异质金属界面的电子结构与超导性

本文引用一系列明确定义的界面平面波轨道(IPWO)与已有的“丸盒轨道”(MTO)能带论方法相结合,可以处理异质金属界面的电子结构.得出Bloch波的反射透射关系,界面附近电子态密度,不同角动量分波的含量等的关系式.联系到电子-声子耦合的超导原理,指出探索高T_c超导体有一个异质杂化原则,即导带角动量杂化程度强的高T_c超导体应该到由价电子结构差异大的组元形成的化合物或复合异质微晶体系中去找.     

杂化能带与电子-声子耦合的本征上限

本文用角动量分波表象的能带论方法研究了杂化导带的一般性质。得出了电子态密度和分波含量的一般关系式。发现引用一个径向波函数φ_l的“相位函数”y_l,可以方便地对球对称势场造成的相移和φ_l的对数微商作解析近似处理。在此基础上研究了电子-声子耦合中的系数η的性质。论证了η有一个“本征上限”,并估计了它的量级。讨论了影响η强弱的一般规律,与实验现象作了比较。指出提高导带角动量杂化程度,特别是提高d导带中的f分波含量,以至试图实现似f导带,可能是强化电子-声子耦合潜力较大的探索方向。 关键词：     

专题导读

正重费米子化合物是一类典型的强关联电子体系,通常存在于含有f电子的镧系或者锕系金属间化合物中,近期在一些过渡金属化合物中也发现了类似的重费米子行为。在该类化合物中,局域电子与导带电子通过近藤效应杂化而形成复合费米子,其电子有效质量可高达自由电子的上千倍,"重费米子"因此得名。此外,局域电子与导带电子杂化会在费米能级附近打开一个能隙,根据能隙的大小以及费米能级的位置,重费米子体系可以是金属、半金属、半导体甚至绝缘体。     

C/SiC纳米管异质结电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算,对扶手椅型(4,4)和(6,6)及锯齿型(8,0)和(10,0)C/SiC纳米管异质结的电子结构进行了研究.结果表明两类异质结结构都表现为半导体特性.扶手椅型纳米管异质结形成了Ⅰ型异质结,电子和空穴都限制在碳纳米管部分.锯齿型纳米管异质结中价带顶主要分布在碳纳米管部分及C/SiC界面处,而导带底均匀分布在整个纳米管异质结上.这两种异质结结构在未来纳米器件中具有潜在的应用价值. 关键词： C/SiC纳米管异质结 第一性原理 电子结构     

钽(Ta)的添加对V_2(HfZr)三元C15化合物超导转变温度的影响的研究

本文研究了四元C15结构化合物V_2(ZrHfTa)超导转变温度的变化规律.发现Ta对V_2(HfZr)超导转变温度T_c的影响与组分有关.在富Hf区和富Zr区,Ta的添加都使T_c升高.而在三元系T_c峰值组分的V_2(Hf_0.5Zr_0.5)附近,T_c随Ta含量增加而下降,这与V_2Hf、V_2Zr加Ta后T_c随Ta含量增加而上升的趋势正相反,测量了样品的低温电阻、低温比热,未发现该四元系存在标志相变的温度反常区. 运用由角动量分波表象的能带论方法分析电子-声子作用得出的杂化能带理论分析本实验结果,认为V_2Hf与V_2(Zr_0.5Hf_0.5)的本质差异和加Ta后T_c完全相反的变化可能与Hf、Ta原子封闭的4f壳层电子与Zr原子的4d导带的相互作用有关.在某些化合物中4d与5d元素的相互作用有利于导带中的d-f杂化,因而对提高T_C可能是有利的.     

MgB2及高温超导体的强电子-声子耦合和输运性质

我们对MgB2和高临界温度的铜氧化合物超导体进行了对比研究.MgB2显示出以声子为媒介的BCS超导体的典型行为.对所谓的McMillan-Hopfield系数的解析表达暗示具有高角动量l的导带l(l±1)杂化对强BCS超导体有利.我们还分析讨论了高临界温度铜氧化物和MgB2的传输性质,发现一个统一的R(T,H)方程,可以很好地描述与MgB2和YBCO无孪单晶在B8T的宽场强范围内的正常态-超导电阻转变.     

