Strong Te Research Articles

The Predicted fcc Superconducting Phase for Compressed Se and Te

In the framework of the ab initio random structure search method, we show that elemental Se and Te undergo pressure-induced structural transition from the bcc to fcc phase, in agreement with the theoretical results previously reported. By means of the pseudopotential plane-wave method based on density functional perturbation theory, the fcc structure for both elements is found to be another phonon-mediated superconducting phase of these materials. With a reasonable value for the Coulomb pseudopotential μ* = 0.12, the maximum superconducting transition temperature Tc in the fcc phase of Se and Te is estimated to be about 5.7 K and 4.6 K, respectively. Furthermore, we show that in the entire fcc phase for Se and Te, the superconducting transition temperature decreases together with the increase in pressure, leading to the final suppression of the superconductivity. It is suggested that such behavior is mainly caused by the rapid increase in the mean-square phonon frequency 〈ω2〉 with pressure. Finally, a very strong electron-phonon coupling value, for both Se and Te in the fcc phase, is found along the G—K high symmetry lines.

Effective radius of heating of the lower ionosphere by intense shortwave radiation

The results of a theoretical analysis of the radial distribution of electron temperature Te in the area of heating of the lower ionosphere by intense shortwave radiation are presented. It was established that effective radius reff of heating at a certain height may differ significantly from the characteristic radius of illumination of the ionosphere (a) by radiation at this height. At the boundary of the heating area (r = reff), the characteristic radial scale of Te changes is less than the corresponding scale of changes in the squared amplitude of the radiation electric field, and it is almost independent of the amplitude value; i.e., the formation of a relatively strong Te gradient at such a boundary is a common feature of heating of the lower ionosphere by intense shortwave radiation.

Study of thermal, structural and optical properties of tellurite glass with different TiO2 composition

Tellurite glasses with different TiO2 composition are elaborated and characterized. Differential Scanning Calorimetry (DSC) measurements show an improvement of the stability factor ΔT of the glass with the increase of TiO2 composition which can indicate a reinforcement of the network. Both Raman and FTIR results prove that the Te–O–Te inter-chain linkages are progressively substituted by stronger Te–O–Ti bridges that are at the origin of the increase of the thermal stability of the glass. However, for glasses with high TiO2 composition, some TiO4 polyhedron and crystalline phases of TiTe3O8 can be formed in the amorphous phase.It was found that TiO2 contributes in the highest extent to decrease the glass energy gap and to increase the refraction index due to Ti4+ that could have a role as network modifier. The Urbach energy Eu was found to decrease with TiO2 compositions which suggest the possibility of long range order locally arising from the formation of TiTe3O8 groups.

First-principles study of the ferroelectric and optical properties of BaTeMo 2O 9

Electronic structure, spontaneous polarization and optical properties of single crystal BaTeMo 2O 9 have been investigated based on density-functional theory. It is found that BaTeMo 2O 9 has a direct-band-gap of 2.78 eV and there exists obvious hybridization between O 2p–Te 5p and O 2p–Mo 4d states. The results show that this compound is a good ferroelectric with large spontaneous polarization, which mainly arises from the strong Te–O and Mo–O hybridization. The interband contributions to the peaks of the optical spectra are discussed in details. Furthermore, the nonlinear optical properties are calculated by using 2 n + 1 theorem applied to an electric-field dependent energy functional. The large NLO susceptibilities reveal that BaTeMo 2O 9 is a high-performance NLO crystal.

