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1.
The coupling of 4f electrons and longitudinal optical phonons and its implications on different types of structural phase transitions in intermediate valence compounds is discussed within the framework of the periodic Anderson model. Two types of interactions are considered. First, the usual density type of coupling of 4f electrons and phonons leads to a weakly temperature dependent renormalization of the positionE
0 of the 4f level with respect to the 5d band. Secondly, phonon induced 4f–5d interband transitions lead to a renormalization of the hybridization energyV of 4f and conduction electrons. For appropriate parameter values the latter effect gives rise to discontinuous changes of the occupation of the 4f state.The tendency towards a ferromagnetic or antiferromagnetic instability is essentially suppressed by the presence of phonons.The dependence of the susceptibility on temperature and onE
0 is calculated and a jump is found whenever a discontinuous change of the occupation of the 4f state occurs.Work performed within the research program of the Sonderforschungsbereich 125, Aachen-Jülich-Köln 相似文献
2.
Phonon anomalies in two intermediate valence compounds (IVC), SmS and Sm0·75Y0·25S have been investigated using breathing shell model (BSM). The BSM includes breathing motion of electron shells of the rare
earth atom due tof−d hybridization. The phonon dispersion curves of IVC, calculated from the present model, agree well with the measured data.
One-phonon density of states calculated from the present model compares well with the Raman spectra. 相似文献
3.
Shinoo Srivastava Seema Srivastava M. K. Pandey S. K. Naman Shweta Srivastava 《Journal of Macromolecular Science: Physics》2013,52(3):617-636
A study of the normal modes of vibration and their dispersion in polyadenylic acid based on the Urey–Bradley force field is reported. It gives a better interpretation of FTIR spectra as compared with the valence force field. Characteristic features of dispersion curves such as regions of high density‐of‐states, repulsion, and character mixing of dispersive modes are discussed. Predictive valuzes of heat capacity as a function of temperature are reported. 相似文献
4.
The low-temperature heat capacities are studied for antiperovskite compounds AX M_3(A = Al, Ga, Cu, Ag, Sn, X = C,N, M = Mn, Fe, Co). A large peak in(C- γ T)/T~3 versus T is observed for each of a total of 18 compounds investigated,indicating an existence of low-energy phonon mode unexpected by Debye T~3 law. Such a peak is insensitive to the external magnetic field up to 80 k Oe(1 Oe = 79.5775 A·m-1). For compounds with smaller lattice constant, the peak shifts towards higher temperatures with a reduction of peak height. This abnormal peak in(C- γ T)/T~3 versus T of antiperovskite compound may result from the strongly dispersive acoustic branch due to the heavier A atoms and the optical-like mode from the dynamic rotation of X M_6 octahedron. Such a low-energy phonon mode may not contribute negatively to the normal thermal expansion in AX M_3 compounds, while it is usually concomitant with negative thermal expansion in open-structure material(e.g., ZrW_2O_8, Sc F_3). 相似文献
5.
The collision-generated hybridization which has been found responsible for the on-site mixing of the atomic-likef-state and the band-liked states in mixed valence solids has been studied for the cerium solid. A practical expression which depends on the lattice
constant and temperature has been obtained for the collision-generated hybridization. Numerical calculations show that the
valence varies continuously with lattice constant and that temperature makes the transition smoother. The collision-generated
hybridization is found to be of significant strength in the intermediate valence regime; but over a wide range of the valence
near 3.5 it varies rather slowly without preferring a particular valence. Factors which can assist the collision-generated
hybridization in stabilizing the mixed valence phase at a particular lattice constant are discussed. 相似文献
6.
We explore the possibility of magnetic re-entrance in intermediate valence compounds. Using a simplified Anderson-Lattice model we obtain the pressure-temperature magnetic phase diagram. This diagram shows that for some value of the microscopic parameters the temperature induced two transitions (non-magnetic to magnetically ordered to paramagnetic).We calculate the magnetization and the average occupation number of the localized state. Estimations of the observability of the effect in systems like CeAl2 are made. 相似文献
7.
Superconductivity in CeRu2 was discovered 40 years ago, and was extensively studied because alloying with magnetic elements showed the coexistence of
superconductivity and magnetic order. The normal state of CeRu2 has been of interest because of its intermediate valuence character. The superconducting state has been studied extensively
because of its paramagnetic nature and anomalous pinning properties. This review presents the present status of knowledge,
and discusses the puzzling features of CeRu2. 相似文献
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9.
Magnetic susceptibility of mixed-valence compounds has been calculated as a function of pressure using Falicov-Kimball model
wherein thef-s hybridisation has been taken into account. This model can very well explain the continuous as well as discontinuous transitions
from ground states of integral to intermediate valence. Our results for magnetic susceptibility are in qualitative agreement
with recent experimental results on some mixed-valence compounds. 相似文献
10.