AlxGa1-xN/GaN异质结构中Al组分对二维电子气性质的影响

通过用数值计算方法自洽求解薛定谔方程和泊松方程,研究了Al组分对AlxGa1-xN/GaN异质结构二维电子气性质的影响,给出了AlxGa1-xN/GaN异质结构二维电子气分布和面密度,导带能带偏移以及子带中电子分布随AlxGa1-xN势垒层中Al组分的变化关系,并用AlxGa1-xN/GaN异质结构自发极化与压电极化机理和能带偏移对结果进行讨论分析.     

硫钒铜矿化合物电子结构和电致变色特性的第一原理研究

基于密度泛函的第一原理赝势平面波方法,计算晶体结构、电子结构和光学性质,研究硫钒铜矿化合物Cu₃VS₄、Cu₃NbS₄和Cu₃TaS₄的电子输运及电致变色特性,探讨作为透明半导体材料应用于太阳能电池和电致变色器件的可能性.电子结构的计算表明这类化合物是间接带隙半导体,其电子能带的导带底和价带顶分别位于布里渊区的X点和R点.价带顶的电子本征态主要来自于Cu原子的d电子轨道,而导带底电子态主要来源于VB族元素原子的d电子轨道.能带结构、电荷布居分析、电子局域化函数和光吸收及反射谱的计算表明这些硫钒铜矿化合物属于极性共价半导体,具有较高的电荷迁移率和优良的电致变色特性,可应用于高效电致变色器件.     

Au-Sn金属间化合物的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势法,计算研究了Au-Sn二元系金属间化合物的生成焓、结合能、电子结构、弹性性质和结构稳定性. 计算结果表明：Au₅Sn合金的生成焓最小,说明Au₅Sn较容易生成,但Au₅Sn在热力学和力学上是不稳定的;AuSn₂和AuSn₄的键合作用较强,弹性模量、剪切模量均大于AuSn和Au₅Sn;从电子结构的角度,AuSn₂和AuSn₄ 的成键主要来自于Au原子d 轨道与Sn原子p轨道的杂化;而AuSn以Sn–Sn键的相互作用为主,Au₅Sn相中Au 的占比较大,导致Au–Au共价键发挥作用,抑制了Sn导带p电子的成键. 关键词： 电子结构 弹性性质 第一性原理 Au-Sn金属间化合物     

过渡金属二硫化物/三卤化铬范德瓦耳斯异质结的反折叠能带

过渡金属二硫化物MX₂/三卤化铬CrX₃组成的范德瓦耳斯异质结能有效操控MX₂的谷极化,在能谷电子学中有广泛的应用前景.本文结合第一性原理和k投影能带反折叠方法比较研究了MoSe₂/CrI₃, MoSe₂/CrBr₃和WS₂/CrBr₃三种磁性范德瓦耳斯异质结的堆垛和电子结构,探索了体系谷极化产生的物理机理.计算了异质结不同堆垛的势能面,确定了稳定的堆垛构型,阐明了时间/空间反演对称破缺对体系电子结构的影响.由于轨道杂化,磁性异质结的导带情况复杂,且MoSe₂/CrI₃体系价带顶发生明显变化,不能与单层MX₂直接对比.而借助于反折叠能带,计算清晰揭示了CrX₃对MX₂电子结构的影响,定量地获得了MX₂的能谷劈裂,并发现层间距和应变可以有效调控能谷劈裂.当层间距减...     

