Character Of Bands Research Articles

Detailed optical spectroscopy of hybridization gap and hidden-order transition in high-qualityURu2Si2single crystals

We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high quality single crystals of URu$_{2}$Si$_{2}$ of $ac$- and $ab$-plane surfaces. Our data demonstrate the itinerant character of the narrow 5f bands, becoming progressively coherent as temperature is lowered below a cross-over temperature $T^*{\sim}75~K$. $T^*$ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy fermion plasmons with $a$-and $c$-axis polarization. Our observation of a suppression of optical conductivity below 50~meV both along $a$ and $c$-axis, along with a heavy fermion plasmon at 18~meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature $T_{HO}=17.5$~K. In the hidden order phase the low frequency optical conductivity shows a single gap at $\sim 6.5$meV, which closes at $T_{HO}$.

Origin and Magnitude of ‘Designer’ Spin-Orbit Interaction in Graphene on Semiconducting Transition Metal Dichalcogenides

We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultra-strong and extremely robust, despite it being merely interfacially-induced, with neither graphene nor the TMD substrates changing their structure. This is found to be the case irrespective of the TMD material used, of the transport regime, of the carrier type in the graphene band, and of the thickness of the graphene multilayer. Specifically, we perform weak antilocalization measurements as the simplest and most general diagnostic of SOI, and show that the spin relaxation time is very short in all cases regardless of the elastic scattering time. Such a short spin-relaxation time strongly suggests that the SOI originates from a modification of graphene band structure. We confirmed this expectation by measuring a gate-dependent beating, and a corresponding frequency splitting, in the low-field Shubnikov-de Haas magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$. These measurements provide an unambiguous diagnostic of a SOI-induced splitting in the electronic band structure, and their analysis allows us to determine the SOI coupling constants for the Rashba term and the so-called spin-valley coupling term, i.e., the terms that were recently predicted theoretically for interface-induced SOI in graphene. The magnitude of the SOI splitting is found to be on the order of 10 meV, more than 100 times greater than the SOI intrinsic to graphene. Both the band character of the interfacially induced SOI, as well as its robustness and large magnitude make graphene-on-TMD a promising system to realize and explore a variety of spin-dependent transport phenomena, such as, in particular, spin-Hall and valley-Hall topological insulating states.

Examples of UV–Vis profiles use as tool for evidence of the metallophthalocyanines transformation

The UV–Vis spectra for a set of MPcs (Mmetal, Pc = phthalocyanine ligand), i.e.: In(III)PcI (1), Hf(IV)PcI2Pht (Pht = phthalonitrile) (2), Sn(II)Pc (3), Sn(IV)PcI2(4), and Ge(IV)PcI2(5) have been examined in two solvents, O-donative acetylacetone, and non-coordinative benzene. The UV–Vis spectra in Hacac solution of 1,2 and 4,5 shows that the axially ligated iodine atoms are replaced by (acac)− anions of the solvent, whereas in 3 the oxygen donors of the solvent causing the auto-oxidation of Sn(II) to Sn(IV) ions and as a result the Sn(II)Pc is transformed into the Sn(IV)Pc(acac)2. The chloride complexes of the 1–5 compounds are formed at Hacac solution after acidification by hydrochloric acid, however each compound solution behaviors specifically. The UV–Vis spectra collected for the studied compounds at benzene solvent both before and after the solution acidization clearly indicate that the respective Q band character (besides 3) remains practically unchanged. The presence of the Cl− ions at the Sn(II)Pc solution in benzene results in the formation of Sn(IV)PcCl2.

Surface Fermi arc connectivity in the type-II Weyl semimetal candidateWTe2

We perform ultrahigh resolution angle-resolved photoemission experiments at a temperature T=0.8 K on the type-II Weyl semimetal candidate WTe$_{2}$. We find a surface Fermi arc connecting the bulk electron and hole pockets on the (001) surface. Our results show that the surface Fermi arc connectivity to the bulk bands is strongly mediated by distinct surface resonances dispersing near the border of the surface-projected bulk band gap. By comparing the experimental results to first-principles calculations we argue that the coupling to these surface resonances, which are topologically trivial, is compatible with the classification of WTe$_{2}$ as a type-II Weyl semimetal hosting topological Fermi arcs. We further support our conclusion by a systematic characterization of the bulk and surface character of the different bands and discuss the similarity of our findings to the case of topological insulators.