Synthesis and structure of new aryltellurenyl halides derived from (dmpTe) 2 (dmp = 2,6-dimethoxyphenyl)

[RTeTeR] (R = dmp = 2,6-dimethoxyphenyl) ( 1) reacts with bromine to give [RTeTe(Br) 2R] ( 2) and [RTeBr 3] ( 3), and with SOCl 2 to yield [RTeTe(Cl) 2R] ( 5) and [RTeCl 3] ( 6). The recrystallization of compound 3 in acetone produces [RTeBr 2(CH 2–C(O)–CH 3)] ( 4). The hydrolysis of 2 in aqueous ammonia and methanol containing media affords the methoxy/oxo-derivative [RTe(μ-O)(OCH 3)] 2 ( 7). All the title compounds were obtained with good yields, and strong Te⋯O (methoxy), as well as Te⋯X (X = Br, Cl) secondary interactions, support the distorted octahedral configurations shown mostly in the polymeric compounds 3, 4, 5 and 6. Complexes 2 and 5 close the series of compounds with the structure [RTeTe(X) 2R] (X = Cl, Br, I), started earlier with [RTeTe(I) 2R].

Influence of Si and N additions on structure and phase stability ofGe2Sb2Te5 thin films

The influence of Si and N in Ge2Sb2Te5 (space group ) on structure and phase stability thereof was studied experimentally by thin film growthand characterization as well as theoretically by ab initio calculations. It was foundthat Si and N most probably accumulate in the amorphous matrix embeddingGe2Sb2Te5 grains. The incorporation of Si and N in these samples causes an increase of thecrystallization temperature and the formation of finer grains. N is more efficient inincreasing the crystallization temperature and in reducing the grain size than Si which canbe understood based on the bonding analysis. The incorporation of both Si and N inGe2Sb2Te5 is energetically unfavourable, leading to finer grains and larger crystallization temperatures.While in the case of Si additions no significant changes in bonding are observed, Nadditions appear to enable the formation of strong Te–N bonds in the amorphous matrix,which are shown to be almost twice as strong as the strongest bonds in unalloyedGe2Sb2Te5.

Dy2Te3O9: Der erste Vertreter von Lanthanoid(III)‐Oxotelluraten der Zusammensetzung M2Te3O9

AbstractDy2Te3O9: The First Representative of Lanthanoid(III)‐Oxotellurates(IV) with the Composition M2Te3O9Single crystals of Dy2Te3O9 could be obtained by reaction of Dy2O3 with TeO2 in a 1 : 4 molar ratio as first representative of the formula type M2Te3O9. In this oxotellurate(IV), the lanthanoid(III) cations are hepta‐ (capped trigonal prism: Dy4) and octacoordinated (bicapped trigonal prisms: Dy1–Dy3) by oxygen atoms, respectively. These dysprosium–oxygen polyhedra build up layers parallel to the (100) plane by sharing common edges as dominating feature of the crystal structure. Because of the proportionally higher Dy2O3 moiety in Dy2Te3O9 (Dy2O3 : TeO2 =1 : 3), these ∞2{[Dy4O17]22–} sheets appear more bulky and corrugated than the almost planar ∞2{[Dy2O11]16–} layers in Dy2Te4O11 (Dy2O3 : TeO2 = 1 : 4). In spite of its rather complex constitution, the tellurium–oxygen partial structure in Dy2Te3O9 shows remarkably different motifs. The six crystallographically independent Te4+ cations are each surrounded by three oxygen atoms, which form together with the lone‐pair electrons [TeO3]2– ions of ψ1‐tetrahedral shape. However, three of them show higher coordination numbers such as CN = 3+3 (Te1) and CN = 3+2 (Te2 and Te3) by strong secondary Te···O interactions with adjacent oxotellurate(IV) units, whilst the other three (Te4–Te6: CN = 3) have no secondary contacts at all. The typical 3+1‐coordination of Te4+ as in Dy2Te4O11 and generally common in other rare‐earth metal(III) oxotellurates(IV) does not occur. Also oxoditellurate(IV) units ([Te2O5]2–) as in Dy2Te4O11 or even more condensed ones ([Te3O8]4–) as in Dy2Te5O13 cannot be observed in Dy2Te3O9, owing to its proportionally smaller TeO2 moiety. Five of the [TeO3]2– ions group to screw‐like strands along the [010] direction according to ∞1{[Te5O15]10–} through strong secondary Te···O contacts, whereas the remaining [(Te6)O3]2– units reside isolated within channelsof the porous ∞2{[Dy4O17]22–} sheets. The linkage of the ∞2{[Dy4O17]22–} partial structure to achieve a framework only takes place by [(Te4)O3]2– ions through Dy3–O–Te4–O–Dy4 bridges. Between the ∞2{[Dy4O17]22–} layers as well as within their channels there is still enough space left to accommodate the stereochemically active lone‐pair electrons at the Te4+ ions.

