Search results for "BAND"
showing 10 items of 2610 documents
Spiral graphone and one sided fluorographene nano-ribbons
2013
The instability of a free-standing one sided hydrogenated/fluorinated graphene nano-ribbon, i.e. graphone/fluorographene, is studied using ab-initio, semiempirical and large scale molecular dynamics simulations. Free standing semi-infinite arm-chair like hydrogenated/fluorinated graphene (AC-GO/AC-GF) and boat like hydrogenated/fluorinated graphene (B-GO/B-GF) (nano-ribbons which are periodic along the zig-zag direction) are unstable and spontaneously transform into spiral structures. We find that rolled, spiral B-GO and B-GF are energetically more favorable than spiral AC-GO and AC-GF which is opposite to the double sided flat hydrogenated/fluorinated graphene, i.e. graphane/fluorographene…
Pressure Dependence of the Bandgap Bowing in Zinc-Blende ZnTe 1− x Se x
2002
We report on the pressure dependence of the bandgap bowing in the ZnTe 1 m x Se x alloy, in the whole composition range. The bandgap bowing parameter is shown to increase almost linearly with pressure from 1.23 at ambient pressure to 1.6 at 7 GPa. Saturation effects observed in the pressure dependence for x =0.1 and x =0.2 are shown to be related to the direct-to-indirect crossover. Results are discussed and interpreted in the framework of structural relaxation models for gap bowing. A prediction of these models (the negative bowing of the o 15 m ;X 1 transition) is shown to be compatible with the fact that the direct-to-indirect crossover pressure increases with the Se content.
Pressure-induced band anticrossing in two adamantine ordered-vacancy compounds: CdGa2S4 and HgGa2S4
2021
Abstract This paper reports a joint experimental and theoretical study of the electronic band structure of two ordered-vacancy compounds with defect-chalcopyrite structure: CdGa2S4 and HgGa2S4. High-pressure optical-absorption experiments (up to around 17 GPa) combined with first-principles electronic band-structure calculations provide compelling evidence of strong nonlinear pressure dependence of the bandgap in both compounds. The nonlinear pressure dependence is well accounted for by the band anticrossing model that was previously established mostly for selenides with defect chalcopyrite structure. Therefore, our results on two sulfides with defect chalcopyrite structure under compressio…
Novel Carbon Nanotubes Rolled from 6,6,12-Graphyne: Double Dirac Points in 1D Material
2017
Two kinds of novel carbon nanotubes, namely, (N, 0) and (0, N) 6,6,12-graphyne nanotubes (6,6,12-GNTs), are constructed by rolling up the rectangular 6,6,12-graphyne sheets along two different sides into cylinders. The mechanical and electronic properties of 6,6,12-GNTs with varied N from 3 to 20 are investigated by using density functional theory. Unlike the single-wall carbon nanotubes, the Young’s moduli of 6,6,12-GNTs do not remain constant in the case of (N, 0), but the (0, N) tubes possess almost the same one around 0.32 TPa. The band structures and density of states are also exhibited in this work. When the tube sizes N are bigger than four, Dirac points appear at Fermi level in the …
First-principles study of the electronic structure of cubicGaS: Metallic versus insulating polymorphs
2007
The electronic structure of different polymorphs of gallium sulphide $(\mathrm{GaS})$ with cubic structure is investigated by means of first-principles band structure calculations in connection with experimental reports on a metastable semiconducting cubic form of this material. The expected metallic character of simple cubic phases containing one $\mathrm{GaS}$ group per unit cell (rocksalt or zinc-blende) is confirmed by the calculations. A cubane-based zinc-blende structure is found to exhibit a band gap which is compatible with experimental results but the unit cell parameter is much larger than the reported ones. We have also studied cubic phases containing hydrogen. It is found that t…
Hall effect and electronic structure of films
2010
Abstract Tunneling experiments have shown that in order to retain half-metallicity at room temperature not only a large gap is required but also a Fermi energy considerably distant from the minority band edges. We correlate the position of the Fermi energy in the spin minority gap obtained from band structure calculations to Hall effect experiments. As a model system we chose Co 2 Fe x Mn 1 - x Si , where the Fermi energy was calculated to move from the valence band edge of the minority states to the conduction band edge with increasing x . On high quality laser ablated epitaxial films we observe a sign change of both the normal and the anomalous Hall effect with doping. The experimental da…
Band structure of indium selenide investigated by intrinsic photoluminescence under high pressure
2004
This paper reports on photoluminescence experiments in $n$-type indium selenide $(T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ under hydrostatic pressure up to 7 GPa at low and high excitation densities. Photoluminescence measurements at low excitation density exhibit a broad band around the energy of the direct band gap of $\mathrm{InSe}$ and with the same pressure dependence. The reversible increase of its linewidth above $1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is associated to a direct-to-indirect band-gap crossover between valence band maxima. The reversible decrease of its intensity above $4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is interpreted as evidence of a direct-to-indirect b…
Resonant Raman characterization of InAlGaN/GaN heterostructures
2006
InAlGaN/GaN heterostructures and thin films with In composition ranging from 0.03 to 0.1 are characterized by means of Raman scattering excited at various energies in the ultra violet range, tuning the laser excitation energy through the band gap of In x Al y Ga 1-x-y N. It is shown that the addition of In to the Al y Ga 1-y N alloy diminishes considerably the vibration energy of the A 1 (LO) phonon mode. The phonon line is asymmetric on the low energy side, and the asymmetry increases with In content, while the main peak shifts to lower energies. A shift of the phonon energy has also been observed when the excitation energy is close to the absorption edge of the In x Al y Ga 1-x-y N layer.…
Picosecond energy relaxation in
2005
Abstract Investigating the reflectance response dynamics of La 0.67 Ca 0.33 MnO 3 thin films after excitation by femtosecond laser pulses, we identify for the first time a picosecond relaxation step which only exists below the Curie temperature T C . The relaxation time increases from zero at T C to several picoseconds at low temperatures. The data can be explained with the existence of a magnetization-related effective energy gap, and assuming relaxation between these states to be mediated by a Frohlich-type electron–lattice interaction.
Hysteresis and change of transition temperature in thin films of Fe{[Me2Pyrz]3BH}2, a new sublimable spin-crossover molecule.
2015
Thin films of the spin-crossover (SCO) molecule Fe{[Me(2)Pyrz](3)BH}(2) (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photo-emission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)(2)(NCS)(2), the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition tempe…