Analysis of heat capacity of platinum: Electron-phonon effects and anharmonic contribution

The heat capacity results for platinum from 80 to 1000 K, reported in our previous paper, have been analyzed. The contributions of bare electrons and harmonic phonons to the heat capacity have been evaluated from the available data on the band structure and the phonon spectrum of platinum. At low temperatures, the present estimates for the temperature dependence of the electron-phonon enhancement agree qualitatively with those derived from theoretical calculations found in the literature. The anharmonic heat capacity obtained is negative and linear with temperature in the vicinity of the Debye temperature (237 K), and becomes constant above 450 K. The enhancement in the heat capacity above 1000 K has been analyzed and found to be due mainly to the enlargement of the dilation term corresponding to the similar enhancement in the thermal expansion.     

Transport theory for quantum crystals

A correlation function approach is developed to treat non-equilibrium phenomena of quantum crystals at low frequency and long wavelength within the renormalized harmonic approximation (RHA). The derivation of the transport equations is carried out by studying the hierarchy of equations of motion for the retarded Green's functions of a pure, nonprimitive, nonionic, anharmonic lattice. Using a factorization technique to take into account the most important terms due to the particle fluctuations and the leading contributions to the hydrodynamic singularities of the phonon self-energy, we find a differential equation for the displacement field and a generalized transport equation for the phonon gas. The microscopic RHA expressions for the local temperature, the local heat density and the energy current are derived; the quasiparticle parameters (elastic constants, generalized Grüneisen parameters, quasiparticle interaction) entering the equations of motion are shown to be consistent with the RHA. In the hydrodynamic regime the general equations are reduced to two coupled differential equations for the lattice deformations and for the local temperature. Then only the displacement-displacement, the displacement-energy density and the energy density-energy density correlation functions show macroscopic fluctuations; for these functions thermodynamical sum-rules are derived.     

Y3Al5O12的热输运性质的第一性原理研究

基于密度泛函微扰理论（DFPT）结合模守恒赝势方法进行晶格动力学模拟.得到了钇铝石榴石（YAG）的声子态密度、分波声子态密度和声子的色散谱.利用第一Brillouin区的特殊点取样方法,计算了YAG的比热容和布局数平均的声子群速度.在非谐相互作用下,利用Fermi黄金公式结合第一Brillouin区的特殊点取样方法,得出了YAG非谐声子平均自由程.综合考虑了两种声子散射机制,得到了YAG陶瓷的热导率.结果表明,对于YAG陶瓷,在低温时,晶界散射将对热阻起主要作用;在高于一定温度时,三声子相互作用对热阻的贡献将占主导地位.同时也从理论上证明了Sato等提出的在室温以上,YAG陶瓷与单晶的热导率的差异可以忽略的观点.所得到的热导率、比热容随温度的变化与实验结果很好地符合. 关键词： 声子平均自由程 密度泛函微扰理论 3Al₅O₁₂声子结构')" href="#">Y₃Al₅O₁₂声子结构 热导率     

Static,anharmonic and phonon properties of ND4I

A detailed investigation of the phonon dispersion and static and anharmonic properties has been carried out for ND₄I using an extended three-body force shell model (ETSM) developed by Singh and Chandra. The ETSM results for phonon dispersion show better agreement with the experimental data than those obtained from the deformation dipole and rigid shell models. Calculated values have also been obtained for some other properties.     

Path integral computation of phonon anharmonicity

The partition function of an oscillator disturbed by a set of electron particle paths has been computed by a path integral formalism which permits to evaluate at any temperature the relevant cumulant terms in the series expansion. The low temperature cutoffs in the anharmonic cumulant series are determined fulfilling the constraint of the third law of thermodynamics. The general method here proposed has been applied to the semiclassical Su-Schrieffer-Heeger model whose time dependent source current is linear in the oscillator displacement field. We find that this peculiar current induces large electron-phonon anharmonicities on the phonon subsystem. As a signature of anharmonicity the phonon heat capacity shows a peak whose temperature location strongly varies with the strength of the e-ph coupling. Since the electron hopping potential provides a sizeable background in the low and intermediate temperature range, such a peak is partly smeared in the total heat capacity. Low energy oscillators are more sensitive to anharmonic perturbations.Received: 7 January 2004, Published online: 3 August 2004PACS: 71.20.Rv Polymers and organic compounds - 31.15.Kb Path-integral methods - 63.20.Kr Phonon-electron and phonon-phonon interactions     

Characteristic features of anharmonic effects in the lattice dynamics of fcc metals