Optically pumped lasing of GaN/AlGaN structures grown along a non‐polar crystallographic direction

AbstractDue to their undesirable spontaneous and piezoelectric polarizations, nitride structures grown along polar directions are subject to high built‐in electric fields. A promising means of overcoming the effects related to built‐in electric fields and, thereby, obtaining higher quantum efficiencies is to grow nitrides along a non‐polar direction. It is our intention to report our recent progress on non‐polar homo‐epitaxial structures grown by PAMBE along the (11$/bar 2$0) non‐polar direction. As a substrate we used GaN bulk crystals grown by the High Pressure Solution method and overgrown by HVPE. As has been previously reported, non‐polar AlGaN/GaN quantum wells possess sharp excitonic lines. We are able to present optically pumped laser action on separate confinement heterostructures. Laser action is clearly proved by spontaneous emission saturation, abrupt line narrowing, and the strong TE polarization of output light. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Iodidomesityltellurium(II) iodidotrimesitylditellurium(II)(Te—Te)

In the title compound, C9H11ITe·C27H33ITe2 or (Mes)TeI·(Mes2Te)TeI(Mes) (Mes is mesityl or 2,4,6-trimethyl­phen­yl), a strong Te⋯I inter­action of 3.3157 (9) A links the Te atom of an iodidomesityltellurium(II) moiety, (Mes)TeI, and an I atom of the iodidotrimesitylditellurium(II) unit, (Mes2Te)TeI(Mes). Further weak Te⋯I contacts of 4.0818 (9) A give rise to one-dimensional chains along the b axis in the crystal structure. An intra­molecular C—H⋯π(arene) inter­action is present in the (TeMes2)TeI(Mes) moiety, with a C⋯Cg distance of 3.497 (9) A and a C—H⋯Cg angle of 142° (where Cg is the centroid of a mesityl ring of the Mes2Te moiety).

Emission intensities and line ratios from a fast neutral helium beam

The emission intensities and line ratios from a fast neutral helium beam is investigated in the Madison Symmetric Torus (MST) [R. N. Dexter, D. W. Kerst, T. W. Lovell, S. C. Prager, and J. C. Sprott, Fusion Technol. 19, 131 1991]. Predicted He I line intensities and line ratios from a recently developed collisional-radiative model are compared with experiment. The intensity of singlet lines comes mostly (>95%) from the contribution of the ground state population and is very weakly dependent on the initial metastable fraction at the observation point in the plasma core. On the other hand, the intensity of triplet lines is strongly affected by the local metastable state (21S and 23S) populations and the initial metastable fraction plays an important role in determining line intensities. The fraction of local metastable states can only be estimated by making use of electron temperature (Te), electron density (ne), and effective ion charge (Zeff) profiles as inputs to the population balance equations. This leads triplet lines to be unusable for the investigation of their local plasma parameter dependence. The ratio of singlet lines at 667.8nm and 492.2nm (I667∕I492) as well as the ratio of 667.8nm and 501.6nm lines (I667∕I501) has been investigated for the dependence on Te and ne both theoretically and experimentally. I667∕I492 shows strong dependence on ne with weak sensitivity to Te. Measurements and predictions agree quantitatively within a factor of 2. There has been no ratio of singlet lines identified to have strong enough Te dependence yet. The ratios are expected to be reasonably insensitive to the variation of Zeff.

