtures, suggest in particular a correlation between the measured growth rates and the ratio of the effective particle sizes originating from quantum delocalization effects. We further show that the crystalline structure of the mixture is also affected to a large extent by the presence of the Ne impurities, which likely initiate the freezing process through the formation of Ne crystallites.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