The dispersion in the entire Brillouin zone and the temperature dependence (right up to the melting temperature) of the anharmonic frequency shift and phonon damping in a number of fcc metals is investigated on the basis of microscopic calculations. It is found that the anharmonic effects depend sharply on the wave vector in the directions Г-X, X-W, and Г-L and, in contrast to bcc metals, the magnitude of the effects is not due to the softness of the initial phonon spectrum. It is shown that the relative frequency shifts and the phonon damping near melting do not exceed 10–20%. The relative role of various anharmonic processes is examined, and the relation between the results obtained and existing experimental data is discussed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 649–652 (10 May 1999)     

纳米晶锐钛矿相TiO2的非简谐效应和声子局域

对晶粒尺寸为194,86和56nm的纳米晶锐钛矿相TiO₂,进行了从83到723K的变温拉曼散射测量,并对E_g(1)模式进行了详细研究.根据非简谐效应和声子局域模型,对E_g(1)拉曼峰进行了拟合与计算.结果表明,以上三种纳米晶粒的晶格振动机理,在本质上是相同的.三声子过程对频率蓝移起主要作用.为了得到很好的拟合,需要同时考虑三声子和四声子过程.随着温度的升高,四声子过程增强,并对三声子过程起抵消作用.与非简谐衰减相关的声子寿命随着晶粒 关键词： 2')" href="#">纳米晶TiO₂ 拉曼散射 非简谐耦合 声子局域     

Anharmonic excitations in the FPU and FK models

The properties of vibrational localized (breathers) and traveling (anharmonic phonons) waves are discussed in an infinite, one-dimensional, monoatomic crystal for the Fermi-Pasta-Ulam and Frenkel-Kontorova models. The shooting and finite difference schemes have been implemented to reckon the displacement fields of breathers and anharmonic phonons, respectively. These tools provide localized and traveling waves proving to be indefinitely stable in simulations carried out by solving the equations of motion. The emphasis is laid on the role of the cubic and quartic terms of the anharmonic potential which turn out to oppose and to shore up the restoring force, respectively. The case of vibrational modes arising in an anharmonic crystal subject to a soft phonon induced instability is also addressed. Received 7 November 2001 and Received in final form 5 February 2002 Published online 6 June 2002     

Lost heat capacity and entropy in the helical magnet MnSi

The heat capacity of MnSi at B = 0 and B = 4 T was measured in the temperature range 2.5-100 K. To analyze the data, calculations of the phonon spectrum and phonon density of states in MnSi were performed. The calculated phonon frequencies were confirmed by means of inelastic neutron scattering. The analysis of the data suggests the existence of negative contributions to the heat capacity and entropy of MnSi at T > T(c) that may imply a specific ordering in the spin subsystem in the paramagnetic phase of MnSi.     

Phonon–phonon interactions in Ce0.85Gd0.15O2−δ nanocrystals studied by Raman spectroscopy

Phonon–phonon interactions and phase stability of Gd‐doped ceria nanocrystals were examined over the temperature range 293–1100 K by Raman spectroscopy. The phonon confinement model (PCM) based on size, inhomogeneous strain and anharmonic effects was used to properly describe the anharmonic interactions in this system. The interplay between size and anharmonic effects influenced different phonon decay channels in nano grains than in larger grains. After the gradual cooling down to room temperature (RT), the Raman study revealed the phase separation in this system pointing to the phase instability of Ce_0.85Gd_0.15O_2−δ nanocrystals after heat treatment. The concentration of extrinsic (intrinsic) oxygen vacancies was also studied by Raman spectroscopy during the heat treatment of the Ce_0.85Gd_0.15O_2−δ nanocrystalline sample. Copyright © 2009 John Wiley & Sons, Ltd.     

On the spin-phonon interaction in anharmonic crystals

In this paper, a theory is developed for examining the influence of the spin-phonon interaction on the phonon dynamics in anharmonic crystals. In contrast to a harmonic approximation such an approach allows for comparing the role of the direct interaction between lattice vibrations with that of the indirect interaction via spins in the propagation and attenuation of sound as well as in the behaviour of thermodynamic quantities. Explicit expressions are derived for the phonon self-energy using successively higher approximations obtained from a cumulant expansion of the density of interparticle distances. It is shown that even in the harmonic approximation the two-spin correlation function and the four-spin correlation function enter into the expression of the generalized dynamical matrix in deviation from previous results. The modification of this harmonic dynamical matrix due to the renormalization of the harmonic phonons and due to phonon-phonon scattering processes is given employing a “weak coupling” approximation between the localized spins and the interparticle-distance density of the crystal. The resemblance of the present approach to an earlier microscopic treatment of electronic contributions to phonon dynamics in anharmonic crystals is pointed out.     

