Superconductivity in novel Ge-based skutterudites: {Sr,Ba}pt4Ge12

Combining experiments and ab initio models we report on SrPt4Ge12 and BaPt4Ge12 as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge atoms. Below T(c)=5.35 and 5.10 K for BaPt4Ge12 and SrPt4Ge12, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-p states dominate the electronic structure at the Fermi energy.     

Coherent optical phonon generation in Bi3Ge4O12

Time-domain spectroscopy of coherent optical phonons in bismuth germinate (Bi4Ge3O12) is presented. Utilizing both impulsive stimulated Raman scattering and time-domain terahertz spectroscopy, more than 12 unique vibrational states ranging in frequency from 2 to 11 THz are identified, each with coherent lifetimes ranging from 1 to 20 ps. These modes are highly sensitive to crystal orientation and demonstrate frequency shifts on picosecond timescales consistent with an anharmonic lattice potential.     

Density functional investigation of structural, electronic and optical properties of Ge-doped ZnO

Recent experiments reported fascinating phenomenon of photoluminescence (PL) blueshift in Ge-doped ZnO. To understand it, we examined the structural, electronic and optical properties of Ge-doped ZnO (ZnO:Ge) systematically by means of density functional theory calculations. Our results show that Ge atoms tend to cluster in heavily doped ZnO. Ge clusters can limit the conductivity of doped ZnO but reinforce the near-band-edge emission. The substitutional Ge for Zn leads to Fermi level pinning in the conduction band, which indicates Ge-doped ZnO is of n-type conductivity character. It is found that the delocalized Ge 4s states hybridize with conduction band bottom, and is dominant in the region near the Fermi level, suggesting that Ge-4s states provides major free carriers in ZnO:Ge crystal. The observed blueshift of PL in Ge-doped ZnO originates from the electron transition energy from the valence band to the empty levels above Fermi level larger than the gap of undoped ZnO. The electron transition between the gap states induced by oxygen vacancy and conduction band minimum may be the origin of the new PL peak at 590 nm.     

Structure of 74Ge from the 72Ge(t,p) reaction

The reaction ⁷²Ge(t, p)⁷⁴Ge has been investigated with a 15 MeV triton beam. Fifty energy levels of ⁷⁴Ge were identified up to about 4.9 MeV excitation, eighteen of which were previously unreported. Angular distributions were measured and compared with DWBA calculations. Two low-lying states at 1.913 and 2.164 MeV have been assigned as J^π = 0⁺, corresponding to the 0₃⁺ and 0₄⁺ states in ⁷⁴Ge. The 4₂⁺ state has been located at 2.674 MeV, and the 2₄⁺ at 2.836 MeV. Many additional spin and parity assignments have been made. The ⁷⁴Ge nucleus shows a considerably different structure of 0⁺ states compared with the neighboring ⁷⁶Ge and ⁷⁸Ge nuclei, perhaps further evidence for the shape transition suggested recently for the Ge isotopes.     

A study of low-lying states of 72As using the 72Ge(p,nγ)72As reaction

Several low-lying states of ⁷²As have been studied via the γ-rays following the bombardment of ⁷²Ge by protons in the energy range 5.1–6.2 MeV. Gamma-ray spectra were obtained using a Ge(Li) detector, and gamma-gamma coincidences between this detector and a NaI crystal were also investigated. Besides the single previously known excited state at 46 keV, 12 additional excited states were determined in ⁷²As at energies of 213, 242, 308, 317, 333, 343, 413, 438, 454, 512, 523 and (561) keV. The angular distributions of the γ-rays from the 213 and 242 keV levels have also been measured, and the results have been analyzed using the compound nucleus statistical model. The excited states at 213 and 242 keV were found to have spins of (2) and 3 respectively.     

密度泛函理论研究Nb_2Ge_n(n=1～4)团簇

运用密度泛函方法在(U)B3LYP/Lan L2DZ水平上对Nb_2Ge_n(n=1~4)团簇进行了系统的理论研究,得到Nb_2Ge_n(n=1~4)团簇的最低能结构的几何构型和电子性质.优化结果表明:Nb_2Ge_n(n=1~4)团簇最低能结构的自旋多重度均为单重态.团簇最低能结构的电子态与团簇的大小有关.当n为奇数时,团簇的电子态为~1A~1,n为偶数时电子态为1A.通过对计算平均束缚能和分裂能发现:Nb_2Ge_n(n=1~4)团簇中热力学稳定性最强的是Nb_2Ge_2团簇;最弱的是Nb_2Ge_4团簇.自然电荷分布的结果说明Nb_2Ge_n(n=1~4)团簇中当n=1-2时,电子转移正常,而当n=3-4时出现电荷反转现象.同时还研究了HOMOLUMO能隙、磁性和红外光谱.     

