Intermediate electron-phonon interaction

Anomalies in the phonon spectra of UTe have also been found (Buyers and Holden 1985) and attempts were made by the authors to use the same theories described above, which essentially relate to an intermediate-valence picture. Unfortunately, as Buyers and Holden describe (see their p. 304) this does not lead to a convincing conclusion. There is, of course, no direct electronic evidence (e.g., from photoemission) that materials such as UTe exhibit valence fluctuations. We must conclude that the electron-phonon interaction is different in detail between the 4f intermediate-valence matmals and tiie actinide compounds. In both cases, however, the result is a negative Poisson ratio (which is directly related to the elastic constant C12). 

A charged state is obtained if the upper row is occupied by two electrons each while the alternant sites in the lower row are left unoccupied. The total number of electrons with x = 0 is 2L, but there are (2L) sites in the system. Apparently, the occupancy of the x = 0 states is just an indicator of a possible instability. We know from the BCS model that the phonon interactions contribute to pair formation. This may lead to an energetically favored charged state if the intermediate valence state is missing. ... 

As described in previous works [2, 3], the o(I) below 4.2 K follows a dependence (see Eq. (3.2)) in both the parallel and the perpendicular directions to the chain axis in oriented I-(CH)x samples. The dependence indicates that the contribution from e-e interactions plays a dominant role at very low temperatures. This is also consistent with the enhanced negative contribution to magnetoconductance (MC), as explained in detail in the next section. For the intermediate temperature range (4-40 K), where inelastic electron-phonon scattering (p = 3/2) is the dominant scattering mechanism, for the parallel and the perpendicular directions to the chain axis [1131,1133]. This is also consistent with the enhanced positive contribution to MC at temperatures above 4 K. This suggests that both interaction and localization play dominant roles in o(7) at low temperatures in metallic (CH) r samples. 

The interaction between light and matter can be viewed as the creation of a coherent quantum superposition of initial and final electron states that has an associated polarization [3], as shown in Figure 1. The coherence between states with different wave vector requires an intermediate virtual state and the presence of a coherent phonon. A transition between the initial and final states may occur when the coherence of the system is broken either due to the finite width of an optical wave packet or by scattering from the environment. The transition results in the absorption of a photon and the creation of a hot electron-hole pair. Otherwise, the photon is re-radiated with a different phase and, perhaps, polarisation. 

Whereas coherence can persist up to the nanosecond range for atomic and molecular systems exposed to dilute gaseous environments, the situation is radically different in liquids and solids. Interactions with neighbouring atoms, with phonons in crystalline materials and with conduction electrons in metals, shift the coherence times down by several orders of magnitude, and local quantum superpositions are usually not observable. Intermediate cases are the electronic states used as qubits in the form of superconducting islands introduced by Y. Nakamura et al. [4]. The latest reports [5] show coherence times up to 10 s for these objects, which would allow time for operations of a quantum computer. The decoherence mechanisms in such circuits have been discussed theoretically by Burkhard et al. [6],... 

Fig, 74. (a) LA phonon dispersion in the [111] direction for intermediate-valent TmSe. An electronic plasmon mode is shown as the dashed horizontal line. Mixed electronic plasmon-phonon modes are shown with dotted lines. The heavy dashed line is the phonon mode without interaction, (b) Phonon density of states with (solid curve) and without (dashed curve) plasmon interaction. (After Treindl and Wachter 1980.)... 

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