Thermal expansion and lattice dynamics of RB66 compounds at low temperatures

Thermal characteristics of the phonon and magnon subsystems of icosahedral borides RB₆₆ (R = Gd, Tb, Dy, Ho, Eu, or Lu) have been studied based on the obtained experimental data on the thermal expansion of the borides and the earlier results on their heat capacity in the range of 2–300 K. The contribution to the expansion of borides containing paramagnetic R ³⁺ ions, which is characteristic of transition to the spin-glass state, has been revealed. The phonon spectrum moments of RB₆₆ compounds and the Grüneisen parameters have been calculated.     

Thermal expansion and lattice dynamics under pressure of ZnS,ZnSe and ZnTe

The thermal expansion against temperature of ZnS, ZnSe and ZnTe is studied theoretically using the experimental pressure dependence of elastic stiffness constants and phonon frequencies. The mode Grüneisen parameters obtained from the high pressure effect on the one- and two- phonon Raman spectra at the metallic transition pressure by Weinstein are used originally, but do not reproduce the experimental linear expansion coefficient at high temperatures. The contributions from optical modes with large phonon frequency are important to the thermal expansion at high temperatures, and a set of mode Grüneisen parameters, which bring good agreement with the observed linear expansion coefficient not only at low temperatures, but also at high temperatures, are obtained. Then, the phonon dispersion curves of ZnS, ZnSe and ZnTe at their metallic transition pressures are quantitatively shown.     

Form-factor representation and multipole expansion of the retarded interatomic dispersion energy

Dispersion-relation methods are used to derive a form-factor representation for the retarded dispersion energy of two hydrogen atoms that are described by relativistic electron theory. By expressing the electromagnetic form factors in terms of atomic transition matrix elements the complete multipole expansion of the interatomic dispersion energy is obtained. The long-range asymptotic limit of the successive multipole interactions is given explicitly.     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况。结果表明：原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态。     

Spin and the honeycomb lattice: lessons from graphene

A model of electrons hopping from atom to atom in graphene's honeycomb lattice gives low-energy electronic excitations that obey a relation formally identical to a 2+1 dimensional Dirac equation. Graphene's spin equivalent, "pseudospin," arises from the degeneracy introduced by the honeycomb lattice's two inequivalent atomic sites per unit cell. Previously it has been thought that the usual electron spin and the pseudospin indexing the graphene sublattice state are merely analogues. Here we show that the pseudospin is also a real angular momentum. This identification explains the suppression of electron backscattering in carbon nanotubes and the angular dependence of light absorption by graphene. Furthermore, it demonstrates that half-integer spin like that carried by the quarks and leptons can derive from hidden substructure, not of the particles themselves, but rather of the space in which these particles live.     

Attractive short-range interatomic potential in the lattice dynamics of niobium and tantalum

Summary It is shown in the framework of the pseudopotential approach that there is a sizable attractive short-range component of the interatomic potential due to thes-d interaction which has the same functional form in real space as the Born-Mayer repulsion due to the overlap of core electron wave functions centred on neighbouring ions. The magnitude of this attractive component is such as to completely cancel the conventional Born-Mayer repulsion, making the resultant short-range interatomic potential attractive rather than repulsive. Numerical calculations show that the attractive interatomic potential, which represents the local-field correction, leads to a better understanding of the occurrence of the soft modes in the phonon dispersion curves of niobium and tantalum.

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Riassunto Si mostra, nell'ambito dell'approccio allo pseudopotenziale, che vi è una componente di considerevoli dimensioni attrattiva a corto raggio del potenziale interatomico, dovuta alla interaziones-d, che ha la stessa forma funzionale nello spazio reale della repulsione di Born-Mayer dovuta al sovrapporsi delle funzioni d'onda degli elettroni del nucleo centrate su ioni confinanti. L'ampiezza di questa componente attrattiva è tale da cancellare completamente la repulsione convenzionale di Born-Mayer, rendendo attrattivo, piuttosto che repulsivo, il risultante potenziale interatomico a corto raggio. I calcoli numerici mostrano che il potenziale attrattivo interatomico, che rappresenta la correzione del campo locale, porta ad una migliore comprensione della presenza dei modi molli nelle curve di dispersione dei fononi del niobio e del tantalio.

