Abstract:The neutron-inelastic-scattering technique has been M to measure the phonon dispersion relations for a fcc single crystal of xenon at 10 K. All branche's in the high-s~~etry directions were determined with special emphasis on the small-wave-vector portions. Careful consideration was given to the effects of instrumental resolution in an effort to achieve the highest possible accuracy. The corrected data were fitted to several harmonic Born-von Khrmin force-constant models. From the force constants the frequency… Show more
“…Using two crystals with a combined volume of 5 mm3, and the doublefocussing triple-axis spectometer at the OrphCe reactor (Saclay, France), scientists from the Karlsruhe group recently succeeded (39) in measuring dispersion curves for translational and librational external modes in C,,. As one might expect, given the van der Waals' nature of the intermolecular bonding, translational phonons in the high temperature phase closely resemble those in one of the fcc rare gas solids, namely xenon (40). Measurements at 200K, in the orientationally ordered phase, show little change in these modes (apart from the zone-folding effect), other than a slight hardening as the temperature is lowered.…”
Section: Phonon Measurements and Molecular Spectroscopymentioning
“…Using two crystals with a combined volume of 5 mm3, and the doublefocussing triple-axis spectometer at the OrphCe reactor (Saclay, France), scientists from the Karlsruhe group recently succeeded (39) in measuring dispersion curves for translational and librational external modes in C,,. As one might expect, given the van der Waals' nature of the intermolecular bonding, translational phonons in the high temperature phase closely resemble those in one of the fcc rare gas solids, namely xenon (40). Measurements at 200K, in the orientationally ordered phase, show little change in these modes (apart from the zone-folding effect), other than a slight hardening as the temperature is lowered.…”
Section: Phonon Measurements and Molecular Spectroscopymentioning
“…The data run from T = 0 to T m for Al [5], Ar [6], Bi [7], Cd [8], Cs [9], Cu [10], In [11], K [12], Na [13], Nb [14], Ne [15], Pb [11], Sn [7], Ta [16], Te [17], Xe [18], and Zn [19].…”
An analytic model of the shear modulus applicable at temperatures up to melt and at all densities is presented. It is based in part on a relation between the melting temperature and the shear modulus at melt. Experimental data on argon are shown to agree with this relation to within 1%. The model of the shear modulus involves seven parameters, all of which can be determined from zero-pressure experimental data. We obtain the values of these parameters for 11 elemental solids. Both the experimental data on the room-temperature shear modulus of argon to compressions of ∼ 2.5, and theoretical calculations of the zero-temperature shear modulus of aluminum to compressions of ∼ 3.5 are in good agreement with the model. Electronic structure calculations of the shear moduli of copper and gold to compressions of 2, performed by us, agree with the model to within uncertainties.
“…Under this condition lattice theories can give reasonable description of the phonon dispersions [3]. The rare gas solids [8][9][10][11] and CsCl [12] are examples of a vanishing Debye boson-phonon interaction. For these solids the acoustic phonons are perfectly described by sine functions of wave vector.…”
A method is described that allows one to construct the dispersion of the Debye bosons (sound waves) from the known temperature dependence of the sound velocities. The method simply assumes that the sound velocity measured at temperature T gives the slope of the dispersion relation at excitation energy E = kBT. The associated reduced wave vector is set equal to q/q0 = a0kBT/hvL/T. In this way the dispersion of the Debye bosons can be constructed for all thermal energies for which sound velocities vL/T(T) are known. This can be up to melting temperature. Surprisingly, for metals and for insulators the dispersion of the Debye bosons can continue up to wave vector values of several times the zone boundary. At melting temperature the wavelength of the Debye bosons is of the order of the atomic diameters. The sources of the Debye bosons therefore must have atomic dimensions. Spontaneous generation of Debye bosons by individual atoms is, however, a completely unexplored process. Interactions with the atomistic background of phonons or lattice defects provide damping to the Debye bosons and make the material specific sound velocity vL/T(T) additionally sample and temperature dependent. For practically all solids it is observed that elastic constants and vL/T(T) decrease as a function of increasing temperature. For high energies the dispersion relation of the Debye bosons therefore becomes visibly lower than linear. Interactions between Debye bosons and phonons can modify the dispersion of the acoustic phonons appreciably. Because of their different symmetries the dispersion relations of Debye bosons and acoustic phonons can attract each other. It is observed that for low wave vector values the dispersion of the acoustic phonons can assume the linear wave vector dependence of the Debye bosons. At the end of the linear section a functional crossover to a sine-like function of wave vector occurs.
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