Electronic structure and spin–orbit coupling in ternary transition metal chalcogenides Cu2TlX2 (X = Se,Te)

Ternary transition metal chalcogenides provide a rich platform to search and study intriguing electronic properties. Using angle-resolved photoemission spectroscopy and ab initio calculation, we investigate the electronic structure of Cu$_{2}$Tl$X_{2}$ ($X=\text{Se, Te}$), ternary transition metal chalcogenides with quasi-two-dimensional crystal structure. The band dispersions near the Fermi level are mainly contributed by the Te/Se p orbitals. According to our ab-initio calculation, the electronic structure changes from a semiconductor with indirect band gap in Cu$_{2}$TlSe$_{2}$ to a semimetal in Cu$_{2}$TlTe$_{2}$, suggesting a band-gap tunability with the composition of Se and Te. By comparing ARPES experimental data with the calculated results, we identify strong modulation of the band structure by spin-orbit coupling in the compounds. Our results provide a ternary platform to study and engineer the electronic properties of transition metal chalcogenides related to large spin-orbit coupling.     

Study of122Te(n, γ)123 Te reaction

-rays emitted after thermal neutron capture in¹²²Te have been investigated for the first time. Energies and intensities of 80 transitions assigned to¹²³Te have been determined. Fortyeight of these transitions have been placed in the¹²³Te level scheme among 27 excited levels. Two new levels of 1808·8 and 2092·3 keV,J=(1/2, 3/2), have been observed. The neutron binding energy has been found to be 6929·1±0·5 keV. The role of direct capture mechanism has been examined.     

LnPO4: RE3+ (La = La, Gd; RE = Eu, Tb) nanocrystals: solvo-thermal synthesis, microstructure and photoluminescence

Through altering the solvents, we have obtained the Eu³⁺/Tb³⁺ ions-doped LnPO₄ (Ln = La, Gd) phosphors with different particle sizes, microstructures and morphologies via a facile solvo-thermal technology. X-ray powder diffraction (XRD), transmission electron microscope (TEM), and scanning electron microscope (SEM) have shown that the products using different solvents have various structures and morphologies. With the increase of DMA/water volume ratio, the microstructure has changed from hexagonal phase to monoclinic one, and the morphology from nanorod to nanoparticle, revealing the decreased oriented growth. The presence of DMA is an important factor in guiding the anisotropic growth of hexagonal lanthanide phosphates. Besides, N-methyl-2-pyrrolidone has been used as solvent to induce the Eu³⁺/Tb³⁺ ions-doped LnPO₄ (Ln = La, Gd) phosphors with different morphologies and structures. Finally, the photoluminescence behaviors of these nanocrystals have been investigated, which are dependent on their microstructures and morphologies.     

Study of the reaction130Te(n, γ)131Te

Energies and intensities of the gamma-ray transitions following the thermal neutron capture in¹³⁰Te are determined. The neutron separation energy for¹³¹Te is evaluated to be 5929.4 ±±0.3 keV. A statistically significant correlation between the reduced transition intensities and the spectroscopic factors from the (d, p) reactions is found. Evidence for the presence of potential capture mechanism is given.On leave fromAl-Faateh University, Physics Department, P.O. Box 13371, Tripoli, Libya.     

Influence of γ-irradiation on the optical and electrical properties of Se(0.85) Te(0.15) films

Abstract

Se_(0.85) Te_(0.15) films were prepared by thermal evaporation under vacuum on glass substrate. The optical and electrical properties of as deposited and irradiated Se_(0.85) Te_(0.15) films with different γ-doses are reported.

The optical constants (absorption coefficient (α), extinction coefficient (k), refractive index (n) and dielectric constants (?, ?) of unirradiated and irradiated films were calculated. The value of allowed direct optical energy gap of Se_(0.85) Te_(0.15) films increased from 1.47 eV. to 1.72 eV. with increasing the γ-doses to 2.5 Mrad. The irradiated films have lower resistivity than those as deposited films (unirradiated). The activation energy (ΔE) increases from 0.72 eV. to 0.86 eV. with increasing γ-doses to 2.5 Mrad.     