Electronic structure of SixGe1−x semiconductor solid solutions

The electronic structure of Si_xGe_1?x solid solutions is studied by means of a semi-ab initio method without involving the virtual crystal approximation. The calculations are performed on large cubic cells of the diamond lattice which contain a total of 64 Si and Ge atoms homogeneously distributed in different compositional ratios. The basis functions and the potentials used in the calculation give good band structures of elemental Si and Ge respectively. The calculated variation of band gap with x is in agreement with optical experiment: the two linear curves of gap vs concentration have different slopes at high and low x values with a crossing-over at about x=0.28. The band gaps are direct for xε (0, 0.28) and indirect for x ≥ 0.28. The density of states (DOS) of the solid solution can be very well approximated by the weighted average of the bulk Si and Ge DOS. There is a very slight charge transfer of about 0.05 electron per atom from Si to Ge.     

Making and breaking covalent bonds across the magnetic transition in the giant magnetocaloric material Gd5(Si2Ge2)

A temperature-dependent, single crystal x-ray diffraction study of the giant magnetocaloric material, Gd5(Si2Ge2), across its Curie temperature (276 K) reveals that the simultaneous orthorhombic to monoclinic transition occurs by a shear mechanism in which the (Si, Ge)-(Si,Ge) dimers that are richer in Ge increase their distances by 0.859(3) A and lead to twinning. The structural transition changes the electronic structure, and provides an atomic-level model for the change in magnetic behavior with temperature in the Gd5(SixGe1-x)(4).     

The electronic,optical,and thermodynamical properties of tetragonal,monoclinic,and orthorhombic M_3N_4(M=Si,Ge,Sn):A first-principles study

A detailed study of the M_3N_4(M = Si,Ge,Sn) nitrides in their tetragonal,monoclinic and orthorhombic phases has been performed with the plane-wave pseudo-potential method combined with the quasi-harmonic approximation,including the phononic effects.We rationalize the main puzzle,i.e.,the fundamental properties of these phases are unclear,by calculating the crystal structures,density of states,and optical properties.The direct band gaps of t-Ge_3N_4,m-Si_3N_4,and o-Ge_3N_4 benefit the opto-electrical properties,t-,m-,and o-Si_3N_4 can be used as refractive materials while m-M_3N_4(M = Si,Ge,Sn) are optically transparent in the visible light region.Our results improve the understanding of the detailed electronic structures of all compounds,as well as the influences of electronic structure on their stabilities.Furthermore,we find that thermodynamic quantities are sensitive to structures and,therefore,depend on various temperature and pressure conditions.     

Analysis and Improvement of Reflection-type Transverse Modulation Optical Voltage Sensors

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Ge，Al掺杂锰硅化合物Mn4Si7的第一性原理计算

基于密度泛函理论（DFT）的第一性原理计算方法,对本征Mn4Si7以及Ge,Al单掺杂和共掺杂Mn4Si7的晶体结构,能带结构,态密度以及光学性质进行了计算和分析.计算结果表明：本征态Mn4Si7的禁带宽度为0.810 eV,为直接带隙半导体材料,掺杂后晶体结构稍微变化,禁带宽度减小,且共掺杂时禁带宽度最小,电导率最好.Al以及Ge,Al共同掺杂时会产生杂质能级.掺杂后光子能量向低能级方向移动,光电导率,光吸收,反射系数都有所增大,说明掺杂改善了Mn4Si7的光学性质,从而可以提高光伏发电效率.     

Electron-phonon interactions of Si100 and Ge100 superconductors with Ba atoms inside

The absolute density of states of isostructural Ba24Si100 and Ba24Ge100 near the Fermi level, which is very relevant for discussion on phonon-mediated superconductivity, is quantitatively evaluated to be 0.28 states eV?1/Ge atom and 0.18 states eV?1/Si atom by combining soft x-ray photoelectron spectroscopy and magnetic susceptibility measurements. The energetics of the anharmonic motions of the endohedral Ba atoms are also discussed on the basis of the 4d-core-level spectra. The discussion based on these important physical parameters concludes that the unique electron-phonon interactions do not occur due to the local Jahn-Teller distortion of the fragile open cage structure in the phonon-mediated BCS formalism. Compounds with a rigid cage would be essential to give rise to anticipated unique interactions as a key ingredient favored for superconducting paring.     

Na_2Ge_2Se_5电子结构和光学性质的第一性原理研究

Na_2Ge_2Se_5是一种优异的红外非线性晶体材料.采用基于第一性原理的密度泛函理论赝势平面波方法对Na_2Ge_2Se_5进行结构优化,并以此为基础计算研究了Na_2Ge_2Se_5的电子结构和光学性质.结果表明:Na2Ge2Se5是宽禁带间接带隙半导体,价带至导带的电子跃迁主要来自于Ge和Se的48,4p态;Na对光学性质的贡献较小,Ge和Se之间的相互耦合作用决定了Na_2Ge_2Se_5的光学性质;该晶体在紫外区有强烈的反射和吸收,静态折射率为2.133,双折射率值适中,为0.145.理论计算结果表明,Na_2Ge_2Se_5是一种性能优良的红外非线性光学晶体材料.     