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Резюме В рамках псевдопотенциального подхода показывается, что заметный короткодействующий вклад в межатомный потенциал притяжения, обусловленныйs-d взаимодействием, имеет такую же функциональную форму в реальном пространстве, как отталкивание Борна-Майера, обусловленное перекрытием электронных волновых функций, центрированных на соседних ионах. Величина этой компоненты притяжения полностью компенсирует обычное отталкивание Борна-Майера, что приводит к результирующему короткодействующему межатомному потенциалу притяжения, а не отталкивания. Численные вычисления показывают, что межатомный потенциал притяжения, который представляет поправку локального поля, приводит к лучшему пониманию существования мягких мод в фононных дисперсионных кривых ниобия и тантала.

     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况.结果表明:原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态.     

量子关联与原子间相互作用距离关系的研究

本文将两个二能级原子注入一个腔中,用共生纠缠的方法来度量两原子之间的纠缠并推出它们之间的量子失协,还给出了原子与环境之间的共生纠缠与量子失协的计算公式,讨论了原子自发衰变率的变化对量子纠缠及失协的影响,及不同的初始状态情况下,纠缠及失协随原子距离的演化情况。结果表明：原子自发衰变率减小,原子与环境之间的量子纠缠及失协增加;选择不同的初态,可以控制原子间出现纠缠死亡的现象或量子失协为零的状态。     

Dependence of the exchange splitting in excitons on the interatomic distance

The exchange splitting observed in excitons depends exponentially on the interatomic distance of the cation and anion of the compound.     

Multipole expansion of the retarded interatomic potential energy: Derivation from relativistic electron theory

The inductive and dispersive retarded interaction energies of two ground state hydrogen atoms described by Dirac theory are obtained up to all multipole orders. The long range terms are given as symmetric expression in the electric and magnetic dipole moments.     

Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials

We review the thermal properties of graphene and multilayer graphene, and discuss graphene’s applications in thermal management of advanced electronics and optoelectronics. A special attention is paid to the use of the liquid-phase-exfoliated graphene and multilayer graphene as the fillers in the thermal interface materials. It has been demonstrated that addition of an optimized mixture of graphene and multilayer graphene to the composites with different matrix materials produces the record-high enhancement of the effective thermal conductivity at the small filler loading fraction (f≤10 vol%). The thermal conductivity enhancement due to the presence of graphene in the composites has been observed for a range of matrix materials used by industry. The hybrid composites where graphene is utilized together with metallic micro- and nanoparticles allow one to tune both the thermal and electrical conductivity of these materials. Theoretical considerations indicate that the graphene-based thermal interface materials can outperform those with carbon nanotubes, metal nanoparticles and other fillers owing to graphene’s geometry, mechanical flexibility and lower Kapitza resistance at the graphene–base material interface.     

Connectivity constant of the kagomé lattice

From the computation of self-avoiding walks on the kagomé lattice, its connectivity constant is found to be 2·569 ± 0·008.     

Prediction of lattice constant in cubic perovskites

Regularities of lattice constant in ideal perovskites are investigated by using a total of 132 ABX₃-type compounds, including oxides and halides. Two atomic properties; the sum of ionic radius of B and X atoms and the well known ‘tolerance factor’ (which is a function of ionic radius of A, B and X), were found very effective in reproducing the measured lattice constant through a linear combination of these two parameters (R²=0.995). It is further indicated that these two parameters are linked to the crystal features of perovskite. The average error limits in predicting lattice constant, by using this empirical equation, are expected within 0.63%. It may be useful to design new substrates/buffer materials for compound semiconductor epitaxy, in which there is a requirement of lattice match between them and adjacent layers.     

Dispersion relations and lattice thermal expansion of dysprosium

The lattice dynamics, second and third order elastic constants and the lattice thermal expansion of dysprosium have been calculated using Keating's approach. The ten third order elastic constants are calculated using four anharmonic parameters. The present model reproduces the measured pressure derivatives of the second order elastic constants of dysprosium well. The low and high temperature limits γ?_I and γ?_H of the lattice thermal expansion are evaluated and the agreement between the calculated γ?_H and that obtained from the thermal expansion and specific heat data is satisfactory.     

Pressure dependence of the lattice constant of diamond: Isotopic effects

The pressure dependence of lattice constants of ¹³C-enriched diamond and diamond of natural isotopic composition have been studied by using x-ray synchrotron radiation and the imaging plate technique. The compression curves reveal features which indicate an inversion of the isotope effect at high pressure. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 73–77 (25 January 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.