SiP化合物结构和电子特性的第一性原理研究

应用第一性原理方法研究了SiP化合物的结构和电子特性,并且将研究推广到其他第四族元素磷化物(IV-P).在研究的各种结构中,SiP单斜晶体结构是能量最低、最稳定的结构.SiP的体弹性模量比CN和CP化合物以及相对应的第三族元素氮化物和磷化物要小.SiP不同的结构间能发生相变,其单斜晶体结构(monoclinic)在压强为6.2 GPa, 15.0 GPa, 19.3 GPa, 20.0 GPa 和 10.3 GPa时分别转变成GeP型结构、Rhom.型结构、β-InS型结构、 CsCl型结构和NaCl型结构.能带计算结果显示SiP单斜晶体结构(monoclinic)和GaSe型结构是间接带隙分别为1.123 eV 和 0.123 eV的半导体,SiP其他结构则显示出金属特性.其他化学比为1:1的第四族元素磷化物(IV-P)具有相同的性质.     

Semiconductor-metal transition in selenium under shock compression

Phase transitions in selenium are studied by time-resolved measurements of the electrical conductivity under shock compression at a pressure of up to 32 GPa. The pressure dependence of the electrical conductivity (σ(P)) has two portions: a sharp increase at P < 21 GPa and a plateau at P > 21 GPa. The experimental data and the temperature estimates indicate that, at P < 21 GPa, selenium is in the semiconductor state. The energy gap of semiconducting selenium decreases substantially under compression. At P > 21 GPa, the electrical conductivity saturates at ～10⁴ Ω^?1 cm^?1. Such a high value of the electrical conductivity shows the effective semiconductor-metal transition taking place in shock-compressed selenium. Experiments with samples having different initial densities demonstrate the effect of temperature on the phase transition. For example, powdered selenium experiences the transition at a lower shock pressure than solid selenium. Comparison of the temperature estimates with the phase diagram of selenium shows that powdered selenium metallizes in a shock wave as a result of melting. The most plausible mechanism behind the shock-induced semiconductor-metal transition in solid selenium is melting or the transition in the solid phase. Under shock compression, the metallic phase arises without a noticeable time delay. After relief, the metallic phase persists for a time, delaying the reverse transition.     

Band gap and electronic structure of an epitaxial, semiconducting Cr0.80Al0.20 thin film

Cr(1-x)Al(x) exhibits semiconducting behavior for x = 0.15-0.26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr(0.80)Al(0.20) thin film show several features in the valence band region, including a gap at the Fermi energy (E(F)) for which the valence band edge is 95 ± 14 meV below E(F). Theory agrees well with the valence band measurements, and shows an incomplete gap at E(F) due to the hole band at M shifting almost below E(F).     

Pressure induced phase transformations and band structure of different high pressure phases in tellurium

We report here high-pressure x-ray diffraction (XRD) studies on tellurium (Te) at room temperature up to 40 GPa in the diamond anvil cell (DAC). The XRD measurements clearly indicate a sequence of pressure-induced phase transitions with increasing pressure. The data obtained in the pressure range 1 bar to 40 GPa fit five different crystalline phases out of Te: hexagonal Te (I) → monoclinic Te(II) → orthorhombic Te (III) → Β-Po-type Te(IV) → body-centered-cubic Te(V) at 4, 6.2, 11 and 27 GPa, respectively. The volume changes across these transitions are 10%, 1.5%, 0.3% and 0.5%, respectively. Self consistent electronic band structure calculations both for ambient and high pressure phases have been carried out using the tight binding linear muffin tin orbital (TB-LMTO) method within the atomic-sphere approximation (ASA). Reported here apart from the energy band calculations are the density of states (DOS), Fermi energy (E _f) at various high-pressure phases. Our calculations show that the ambient pressure hexagonal phase has a band gap of 0.42 eV whereas high-pressure phases are found to be metallic. We also found that the pressure induced semiconducting to metallic transition occurs at about 4 GPa which corresponds to the hexagonal phase to monoclinic phase transition. Equation of state and bulk modulus of different high-pressure phases have also been discussed.     

Chemical analysis of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy

We studied the chemical state of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy (XPS), which were fabricated using dual-target magnetron sputtering by controlling the power applied to both metal and chalcogenide targets. On the basis of the valence band spectra obtained, it was suggested that semiconducting SmS has the final state corresponding to the Sm²⁺(4f⁶) configuration below the Fermi level, and metallic SmS has mainly the Sm³⁺(4f⁵) final state and a virtual band state in the Sm 5d band, contributing to the delocalization of 4f electrons and the emergence of metallic conductivity (4f⁶d⁰-4f⁵d¹). Thus, the spectra of our fabricated SmS thin films correspond to the band structure obtained from the dielectric property. This is the first work performed on the intrinsically prepared metallic SmS while the former works done for the sample transformed from semiconductor to metal phase by hard polishing.     