Charge doping versus impurity scattering in chemically substituted iron pnictides

To reveal the relative importance of charge doping and defect scattering in substitutionally modified 122 iron pnictides, we perform a systematic first principles study on selected bands at the Fermi level. Disorder effects are induced by various substitutions using an orbital based coherent potential approximation (CPA). Pronounced level shifts of individual bands suggest that transition metal substitutions introduce mobile charge carriers into the system. However, important deviations from such a rigid band scenario as well as spectral broadenings due to impurity scattering correlate with the band character. Finally a $T$-matrix analysis exhibits a larger intraband than interband scattering consistent with an $s^{+-}$ pairing state. Comparing different substitutions reveals an increase of pair-breaking along the transition metal series.

Three-dimensional Dirac cone carrier dynamics inNa3BiandCd3As2

Optical measurements and band structure calculations are reported on three-dimensional Dirac materials. The electronic properties associated with the Dirac cone are identified in the reflectivity spectra of ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ and ${\mathrm{Na}}_{3}\mathrm{Bi}$ single crystals. In ${\mathrm{Na}}_{3}\mathrm{Bi}$, the plasma edge is found to be strongly temperature dependent due to thermally excited free carriers in the Dirac cone. The thermal behavior provides an estimate of the Fermi level ${E}_{F}=25$ meV and the $z$-axis Fermi velocity ${v}_{z}=0.3$ eV \AA{} associated with the heavy bismuth Dirac band. At high energies above the $\mathrm{\ensuremath{\Gamma}}$-point Lifshitz gap energy, a frequency- and temperature-independent ${\ensuremath{\epsilon}}_{2}$ indicative of Dirac cone interband transitions translates into an ab-plane Fermi velocity of 3 eV \AA{}. The observed number of IR phonons rules out the $P{6}_{3}/mmc$ space-group symmetry but is consistent with the $P\overline{3}c1$ candidate symmetry. A plasmaron excitation is discovered near the plasmon energy that persists over a broad range of temperature. The optical signature of the large joint density of states arising from saddle points at $\mathrm{\ensuremath{\Gamma}}$ is strongly suppressed in ${\mathrm{Na}}_{3}\mathrm{Bi}$, consistent with band structure calculations that show the dipole transition-matrix elements to be weak due to the very small $s$-orbital character of the Dirac bands. In ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$, a distinctive peak in reflectivity due to the logarithmic divergence in ${\ensuremath{\epsilon}}_{1}$ expected at the onset of Dirac cone interband transitions is identified. The center frequency of the peak shifts with temperature quantitatively consistent with a linear dispersion and a carrier density of $n=1.3\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}3}$. The peak width gives a measure of the Fermi-velocity anisotropy of $10%$, indicating a nearly spherical Fermi surface. The line shape gives an upper bound estimate of 7 meV for the potential fluctuation energy scale.

Preparation and characterization of the semiconductor CuMnO2 by sol-gel route

Nano crystallites of the crednerite CuMnO2 are prepared by sol–gel method with two-step annealing process. The powder heated at 450°C under air flow shows a mixture of CuO, Mn2O3 and CuxMn3−xO4. However, when calcined at 900°C under N2 atmosphere, the crednerite CuMnO2 with a monoclinic structure (space group: C2/m) is obtained. The Raman spectrum shows a single peak at 679cm−1 assigned to A1g mode whereas the infrared analysis confirms the linearity of CuO23− units. The optical transition at 1.70eV, determined from the diffuse reflectance is attributed to the inter-band d-d transition of Cu+ ion. The oxide exhibits semiconducting properties with an activation energy of 0.21eV. The photo-electrochemical measurement shows p-type conduction due to O2− insertion in the two dimensional lattice. The flat band potential (+0.12 VSCE), indicates a cationic character of both valence and conduction bands deriving from Cu+: 3d orbital.

