Energy gap induced by magnetic field in zero-gap semiconductors

The k_z dependence of magnetic sub-bands in α-Sn and HgTe is studied theoretically using the method of Luttinger in the spherical approximation. It is demonsrated that the external magnetic field opens a gap of forbidden energy in these zero-gap semiconductors. Possible experimental consequences of this effect are examined.     

Al和Sn液态结构的温度变化特性

通过对Al和Sn的液态X射线衍射数据的分析，发现Al和Sn的液体结构随温度的变化都存在突变.Al的突变发生在1050—1250 ℃的温度区间内，Sn有两个突变点，一个在800℃左右，另一个在1200℃附近.随着温度的升高，两种液态金属的平均最近邻原子间离r₁都呈现减小的趋势.在短程尺度上Sn的液体结构类似于α-Sn的结构.对相关半径r_c的物理意义进行了探讨. 关键词：     

Magnetic sub-band structure of HgTe under uniaxial stress

The magnetic sub-band structure of HgTe is calculated for the strain-splitting Γ₈ bands, and the lowest Landau levels of the conduction and valence bands at k_H = 0 are shown to cross each other as the magnetic field is increased. It is demonstrated that the crossing means the band inversion for the case of the compressional stress and the semimetal—semiconductor transition for the case of the tensile stress.     

Thermopower of a Two-Dimensional Semimetal in a HgTe Quantum Well

The thermopower in a two-dimensional semimetal existing in HgTe quantum wells 18–21 nm thick has been studied experimentally and theoretically for the first time. It has been found theoretically and experimentally that the thermopower has two components—diffusion and phonon-drag—and that the second component is several times larger than the first. It has been concluded that the electron–hole scattering plays an important role in both mechanisms of the thermopower.     

Wave-vector filtering effect of the electric-magnetic barrier in HgTe quantum wells

Because of the helicity of electrons in HgTe quantum wells(QWs) with inverted band structures,the electrons cannot be confined by electric barriers since electrons can tunnel the barriers perfectly without backscattering in the HgTe QWs.This behavior is similar to Dirac electrons in graphene.In this paper,we propose a scheme to confine carriers in HgTe QWs using an electric-magnetic barrier.We calculate the transmission of carriers in 2-dimensional HgTe QWs and find that the wave-vector filtering effect of local magnetic fields can confine the carriers.The confining effect will have a potential application in nanodevices based on HgTe QWs.     

Wave-vector filtering effect of electric–magnetic barrier in HgTe quantum wells

Because of helicity of electrons in HgTe quantum wells (QWs) with inverted band structure, the electrons cannot be confined by electric barriers since electrons can tunnel the barriers perfectly without backscattering in HgTe QWs. This behavior is similar to Dirac electrons in graphene. In this paper, we propose a scheme to confine carriers in HgTe QWs using an electric-magnetic barrier. We calculate the transmission of carriers in 2-dimensional HgTe QWs and find that the wave-vector filtering effect of local magnetic fields can confine the carriers. The confining effect will have potential application in nanodevices based on HgTe QWs.     

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well nanoparticles

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well (QDQW) nanoparticles synthesized by the colloidal method are investigated in this study. Strong exciton bands were observed in absorption and photoluminescence (PL) spectra taken for the CdTe/HgTe/CdTe QDQW nanoparticles. The energy difference between the exciton absorption and PL bands is larger than those obtained with CdTe and HgTe nanoparticles. Photocurrent-voltage curves and time-dependent photocurrent curves were obtained for the CdTe/HgTe/CdTe QDQW nanoparticles. With regard to the photocurrent mechanism of these QDQW nanoparticles, those charge carriers participating in the formation of excitons may not contribute to the photocurrent, because of the large binding energy of the excitons. Moreover, it is suggested in this paper that free holes in the HgTe quantum-well in the valance band, rather than free electrons, are the main contributors to the photocurrent.     