Structural stability and phase transitions in K8Si46 clathrate under high pressure

The structural stability of type-I K8Si46 clathrate has been investigated at high pressure by synchrotron x-ray diffraction. In contrast to that observed in the Na-doped structure-II analogue [A. San-Miguel, Phys. Rev. Lett. 83, 5290 (1999)]], no phase separation into the beta-Sn Si structure was identified at 11 GPa. Instead, K8Si46 is found to undergo a transition to an isostructural positional disordered phase at around 15 GPa. Ab initio phonon band structure calculations reveal a novel phenomenon of phonon instabilities of K atoms in the large cavities is responsible for this transition. Above 32 GPa, the new structure transforms into an amorphous phase.     

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。     

Dangling bond modulating the electronic and magnetic properties of zigzag SiGe nanoribbon

First-principles nonmagnetic calculations reveal a metallic character in zigzag SiGe nanoribbons (ZSiGeNRs) regardless of their width. The partial DOS projected onto the Si and Ge atoms of ZSiGeNR shows that a sharp peak at the Fermi level is derived from the edge Si and Ge atoms. The charge density contours show the Si–Ge bond is covalent bond, while for the Si–H bond and Ge–H bond, the valence charges are strongly accumulated around H atoms due to their stronger 1 s potential and the higher electronegativity of 2.20 than that of 1.90 for Si atom and 2.01 for Ge atom, so that a significant charge transformation from Si or Ge atoms to H atoms and thus an ionic binding feature. Spin–polarization calculations show that the band structures of ZSiGeNR are modified by the dangling bonds. Compared with perfect ZSiGeNR which is a ferrimagnetic semiconductor, the bands of the ZSiGeNRs with bare Si edge, bare Ge edge, and bare Si and Ge edges shift up and nearly flat extra bands appear at the Fermi level. The ZSiGeNR with bare Si edge or bare Ge edge is a ferrimagnetic metal, while ZSiGeNR with bare Si and Ge edges is a nonmagnetic metal.     

Superconductivity of bilayer phosphorene under interlayer compression

According to first-principles calculations, it is our prediction that bilayer phosphorene(BLP) will become a quasitwo-dimensional superconductor under a certain degree of interlayer compression. A decreasing interlayer distance may realize the transition in the BLP from a semiconducting phase to a metallic phase. On the other hand, a severe vertical compression may make the BLP lattice become dynamically unstable. It is found that in the stable metallic phase of the BLP, interlayer phonon modes dominate the electron-phonon coupling λ. The obtained λ can be greater than 1 and the superconducting temperature T_c can be higher than 10 K.     

Can freestanding Xene monolayers behave as excitonic insulators?

We predict that a phase transition in freestanding monolayer Xenes from the semiconducting phase to the excitonic insulating (EI) phase can be induced by reducing an external electric field below some critical value which is unique to each material. The splitting of the conduction and valence bands due to spin–orbit coupling at non-zero electric fields leads to the formation of A and B excitons in the larger or smaller band gap, with correspondingly larger or smaller binding energies. Our calculations show the coexistence of the semiconducting phase of A excitons with the EI phase of B excitons for a particular range of electric field. The dielectric environment precludes the existence of the EI phase in supported or encapsulated monolayer Xenes.     

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.     

Compression behavior of Zr_(41)Ti_(14)Cu_(12.5)Ni_(10)Be_(22.5) bulk metallic glass up to 24 GPa

With the development of multicomponent Zr-based glass bulk metallic glasses(BMGs) with a larger geometry by a conventional casting process with a low coolingrate[1—4], high pressure, which is the simplest way to vary the interatomic distance ofsubstance and thus change its physical properties, has been employed as an importantmeans to investigate structural relaxation and the crystallization of the BMGs[5]. Relativeto crystalline, metallic glasses have excess free volume, which will annihil…     

Magnetoresistance and thermopower of tellurium at high pressures up to 30 GPa

Measurements are reported of the transverse magnetoresistance MR and of the thermopower S, carried out at high pressures P on Te single crystals in synthetic-diamond chambers. The MR is found to increase with decreasing gas width under a pressure up to 4 GPa as one approaches the semiconductor-metal phase-transition point, to fall off subsequently in the high-pressure metallic phase. The behavior of S(P) correlates with the pressure dependences of the measured MR. A negative MR at T=77 K was found within a narrow interval P=1.5–2 GPa, where the valence band of Te is assumed to undergo rearrangement. Above the point of the phase transition to the β-Po structure, MR is established to increase with pressure for P>12 GPa. The MR data are used to estimate the hole mobility μ for various Te phases. A comparison is made of the mobilities in Te, Se, and high-pressure phases of mercury chalcogenides, which are their structural and electronic analogs, for pressures of up to 30 GPa.     

High pressure phase transition in metallic LaB6: Raman and X-ray diffraction studies

High pressure Raman and angle dispersive X-ray diffraction (ADXRD) measurements on the metallic hexaboride LaB₆ have been carried out upto the pressures of about 20 GPa. The subtle phase transition around 10 GPa indicated in Raman measurements is confirmed by ADXRD experiments to be a structural change from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full potential linear augmented plane wave method carried out as a function of pressure show that this transition is driven by the interception of Fermi level by electronic band minimum around the transition pressure.     