The Optoelectronic Properties of Sb Doped BBi Compounds

We have determined structural, electronic and optical properties of BBi1 xSbx alloy by using density functional theory based on the full potential linearized augmented plane wave method. For the exchange correlation potential, the generalized gradient approximation of Perdew, Burke, and Ernzerhof has been performed. The structural properties, including lattice constants and bulk modulus have been calculated by changing x concentration. We have investigated the eect of composition on lattice constant, bulk modulus and band gap. Properly, direct/indirect band character of BBi1 xSbx has been investigated depending on the x concentration. Then we have determined some basic linear optical properties BBi1 xSbx alloy in direct band gap region. The obtained results have been compared with available studies. All the calculations have been performed after geometry optimization. As far as we know, no experimental or theoretical data are presently available for the studied ternary alloy BBi1 xSbx (0 < x < 1).

Lone-Pair Stabilization in Transparent Amorphous Tin Oxides: A Potential Route to p-Type Conduction Pathways

The electronic and atomic structures of amorphous transparent tin oxides have been investigated by a combination of X-ray spectroscopy and atomistic calculations. Crystalline SnO is a promising p-type transparent oxide semiconductor due to a complex lone-pair hybridization that affords both optical transparency despite a small electronic band gap and spherical s-orbital character at the valence band edge. We find that both of these desirable properties (transparency and s-orbital valence band character) are retained upon amorphization despite the disruption of the layered lone-pair states by structural disorder. We explain the anomalously large band gap widening necessary to maintain transparency in terms of lone-pair stabilization via atomic clustering. Our understanding of this mechanism suggests that continuous hole conduction pathways along extended lone pair clusters should be possible under certain stoichiometries. Moreover, these findings should be applicable to other lone-pair active semiconductors.

New opportunities for materials informatics: Resources and data mining techniques for uncovering hidden relationships

Abstract

Correlation between topological band character and chemical bonding in a Bi14Rh3I9-based family of insulators

Recently the presence of topologically protected edge-states in Bi14Rh3I9 was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi14Rh3I9 parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi14Rh3I9 and graphene in terms of -molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt- and Pd-based systems, whereas the Os- and Ru-systems remain trivial. Furthermore, the energy position of the metal -band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The -band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.

Dielectric matrices with air cavities as a waveguide photonic crystal

Frequency dependences of the transmission coefficient of a microwave photonic crystal that represents a structure containing alternating layers of ceramic material (Al2O3) with a relatively large number of cavities and foam plastic are studied in the presence and absence of distortions of the periodicity of a photonic structure. The frequency dependences of the transmission coefficient can be analyzed using a model of effective medium that makes it possible to consider the interaction of electromagnetic wave and photonic crystal using a transfer matrix of a 1D photonic crystal. The band character of the frequency dependence of the transmission coefficient of the photonic crystal related to the periodicity of the photonic crystal in the transverse plane for the waveguide with a standard cross section is not manifested in a certain range of material permittivities.

Theoretical study of defect impact on two-dimensional MoS2

Our theoretical findings demonstrate for the first time a possibility of band-gap engineering of monolayer MoS2 crystals by oxygen and the presence of vacancies. Oxygen atoms are revealed to substitute sulfur ones, forming stable MoS2−xOx ternary compounds, or adsorb on top of the sulfur atoms. The substituting oxygen provides a decrease of the band gap from 1.86 to 1.64 eV and transforms the material from a direct-gap to an indirect-gap semiconductor. The surface adsorbed oxygen atoms decrease the band gap up to 0.98 eV depending on their location tending to the metallic character of the electron energy bands at a high concentration of the adsorbed atoms. Oxygen plasma processing is proposed as an effective technology for such band-gap modifications.

