First-principles study of structural,elastic, electronic and thermodynamic properties of topological insulator Bi2Se3 under pressure

The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Bi₂Se₃ are investigated by the generalized gradient approximation (GGA) with the Wu–Cohen (WC) exchange-correlation functional. The calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA calculations indicate that Bi₂Se₃ is a 3D topological insulator with a band gap of 0.287 eV, which are well consistent with the experimental value of 0.3 eV. The pressure dependence of the elastic constants C_ij, bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio σ of Bi₂Se₃ are also obtained successfully. The bulk modulus obtained from elastic constants is 53.5 GPa, which agrees well with the experimental value of 53 GPa. We also investigate the shear sound velocity V_S, longitudinal sound velocity V_L, and Debye temperature Θ_E from our elastic constants, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity C_v and the thermal expansion coefficient α at 0 GPa and 300 K are 120.78 J mol^?1 K^?1 and 4.70 × 10^?5 K^?1, respectively.     

β-Zn4Sb3/Zn1-δAlδO复合材料的制备及热电性能研究

采用真空熔融淬火结合放电等离子烧结工艺制备了单相β-Zn₄Sb₃以及β-Zn₄Sb₃/Zn_1-δAl_δO复合热电材料,其中第二相Zn_1-δAl_δO(AZO)的含量分别为0.2 wt%,0.4 wt%和0.7 wt%.对材料的相组成和微结构进行了表征,并在300—673 K的温度 关键词： 4Sb₃')" href="#">β-Zn₄Sb₃ 1-δAl_δO')" href="#">Zn_1-δAl_δO 复合材料 热电性能     

有leader的二阶动态多智能体网络的L2—L∞控制

考虑系统噪声和时滞,利用L₂—L_∞控制理论研究了有leader的二阶定拓扑多智能体系统的协调一致问题.工程应用中控制输出的极大值经常需控制在一定范围内,并且考虑系统对位置变量和速度变量限制的范围不同,分别针对位置和速度的控制输出设计了加权系数;进而建立了多智能体系统的数学模型.对有时滞和无时滞的两种网络拓扑,利用李亚普诺夫函数,分析了多智能体系统在满足L₂—L_∞性 关键词： L₂—L_∞控制')" href="#">L₂—L_∞控制 多智能体系统 一致问题 时滞     

熔体旋甩法合成n型(Bi_(0.85)Sb_(0.15))_2(Te_(1-x)Se_x)_3化合物的微结构及热电性能

采用熔体旋甩结合放电等离子烧结(MS-SPS)技术制备了单相n型四元(Bi0.85Sb0.15)2(Te1-xSex)3(x=0.15,0.17,0.19,0.21)化合物,并对所得样品的微结构和热电传输性能进行了系统研究.样品自由断裂面的场发射扫描电子显微镜及抛光面的背散射电子成分分析表明:块体材料晶粒细小,晶粒排列紧密,成分分布均匀且相结构单一,样品中存在大量10—100nm的层状结构.随着Se含量x的增加,样品的电导率和热导率逐渐增加,而Seebeck系数逐渐降低.相比商业应用的区熔材料,MS-SPS方法合成的高Se组成的样品均在425K后表现出更高的ZT值,其中(Bi0.85Sb0.15)2(Te0.83Se0.17)3样品具有最高的ZT值,在360K可达到0.96,并在320—500K均保持较高的ZT值,500K时其ZT值相比区熔材料提高了48%.此外,通过调节Se的含量,可以有效地调控材料的ZT峰值出现的温度段,这对多级或梯度热电器件的制备具有重要意义.     