Surface state on the SiC(0001)-(√3 × √3) surface

An angle resolved photoemission study of a surface state on the SiC(0001)-(√3 × √3) surface is reported. Experiments carried out on the 6H and 4H polytypes give essentially identical results. A surface state band with semiconducting occupation is observed, centered around 1.0 eV above the valence band maximum (VBM) and with a width of about 0.2 eV. Recently calculated results for a Si-adatom-induced √3 × √3 reconstruction give a metallic surface state band centered about 1.2 eV above the VBM and with a width of 0.35 eV. The dispersion determined experimentally is smaller than calculated but exhibits the same trend, the surface state disperses downwards towards the VBM with increasing parallel wavevector component along both the qG--- and qG--- directions of the √3 × √3 surface Brillouin zone. The VBM is determined to be located at about 2.3(±0.2) eV below the Fermi level. The results indicate that Si adatoms on top of an outermost Si---C bilayer may be an inadequate structural model for explaining recent experimental findings for the SiC(0001)-(√3 × √3) surface.     

Evidence of a pressure-induced metallization process in monoclinic VO2

Raman and combined infrared transmission and reflectivity measurements were carried out at room temperature (RT) on monoclinic VO2 over the 0-19 GPa and 0-14 GPa pressure ranges. Both lattice dynamics and optical gap show a remarkable stability up to P* approximately 10 GPa whereas subtle modifications of V ion arrangements within the monoclinic lattice, together with the onset of a metallization process via band gap filling, are observed for P >P*. Differently from P=0, where the VO2 metallic phase is found only in conjunction with the rutile structure above 340 K, a new RT metallic phase within a monoclinic structure appears accessible in the high pressure regime.     

Pressure-induced polyamorphism in Nd60Fe30Al10 and Ce70Al10Cu20 metallic glasses by high-energy X-ray diffraction and electrical resistance measurements

In situ high-energy X-ray diffraction of Nd₆₀Fe₃₀Al₁₀ and Ce₇₀Al₁₀Cu₂₀ metallic glasses is carried out under high pressure. During compression, Ce₇₀Al₁₀Cu₂₀ and Nd₆₀Fe₃₀Al₁₀ exhibit a polyamorphic transition from low-density state below 2.0 and 10.0?GPa, to high-density state above 8.4 and 21.1?GPa, respectively. The intermediate hysteresis regions are the mixture of both phases. Electrical resistance measurements under high pressure show that Ce₇₀Al₁₀Cu₂₀ display a discontinuous change in pressure dependence curve of resistivity at around 1.7?GPa. The addition of Fe atom gives a significant standoff of phase transition pressure in Nd₆₀Fe₃₀Al₁₀. The results in this work suggest that the solute element and microstructure of lanthanide solvent aggregates have implications on the polyamorphic transition in metallic glasses.     

锯齿型石墨烯纳米带的能带研究

运用π电子紧束缚模型,具体研究了锯齿型石墨烯纳米带(ZGNRs)的边界结构对能带,特别是费米面附近的导带和价带电子的影响.计算了七种不同边界结构的ZGNRs的能带色散关系及费米面附近价带电子在原胞中各原子上的分布情况.计算结果表明:两边界都无悬挂原子的NN-ZGNRs,只有一边界有悬挂原子的DN-ZGNRs,两边界都有五边形环的SPP-ZGNRs和ASPP-ZGNRs为金属性.两边界都有悬挂原子的DD-ZGNRs,一边界为五边形环另一边界无悬挂原子的PN-ZGNRs和一边界为五边形环另一边界有悬挂原子的P 关键词： 锯齿型石墨烯纳米带 紧束缚模型 电子密度分布 缺陷结构     

锗在高压下固化时的相演化及动力学成因

 报道了锗在高压下固化时亚稳相的形成和转化。通过原位观测锗在高压下固化过程中的电阻变化、差热信号，并结合X射线衍射分析，详细讨论了锗在高压下固化时的相演化行为。在我们所进行的实验条件下，当压力大于4 GPa时，熔态锗首先固化成金属锗，然后再向GeⅠ或GeⅢ相转变。还讨论了压力对锗的固化动力学参数的影响。     

MgS晶体结构性质的密度泛函研究

采用基于密度泛函理论(DFT)基础上的第一性原理赝势平面波方法对MgS晶体四种构型(B1，B2，B3，B4)的体相性质进行了系统研究.计算结果表明，B1构型的晶体是间接带隙型半导体，而B2，B3和B4构型的晶体则是直接带隙型材料，其中B2构型的带隙宽度最窄，其值为0.42eV.在压力不超过200.3GPa时，B1构型的MgS 晶胞是最稳定的，当压力大于该值时，会发生B1构型到B2构型的转化. 关键词： MgS 第一性原理赝势平面波方法 电子结构 转化压力