Phonon mechanisms

The original specific heat experiments on BaPb xB Og by Methfessel et al (60) immediately raised the prospect that an unusual mechanism was operative in this newly found system. Their finding of no heat capacity anomaly at Tc could actually have a number of possible interpretations, including an impurity phase giving rise to superconductivity, a non-phonon mechanism, or some new form of conductivity. 

In this paper, we investigate our two-band model for the explanation the multi-gap superconductivity of MgB2. We apply the model to an electron-phonon mechanism for the traditional BCS method, an electron-electron interaction mechanism for high- Tc superconductivity, and a cooperative mechanism in relation to multi-band superconductivity. 

While it is clear that the conventional BCS mechanism does not explain the high-temperature superconductivity, the unconventional nature of the e-p coupling suggests that the phonon mechanism cannot be discarded without careful examination. In addition the electronic inhomogeneity observed by the STM/STS measurement4,5 suggests that the inhomogeneity may be an intrinsic nature of the HTSC phenomenon, and any theory on the mechanism has to incorporate this aspect in a fundamental manner. At present no theory can explain both simultaneously. 

Tagantsev AK (1993) Phonon mechanisms of intrinsic dielectric loss in crystals. In Setter N, CollaEL (eds) Ferroelectric ceramics. Birkhauser, Basel, p 127... 

In the case of solid crystalline oxides, thermal conductivity decreases with increasing temperature but begins to rise above 1500— 1600 °C because transmission of heat by radiation (photons) begins to take a significant part besides the conduction of heat (phonon mechanism). In completely transparent materials (the coefficient of absorption a = O), no interaction with the radiation occurs in an opaque body (a = oo) the heat is transferred by conduction alone. With translucent materials, each element of the substance absorbs some of the incident radiation, and emits simultaneously,This internal radiation mechanism of heat transmission is characteristic for glasses. At high temperatures, a considerable proportion of heat is therefore transmitted by radiation the so-called apparent thermal conductivity is a sum of true conductivity with radiation conductivity ... 

The first conclusion to be drawn from this work is that calculations based on the BCS model cannot presently explain the Tc s of order 90 K given the phonon frequencies and band structures calculated. This points out that perhaps a mechanism other than an electron-phonon mechanism is involved, which depends on parameters with larger energy scales. A possibility is an electronic mechanism, many of which have been proposed. We will discuss one of these in the next section. 

Although the phonon mechanism has not been proven to be the operative one, this exciting discovery does fit well within our current understanding of phonon mediated superconductivity, in which electronic motions and lattice deformations are strongly correlated. This mechanism is easily defensible up to transition temperatures (Tc s) of 40K (2), and perhaps to slightly higher... 

Therefore whenever the spin-phonon mechanism is the dominant relaxation process, the magnitude of the linewidth, with proper normalization, can be... 

The defects of the Ceo structure lower its symmetry. As a consequence some transitions between vibrational states that are foAidden for an ideal molecule become IR-active. Several works were devoted to the study of vibrational s tra of fixllerene molecules for which some of the atoms were replaced by isotopes. They tried to address an important question concerning whether such replacement influences the physical properties of Cgo- First of all, in view of the electron-phonon mechanism of superconductivity in doped fiillerite, the question whether isotopical replacement results in a change in temperature of transition into a superconductive state was investigated. 

Describing the mechanisms of conduction in solids is not easy. Here only a brief qualitative sketch of some of the physical phenomena is given. In general, thermal energy in solids is transported by lattice vibrations, i.e. phonons, free electrons, and radiation. Given that the concentration of free electrons in ceramics is low and that most ceramics are not transparent, phonon mechanisms dominate and are the only ones discussed below. 

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