Calculation of astrophysical reaction rate of 82Ge(n,γ)83Ge

The neutron capture reaction on a neutron-rich near closed-shell nucleus 82Ge may play an important role in the r-process following the fallout from nuclear statistical equilibrium in core-collapse supernovae.By carrying out a DWBA analysis for the experimental angular distribution of 82Ge(d, p)83Ge reaction we obtain the single particle spectroscopic factors, S2,5/2 and S0,1/2 for the ground and first excited states of 83Ge=82Ge(⊕)n, respectively. And then these spectroscopic factors are used to calculate the direct capture cross sections for the 82Ge(n, γ)83Ge reaction at energies of astrophysical interest. The optical potential for neutron scattering on unstable nucleus 82Ge is not known experimentally. We employed a real folding potential which was calculated by using the proper 82Ge density distribution and an effective nucleon-nucleon force DDM3Y.The neutron capture reactions on neutron-rich closed-shell nuclei are expected to be dominated by the direct capture to bound states. We will show that the direct capture rates on these nuclei are sensitive to the structure of the low-lying states.     

An alternative method to calculate surface states in crystals

An alternative method to calculate surface states in crystals is presented based on solving a one-dimensional problem in the direction perpendicular to the crystal surface and a nearly-free-electron (N.F.E.) model in the direction parallel to the surface. The potential is calculated making use of pseudopotential and a study of the structure factor. As an example we consider the case of the (111) surface orientation of Ge and Si, obtaining two bands of surface states separated 0.55 eV and 0.43 eV at the X and L points of the two-dimensional B.Z. This splitting is achieved without considering surface rearrangement.     

Structural phase transitions in Si and Ge under pressures up to 50 GPa

The crystal structures of Si and Ge were studied by energy dispersive X-ray diffraction at room temperature and pressures up to 50 GPa. Si transforms to a primitive hexagonal (Si-V) structure around 16 GPa, to an intermediate phase Si-VI between 35 and 40 GPa, and to hcp (Si-VII) around 40 GPa. In contrast, Ge remains in the β-tin structure up to at least 51 GPa. The pseudopotential method reproduces these differences in the high-pressure behavior of Si and Ge.     

Ge/SiO2纳米晶玻璃的制备及其荧光性质

以正锗酸乙酯和正硅酸乙酯为原料 ,合成了Ge/SiO2 纳米晶玻璃。探索了用溶胶凝胶法合成GeO2 /SiO2 玻璃 ,进而在高温 70 0℃下用氢气将其还原成Ge/SiO2 纳米晶玻璃的全过程。研究了Ge/SiO2 纳米晶玻璃的紫外吸收光谱、X射线衍射谱及荧光光谱。紫外吸收光谱显示由GeO2 /SiO2 玻璃向Ge/SiO2 纳米晶玻璃转变中有组成和结构的改变。X射线衍射谱明显显示有立方相Ge晶体颗粒在玻璃中析出。荧光光谱显示其具有荧光发射效应。根据其荧光发射峰值计算出Ge晶体颗粒平均大小为 3nm     

Nonlinear absorption in undoped and Ge doped layered GaSe semiconductor crystals

P type-GaSe and 0.01 at% Ge doped GaSe crystal were grown using the conventional Bridgman–Stochbarger method from a stoichiometric mixture of starting materials in quartz ampoules, and the nonlinear absorption properties of both crystals have been studied by using an open aperture Z-scan technique under 1064 nm wavelength with 4 ns or 65 ps pulse durations. GaSe crystal exhibits two-photon absorption (TPA) at all input irradiances. On the other hand, at low input irradiance the Ge doped GaSe crystal exhibits saturable absorption (SA). At higher input irradiances TPA becomes dominant. A monotonic increase of the nonlinear absorption coefficient with increasing laser pulse duration from 65 ps to 4 ns is observed for the GaSe and Ge doped GaSe crystals.     

Photoconductivity of Si/Ge/Si structures with 1.5 and 2 ML of Ge layer

A photoconductivity (PC) of Si/Ge/Si structures with narrow Ge layer [thickness's 1.5 and 2 monolayers (ML)] on interband light intensity has been investigated for the different values of lateral voltage U, and temperature T. In contrast to the Si/Ge structure with 2 ML, where only monotonous PC growth was registered, for the 1.5 ML structure a stepped and a fluctuated PC were observed. These PC features are explained by a percolation of photoexcited carriers via the localized states induced by one monolayer scale Si/Ge interface roughnesses.     

A study of the 70, 72, 74, 76Ge(d,p)71, 73, 75, 77Ge reactions using polarized deuterons

Differential cross sections and vector analyzing powers were measured at an incident deuteron energy of 12.0 MeV for the ^{70, 72, 74, 76}Ge(d, p)^{71, 73, 75, 77}Ge reactions. The data were analyzed with the distorted-wave Bom approximation. Spin and parity assignments were made to 13 states in ⁷¹Ge, 19 states in ⁷³Ge, 13 states in ⁷⁵Ge and 11 states in ⁷⁷Ge. Spectroscopic factors were extracted and a simple sum rule analysis was applied to the data.