AlM(M=O,S,Se,Te,Po)势能函数和光谱研究

本文用B3LYP方法,相对论有效实势,其中铝原子用6-311 G(d,p)基组,其它用sddall基组分别对AlM(M=O,S,Se,Te,Po)分子进行了优化计算,得出了它们的平衡结构和双原子分子光谱常数.计算结果表明:它们的基态结构分别为AlO(2Σ),AlS(2Σ),AlSe(2Σ),AlPo(2Σ),其中AlO和AlS的计算数据与实验结果是一致的.氧簇元素原子与铝结合的中性双原子分子的平衡距离,随原子序数的增加而增长,离解能随原子序数的增加而减小,但它们都能形成稳定的双原子中性分子.它们的谐振频率随原子序数的增加而减小,它们的最低空轨道和最高占据轨道能量随原子序数的增加而略有增加,但变化不大.其能隙随原子序数的增加而略有增加.本文从理论上系统地研究了氧簇元素的原子(包括放射性较强,不太稳定的Po)与铝原子形成的中性双原子分子的光谱和分析势能函数.     

Pre-exponential factor for the crystallization of glassy Se80Te20 and Se75Te20M5 alloys: applicability of Meyer–Neldel rule

The Meyer–Neldel rule or MN rule (also known as compensation effect) is an empirical law known since 1937. This rule is observed in wide range of phenomena in physics, chemistry, biology and electronics. Many activated phenomena, including solid state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers, and electronic conduction in amorphous semiconductors obey the MN rule. In the present article, we report the MN rule in the non-isothermal crystallization in glassy Se₈₀Te₂₀ and Se₇₅Te₂₀M₅ (M = Ag, Cd, In, Sb) alloys. We have observed MN rule between pre-exponential factor K _o and activation energy of crystallization E _c in the present case for thermally activated non-isothermal crystallization.     

Electrical and thermal properties of a-(Se70Te30)100−x(Se98Bi2)x (0⩽x⩽20) alloys

In electrical properties, the dc conductivity and photoconductivity measurements have been made in vacuum evaporated thin films of a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system, in the temperature range (308–355 K). It has been observed that dc conductivity and activation energy depend on the Bi concentration. Photocurrent dependence on incident radiation has also been observed which follow the power law (I_ph∝F^γ). Transient photocurrent exhibits the non-exponential decay time. All these parameters show that the recombination within the localized states is predominant. In crystallization kinetics, the heating rate dependence of glass transition and crystallization temperatures is studied to calculate the activation energy for thermal relaxation and activation energy for crystallization. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te–Bi glassy system.     

Half-metallic behavior of Co2YZ (Y = V,Cr; Z = Al,Ga) under pressure: a DFT study

The structural, electronic and magnetic properties of Co-based Heusler compounds Co₂YZ (Y = V, Cr; Z = Al, Ga) under pressure are studied using first principles density functional theory. The calculations are performed within generalized gradient approximation. The total magnetic moment decreases slightly on compression. Under application of external pressure, the valence band and conduction band are shifted downward which leads to the modification of electronic structure. There exists an indirect band gap along Г–X for all the alloys studied. Co₂CrAl shows half-metallic nature up to 85 GPa. After this pressure transition from true half-metallic behavior to nearly half-metallic behavior is observed and at 90 GPa it shows metallic behavior. Co₂CrGa shows nearly half-metallic behavior at ambient pressure, but true half-metallic behavior is observed as pressure is increased to 100 GPa. For Co₂VGa, true half-metallic to nearly half-metallic transition is observed at 40 GPa and around 100 GPa, Co₂VGa shows metallic behavior. For Co₂VAl, true half-metallic behavior is not observed at ambient as well as higher pressures. The half metal-to-metal transition in Co₂VAl and Co₂CrAl is accompanied by quenching of magnetic moment.     

A study of the125Te(n, γ)126Te reaction with thermal neutrons

The¹²⁵Te(n, )¹²⁶Te reaction has been studied with thermal neutrons. Totally 113 lines in the -ray spectrum were assigned to radiative transitions in the¹²⁶Te isotope. The neutron separation energy of¹²⁶Te was evaluated to be 9113.8±0.1 keV. The experimental values of the partial cross sections of the primary E1-transitions were compared with predictions of the Lane-Lynn theory.     

Map of Two-Dimensional Tungsten Chalcogenide Compounds (W–S,W–Se,W–Te) Based on USPEX Evolutionary Search

JETP Letters - New two-dimensional nanostructures of W–X composition (X = S, Se, Te) are predicted using the evolutionary algorithm implemented in the USPEX software package. Based on the...     

