Phonon Mechanism Of Superconductivity Research Articles

The Formative Years of Ideas on High T c Organic Superconductivity

This paper is a historical review, told from a personal perspective, of the debates and of the research efforts generated by Little’s original idea of replacing the phonon mechanism of superconductivity by an electronic polarization mechanism, specifically the excitonic mechanism implemented in a one-dimensional structure. It discusses the main issues and problems related to this idea and their theoretical exploration, and it highlights the influence of Little’s work on the development of organic conductivity and superconductivity.

Kohn anomaly in phonon driven superconductors

Anomalies often occur in the physical world. Sometimes quite unexpectedly anomalies may give rise to new insight to an unrecognized phenomenon. In this paper we shall discuss about Kohn anomaly in a conventional phonon-driven superconductor by using a microscopic approach. Recently Aynajian et al.'s experiment showed a striking feature; the energy of phonon at a particular wave-vector is almost exactly equal to twice the energy of the superconducting gap. Although the phonon mechanism of superconductivity is well known for many conventional superconductors, as has been noted by Scalapino, the new experimental results reveal a genuine puzzle. In our recent work we have presented a detailed theoretical analysis with the help of microscopic calculations to unravel this mystery. We probe this aspect of phonon behaviour from the properties of electronic polarizability function in the superconducting phase of a Fermi liquid metal, leading to the appearance of a Kohn singularity. We show the crossover to the standard Kohn anomaly of the normal phase for temperatures above the transition temperature. Our analysis provides a nearly complete explanation of this new experimentally discovered phenomenon. This report is a shorter version of our recent work in JPCM.

Superconductivity in doped nondegenerate insulators

An analysis of experimental data on the change in the electronic structure and the position of the chemical potential depending on the doping level in superconducting tungsten bronze NaxWO3 and cuprates La2−xSrxCuO4 and Nd2−xCexCuO4 demonstrates the importance of the general problem of superconductivity in the impurity bands of doped nondegenerate insulators. The theory of superconductivity in the impurity bands of doped nondegenerate insulators is formulated. The approach to a description of a doped insulator is substantiated on the basis of Holstein-Anderson (Frohlich-Anderson) model with electron correlations at impurity sites distributed at random in the initial lattice. An insulator-metal transition observed in the normal phase is caused by attenuation of spin fluctuations in the doped system upon an increase in the doping level and/or temperature. This transition is characterized by the presence of narrow allowed bands in the initial insulator. In contrast to the BCS theory, the equation for the superconducting gap does not arise in the description of such a peculiar superconductivity. Instead, a nontrivial solution to the equation for singlet bosons localized at lattice sites must exist for a superconducting transition. The formation of such bosons is a precursor to the emergence of charged extended bosons in the doped system. In the general case, both singlet and triplet pairing channels are possible in the superconducting state. The spin-triplet channel exists only in the case of a finite spectral density of spin fluctuations in a doped compound. In this case, single-particle Green functions are nondiagonal in the spin index. The results of analysis of the phonon mechanism of superconductivity with a spin-singlet pairing channel are considered. The superconductor-metal transition in a doped compound, which is induced by changes in the temperature and/or the doping level, as well as the isotopic effect, is studied using numerical methods. The results are compared with the available data for HTSC materials.

Phonon mechanism of high-Tc superconductivity

For a long time the majority view has been that phonons are irrelevant to the mechanism of high-temperature superconductivity. However, recent experimental results, including the neutron inelastic scattering measurements reviewed here, seriously challenge this view. We point out that the electron-phonon coupling in the cuprates can be substantially different from that in simple metals because of the covalency and strong electron correlation. In particular certain phonon branches induce substantial intersite charge transfer that can result in a negative electronic dielectric susceptibility. Such a strong electron-phonon coupling of certain modes could form the basis for the phonon mechanism of superconductivity in the cuprates.

ON PHONON MECHANISM OF HIGH TC SUPERCONDUCTIVITY

This paper explains simultaneously dTc/dP > 0, dλ/dP < 0 and the isotope effect observed for Y1Ba2Cu3O6.9 and La1.85Sr0.75CuO4 in terms of unified parameters in phonon mechanism of superconductivity.

Model for an Exciton Mechanism of Superconductivity

The exciton mechanism of superconductivity is discussed with respect to a particular model, a thin metal layer on a semiconductor surface. In this model, the metal electrons at the Fermi surface tunnel into the semiconductor gap where they interact with virtual excitons, producing a net attractive interaction among the electrons in direct analogy with the phonon mechanism of superconductivity. The physical requirements for successful realization of the exciton mechanism in a metal-semiconductor system are explored in detail, and the relevant parameters are described. Estimates are made for electron tunneling and band-bending effects, and an electron-exciton coupling constant is defined and estimated. Finally, an appropriately modified integral equation for the superconducting energy gap is solved numerically to yield transition temperatures both for a pure-exciton mechanism and for the exciton and phonon mechanisms acting simultaneously.