Optically pumped GaN∕AlGaN separate-confinement heterostructure laser grown along the (112¯0) nonpolar direction

This letter concerns experiments on optically pumped GaN∕AlGaN separate-confinement heterostructure laser structures grown by plasma assisted molecular beam epitaxy. The structures were grown along the (112¯0) nonpolar crystallographic direction on a bulk GaN substrate. Different widths of GaN quantum wells were applied in the studied structures. Laser action is clearly demonstrated by the spontaneous emission saturation, abrupt line narrowing, and strong TE polarization of output light. A lasing threshold was reached at an excitation power density of 260kW∕cm2 for a 700-μm-long cavity at room temperature.

Polarization characteristics of ZnO rib waveguide random lasers

Ultraviolet ZnO rib waveguide random lasers with stripe width ranging from 1.7 to 6.5μm have been fabricated. It is found that the ZnO random lasers demonstrate strong TE lasing emission especially for rib waveguide with a narrower width. Rate equation analysis has shown that the dominant TE lasing emission is due to the corresponding large confinement factor and scattering strength inside the random media with rib waveguide. In addition, Fourier transform studies of the lasing spectra show that the width of rib waveguide constrains the formation of closed-loop paths of lights and, hence, reduces the number of lasing peaks.

Lithospheric domains and controls on kimberlite emplacement, Slave Province, Canada: Evidence from elastic thickness and upper mantle composition

We have mapped the deep structure of the Slave craton by combining analysis the effective elastic thickness (Te) with data on mantle samples from numerous kimberlites. Three‐dimensional mapping of the subcontinental lithospheric mantle (SCLM), using mantle‐derived xenoliths and xenocrysts in kimberlites, has shown that much of the craton is underlain by a strongly layered SCLM; a highly depleted upper layer (low in basaltic components Ca, Al, Fe) is separated from a relatively fertile lower layer by a sharp boundary. This boundary lies at 140–150 km depth in the Lac de Gras area and shallows to ≤100 km in the northern and southern parts of the Craton. Weak lithosphere (Te < 25 km) on the northern edge of the craton may reflect the intrusion of the Mackenzie Plume (circa 1270 Ma). The strongest lithosphere (Te > 56 km), in the younger eastern part of the craton, is separated from the older western part by a zone of steep Te gradient parallel to the major locus of kimberlite intrusion, which may map the deep extension of the boundary between the two domains. Another strong Te gradient across the Kilohigok Basin accompanies a marked compositional change in the upper layer of the lithospheric mantle; the Basin probably marks a major translithospheric fault. Correlations between Te and mantle composition suggest that Te is strongly influenced by the rheology of the upper mantle.

Lutetium(III) oxotellurate(IV), Lu2Te4O11

Rare-earth(III) oxotellurates(IV) with the composition M2Te4O11 are known for yttrium and all lanthanides except promethium and lutetium from single-crystal X-ray structure determinations. Single crystals of the last missing non-radioactive isostructural compound, Lu2Te4O11, can now be obtained by modifying the common method of synthesis from the binary oxides (Lu2O3 and TeO2; 1:4 molar ratio) using torch-sealed non-evacuated silica ampoules as reaction containers. The structure contains layers of edge-sharing [LuO8] polyhedra connected by oxotellurate(IV) chains. These consist of [TeO3]2− and [Te2O5]2− anions (with ψ1-tetra­hedral oxygen coordination for all central Te4+ cations) linked by strong secondary Te—O inter­actions.

Electronic structure of layered transition-metal dichalcogenidesNb1−xTixXc2 (Xc = S,Se, Te) studied by angle-resolved photoemission spectroscopy

We have performed systematic high-resolution angle-resolved photoemissionspectroscopy (ARPES) on layered transition-metal dichalcogenidesNb1−xTixXc2 (Xc = S, Se, Te) to study the mechanism of metal–semiconductor transition as a function ofx and Xc. We founda pseudogap near EF in the S- and Se-based compounds while a clear Fermi edge is observed in the Te-basedcompound. The existence of the pseudogap indicates the strong scattering of electrons by therandom potential caused by substitution, which prevents formation of the charge densitywave and at the same time decreases the metallic conductivity. The metallic character ofNb1−xTixTe2 is explained in terms of the relatively strong Te 5p–Nb 4d (Ti 3d) hybridization nearEF.