NiAs、PbO型FeX(X=S,Se,Te)结构的稳定性与电子特征

利用基于密度泛函理论的第一性原理方法,Ni As与Pb O型Fe X(X=S,Se,Te)结构的稳定性与电子特征得到了研究.计算结果显示Fe的内聚能与X-Fe元素之间的相互作用是影响Fe X结构稳定性的重要因素.当X原子半径较小、电负性较大时(X=S),Fe X趋向于形成Fe-X相互作用较强、密度较大的Ni As型结构;当X原子半径较大、电负性较小时(X=Se,Te),Fe X趋向于形成Fe-Fe相互作用较强、密度较小的Pb O型结构.此外,压强使得Pb O型Fe X结构的稳定性降低.当压强分别大于5、9 GPa时,Fe Te、Fe Se趋向于形成Ni As型结构.Pb O型Fe Se中Fe原子周围的电子密度随压强的增大而增大.     

Effect of chemical ordering on the crystallization behavior of Se90Te10−xSnx (x=2, 4, 6, and 8) chalcogenide glasses

Ternary Se₉₀Te_10−xSn_x (x=2, 4, 6, and 8) chalcogenide glassy alloys have been prepared by melt quenching technique. Various crystallization parameters, such as onset (T_c) and peak (T_p) crystallization temperatures, activation energy of crystallization (E_c) and Avrami exponent (n) have been determined for these alloys. T_c and T_p have been determined directly from the non-isothermal differential scanning calorimeter (DSC) thermograms. The value of E_c has been calculated from the variation of both T_c and T_p with the heating rate (β) according to Kissinger, Takhor, Augis–Bennett and Ozawa models while Augis–Bennett method has been used to deduce the value of n for the studied samples. The obtained values of the crystallization parameters have been correlated with the character and the energy of the chemical bonds through the calculation of the heteronuclear bond energies of the constituent atoms using Pauling principle. In addition to that, Tichy–Ticha model was used to estimate the mean bond energy of the average cross-linking per atom 〈E_cl〉, the average bond energy per atom of the remaining matrix 〈E_rm〉, and the overall mean bond energy 〈E〉 of the studied glasses. Results reveal that both of T_c and T_p decreases with increases Sn content. This is may be attributed to the decreasing in the overall mean bond energy 〈E〉. Besides, the plot of E_c (and also T_g) against 〈E〉 was found to be non linear, which contradicts the well known linear correlation between E_c and T_g with 〈E〉 as suggested by Tichy–Ticha model. This discrepancy may be due to the fact that the Tichy–Ticha linear correlation model was based on the assumption of covalent glassy network, while in the present glassy alloys, Se–Te binary doped with heavy elements such as Sn exhibit iono-covalent bonding. The calculated values of the ionicity are in support of this argument.     

Optical properties of polyvinyl alcohol doped (Se80Te20)100–xAgx (0 ≤ x ≤ 4) composites

Polyvinyl alcohol (PVA) doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films were prepared by the spin-coating technique on a quartz substrate. The optical parameters of PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) composites at the same chalcogen concentration (S₀ = 0.1 mg ml^?1) and PVA/(Se₈₀Te₂₀)₉₆Ag₄ composites at three different chalcogen concentrations viz. S₁ = 0.3 mg ml^?1, S₂ = 0.6 mg ml^?1 and S₃ = 1 mg ml^?1 have been studied. The semi-crystalline nature of the as-deposited thin filmsisdetermined by X-ray diffraction. The transmission and reflection spectra of PVA-doped Se–Te–Ag thin films were obtained in a 350–650 nm spectral region. The optical-band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical-band gap increases, but the refractive index, extinction coefficient, and the real and imaginary parts of the dielectric constant decrease, with increase in Agcontent in PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films. Such type of behavior is explained on the basis of decrease in density of the defect states. However, the optical-band gap has been found to be decreased and all other optical parameters show increase in their values with increase in concentration of (Se₈₀Te₂₀)₉₆Ag₄ glass in PVA-doped composites. The results have been explained on the basis of cluster-size formation at the time of dissolution. This study shows that the optical properties of new composites are affected by the change in silver and chalcogen concentration.     

二维BX(X=S,Se,Te)结构预测与性质的第一性原理研究

采用基于粒子群优化算法的结构预测程序CALYPSO结合密度泛函理论的VASP软件包,预测得到二维BX(X=S、Se、Te)的最低能量结构,该结构是由B和X原子形成的双层褶皱的六角密堆积结构,层与层之间较强的B-B键是稳定结构的关键．凝聚能和声子谱计算结果表明二维BX(X=S、Se、Te)在热力学和动力学上均是稳定的．能带结构和电子态密度的计算分析发现三种二维材料均呈现间接带隙半导体行为,带隙分别为3.98 eV（BS）、3.84 eV（BSe）和2.31 eV（BTe）. 采用“应力应变”方法,计算得到二维BX(X=S、Se、Te)的弹性常数、杨氏模量、泊松比以及杨氏模量和泊松比随方向变化的关系并进行了详细的讨论, 发现杨氏模量和泊松比呈现各向同性. 除此之外,我们还研究了二维BX(X=S、Se、Te)的应力-应变关系,发现BTe较BS和BSe有较强的抗拉伸性．由于BS和BSe在价带顶有效质量较大,为此,我们采用形变势理论研究了BTe的载流子迁移率. 结果发现BTe在ao1和ao2方向上的电子迁移率分别为20.8和122.6 cm2V-1s-1,而空穴在两个方向上的迁移率分别为673.4和65.0 cm2V-1s-1, 空穴在ao1方向上的较高的迁移率说明二维BTe具有较好的输运性质．     

