Electronic structure of rectangular HgTe quantum dots

We theoretically investigate the single- and few-electron ground-states properties of HgTe topological insulator quantum dots with rectangular hard-wall confining potential using configuration interaction method. For the case of single electron, the edge states is robust against the deformation from a square quantum dot to a rectangular ones, in contrast to the bulk states, the energy gap of the QDs increased due to the coupling of the opposite edge states; for the case of few electrons, the electrons first fill the edge states in the bulk band gap and the addition energy exhibit universal even-odd oscillation due to the shape-independent two-fold degeneracy of the edge states. The size of this edge shell can be controlled by tuning the dot size, shape or the bulk band gap via lateral or vertical electric gating respectively of the HgTe quantum dot.     

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.     

Crystal structure of a new superconducting high-pressure phase in the YBaCuO-System

A superconducting (T _c=40K) high-pressure phase recently discovered in the system of perovskite type YBaCuO structures is investigated by high-resolution transmission electron microscopy and electron diffraction in order to find its crystal structure. A structure model is proposed on the basis of a comparison between the observed images and image simulations based on crystal chemical considerations. The new phase has aB-centered orthorhombic cell with a monoclinic primitive cell. The primitive cell is composed of two subunits. The first of these is identical with the unit cell of orthorhombic YBa₂Cu₃O_7–x (1-2-3 structure) withT _c=92K. In contrast to the first subunit, the second one contains two adjacent Cu–O chains but is identical otherwise. This second subunit has been observed as one type of planar defect in the 1-2-3 structure. It is therefore concluded, that other stacking polytypes composed of these two different units could exist. The structure of the new phase is compared to the structures of the other known high-T _c superconductors. The chemical formula for the new phase can be written as Y₂Ba₄Cu₇O_14+x, with x0.5±0.2.     

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.     

Electronic structure of HgTe nanocrystals: An observation of p-d weakening

Photoemission studies to identify the electronic structure of the HgTe nanocrystals revealed a new phenomenon of p-d weakening, as a consequence of size quantization effect associated with the mean crystalline size, 5.35 ± 0.83 nm. The weakening of the p-d hybridization by a factor of 0.33, to that of the bulk HgTe suggests the valence band maxima and core level shifts toward higher binding energy. The widening of the band gap due to size quantization is confirmed from optical absorption and photoluminescence measurements. The upward and downward shift of the conduction band minima and the valence band maxima with respect to the bulk value of HgTe are found to be 1.6 eV and 0.54 eV respectively.     

Crystal structure prediction of ReN at high pressure: a new incompressible phase

Based on the swarm-intelligence-based CALYPSO method the NbO, R3m and NiAs phases for ReN are predicted. The R3m phase of ReN at high pressure is firstly found. The structural, mechanical and electronic properties of ReN with the three phases are studied systematically. Moreover, it is also firstly found that pressure stimulated ReN to undergo twice phase transitions, from NbO to R3m phase at 43.3?GPa and from R3m to NiAs phase at 53.6?GPa. The three phases of ReN are verified to be mechanically stable and a promising low-compressible material at ambient conditions. According to the electron density of states and electron localization functions we have found that their structural stability and high hardness is on account of the strong covalent bonding of Re-N and N-N.     

Crystal structure prediction of uranium hydrides at high pressure: A new hydrogen-rich phase

Uranium hydride is a novel hydrogen-rich system which contains 5f electrons. Uranium hydride can not only be used in the nuclear fuel industry, but also be a candidate of high superconducting-temperature materials. In this paper, we have searched the stable uranium hydride structures by using particle swarm optimization method and first-principles calculations. Besides UH₈ and UH₉, we find that UH₁₇, which contains larger hydrogen content than most hydride materials reported before, is also stable at high pressure. The atomic structures, electronic structures and phase diagram of uranium hydrides are provided, and we find that all of the discovered uranium hydrides are metals with negligible magnetic moments.     

