Phonon anomaly

AB INITIO INVESTIGATIONS OF PHONON ANOMALIES AND MARTENSITIC TRANSFORMATIONS... 

In a general concept of a symmetry-restricted anharmonic theory Krumhansl relates the phonon anomalies to the electron band topology. The latter is directly determined by the competition of nearest neighbour interactions which in turn can be a function of stress, composition and temperature Nagasawa, Yoshida Makita simulated the <110> ... 

A. Zheludev, S.M. Shapiro, P. Wochner, A. Schwartz, M. Wall and L.E. Tanner, Phonon anomaly, central... 

A. Kmmhansl, Phonon anomalies, stmctural transformations and precursor stmcture in the martensitic... 

A. Kmmhansl, Competing displacive interactions, phonon anomalies and stmctural transitions which do... 

It is noteworthy that the phonon anomaly, due to the dynamical coupling between substrate Rayleigh wave and adlayer mode, is likewise present in the bi- and even the trilayer films. It is only the Q range of the anomaly which... 

There are several arguments that support a causality between the phonon anomalies and the pseudo gap formation in the spin fluctuation spectrum. The connecting element is a coupling of the lattice displacement of the a phonon to the occupancy and admixture of interlayer dyZ and dxz orbitals. The energy of this mode softens from 155 to 130 cm-1 in the temperature range from 200 to... 

At Tc, infrared spectra also exhibit various phonon anomalies. Fig. 4.8-24 demonstrates this for a Cu/Ga substituted 123-superconductor (a) and a Tl-based superconductor (b). The Y compound clearly shows spontaneous softening of the mode at T,., whereas in the spectrum of the T1 superconductor the mode at 305 cm bleaches below Tc (110 K), while another mode at lower energy gains oscillator strength. Interestingly, IR-active phonons do not only soften but may also harden as a consequence of phase transition. This led to the conclusion that there are two different gap energies, one around 30 meV ( 5/cbT,.) for electrons in the a, b plane, and one around 10 meV (s 1,7 b7 c) in c-direction. 

Figure 4.8-24 Phonon anomalies from IR-reflectance on YBa2Cu307 (a), according to Kuzmany et al., 1990b, and from IR-conductivity in Tl2Ba2Sr2Cu30io (b), according to Zetterer et al. 1990. The bold line in part (a) labels the transition temperature.

As in the case of the noble metals above, it is the sensitivity of the helium atom scattering to the distribution of electrons in the conduction energy states which has led to the detailed explanation of the surface properties of these metals. Intriguingly, for the W( 110) and Mo(llO) surfaces, it is still uncertain why saturation with hydrogen leads to just the opposite consequences, namely giant phonon anomalies, whereas the clean surfaces have normal dispersion curves [113]. 

It has been emphasized (Mock et al. 1986) that besides a systematic understanding of the occurrence or absence of elastic and phonon anomalies in different IV compounds the concept introduced above allows for a first experimental estimate of charge fluctuation rates. The direct experimental investigation of these has not been feasible, unlike the magnetic relaxation rates, which have been investigated intensively by quasielastic neutron scattering (Holland-Moritz et al. 

However, contrary to CeBCu this mode softening in Cei La Bei3 for 0.8 is also observed for all other symmetry modes with respect to the average behavior of the reference materials. The phonon softening in Cej La, Be,3 for 0.1 X 0.8, independent of the mode symmetry is also reflected by the behavior of the Debye temperature 0 (Besnus et a. 1983), which is displayed at the bottom of fig. 35. No temperature dependent phonon anomaly has been observed for the optical phonons of CeBcij, contrary to the anomalous softening of the bulk modulus upon cooling down below 350 K (Lenz et al. 1984). 

The very same charge deformability of the mixed-valence Sm ion due to 4f" - 4f"5d excitations used for the description of the Raman intensities in fig. 37 has been used to describe the phonon anomalies (Bilz et al. 1979). Therefore we can conclude that the dominant F scattering intensities of Sm 25S near 250 cm" and 85 cm , respectively, arise mainly from the LO and LA phonon anomalies in the [111] direction, emphasizing scattering from L-point phonons. The available data on the LO(L) phonon frequencies of RS are depicted in fig. 38 as a function of the lattice parameter. The LO(L) phonons of intermediate-valence metallic SmS and Sm jY 25S lie between the divalent reference line given by YbS and EuS, and the trivalent reference line spanned by YS, GdS, PrS and LaS, thus exhibiting the behavior of an alloy of divalent and trivalent Sm ions. Figure 39 shows the bulk modulus of several RS compounds at room... 

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