Structure and dynamics of liquid MgO under high pressure

Microstructure, dynamics, and diffusion mechanism in liquid MgO have been studied by molecular dynamics simulation. Models consisting of 2000 atoms were constructed under a wide range of pressure and at a temperature of 3800 K. The local structure is analyzed through the coordination number distribution and topology statistics of coordination units (basic structural units) MgO _x (x=2, 3, 4, 5, 6, 7). As regards the structural dynamics, the nearest-neighbor atomic exchange among coordination units, spatially heterogeneous dynamics, clustering, and structural stability (lifetime of basic structural units) are investigated in detail. Investigation of structural dynamics allows us to gain insight into various important atomic (molecular) properties and to clarify the diffusion mechanism in liquid MgO under high pressure.     

Crystal face specificity of xenon adsorption on iridium field emitters

Fundamental problems of the adsorption of noble gas atoms on metal surfaces are discussed on the basis of new data of xenon adsorption on well-defined crystal faces of iridium. These data include surface potentials ( = changes in work function), heats of adsorption and their decrease with increasing coverage; they have been obtained by using a field emitter probe-hole assembly. It is found that the heat of adsorption Q_hkl is not simply additive in the number of Ir atoms contacting a Xe atom on a given site; in particular for the close-packed faces, Q₁₁₁ and Q₁₀₀ are relatively too high. Apparently, strong bonding is favoured by high work function of the adsorbing crystal face. This proves a significant contribution of a charge-transfer no-bond interaction to the adsorption bond. A model of Xe polarization by an electric surface field is rejected, as it predicts the wrong sign for the adsorption dipole. While at low coverage adsorption is confined to sites determined by the atomic topography of the adsorbing surface, several possibilities exist for high coverages. Either a two-dimensional close-packed layer is formed with little or no epitaxial relation to surface topography, or adsorption remains confined to certain sites. The present data favour the former possibility for atomically smooth faces in agreement with recent LEED results. For atomically rough faces however, the smallness of the decrease of Q_hkl with coverage seems to favour site adsorption even at high coverage. The latter result is of relevance for surface area determinations by means of “physical” adsorption.     

Cleavage of C<Subscript>60</Subscript> fullerite crystals

It is found that, under certain conditions, C₆₀ fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τ_y = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ? 60b³ is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.     

Structure and physical properties of titanium diselenide intercalated with nickel

The structure, electrical resistivity, thermopower, and magnetic susceptibility of titanium diselenide intercalated with nickel (N_xTiSe₂) are studied systematically in the nickel concentration range x=0–0.5. In accordance with a model proposed earlier, strong hybridization of the Ni3d/Ti3d states is observed, giving rise to suppression of the magnetic moment because of delocalization of the nickel d electrons. It is shown that the strain caused by the Ni3d/Ti3d hybridization does not change the local coordination of a titanium atom.     

Influence of (In,Ni, Al,Ga)-ions Substitution on the Magnetic Properties of ZnCr<Subscript>2</Subscript>Se<Subscript>4</Subscript>

Magnetic properties of Zn_1-xM_xCr₂Se₄ (M=In, Ni, Al, Ga) in high magnetic field up to 14 T and at low temperatures are presented. ZnCr₂Se₄ belongs to the group of normal spinels with Zn²⁺ located at tetrahedral (A) sites and Cr³⁺ occupying (B) sites of the cubic close-packed array of selenium anions. Magnetic order in the compounds is connected with superexchange interactions, which are positive between the nearest neighbors and negative between the next-nearest neighbors. Substitution of one of the cations of spinel with another 3d metal causes essential changes in the cation distribution and physical properties.     

