Phase transition,elastic and electronic properties of topological insulator Sb_2Te_3 under pressure: First principle study

The phase transition, elastic and electronic properties of three phases(phase Ⅰ,Ⅱ, and Ⅲ) of Sb_2Te_3 are investigated by using the generalized gradient approximation(GGA) with the PBESOL exchange–correlation functional in the framework of density-functional theory. Some basic physical parameters, such as lattice constants, bulk modulus, shear modulus,Young's modulus, Poisson's ratio, acoustic velocity, and Debye temperature Θ are calculated. The obtained lattice parameters under various pressures are consistent with experimental data. Phase transition pressures are 9.4 GPa(Ⅰ→Ⅱ) and 14.1 GPa(Ⅱ→Ⅲ), which are in agreement with the experimental results. According to calculated elastic constants, we also discuss the ductile or brittle characters and elastic anisotropies of three phases. Phases Ⅰ and Ⅲ are brittle, while phaseⅡ is ductile. Of the three phases, phaseⅡ has the most serious degree of elastic anisotropy and phase Ⅲ has the slightest one.Finally, we investigate the partial densities of states(PDOSs) of three phases and find that the three phases possess some covalent features.     

Structures and phase transitions of ScH3 under high pressure

The structures and the phase transitions of ScH3 under high pressure are investigated using first-principles calculations. The calculated structural parameters at zero pressure agree well with the available experimental data. With increasing pressure, the transition sequence hcp (GdH3 -type)→ C2/m →fcc→hcp (YH3-type)→Cmcm of ScH3 is predicted first; the corresponding transition pressures at 0 K are 23 GPa, 25 GPa, 348 GPa, and 477 GPa, respectively. The C2/m symmetry structure is a possible candidate but not a good one as the intermediate state from hexagonal to cubic in ScH3 . On the other hand, via the analysis of the structures of hexagonal ScH2.9 , cubic ScH3 , and cubic ScH2 , we find that the repulsive interactions of H-H atoms must play an important role in the transition from hexagonal to cubic.     

Effect of Mo doping on phase change performance of Sb2Te3

Mo, as a dopant, is doped into SbTe to improve its thermal stability. It is shown in this paper that the Mo-doped Sb₂Te₃ (Mo_0.26Sb₂Te₃, MST) material possesses phase change memory (PCM) applications. MST has better thermal stability than Sb₂Te₃(ST) and will crystallize only when the annealing temperature is higher than 250 ℃. With the good thermal stability, MST-based PCM cells have a fast crystallization time of 6 ns. Furthermore, endurance up to 4×10⁵ cycles with a resistance ratio of more than one order of magnitude makes MST a promising candidate for PCM applications.     

Density functional study of α–β phase transition of polyvinylidene difluoride

We perform density functional calculations to investigate structural and dynamical properties of crystalline polyvinylidene difluoride (PVDF) associated with the transition from α to β phase. We examine the change of the conformational energy and the corresponding structure of each phase depending on the lattice parameters of the orthorhombic crystalline structure. From this information, we construct the path that connects the point where the α phase is most stable to the point where the β phase is most stable, and identify the sub‐ region in the lattice parameter space where α and β phases have the same energy. In this sub‐region, we locate the point which gives the lowest conformation energy for both α and β phases, and examine the behaviour of the lowest energy profile and corresponding change of intermediate structures as the conformation of the PVDF chain transforms from α phase to β phase. Finally we perform ab‐initio molecular dynamics simulations and analyse the characteristic dynamics associated with transition from α to β phase. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

Migration of nitrogen in hexagonal Ge2Sb2Te5: An ab‐initio study

The migration barrier energies of the nitrogen atom and N₂ molecule, and the activation barriers for the dissociation and formation of N₂ in Ge₂Sb₂Te₅ were calculated by ab‐initio methods. Various transition and metastable states were found along the migration pathway. Migration barrier energies up to 1.19 eV for the nitrogen atom suggest that it is difficult for it to move from one site to any other site or diffuse out from Ge₂Sb₂Te₅ although doped nitrogen is energetically less stable with respect to N₂ in vacuum. N₂ in Ge₂Sb₂Te₅ was hardly expected to dissociate into nitrogen atoms and vice versa. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

氮掺杂Ge2Sb2Te5相变存储器的多态存储功能

通过反应溅射的方法,制备了N掺杂的Ge2Sb2Te5(N-GST)薄膜,用作相变存储器的存储介质.研究表明,掺杂的N以GeN的形式存在,不仅束缚了Ge2Sb2Te5 (GST)晶粒的长大也提高了GST的晶化温度和相变温度.利用N-GST薄膜的非晶态、晶态面心立方相和晶态六方相的电阻率差异,能够在同一存储单元中存储三个状态,实现相变存储器的多态存储功能.     