HgTe/CdS/ZnS多壳层量子点的制备与表征

采用一种简单的方法合成HgTe/CdS/ZnS多壳层量子点。首先,以1-硫代甘油为稳定剂,在水相溶液中制备出HgTe核量子点;然后,采用外延生长法依次在HgTe核量子点表面包覆CdS和ZnS壳层,合成出最终具有稳定近红外发光的HgTe/CdS/ZnS多壳层量子点。该合成方法仅需3个步骤,具有操作简单、成本低廉的优点。实验结果显示,当反应温度为90 ℃、反应溶液pH为11.0、反应加热回流时间为4 min时,HgTe/CdS/ZnS多壳层量子点具有最高荧光量子产率36%。     

银—锡—铝三元系合金相图

用X射线方法测定了银-锡-铝三元系合金相图。合金含量从30wt.％Sn到纯锡,从30wt.％Al到纯铝。同时,对此三元系的富银角(<30wt.％Sn,<30wt.％Al)相图的γ相区,ζ+δ相区和β+γ+δ相区重新做了测定。室温相截面由六个单相(即α,μ,β,γ,Sn和δ)相区,八个双相(即α+μ,α+β,β+μ,β+γ,γ+Sn,β+Sn,β+δ及Sn+δ)相区,和三个三相(即α+β+μ,β+γ+Sn及β+Sn+δ)相区所构成。所有单相与三个二元系的单相一致,没有新相出现。     

Effect of precipitates on thermal conductivity of aged Mg-5Sn alloy

Abstract

The influence of precipitates on thermal conductivity of aged Mg-5Sn alloy has been investigated at different heat treatment temperatures. The results show that the thermal conductivity of aged Mg-5Sn alloy increases from 87.5 to 92.8 W·m^?1·K^?1 at 433 K for 720 h and from 87.5 to 122 W·m^?1·K^?1 at 513 K for 120 h with the increasing ageing time. The increasing rate of the former is obviously lower than that of the latter. Meanwhile, the Sn content of precipitates at 433 K is considerably below that of aged Mg-5Sn at 513 K. The interface between precipitates and α-Mg matrix is completely coherent at 433 K for 720 h. The increase in thermal conductivity is mainly attributed to the remaining Sn solutes in α-Mg matrix, and the interface relationship between precipitates and α-Mg matrix.     

The upper critical field of quenched condensed Sn,Tl, In and Hg with extremely short mean free path

Quenched condensed films of Sn, Tl, In and Hg are fine crystalline and have very short mean free path. The upper critical field of these disordered superconductors is measured. The initial slope,dB _c 2/dT is compared with a strong coupling calculation. We use α² F(ω) which is measured for these superconductors in the pure state and for Sn in the quenched state. The strong coupling behavior enhances the upper critical field by 15–20%. We find a fair (about 10%) agreement between the experimental and theoretical values ofdB _c 2/dT except for Hg. In Hg one has either to consider band structure effects or the α² F(ω) of quenched condensed Hg is quite different from that of pure Hg.     

Giant magneto-optical faraday effect in HgTe thin films in the terahertz spectral range

We report the observation of a giant Faraday effect, using terahertz (THz) spectroscopy on epitaxial HgTe thin films at room temperature. The effect is caused by the combination of the unique band structure and the very high electron mobility of HgTe. Our observations suggest that HgTe is a high-potential material for applications as optical isolator and modulator in the THz spectral range.     

Self consistent energy band structures for HgTe and CdTe

Self consistent energy band calculations have been performed for HgTe and CdTe with local density functional potentials, using the LMTO method in the atomic sphere approximation. Equilibrium volumes and bulk moduli are obtained in good agreement with experiment. In the case of HgTe the effect of spin-orbit interation is found to be important in obtaining the equilibrium volume. For both materials the energy bands (although in qualitative agreement with experiment) show a semiconducting gap (inverted in the case of HgTe) which is almost 1 eV lower than experiment. This seems to be a feature common to all calculations for semiconductors using existing local exchange and correlation potentials.     

NMR Knight shift studies in SmSn3 — Extension to low temperatures

The ¹¹⁹Sn Knight shift in SmSn3 has been measured in the temperature range 15—300 K, and it is found that the 4f-contribution to the Knight shift does not show a sign reversal in this temperature interval. The range of crystal field parameters for which simultaneous fit to the magnetic susceptibility and the extended Knight shift results in SmSn3 is obtained can be substantially narrowed from that reported earlier.     