Universal relationships for the phonon spectra in BCC,FCC, and HCP crystals with a short-range interatomic interaction

The frequencies of the phonon branches that correspond to the vibrations of the close-packed atomic planes in bcc, fcc, and hcp crystals with short-range interatomic interaction are shown to be described by a universal relationship, which only contains two parameters for each branch, for any polarization λ. These phonon branches correspond to the (ξ, ξ, 0) direction in bcc crystals, the (ξ, ξ, ξ) direction in fcc crystals, and the (0, 0, ξ) direction in hcp crystals. This universal relationship can only be violated by long-range interactions, namely, the interactions outside the sixth coordination shell in a bcc crystal, the fifth coordination shell in an fcc crystal, and the eleventh or tenth coordination shell in an hcp crystal. The effect of these long-range interactions for each phonon branch can be quantitatively characterized by certain parameters Δ _nλ, which are simply expressed in terms of the frequencies of three phonons of the branch. The values of these parameters are presented for all bcc, fcc, and hcp metals whose phonon spectra are measured. In most cases, the proposed relationships for the frequencies are found to be fulfilled accurate to several percent. In the cases where the Δ _nλ parameters are not small, they can give substantial information on the type and scale of long-range interaction effects in various metals.     

Fast and accurate SCFT calculations for periodic block-copolymer morphologies using the spectral method with Anderson mixing

We study the numerical efficiency of solving the self-consistent field theory (SCFT) for periodic block-copolymer morphologies by combining the spectral method with Anderson mixing. Using AB diblock-copolymer melts as an example, we demonstrate that this approach can be orders of magnitude faster than competing methods, permitting precise calculations with relatively little computational cost. Moreover, our results raise significant doubts that the gyroid (G) phase extends to infinite c \chi N . With the increased precision, we are also able to resolve subtle free-energy differences, allowing us to investigate the layer stacking in the perforated-lamellar (PL) phase and the lattice arrangement of the close-packed spherical ( S _cp phase. Furthermore, our study sheds light on the existence of the newly discovered Fddd ( O⁷⁰ morphology, showing that conformational asymmetry has a significant effect on its stability.     

Intercalative route to heterostructured nanohybrid☆

We have successfully synthesized inorganic–inorganic, organic–inorganic and bio-inorganic nanohybrids by applying a chimie douce intercalation technique systematically to layered titanate, to Bi-based cuprate superconductors, Bi₂Sr₂Ca_m−1Cu_mO_y (m=1, 2, and 3; BSCCO), and to layered double hydroxides (LDHs), those which are of high importance in terms of basic understanding of intercalation reactions and also having practical application. The inorganic/inorganic hybrids were synthesized by exfoliation-restacking methods to obtain TiO₂-pillared titanate. A novel pillaring procedure using an osmotic swelling had been developed to prepare TiO₂-pillared layered titanate with a large surface area, high thermal stability, and enhanced photocatalytic activity. On the other hands, the organic/inorganic hybrids were achieved via intercalative complexation of iodine intercalated BSCOO with an organic salt of Py–C_nH_2n+1I (Py=pyridine). The high-T_c superconducting intercalate with its remarkable lattice expansion can be applied as a precursor for superconducting colloids when dispersed in an appropriate solvent. We were also able to demonstrate that the biomolecules stabilized in the interlayer space of layered double hydroxides (LDH), bio-inorganic hybrids, retain their chemical and biological integrity. If necessary, LDH, as a reservoir, can be intentionally removed by dissolving it in an acidic medium or interlayer biomolecules in LDH can be released via ion-exchange reaction in electrolyte. It can, therefore, be concluded that the inorganic LDH can play a role as a good host lattice for gene reservoir or carrier. These intercalative methods will provide remarkable synthetic routes to design new heterogeneous hybrid materials, which have the intrinsic properties starting materials and create synergetic effects by hybridization.     

Supersonic <Emphasis Type="Italic">N</Emphasis>-Crowdions in a Two-Dimensional Morse Crystal

An interstitial atom placed in a close-packed atomic row of a crystal is called crowdion. Such defects are highly mobile; they can move along the row, transferring mass and energy. We generalize the concept of a classical supersonic crowdion to an N-crowdion in which not one but N atoms move simultaneously with a high velocity. Using molecular dynamics simulations for a close-packed two-dimensional Morse crystal, we show that N-crowdions transfer mass much more efficiently, because they are capable of covering large distances while having a lower total energy than that of a classical 1-crowdion.