On the special points on the (P,T, x)-phase diagram for Sn2P2(Se x S1− x )6 crystals: acoustic studies of Sn2P2(Se0.28S0.72)6

On the basis of temperature dependences of the acoustic wave velocity and the acoustic attenuation in Sn₂P₂(Se_0.28S_0.72)₆ crystals under normal conditions and hydrostatic pressures, we show that increasing pressure causes these crystals to move away from the tricritical point and enter into the region of first-order phase transitions (PTs) in (P,?T,?x)-space. Still higher hydrostatic pressures lead to an increase in the attenuation of the acoustic wave with the velocity v ₂₂ and broadening of its anomaly in the PT region, thus hinting at splitting of the PT, with the appearance of an intermediate incommensurate phase approximately at P?=?3.3?kbar. We offer a (P,?T,?x)-phase diagram for the solid solutions Sn₂P₂(Se _x S_{1? x} )₆, which includes two special lines of PTs, tricritical and triple ones, which can intersect at the Lifshitz point at negative hydrostatic pressures. The variations of the surface of acoustic wave velocities occurring with changing temperature are obtained for Sn₂P₂(Se_0.28S_0.72)₆ crystals under the atmospheric pressure.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.     

Far IR transmission spectra of an YBa2Cu3O7−δ thin film

Abstract

The FIR transmission of an YBa₂Cu₃O_7-δ film 1000 Å thick deposited on an MgO plate has been studied from 20 cm^?1 to 4000 cm^?1 at T = 300 K, and at 120 K, 80 K and 7 K. i) The spectra for the normal state are well fitted if a mid-IR oscillator of high strength and high damping is added to the simplest Drude model. ii) The spectra for the superconductive state do not show significant variations of transmission vs. temperature for ω > 120 cm^?1, which should be in agreement with a weak BCS coupling 2Δ = 3.5 kT_c . iii) The FIR transmission at 7 K for ω = 20 cm^?1 is not zero (around 1%) and seems to confirm that the low-temperature perovskite is made of two phases: a superconducting, and a normal one, the proportion of the first one increasing when the film temperature is decreased.     

Effects of RE2O3 (RE = Tm, Sc, Yb) addition on the superconducting properties of ErBa2Cu3Oy

We investigated the effects of added Tm₂O₃, Sc₂O₃, and Yb₂O₃ on the superconducting properties of sintered Er123 samples. Tm₂O₃ addition caused the least T_c degradation, exhibiting a T_c above 90 K even for 17 vol% addition. Samples with added Sc₂O₃ maintained a T_c at above 90 K up to an addition of 7.2 vol%, while Yb₂O₃-containing samples showed a monotonic decrease in T_c with increased vol% of added Yb₂O₃. Tm₂O₃-containing samples exhibited a slight increase in J_c(0.1 T)/J_c(0) and had constant J_c values even for 17 vol% addition. XRD and SEM results indicate that the Tm₂O₃ is very stable in the superconducting matrix.     

Magnetic entropy change and large refrigerant capacity of Ce6i2Si3-type GdCoSiGe compound

Magnetic entropy change (Δ S_M) and refrigerant capacity (RC) of Ce₆Ni₂Si₃-type Gd₆Co_1.67Si_2.5Ge_0.5 compounds have been investigated. The Gd₆Co_1.67Si_2.5Ge_0.5 undergoes a reversible second-order phase transition at the Curie temperature T_C = 296 K. The high saturation magnetization leads to a large Δ S_M and the maximal value of Δ S_M is found to be 5.9 J/kg,cdot,K around T_C for a field change of 0--5 T. A broad distribution of the Δ S_M peak is observed and the full width at half maximum of the Δ S_M peak is about 101 K under a magnetic field of 5 T. The large RC is found around T_C and its value is 424 J/kg.     

Some characterizations of a new metal–organic framework (n-C14H29NH3)2CdCl4 and the role of hydrogen bonding

A cadmium-based organic–inorganic hybrid (n-C₁₄H₂₉NH₃)₂CdCl₄ is synthesized and characterized, thermally and dielectrically. The differential scanning calorimetery (DSC) and the differential thermal analysis (DTA) thermograms were performed in a suitable range of temperature. The general feature of each thermogram indicates that the hybrid undergoes two structural phase transitions at T_major ≈ 351.5?K and T_minor ≈ 344.66?K in addition to an intermediate temperature which is located at ≈346.8?K. For further confirmation of the observed phase transitions, the complex dielectric permittivity ε* (ω,T)?=?ε′(ω,T) – iε″(ω,T) and ac conductivity σ_ac(ω,T) were accurately measured in the wide range of temperature 100?K<T<400?K at some suitable range of frequencies. The data evidenced the existence of such transitions. Comparison with other hybrids reveals the absence of the odd–even effect whereas the transition temperature increases with the increase of the chain length. The mechanism of proton transfer and kink defects was outlined.     