高压下稳态六方Ge2Sb2Te5结构、电子和光学性质的第一性原理研究

采用基于密度泛函理论的广义梯度近似方法研究了稳态六方petrov原子序列结构Ge2Sb2Te5的结构、电子和光学性质。计算所得的平衡态晶格参数与实验数据和先前的理论结果吻合很好。基态的能带结构和态密度表明了稳态六方petrov原子序列结构的Ge2Sb2Te5持有金属性。从压强影响下体积的变化趋势发现稳态六方Ge2Sb2Te5在17 GPa和34 GPa 出现不稳定,暗示在此压强下的相变发生,这与2009年Krbal等人的实验结果相吻合。同时,还系统地研究了稳态六方petrov原子序列结构的Ge2Sb2Te5高压下的光学性质,得到了高压下介电函数、吸收率、光反射率、折射率、消光系数和电子能量损失谱在20 eV内的变化情况。     

Growth behavior of Bi2Te3 and Sb2Te3 thin films on graphene substrate grown by plasma‐enhanced chemical vapor deposition

A comparative study of the substrate effect on the growth mechanism of chalcogenide Bi₂Te₃ and Sb₂Te₃ thin films was carried out. Obvious microstructural discrepancy in both the as‐deposited Bi₂Te₃ and Sb₂Te₃ thin films was observed when grown on graphene or SiO₂/Si substrate. Bi₂Te₃ and Sb₂Te₃ thin films deposited on the graphene substrate were observed to be grown epitaxially along c‐axis and show very smooth surface compared to that on SiO₂/Si substrate. Based on the experimental results of this study, the initial adsorption sites on graphene substrate during deposition process, which had been discussed theoretically, could be demonstrated empirically.     

Proximity-Effect-Induced Superconductivity in Nb/Sb2Te3-Nanoribbon/Nb Junctions

Nanohybrid superconducting junctions using antimony telluride (Sb₂Te₃) topological insulator nanoribbons and Nb superconducting electrodes are fabricated using electron beam lithography and magnetron sputtering. The effects of bias current, temperature, and magnetic field on the transport properties of the junctions in a four-terminal measurement configuration are investigated. Two features are observed. First, the formation of a Josephson weak-link junction. The junction is formed by proximity-induced areas in the nanoribbon right underneath the inner Nb electrodes which are connected by the few tens of nanometers short Sb₂Te₃ bridge. At 0.5 K a critical current of 0.15 µA is observed. The decrease of the supercurrent with temperature is explained in the framework of a diffusive junction. Furthermore, the Josephson supercurrent is found to decrease monotonously with the magnetic field indicating that the structure is in the small-junction limit. As a second feature, a transition is also observed in the differential resistance at larger bias currents and larger magnetic fields, which is attributed to the suppression of the proximity-induced superconductive state in the nanoribbon area underneath the Nb electrodes.     

Characterization of Cr‐doped Sb2Te3 films and their application to phase‐change memory

Phase‐change memory (PCM) is regarded as one of the most promising candidates for the next‐generation nonvolatile memory. Its storage medium, phase‐change material, has attracted continuous exploration. Along the traditional GeTe–Sb₂Te₃ tie line, the binary compound Sb₂Te₃ is a high‐speed phase‐change material matrix. However, the low crystallization temperature prevents its practical application in PCM. Here, Cr is doped into Sb₂Te₃, called Cr–Sb₂Te₃ (CST), to improve the thermal stability. We find that, with increase of the Cr concentration, grains are obviously refined. However, all the CST films exhibit a single hexagonal phase as Sb₂Te₃ without phase separation. Also, the Cr helps to inhibit oxidation of Sb atoms. For the selected film CST_10.5, the resistance ratio between amorphous and crystalline states is more than two orders of magnitude; the temperature for 10‐year data retention is 120.8 °C, which indicates better thermal stability than GST and pure Sb₂Te₃. PCM cells based on CST_10.5 present small threshold current/voltage (4 μA/0.67 V). In addition, the cell can be operated by a low SET/RESET voltage pulse (1.1 V/2.4 V) with 50 ns width. Thus, Cr–Sb₂Te₃ with suitable composition is a promising novel phase‐change material used for PCM with high speed and good thermal stability performances. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