Crystal structure and thermochemical properties of phase change materials bis(1-octylammonium) tetrachlorochromate

A new crystalline complex (C₈H₁₇NH₃)₂CdCl₄(s) (abbreviated as C₈Cd(s)) is synthesized by liquid phase reaction. The crystal structure and composition of the complex are determined by single crystal X-ray diffraction, chemical analysis, and elementary analysis. It is triclinic, the space group is P-1 and Z = 2. The lattice potential energy of the title complex is calculated to be U_POT (C₈Cd(s))=978.83 kJ·mol^-1 from crystallographic data. Low-temperature heat capacities of the complex are measured by a precision automatic adiabatic calorimeter over a temperature range from 78 K to 384 K. The temperature, molar enthalpy, and entropy of the phase transition for the complex are determined to be 307.3± 0.15 K, 10.15± 0.23 kJ·mol^-1, and 33.05± 0.78 J·K^-1·mol^-1 respectively for the endothermic peak. Two polynomial equations of the heat capacities each as a function of temperature are fitted by the least-square method. Smoothed heat capacity and thermodynamic functions of the complex are calculated based on the fitted polynomials.     

Crystal symmetry of the low temperature phase of magnetite

A neutron diffraction study of magnetite below the Verwey transition (123 K) reveals that the superlattice reflections are observed for (hhl), with l = integers $\text{+1}\text{2}$ but not for (hhl). This determines a (110) glide plane in the direction of [100] and implies that only one of the doubly degenerate modes, proposed recently by Yamada, condenses at the transition.     

Crystal shape near the faceting phase transition point

The critical behavior of an equilibrium shape of a crystal with the size much larger than the capillary length is studied near the faceting phase transition point.     

Crystal structure of ice accretions

Summary The crystal size in accretions grown in an icing wind tunnel at ambient temperatures −18≥T _a ≥−27°C and different substrate temperatureT _s <0°C is studied. For constantT _s the mean area of crystal sections shows an approximately exponential dependence onT _a which may be written as , where ΔT≃−T _a represents the droplet supercooling. On the other hand, for fixedT _a , the crystal size is observed to decrease markedly withT _s . By considering previous results on the orientation of crystals forming the accretions, it is shown that the value of the mean angle formed by the crystalc-axis with the growth direction depends as well both onT _a andT _s . The exponential dependence of andT _a and the observed correspondence between the behaviour of and are correlated with a nucleation mechanism, accompanied by a process of orientation selection, which would be responsible of both the crystal size and the orientation. The application of the crystal structure analysis to the determination of the growth conditions of hailstones is discussed.

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Riassunto Si studia la dimensione di un cristallo in accrezioni cresciute in una galleria a vento congelante a temperature ambienti −18≥T _a ≥−27 °C e diversa temperatura di substratoT _s <0 °C. PerT _s costante, l’area media delle sezioni di cristallo mostra una dipendenza daT _a approssimativamente esponenziale che potrebbe essere scritta così: , dove ΔT≃−T _a rappresenta il superraffreddamento della gocciolina. D’altra parte, perT _a fisso, si osserva che la dimensione del cristallo decresce marcatamente conT _s . Considerando precedenti risultati sull’orientamento dei cristalli che formano le accrezioni, si mostra che anche il valore dell’angolo medio formato dall’assec del cristallo con la direzione di accrescimento dipende sia daT _a che daT _s . La dipendenza esponenziale di daT _a e la corrispondenza osservata tra il comportamento di e di sono collegate con il meccanismo di nucleazione, accompagnato da un processo di selezione dell’orientamento, che dovrebbe essere responsabile sia della dimensione che dell’orientamento del cristallo. Si discute l’applicazione dell’analisi della struttura del cristallo alla determinzione delle condizioni di accrescimento dei chicchi di grandine.

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To speed up publication, the authors of this paper has agreed to not receive the proofs for correction.