The self-associating behavior of pyrrole in liquid xenon

The self-associating behavior of pyrrole in liquid xenon was investigated by analyzing a data set of 185–113 infrared spectra obtained for different concentrations recorded at a constant temperature of 203 K. Analysis of the data using a recently developed least-squares approach allows the vibrational spectra of the monomer and of the different oligomers to be isolated. Apart from the monomer transitions, intense absorption bands originating from pyrrole trimers are observed in almost every spectral region including regions for which no data have yet been reported. Apart from these bands, weak features proving the presence of pyrrole dimer and pyrrole tetramer in the solutions are also reported. The weak character of the dimer bands observed and the low concentrations of these species deduced are explained by the fact that the cryosolutions studied are in chemical equilibrium and by the fact that due to strong cooperative effect present in the trimer, the complexation equilibria are strongly shifted towards the latter species, thereby strongly reducing the equilibrium concentrations of dimer and tetramer.

Interface effect on structural and electronic properties of graphdiyne adsorbed on SiO2 and h-BN substrates: A first-principles study**Project supported by the National Natural Science Foundation of China (Grant No. 51331006), the Key Research Program of Chinese Academy of Sciences (Grant No. KGZD-EW-T06), and the IMR SYNL-Young Merit Scholars Research Grant, China.

We use the first-principles calculation method to study the interface effect on the structure and electronic properties of graphdiyne adsorbed on the conventional substrates of rough SiO2 and flat h-BN. For the SiO2 substrate, we consider all possible surface terminations, including Si termination with dangling bond, Si terminations with full and partial hydrogenation, and oxygen terminations with dimerization and hydrogenation. We find that graphdiyne can maintain a flat geometry when absorbed on both h-BN and SiO2 substrates except for the Si termination with partial hydrogenation (Si-H) SiO2 substrate. A lack of surface corrugation in graphdiyne on the substrates, which may help maintain its electronic band character, is due to the weak Van der Waals interaction between graphdiyne and the substrate. Si-H SiO2 should be avoided in applications since a covalent type bonding between graphdiyne and SiO2 will totally vary the band structure of graphdiyne. Interestingly, the oxygen termination with dimerization SiO2 substrate has spontaneous p-type doping on graphdiyne via interlayer charge transfer even in the absence of extrinsic impurities in the substrate. Our result may provide a stimulus for future experiments to unveil its potential in electronic device applications.

Direct characterization of photoinduced lattice dynamics in BaFe2As2.

Ultrafast light pulses can modify electronic properties of quantum materials by perturbing the underlying, intertwined degrees of freedom. In particular, iron-based superconductors exhibit a strong coupling among electronic nematic fluctuations, spins and the lattice, serving as a playground for ultrafast manipulation. Here we use time-resolved X-ray scattering to measure the lattice dynamics of photoexcited BaFe2As2. On optical excitation, no signature of an ultrafast change of the crystal symmetry is observed, but the lattice oscillates rapidly in time due to the coherent excitation of an A1g mode that modulates the Fe–As–Fe bond angle. We directly quantify the coherent lattice dynamics and show that even a small photoinduced lattice distortion can induce notable changes in the electronic and magnetic properties. Our analysis implies that transient structural modification can be an effective tool for manipulating the electronic properties of multi-orbital systems, where electronic instabilities are sensitive to the orbital character of bands.

First-Principles GGA+U Study of Intermediate-Band Characters from Zn1−x M x O (M = 3d Transition-Metal) Alloys Suitable for High Efficiency Solar Cell* *Supported by the State Key Program for Basic Research of China under Grant No. 2006CB921803, Project of High Technology Research & Development of China (Project No. 2007AA03Z404), National Natural Science Foundation of China under Grant Nos. 61274058, 60990312,

The electronic structure characters are calculated for the Zn1−xMxO alloys with some Zn atoms in ZnO substituted by 3d transition-metal atoms (M), in order to find out which of these alloys could provide an intermediate band material used for fabricating high efficiency solar cell. Especially, among of these alloys, the electronic structure character and optical performance of Zn1−xCrxO alloys clearly show an intermediate band filled partially and isolated from the VB and the CB in energy band structure of ZnO host, and the intermediate band characters can be preserved with increasing Cr concentrations no more than 8.33% in Zn1−xCrxO alloys, at the same time, the ratio 0.52 of Eg FC to Eg VF in Zn1−xCrxO, (x = 4.16%) alloy is closest to the optimal ratio of 0.57. Besides, compared to the ZnO, the optical absorption does indicate a great improved absorption below the calculated band gap of the ZnO and an enhancement of the optical absorption in the whole solar spectral energy range.