An ab initio investigation of vibrational,thermodynamic, and optical properties of Sc_2AlC MAX compound 下载免费PDF全文
The structural vibrational, thermodynamical, and optical properties of potentially technologically important, weakly coupled MAX compound, Sc2 Al C are calculated using density functional theory(DFT). The structural properties of Sc_2AlC are compared with the results reported earlier. The vibrational, thermodynamical, and optical properties are theoretically estimated for the first time. The phonon dispersion curve is calculated and the dynamical stability of this compound is investigated. The optical and acoustic modes are observed clearly. We calculate the Helmholtz free energy(F), internal energy(E), entropy(S), and specific heat capacity(Cv) from the phonon density of states. Various optical parameters are also calculated. The reflectance spectrum shows that this compound has the potential to be used as an efficient solar reflector. 相似文献
11.
We report for the first time the complete phonon dispersion curves for the ytterbium pnictide compounds (YbN, YbP and YbAs)
using a breathing shell model to establish their predominant ionic nature. The calculated results also show that this group
of rare earth compounds does not show any elastic and phonon anomalies which are the characteristic features of other rare
earth compounds. We emphasize the need for further Raman and neutron scattering measurements. 相似文献
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14.
This work reports on the phonon energy inversion in graphene nanoribbons: after initial localized thermal excitation, the energy of initial cold phonons (flexural mode: FM) becomes higher than that of local hot phonons (longitudinal and transverse modes: LM/TM). Such energy inversion holds for about 50 picoseconds. Two physical factors combine together to give rise of this phenomenon: one is the much faster heat conduction by FM phonons than that by LM/TM phonons, and the other factor is the strongly temperature-dependent energy exchange rate between FM and LM/TM phonons: 3.7×1010 s−1 at 84 K to 20.3×1010 s−1 at around 510 K. 相似文献
15.
在二维正方Heisenberg铁磁系统的基础上建立了磁振子-声子相互作用模型. 利用松原格林函数理论研究了系统的声子衰减,计算了布里渊区的主要对称点线上的声子衰减曲线. 发现在第一布里渊区,在Δ线上,横向声频支声子无衰减,在Z线上,纵向声频支声子无衰减;横向声频支声子衰减比纵向声频支声子衰减至少大一个数量级,并讨论了各项参数的变化对横向声频支声子衰减与纵向声频支声子衰减的影响. 根据声子衰减与声子寿命的关系,声子衰减与声子态密度的关系,可以讨论横向声频支声子与纵向声频支声子的寿命与态密度.
关键词:
磁振子-声子相互作用
横向声频支声子衰减
纵向声频支声子衰减
声子寿命 相似文献
16.
Shweta Srivastava Shinoo Srivastava Seema Srivastava Vishwambhar Dayal Gupta Sanjeev John La'Verne 《Journal of Macromolecular Science: Physics》2013,52(1):157-173
A study of the normal modes of vibration and their dispersion in polypyrrole (PPY) based on the Urey-Bradley force field is reported. It gives a fuller interpretation of previously reported FTIR spectra. Characterstic features of dispersion curves, such as regions of high density-of-states, repulsion, and character mixing of dispersive modes, are discussed. Predictive values of heat capacity as a function of temperature are reported. 相似文献
17.
It is shown that in some stoichiometric intermediate valence compounds a symmetry breaking distortion leading to a hybridization gap in the electronic density of states can exist. We study this possibility for the high pressure phase of SmS and the paramagnetic phase of TmSe. 相似文献
18.
The thermodynamic potential of rare earth compounds in a mixed valent state is investigated by perturbation theoretical methods. Two types of contributions must be distinguished, the impurity terms and terms corresponding to interactions between different rare earth sites. For a large class of substances the latter are shown to be of minor importance. The susceptibility of the resulting theory is compared directly with the renormalization group calculations of Krishna-murthy, Wilson and Wilkins and with measured susceptibilities of-Ce, SmB6, YbAl3. 相似文献
19.
By using determinant method as in our recent work, the IO phonon modes, the orthogonal relation forpolarization vector, electron-IO phonon Frohlich interaction Hamiltonian, the dispersion relation, and the electron-phonon coupling function in an arbitrary layer-number quantum well system have been derived and investigated withinthe framework of dielectric continuum approximation. Numerical calculation on seven-layer Alx Ga1-x As/GaAs systemshave been performed. Via the numerical results in this work and previous works, the general characters of the IO phononmodes in an n-layer coupling quantum well system were concluded and summarized. This work can be regarded as ageneralization of previous works on IO phonon modes in some fixed layer-number quantum well systems, and it providesa uniform method to investigate the effects of IO phonons on the multi-layer coupling quantum well systems. 相似文献
20.
Y. Kuramoto 《Zeitschrift für Physik B Condensed Matter》1981,40(4):293-299
We study the mechanism by which the magnetic order is suppressed in intermediate valence compounds. By a diagrammatic perturbation theory in the hybridization interaction, the effective inter-site exchange interaction is derived for the periodic Anderson model. The result includes the effect of charge fluctuations and is valid for arbitrary position of the local electron level relative to the Fermi level. It is shown that the intersite interaction is destructive to magnetic fluctuations with any wavenumber in the case of intermediate valence.Work supported by the Deutsche Forschungsgemeinschaft through the research program SFB 125 Aachen-Jülich-Köln 相似文献