高温高压下掺杂N型PbTe的热电性能

 以Sb₂Te₃作为掺杂剂,利用高温高压技术,成功合成出N型PbTe。在常温下对其热电性能的测试结果表明：掺杂微量的Sb₂Te₃后,PbTe的赛贝克系数绝对值和电阻率大幅度下降,热导率随掺杂浓度的增加缓慢升高。掺杂后PbTe的品质因子先大幅度增加,后逐渐降低,最高达到8.7×10^-4 K^-1,它比常压合成的PbTe掺杂PbI₂高一倍以上。结果表明,将高温高压方法与掺杂相结合,能有效地改善PbTe的热电性能。     

Al_2O_3从CaIrO_3结构转变为U_2S_3结构的相变对其高压下光学性质的影响

采用基于密度泛函理论(DFT)的第一性原理方法,计算了Al_2O_3晶体在高压下的光学性质.结果表明:(1)Al_2O_3从CaIrO_3结构转变为U_2S_3结构:将使得其吸收谱主峰值强度增强、副峰值强度显著减弱、主副谱峰均红移以及光谱吸收边出现巨大的红移.(2)结构相变将引起Al_2O_3折射率谱峰值强度减弱和谱峰数增加;同时,在波长为400-2000 nm的范围内,结构相变将导致Al_2O_3折射率显著增大.本文的计算结果为未来进一步的实验研究提供了参考信息.     

溅射气压对Ge2Sb2Te5薄膜光学常数的影响

实验研究了氩气气压对溅射制备的Ge2 Sb2 Te5 薄膜的光学常数随波长变化的影响 ,结果表明 :随薄膜制备时氩气气压的增加 ,Ge2 Sb2 Te5 薄膜的折射率n先增大后减小 ,而消光系数k先减小后增大。二者都随波长的变化而变化 ,且在长波长范围变化较大 ,短波长范围变化较小 ,解释了溅射气压对Ge2 Sb2 Te5 薄膜的光学常数影响的机理     

高压下MgS的弹性性质、电子结构和光学性质的第一性原理研究

采用基于密度泛函理论(density functional theory)基础上的第一性原理赝势平面波方法, 计算研究了MgS晶体B2构型在不同压强下的几何结构、弹性性质、电子结构和光学性质. 计算结果表明, 在高压作用下, 该结构的导带能级有向高能级移动的趋势, 而价带能级有向低能级移动的趋势. 同时, 对照态密度分布图及高压下能级的移动情况, 分析了MgS B2构型在高压作用下的光学性质, 发现高压作用下, 吸收光谱发生了明显的蓝移.     

短波长相变光盘记录介质Ge2Sb2Te5薄膜的制备及静态性能研究

研究了６３２．８ｎｍ波长下适用的相变光盘介质Ｇｅ２Ｓｂ２Ｔｅ５薄膜的制备方法和静态光存诸记录特性，发现该薄膜可在１００ｎｓ条件下实现直接重写，在优化膜层结构后，写擦循环次数高达１０＾６，反射率对比度在１５％以上。     

Si掺杂Al_2O_3电子结构的第一性原理计算

本文采用第一性原理对纯Al2O3和Si掺杂的Si0.167Al0.833O1.5,Si0.25Al0.75O1.5晶体体系的能带结构、态密度进行了计算分析.结果发现:随着Si在Al2O3晶体中所占比例的增加,体系能隙变小,在Si0.25Al0.75O1.5晶体体系中能隙已降到2.5 e V,表明该体系为半导体材料;而在掺杂的体系中有数条分散的能带穿过了费米能级,即可以预测该掺杂体系有特别的光电性质;同时对比纯Al2O3和Si掺杂的Si0.167Al0.833O1.5,Si0.25Al0.75O1.5晶体体系的总态密度,发现掺杂体系的价带和导带向低能区域移动.     

Si掺杂Al2O3电子结构的第一性原理计算

本文采用第一性原理对纯Al2O3和Si掺杂的Si 0.167Al0.833O1.5, Si 0.25Al0.75O1.5晶体体系的能带结构、态密度进行了计算分析. 结果发现：随着Si在Al2O3晶体中所占比例的增加,体系能隙变小,在Si 0.25Al0.75O1.5晶体体系中能隙已降到2.5eV,表明该体系为半导体材料;而在掺杂的体系中有数条分散的能带穿过了费米能级,即可以预测该掺杂体系有特别的光电性质;同时对比纯Al2O3和Si掺杂的Si 0.167Al0.833O1.5, Si 0.25Al0.75O1.5晶体体系的总态密度,发现掺杂体系的价带和导带向低能区域移动.