CdTe和HgTe电子结构的紧束缚模型计算

基于局域密度近似(LDA或GGA)的密度泛函理论计算往往低估体系的禁带宽度,而这一低估对窄带隙半导体尤为严重.尽管基于混合泛函的密度泛函理论能有效地修正这一误差,但是由于计算量较大仍无法用于计算较大体系.本文发展了一组能够比较准确描述CdTe和HgTe晶体电子结构的紧束缚参数.将基于混合泛函的密度泛函计算结果作为输入,我们构建了正交的sp~3s~*基组下的紧束缚模型.此模型能够比较准确地描述能带结构在费米面附近4 eV范围内的色散关系.利用当前模型计算了CdTe和HgTe非晶的电子态密度,计算结果与他人的理论计算和实验值符合较好.     

Quantum Hall effect from the topological surface states of strained bulk HgTe

We report transport studies on a three-dimensional, 70-nm-thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate. The strain induces a band gap in the otherwise semimetallic HgTe, which thus becomes a three-dimensional topological insulator. Contributions from residual bulk carriers to the transport properties of the gapped HgTe layer are negligible at mK temperatures. As a result, the sample exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states.     

14T全身超导MRI磁体的技术挑战——大规模应用强场超导磁体未来十年的发展目标之一

本文首先综述了大规模应用的超导磁体,依赖并推动铌三锡Nb3Sn导线技术进步,向更强磁场发展的趋势.着重分析了超高场14 T全身MRI磁体的挑战性技术.选择青铜Nb3Sn导线,采用Nb3Sn线圈和NbTi线圈相结合的混合结构,对14 T全身MRI磁体进行了电磁概念设计和热稳定性及失超保护仿真分析,并简要阐述了14 T全身MRI磁体在应力、接头和匀场方面的关键问题.根据分析结果认为:1)Nb3Sn导线是14 T全身MRI磁体需要面临的首要挑战性问题—作为最佳选择的青铜Nb3Sn导线,其现有产品的性能指标离14 T全身MRI磁体的要求尚存在有一定的差距;2)14 T全身MRI磁体的失超保护涉及线圈的铜超比设计、运行电流同线圈电感的协调配置、被动保护的分段策略和主动保护的失超触发控制以及主动屏蔽结构磁体在失超过程中的逸散磁场限制等多个十分复杂的环节,是最具挑战性的综合性技术.     

Optoelectronic characteristics of close-packed HgTe nanoparticles in the infrared range

Absorption, photoluminescence (PL), photoresponse, and I-V measurements were made for a close-packed HgTe nanoparticle film without organic capping materials to investigate its optoelectronic characteristics in the infrared (IR) range. In the absorption and PL spectra taken for the close-packed nanoparticle film, the wavelength of exciton peak was red-shifted, compared with 1-thioglycerol capped HgTe nanoparticles dispersed in solution. For the HgTe nanoparticle film, dark current was below several pA level, current was increased by about three orders of magnitude at a biased voltage of 3 V under the illumination, and photoresponse was very rapid compared with 1-thioglycerol capped HgTe nanoparticles. These optoelectronic characteristics illustrate that HgTe nanoparticles are one of promising materials for the photodetector in the IR range. Finally, the origin for the increase of photocurrent with increasing temperature observed in this study will be discussed.     

Two-dimensional semimetal-insulator transition in HgTe-based quantum wells induced by a longitudinal magnetic field

A metal-insulator transition in a two-dimensional semimetal based on HgTe quantum wells is discovered. The transition is induced by a magnetic field applied parallel to the plane of the quantum well. The threshold behavior of the activation energy as a function of the magnetic-field strength and an abrupt reduction of the Hall resistance at the onset of the transition suggest that the observed effect originates from the formation of an excitonic insulator.     

The velocity of sound and compressibility in the melts of Hg-Te system

Liquid Hg-Te system has been investigated as a function of composition and temperature by measuring the velocity of sound. A minimum in the sound velocity as a function of temperature observed for Te persists up to 50 at.% Hg, of which the position increases rather steeply between 20 and 30 at.% Hg. This suggests that the molten HgTe and alloys in the Te-rich side undergo structural changes involving the change in coordination number with increasing temperature, and the crossover of the nature of the chemical bond from p-p to sp³ covalent bond takes place with changing composition. It is concluded that the structural change in liquid HgTe is due to intrinsic nature of bonding of HgTe as a compound. Received 2 September 1998 and Received in final form 16 March 1999