Phonon softening

Figure 15.1 shows typical examples of size-softened optical phonons, which justifies the validity of the BOLS with derived information about the corresponding dimer vibration is given in Table 15.1. The size-softened phonons arise from the involvement of the interaction between the specific atom and it surrounding neighbors [32, 33]. 

This structural instability is associated with the occurrence of high 7 i.e., a softened phonon spectrum in the normal state strongly influences 7 and is a prelude to the structural transforma-tion. " It is thought that the conditions favorable for high-7 superconductivity also lead to structural instabilities, and that the structural transformation which results may also lead to a lowering of 7. In the case of VjSi a reduction of 0.38°K was calculated. ... 

FegNi. Frozen phonon calculations combined with the determination of the electron-phonon matrix in the framework of the theory of Varma and Weber have been carried out for the ferrous alloy. The resulting phonon dispersion for the bet phase was already presented elsewhere . As expected, no softening or anomalous curvatures have been detected. This confirms the existence of a bet ground state for FesNi. 

According to the concept of the displacive-type ferroelectric phase transition [10], an increase in the dielectric constant corresponds directly to the softening of the IR-active transverse phonon. When the crystal can be regarded as an assembly of the vibrators of normal coordinates, the soft phonon... 

A AH < kT has important consequences. As the temperature is lowered to where AHg, kT, strong electron-phonon interactions must manifest themselves. Direct evidence for mode softening and strong electron-phonon coupling in the internal Ty < T < 250 K has been provided by measurements of the Mdssbauer recoiless fraction and the X-ray Debye-Waller factor as well as of muon-spin rotation Therefore, it would be... 

Classical BCS theory dictated that superconductors with the highest Tc s would not be thermodynamically stable phases. Softening the phonons would raise Tc but would ultimately lead to structural instabilities. Increasing the density of states at the Fermi level would raise Tc but would eventually lead to an electronic instability. 

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