Large polarization change in two-dimensional metallic photonic crystals in subterahertz region

Interesting polarization characteristics of two-dimensional metallic photonic crystals (2D-MPC) in the subterahertz region have been found. We measured the temporal wave form of the THz wave transmitted thorough the 2D-MPC using a THz time domain spectroscopic system. The linear polarization of the incident THz wave transmitted through the 2D-MPC becomes elliptical with a slight tilting of the incident angle from the normal incidence. This result indicates that the 2D-MPC behaves not only as a band-pass filter already reported, but also as a wave plate. This large polarization change is attributed to the strong TE–TM polarization mixing in the 2D-MPC.

Analysis of internal transport barrier heat diffusivity from heat pulsepropagation induced by an ITB event in JT-60U reverseshear plasmas

Internal transport barrier (ITB) formation in the positive shear zone ofreverse shear (RS) plasmas in the JT-60U tokamak is described as a series ofITB events, which are abrupt in time and widein space variations of heat diffusivity, observed as the simultaneous rise and decay of the electrontemperature Te in two zones. A new source of heat pulse propagation(HPP) is found in RS plasmas. HPP is created by an ITB event. For the last ITBevent described in the present paper, the region of strong(~20 keV s-1) Te rise is initially well localized(~4 cm) in space. Later, a slow diffusive broadening of the risingTe perturbation is seen. Outward HPP is analysed in the region with~8 cm width fully located in the positive shear space zone. Values ofthe electron dynamic heat diffusivity as low as ~0.1 m2 s-1 arefound. A similar low value of the ion dynamic heat diffusivity (close to theneoclassical value) is obtained for ion HPP. An important consequence of HPPanalysis is the absence of electron and ion `heat pinch' in the ITB region.

Mode coupling in tilted planar waveguide gratings

Three different grating tilting formulas for predicting the optimum grating tilt angle for strong mode coupling in planar waveguide gratings are derived. The optimum tilt angles obtained by the ray-optics approach deviate ~1 degrees for transmissive mode coupling and ~1.5 degrees for reflective mode coupling from those computed by the coupled-mode approach. The coupled-mode analysis and ray-optics analysis of the tilted planar waveguide gratings show that the transmissive planar waveguide gratings should be tilted more than 84 degrees for strong TE(0)-to-TE(mu) mode coupling near 1550 nm.

Influence of the Te–Te bonds on the valence bands of Tl–Te compounds

The electronic structure of Tl 5Te 3, TlTe and Tl 2Te 3 is theoretically investigated. We show that the valence band of Tl 5Te 3 is formed by 2 peaks at about 6 eV and 1 eV below the top of the valence band. The peak at 6 eV has a Tl 6 s character and its shape can be explained by the Tl–Tl interactions. The peak at 1 eV has a strong Te 5 p character and is asymmetric because of the Tl–Te bond length distribution. In addition to these 2 peaks the DOS of TlTe and Tl 2Te 3 exhibit a third peak at about 3 eV below the top of the valence band which can be related to the existence of short Te–Te bonds in these 2 compounds. The theoretical results explain the observed main changes in the XPS valence bands for the sequence Tl 5Te 3–TlTe–Tl 2Te 3.

Emission spectroscopy as a noninvasive probe for process control during radio frequency sputter deposition of PbTe

An rf magnetron discharge used to sputter PbTe in Ar is analyzed using emission spectroscopy. The intensity (I) of the strong Pb, Te, and Ar emission lines is determined near the target and near the substrate as the rf power or gas pressure is varied. It is shown that as the rf power is varied at pressures below 0.5 Pa, the electron temperature is not affected, so that the sputtered atom density NPb is proportional to IPb/IAr. The electron temperature is, however, sensitive to pressure changes. Application of a substrate bias voltage has a large effect on the emission spectra which are not understood.