Calorimetric study of Te15(Se100− x Bi x )85 glassy alloys using differential thermal analysis

A calorimetric study of Te₁₅(Se_{100? x} Bi _x )₈₅ glassy alloys (x = 0, 1, 2, 3 and 4 at. %) is reported. Differential thermal analysis (DTA) was performed at heating rates of 10, 15, 20 and 25 K/min. The spectra were used to determine the glass transition temperature, T_g , the crystallisation temperature, T_c and the melting temperature, T_m . All these parameters shift to higher values with increasing heating rate, β. The glass transition temperature and the melting temperature increase, and the crystallisation temperature decreases, with increase in the Bi content, x. The activation energy of the glass transition, E_g , was evaluated using the Moynihan and Kissinger methods. The activation energy of crystallisation, E_c , was calculated using modified Kissinger and Matusita approaches. The thermal stability of these glasses has been studied and found to decrease with increase in Bi content. The results obtained are explained on the basis of a chemically ordered network model and an average coordination number.     

Thermo-optical properties of uniaxial NaT(XO4)2 laser host crystals (where T = Y, La, Gd or Bi, and X = W or Mo)

Thermo-optic coefficients dn _o/dT and dn _e/dT were measured in tetragonal double tungstate and double molybdate crystals NaT(XO₄)₂ (where T = Y, La, Gd or Bi and X = W or Mo) by a laser beam deviation method in the spectral range 0.4–1.1 μm. Thermal expansion coefficients in the directions of a and c crystallographic axes were also measured. Analytical expressions for thermo-optic dispersion formulas were derived as series in 1/λ ². All dn/dT values for NaT(XO₄)₂ crystals were found to be negative. Their absolute values satisfy the relation |dn _e/dT| > |dn _o/dT| for crystals without Bi and |dn _o/dT| > |dn _e/dT| for crystals with Bi. A clear tendency for dn/dT values to decrease with the increase of the volumetric thermal expansion coefficient α _vol of the crystal was observed. This is related with dominant contribution of volumetric thermal expansion effect to the temperature dependence of the refractive index. Thermal coefficients of the optical path W = dn/dT + (n ? 1)α _T governing thermal lensing effect were calculated for different light propagation directions and polarizations as well as crystal athermal directions.     

Investigation of pulsed laser ablation process of Hg_(0.8)Cd_(0.2)Te

The vaporization threshold was measured under the irradiation of 1.064-μm,10-ns pulsed laser.Then we calculated the vaporization temperature based on the conservation law of energy and analyzed the vaporization time based on our established model.These results coincided well with the information from the micrograph of scanning electron microscope(SEM)and the spectra of the plasma.Besides,the laser ablation rate was also computed and discussed theoretically.     

H2X(X=O,S,Se,Te)分子的双光子激发

本文采用全相对论量子力学计算了H₂X (X=O, S, Se, Te) 分子的双光子过程, 并考虑相对论效应对双光子过程的影响. 结果表明, 各个不可约表示对称态下激发能有着明显的差异,它反应了双光子吸收过程中选择能级的特点. 同时, 采用非相对论的对称匹配簇/组态相互作用方法 (SAC-CI) 计算其分子的单光子激发, 并与之比较. 双光子跃迁概率要比单光子跃迁概率小2–5个数量级; 同一主族, 随着原子序数的增加, 相对论效应对分子体系的激发能量、跃迁概率、振子强度的大小都有显著地影响; 除此之外,每个分子遵守分子对称群的选择原则. 本文中, 分子H₂X (X=O, S, Se, Te) 的个别不可约表示对称态的跃迁矩分量和振子强度远远大于其他对称态下的跃迁矩分量和振子强度, 甚至大于单光子激发. 这不仅与分子的对称性有一定的关系, 而且应该是选择双光子跃迁能级的重要依据. 关键词： 全相对论量子力学 双光子激发 电偶极跃迁矩 振子强度