Luminescence Properties of Phosphor (Sr&lt;sub&gt;1-x&lt;/sub&gt;Ba&lt;sub&gt;x&lt;/sub&gt;)&lt;sub&gt; 1.95&lt;/sub&gt; SiO&lt;sub&gt;4&lt;/sub&gt;: 0.05Eu&lt;sup&gt;2+&lt;/sup&gt;

The luminescence properties of green-yellow emitting (Sr1-xBax)1.95SiO4:0.05Eu2+ (0≤x≤1) phosphors were discussed in this paper. These phosphors were synthesized through the high temperature solid-state reaction technique and characterized by X-ray diffraction. The experimental results indicated that all the major phase of prepared phosphors gradually varied from α-Sr2SiO4 (x=0) phase to Ba2SiO4 (x=1) phase, and an intermediate phase between the two phases was observed at x=0.5. The excitation spectra (λem λem=524 nm) and emission spectra (λex λex=400 nm) showed a broad band character with peak located at 365 nm and range from 579 nm to 508 nm, respectively. With the increase of concentration of barium, the thermal stability of the phosphors increased first when x is less than 0.5, and then it decreased when x surpasses 0.5. In addition, luminescence intensity was enhanced with the barium concentration. Under the 400 nm NUV light excitation the (Sr1-xBax) 1.95SiO4:0.05Eu2+ phosphors have a potential application in white LED.

Silicene field-effect transistors operating at room temperature.

Free-standing silicene, a silicon analogue of graphene, has a buckled honeycomb lattice and, because of its Dirac bandstructure combined with its sensitive surface, offers the potential for a widely tunable two-dimensional monolayer, where external fields and interface interactions can be exploited to influence fundamental properties such as bandgap and band character for future nanoelectronic devices. The quantum spin Hall effect, chiral superconductivity, giant magnetoresistance and various exotic field-dependent states have been predicted in monolayer silicene. Despite recent progress regarding the epitaxial synthesis of silicene and investigation of its electronic properties, to date there has been no report of experimental silicene devices because of its air stability issue. Here, we report a silicene field-effect transistor, corroborating theoretical expectations regarding its ambipolar Dirac charge transport, with a measured room-temperature mobility of ∼100 cm(2) V(-1) s(-1) attributed to acoustic phonon-limited transport and grain boundary scattering. These results are enabled by a growth-transfer-fabrication process that we have devised--silicene encapsulated delamination with native electrodes. This approach addresses a major challenge for material preservation of silicene during transfer and device fabrication and is applicable to other air-sensitive two-dimensional materials such as germanene and phosphorene. Silicene's allotropic affinity with bulk silicon and its low-temperature synthesis compared with graphene or alternative two-dimensional semiconductors suggest a more direct integration with ubiquitous semiconductor technology.

On the influence of tetrahedral covalent-hybridization on electronic band structure of topological insulators from first principles

Based on first-principles calculations, we investigate the influence of tetrahedral covalent-hybridization between main-group and transition-metal atoms on the topological band structures of binary HgTe and ternary half-Heusler compounds, respectively. Results show that, for the binary HgTe, when its zinc-blend structure is artificially changed to rock-salt one, the tetrahedral covalent-hybridization will be removed and correspondingly the topologically insulating band character lost. While for the ternary half-Heusler system, the strength of covalent-hybridization can be tuned by varying both chemical compositions and atomic arrangements, and the competition between tetrahedral and octahedral covalent-hybridization has been discussed in details. As a result, we found that a proper strength of tetrahedral covalent-hybridization is probably in favor to realizing the topologically insulating state with band inversion occurring at the Γ point of the Brillouin zone.