Scanning tunneling microscopy/spectroscopy on multi-layered cuprate superconductor Ba2Ca5Cu6O12 (O1−x Fx)2

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on multi-layered cuprate superconductor Ba₂Ca₅Cu₆O₁₂ (O_1−x F_x)₂ are carried out. STM topographies show randomly distributed bright spot structures with a typical spot size of 0.8 nm. These bright spots are occupied about 28% per one unit cell of c-plane, which is comparable to the regular amount of apical oxygen of 20% obtained from element analysis. Tunneling spectra simultaneously show both the small and the large gap structures. These gap sizes at 4.9 K are about Δ 15 meV and 90 meV, respectively. The small gap structure disappears at the temperature close to T_C, while the large gap persists up to 200 K. Therefore, these features correspond to the superconducting gap and pseudogap, respectively. These facts give evidence for some ordered state with large energy scale even in the superconducting state. For the superconducting gap, the ratio of 2Δ/K_BT_C = 4.9 is obtained with T_C = 70 K, which is determined from temperature dependence of the tunneling spectra.     

Intrinsic tunneling study of underdoped Bi2Sr2-xLaxCuO6+δ superconductors

We report on a tunneling study of underdoped submicron Bi₂Sr_2-xLa_xCuO_6+δ (La-Bi2201) intrinsic Josephson junctions (IJJs), whose self-heating is sufficiently suppressed. The tunneling spectra are measured from 4.2 K up to the pseudogap opening temperature of T^* = 260 K. The gap value found from the spectral peak position is about 35 meV and has a weak temperature dependence both below and above the superconducting transition temperature of T_c = 29 K. Since the superconducting gap should have a value of 10-15 meV, our results indicate that the pseudogap (～35 meV) plays an important role in the underdoped La-Bi2201 intrinsic tunneling spectroscopy down to the lowest temperature of 4.2 K. However, the contribution of the superconducting gap can be separated by normalizing the spectra to the one near and above T_c, which shows that the IJJs can be a useful tool for the study of the electronic properties of the La-Bi2201 cuprate superconductors.     

GeI2 monolayer: a model thermoelectric material from 300 to 600 K

ABSTRACT

In this work, the electronic structure, optical properties and thermoelectric properties of the GeI₂ monolayer are calculated by the first principles with the Boltzmann transport equation. The monolayer is calculated as an indirect band gap semiconductor with an indirect band gap of a value 2.19?eV. This GeI₂ monolayer is good for absorbing low-energy photons, and it is insensitive to high-energy photons. The material is stable at temperatures up to 600?K, so we calculated the thermal conductivity (K_L), Seebeck coefficient (S), power factor (PF) and thermoelectric figure of merit (ZT) of the GeI₂ monolayer at various carrier concentrations from 300 to 600?K. Due to the lower group velocity, the GeI₂ monolayer has a lower thermal conductivity of 0.48?W/m?K at 300K. In P-type doping, the power factor can up to 0.11?mW/m?K², and its ZT value is 4.04 at 600?K of the GeI₂ monolayer, indicating that the GeI₂ monolayer is a potential thermoelectric material.     

High temperature thermoelectric properties of highly c-axis oriented Bi2Sr2Co2Oy thin films fabricated by pulsed laser deposition

High-temperature thermoelectric transport property measurements have been performed on the highly c-axis oriented Bi2Sr2Co20v thin films prepared by pulsed laser deposition on LaA1Oa （001）. Both the electric resistivity p and the seebeck coefficient S of the film exhibit an increasing trend with the temperature from 300 K-1000 K and reach up to 4.8 m. cm and 202 V/K at 980 K, resulting in a power factor of 0.85 mW/mK which are comparable to those of the single crystalline samples. A small polaron hopping conduction can be responsible for the conduction mechanism of the film at high temperature. The results demonstrate that the Bi2Sr2Co2Oy thin film has potential application has high temperature thin film thermoelectric devices,     

