Phonon-mediated superconductivity

Several transitions have been proposed to occur within ICs. One of them is condensation into an anisotropic liquid phase, due to interactions between molecules in different channels [148]. This has been claimed to occur at remarkably high temperature when the phenomenon of dilation of the lattice of tubes is taken into account [142, 143]. That is a collective phenomenon, analogous to the phonon-mediated superconductivity, but with a higher energy scale associated with the enhanced binding within the channel. 

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... 

Aq is a constant except for the Cu close to Ni. The relation (26) is direct evidence for the attractive force being due to spin fluctuation, irrespective of any theoretical model. In the conventional superconductor, the isotope effect provides direct evidence for phonon-mediated superconductivity. The relation (26) is considered to correspond to the isotope effect in a phonon-mediated superconductivity model. 

Pickett, W. E. (2008) The next breakthrough in phonon-mediated superconductivity. Physica C (Superconductivity),46, 126-135. 

Tunneling Spectroscopy has revealed electron - phonon coupling strengths for optical phonons in Ba K BiOg which suggest that phonon mediated coupling is responsible for superconductivity in this system (59). 

B., Mitchell, A.W. and Richards, D.R., Tunneling Spectroscopy in Superconducting Baj xKxBiOs Direct Evidence for Phonon-Mediated Coupling. Physica C 158 519 (1989). 

Perhaps the Mg intermetallic compound which attracted most interest in the past 10 years is MgB2. The reason for this lies in its superconductivity below 39 K, which is an unusually high value for a phonon-mediated... 

The problematic nature of the melting transition can be illustrated by comparison with other well-known first-order phase transitions, for instance the normal metal-(low T ) superconductor transition. The normal metal-superconductor and melting transitions have similar symptomatic definitions, the former being a loss of resistance to current flow, and the latter being a loss of resistance to shear. However, superconductivity can also be neatly described as a phonon-mediated (Cooper) pairing of electrons and condensation of Cooper pairs into a coherent ground state wave function. This mechanistic description of the normal metal-super-conductor transition has required considerable theoretical effort for its development, but nevertheless boils down to a simple statement, indicat-... 

The electron-phonon interaction is the capital ingredient in superconductivity. Phonons mediate the coupling of two electrons at a very low temperature, although the electrons have minus charge repelling each other. The Cooper pair and phonon move across the crystal lattice without a resistance. 

In the microscopic theory of Bardeen et al. (1957) the electrons are treated as a nearly free electron gas, individual electrons interacting pairwise via an attractive phonon-mediated potential. The superconducting transition temperature is given by the well-known expression... 

In 1957, the American physicists J. Bardeen, L. N. Cooper, and J. R. Schrieffer (1957) developed a microscopic theory of superconductivity (the BCS theory), and were subsequently awarded the Nobel Prize in physics in 1972. The BCS theory is applicable to metals and alloys (conventional low-Tc superconductors). The Coulombic repulsion of electrons is thought to be overcome by a phonon-mediated mechanism, whereby two electrons with their spins aligned in opposite directions strongly attract each other (Cooper pairing). Cooper pairs condense to a remarkably stable macroscopic quantum state, described by identical wave functions (see Appendix E). 

We speculated previously that superconductivity in KjCso and Rb3Ceo resulted from electron pairing mediated by high-frequency intramolecular phonons of the Qo molecule, with the change in between KaCeo and RbaCso explained by a change of the density of states at the Fermi level by about 10%. In this weak-coupling BCS model, the variation of is given by... 

The RVB theory has been amplified or modified In several ways [5,6,7], the details of which we won t go Into here. Rice and Vang [8] have proposed a model In which superconductivity Is due to condensation of a pair of bosons which gives rise to qiiasl-particle excitation energies which are similar to those of RVB theory, but different from BCS theory. Rice and Wang, however, favor a phonon Interaction which mediates the attraction between the boson pair In question. Coffey and Cox [9] have given a nice summary of the essential points of the RVB theory In Section II of their paper. 

The question of the nature of the superconducting state in the radical-ion salts can still not be unambiguously answered even after more than 20 years of intensive research. In the conventional superconductors, the phenomenon can be explained through the BCS theory by the formation of Cooper pairs. Here, the attractive interaction between two electrons is mediated by phonons. The total spin of the Cooper pairs is S = 0, and their total orbital angular momentum is I = 0. The pair wave-function is an s-function. The concept of the Cooper pairs as spin singlets can evidently also be applied to the radical-ion salts. This can for example be shown by measurements of the NMR spin-lattice relaxation time in the superconducting state [3]. 

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