Calculation of the lattice dynamical properties of Ge

The lattice dynamical properties of Ge are studied employing a frozen phonon approach within an ab initio density-functional pseudopotential formalism. Using the atomic number, atomic mass, and crystal structure as the only input information, we calculate phonon frequencies and mode Grüneisen parameters at Γ and X and the third-order anharmonic parameter for the zone center optical mode. The results are in excellent agreement with available experimental data. An analysis of various energy contributions to the TA (X) mode shows that the proper inclusion of the exchange-correlation energy is crucial for accurate microscopic studies of lattice dynamics.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Low-temperature heat capacity of metals

The low-temperature heat capacities of transition metals and alloys were reviewed by Heiniger, Bucher, and Muller¹ in 1966, with particular emphasis on the dependence of the electronic heat capacity on the number of conduction electrons. Nuclear contributions to the heat capacity of metals and alloys have been reviewed recently by Lounasmaa.² The most recent general review of the low-temperature heat capacities of metals, however, was that by Keesom and Pearlman³ in 1956. Since that time there have been important advances in experimental techniques and also in theory related to certain of the heat capacity contributions.     

Magnetic and magnetocaloric properties of Gd1−xScxNi2 solid solutions

Magnetic and heat capacity measurements have been carried out on the polycrystalline Gd_1−xSc_xNi₂ solid solutions (0≤x≤1), which crystallize in the cubic C15 Laves phases superstructure (space group F4?3m). These solid solutions are ferromagnetic with a Curie temperature below 76 K. Their Curie temperature decreases from 75.4 K for GdNi₂ to 13.6 K for Gd_0.2Sc_0.8Ni₂. At high temperatures, all solid solutions, except ScNi₂, are Curie-Weiss paramagnets. The Debye temperature as well as phonon, conduction electron and magnetic contributions to the heat capacity have been determined from heat capacity measurements. The magnetocaloric effect has been estimated both in terms of isothermal magnetic entropy change and adiabatic temperature change for selected solid solutions in magnetic fields up to 3 T.     

Anomalous abrupt resonance-type change in the low-temperature heat conductivity in II–VI semiconductors doped with 3<Emphasis Type="Italic">d</Emphasis> transition metals

The phonon heat conductivity in ZnSe: Ni and ZnS: Ni compounds is investigated. The temperature dependences of the heat conductivity coefficient for these crystals are measured by the method of stationary heat flux. It is found that the heat conductivity exhibits minima in the temperature range 15–20 K. It is assumed that the quasi-resonant behavior of the low-temperature heat conductivity is associated with the umklapp processes due to phonon scattering by anharmonic vibration modes of the cluster.     

纳米晶体线的热膨胀性质

将原子间相互作用势的非谐项作为微扰,运用声子数表象中的晶格原子振动位移和晶格振动哈密顿公式,推导了纳米晶体线的热膨胀系数公式,并进行了数值计算.研究结果表明,纳米晶体线越细,其热膨胀系数越大.     

Lattice dynamics of solid molecular hydrogen for densities 26.18 + 60.78 nm−3, as seen from total neutron interaction cross section measurements

Abstract

The total neutron interaction cross sections versus energy measurements have been carried out on solid parahydrogen for almost 100% change of its density. Some integral parameters of hydrogen very anharmonic phonon spectrum have been determined fro. an analysis of these data.     

Anharmonic stabilization of the high-pressure simple cubic phase of calcium

The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approximation, shows imaginary branches that make it mechanically unstable. In this Letter, the phonon spectrum is recalculated by using density-functional theory ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that the perturbation theory cannot be employed with imaginary harmonic frequencies, a variational procedure based on the Gibbs-Bogoliubov inequality is used to estimate the renormalized phonon frequencies. The results show that strong quantum anharmonic effects make the imaginary phonons become positive even at zero temperature so that the simple cubic phase becomes mechanically stable, as experiments suggest. Moreover, our calculations find a superconducting T(c) in agreement with experiments and predict an anomalous behavior of the specific heat.     

Temperature dependence of the local mode in the vibronic spectrum of SrTiO3:Cr3+

The local vibrational mode exhibited by chromium impurity ions in strontium titanate has been investigated using vibronic spectroscopy. The temperature dependent characteristics of the mode are attributed to anharmonic interactions with lattice phonons. Line broadening contributions are made by both a decomposition mechanism and inelastic phonon scattering. Elastic phonon scattering processes can explain the line shift while multiphonon emission causes the integrated intensity to decrease. The results are compared with previous local mode investigations made on alkali halides by i.r. absorption spectroscopy.