NQR Study of Phase Transition and Cationic Motion in 4-NH2C5H4NHBiBr4·H2O

Four ⁸¹Br NQR lines in 4-NH₂C₅H₄NHBiBr₄·H₂O were observed in the temperature range between 77 and ca. 380 K; with increasing temperatures the respective sets of higher and lower two resonance lines coalesced into single lines discontinuously at 274 K, showing the occurrence of a first-order type phase transition of this crystal. The transition was confirmed with heat anomaly on a DTA curve. Each higher and lower line of high-temperature phase is assignable to the terminal Br atoms and the bridging ones of one-dimensional poly anions (BiBr₄ ⁻) _n in the crystal structure (C2/c), which was investigated by a X-ray structure analysis at room temperature. The 1/T ₁ temperature dependence of ⁸¹Br NQR follows the usual T ² law in the temperature range between 77 and ca. 140 K, being explained by fluctuation of the EFG at Br nucleus due to lattice vibrations. The T ₁ vs. 1/T curve in the temperature range between about 160 and 190 K was describable by the exponential curves, allowing us the estimation of activation energies. These exponential behaviors of T ₁ of ⁸¹Br NQR are attributable to the fluctuations caused by the thermal motion of 4-NH₂C₅H₄H⁺ ions. Echo signals of the ⁸¹Br NQR could not be detected above 190 K owing to poor S/N with very short T ₂.     

z切LiTaO3单晶的冲击相变研究

利用二级轻气炮加载下的冲击Hugoniot线(冲击波速度D-粒子速度u关系)和粒子速度剖面测量，结合基于密度泛函理论的平面波赝势计算研究了z切LiTaO₃单晶的高压相变.实验发现，D-u关系在u=0.95km/s附近出现明显拐折；实测波剖面中25.9 GPa和32.6 GPa时观测到弹-塑性双波结构，而终态压力为42.7 GPa和53.0 GPa时则为三波结构.上述结果都清楚地表明z切LiTaO₃单晶冲击相变的发生，相变起始压力约为37.9 GPa.同时，理论计算的菱形相(R3c对称群)压缩线与低压实验数据符合较好，而正交相(Pbnm对称群)压缩线则与扣除热压贡献的高压实验数据相符，由此推断z-切LiTaO₃的高压相为正交结构.从实验和理论上澄清了z切LiTaO₃的相变起始压力和高压相晶体结构的认识，研究工作亦对类似单晶材料的冲击相变研究有参考价值.     

Bi0.5Ba0.5Fe0.5Ti0.49Nb0.01O3热敏陶瓷的微结构和电学性能研究

采用固相合成工艺,制备了Bi_05Ba_05Fe_05Ti_049Nb_001O₃（BBFTN）热敏陶瓷,借助X射线衍射仪、扫描电子显微镜、阻温测试仪和交流阻抗谱考察其微结构、直流电阻、介电特性、阻抗和电学模量方面的电学性能. 结果表明：BBFTN材料依然为立方钙钛矿结构,平均晶粒尺寸约为10 μm,晶格常数相对于BaTiO₃的晶格常数有所变大;室温电 关键词： 05Ba_05Fe_05Ti_049Nb_001O₃')" href="#">Bi_05Ba_05Fe_05Ti_049Nb_001O₃ 微结构 电学性能     

Structure determination of Cs0.864Rb1.136SeO4

The present structure determination of Cs _x Rb_2-x SeO₄ shows that the crystal belongs to the β - K₂SO₄ family, like the related compounds Cs₂SeO₄ and Rb₂SeO₄. In β - K₂SO₄ compounds the cations occupy two different sites; one of these sites, which is surrounded by 11 anionic atoms, is occupied preferentially by Cs, while the other one with 9 neighbours by Rb. A slight excess of Rb has been determined by X-ray structure analysis in a single crystal prepared at 292 K; its formula is Cs_0.864(9)Rb_1.136(9)SeO₄.

Crystals were grown by evaporation from equimolar water solutions of Cs₂SeO₄ and Rb₂SeO₄ at 292 and 301 K. No phase transition was detected by DSC in the range 103-323 K for either of the samples. Lattice parameters of different crystals isolated at various stages of crystallization indicate formation of crystals with a different proportion of Cs/Rb cations. The lattice parameters as well as their ratios differed significantly from those given by Endo et al., (1983).     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.