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Comisión Nacional de Energía Atómica and Consejo Nacional de Investigaciones Científicas y Técnicas.     

Crystal structure of diserinesulfate monohydrate

The crystal structure of DL-Serine H₂SO₄ · H₂O is determined by single-crystal x-ray diffraction. The intensities of x-ray reflections are measured at three temperatures, namely, 295, 343, and 233 K. The crystal structure of the studied compound is refined using all three sets of intensities. It is demonstrated that, at a temperature of 233 K, the structure transforms into an incommensurately modulated state. This transformation is assumed to be responsible for the disappearance of the piezoresponse at this temperature.     

Crystal structure of N-m-tolyl phthalimide

N-m-tolyl phthalimide, C₁₅NO₂H₁₁ crystallizes in the space group Cc with unit cell dimensions,a=8·54(1),b=19·89(2),c = 7·59(1)A, β=114·53(1)° andZ=4.V=1173(2)A³,D _m =1·35(1),D _c = 1·344 mg.m⁻³,M _r =237 λCoK_a=1·7903A. The structure was solved byMULTAN and refined to an R-factor of 0·116 for 632 counter reflections. The molecules are held together by van der Waal’s forces. The angle between the tolyl plane and the plane through the phthalimide group is 53·4(4)°. Contribution No. 607.     

Characterization of solution-synthesized CdTe and HgTe

We report the characterization of solution-synthesized CdTe and HgTe nanocrystals by X-ray diffraction, transmission electron microscopy, and photoluminescence. Methanol solutions of sodium telluride and cadmium iodide or mercury iodide, respectively, are reacted to precipitate the nanocrystalline metal tellurides, while the sodium iodide byproduct remains in solution. The existence of crystalline CdTe, HgTe, and ternary HgCdTe compounds has been demonstrated by powder X-ray diffraction after a post-synthesis sintering process. Precipitated crystallites from this synthesis were analyzed by transmission electron microscopy, which revealed that crystal diameters can vary from approximately 1 nm to 100 nm and that crystals are stoichiometric within the detection limit of the electron microprobe technique. Narrow size ranges can be selected and investigated due to an in-situ separation process in the electron microscope. Photoluminescence is found at energies above the bulk exciton energy for CdTe and is attributed to near-band-gap recombination which is blue-shifted due to quantum confinement. Both low defect luminescence and dark field imaging suggest a high crystalline quality. A comparative characterization by photoluminescence, transmission electron microscopy, and X-ray diffraction evaluates the effects of heat treatments during and after synthesis.     

Crystal chemistry of phosphates with the eulytine structure

A crystal chemical study has allowed us to identify new phosphates with the eulytine structure and the formulae A_3-x Cd _x Bi(PO ₄)₃ and A_3-2x Na _x Bi_1+x (PO₄)₃ (A = Cd, Ca, Sr, Pb, Ba). Second harmonic generation tests carried out on powder samples have confirmed the non-centro-symmetrical character of their space group I43d. Bismuth cations were found necessary for the stability of the eulytine Cd₃Bi(PO₄)₃. The study has also confirmed that the eulytine structure is easily stabilized with large cations.     

DNA template-driven synthesis of HgTe nanoparticles

Nanoparticle (mercury telluride (HgTe))–single-stranded (ss) DNA complex has been synthesized using an electrodeposition technique. As demonstrated by transmission electron microscopic measurements, the morphology of these particles is found to depend upon the ssDNA sequence. The surface feature of HgTe–ssDNA deposit gives an appearance of nanostars with an average size as 1.4 nm and exhibits the single crystalline cubic phase, contrary to the polydispersity and polycrystallinity observed with HgTe nanoparticles alone. The change in infrared absorption band of ssDNA as a consequence of its association with HgTe nanoparticles have been confirmed from Fourier transform infrared spectroscopic measurements. Photoluminescence (PL) at 300 K of these nanostars have yielded a unique single PL of line width 8 nm at wavelength 548.4 nm