1,721,016 research outputs found
Electron–phonon interaction and breakdown of the adiabatic principle in fullerides and MgB2
No full textNovel materials and superconductors are often characterized by small electron Fermi energies EFEF. This situation is expected to give rise to an unconventional electron–phonon phenomenology when the energy scale EFEF becomes comparable with other electron–phonon energy scales as the phonon frequencies ωphωph or the electron–phonon matrix elements gel–phgel–ph. In this contribution we show how this scenario is intrinsically related to the violation of two different adiabatic assumptions on which the conventional electron–phonon picture relies, namely the Migdal's theorem and the Born–Oppenheimer principle. Focusing on this latter, and using MgB2 as a textbook example, we show that the Born–Oppenheimer principle can be violated even in quasi-adiabatic systems (ωph/EF⪡1)(ωph/EF⪡1) when strong lattice fluctuations are present. Unlike the Migdal's theorem which is related to the ratio ωph/EFωph/EF, we show that these unconventional nonadiabatic effects are ruled by the ratio κ=gel–ph/EFκ=gel–ph/EF, which in MgB2κ=0.91κ=0.91
Superconductivity, nonadiabaticity and strong correlation in the light of recent experiments
No full textRecently a series of experiments in Scanning Tunneling Spectroscopy, angle-resolved photoemission spectroscopy (ARPES) and isotope effects have provided a great deal of information about the nature of the superconducting state, even thought many questions are still open. In this paper we present a discussion of the various theoretical frameworks in the light of these new experiments. Strong correlations started as the possible path to an entirely novel physics but, in many recent versions, they are reduced to the modest role of just raising the coupling in a BCS gap equation with phonon mediators. We argue instead that a conceptual generalization of the Migdal-Eliashberg (ME) theory is necessary and we discuss the consequences arising from the breakdown of the adiabatic assumption. The Nonadiabatic Theory of Superconductivity represents a complex generalization of the ME theory which is natural and, in many respects, unavoidable. In fact Migdal's theorem is strongly violated in these materials for a variety of reasons. The quantification of these effects is technically complicated and it is rather model dependent. In this framework strong correlations can have an important role in locating the system in a favourable range of parameters (forward scattering). We present a critical discussion of these developments in the light of the recent experiments and we also propose some crucial tests
Small Fermi energy, strong electron-phonon effects and anharmonicity in MgB2
No full textThe investigation of the electron-phonon properties in MgB2 has attracted a huge interest after the discovery of superconductivity with T-c = 39 K in this compound. Although superconductivity is often described in terms of the conventional Eliashberg theory, properly generalized in the multiband/multigap scenario, important features distinguish MgB2 from other conventional strong-coupling superconductors. Most important it is the fact that a large part of the total electron-phonon strength seems to be concentrated here in only one phonon mode, the boron-boron E-2g stretching mode. Another interesting property is the small Fermi energy of the a bands, which are strongly coupled with the E-2g mode. In this contribution, we discuss how the coexistence of both these features give rise to an unconventional phenomenology of the electron-phonon properties. (c) 2007 Elsevier B.V. All rights reserved
Nonadiabatic phenomenology in small Fermi energy superconductors
No full textHigh-T-c and unconventional superconductivity appears in a number of different materials with different physical properties. A common characteristics shared by almost all these compounds is that they are 'bad metal' in the sense that their effective charge carrier density is quite small, leading to a small Fermi energy E-F. In such a situation the dynamics of the electronic and lattice degrees of freedom becomes comparable (omega(ph) similar to E-F) driving these systems towards a breakdown of the Born-Oppenheimer adiabatic principle (omega(ph) << E-F) ruled by the comparison between the phonon frequency scale omega(ph) and E-F. The standard concept of Fermi liquid picture applied to the electron-phonon interaction needs thus to be deeply revised. We discuss the interesting implications of the onset of nonadiabatic effects on many properties of systems with small Fermi energy. (c) 2005 Elsevier Ltd. All rights reserved
Nonadiabatic breakdown and pairing in high-Tc compounds
No full textThe electron-phonon interaction plays a fundamental role in the superconducting and normal-state properties of all the high-Tc materials, from cuprates to fullerenes. Another common element of these compounds is in addition the extremely small Fermi energy EF, which is comparable with the range ωph of the phonon frequencies. In such a situation the adiabatic principle ωph/EF⪡1, on which the standard theory of the electron-phonon interaction and of the superconductivity relies, breaks down. In this contribution we discuss the physical consequences of the breakdown of the adiabatic assumption, with special interest on the superconducting properties. We review the microscopic derivation of the nonadiabatic theory of the electron-phonon coupling which explicitly takes into account higher-order electron-phonon scattering not included in the conventional picture. Within this context we discuss also the role of the repulsive electron-electron correlation and the specific phenomenology of cuprates and fullerides
NONADIABATIC ELECTRON-PHONON EFFECTS IN LOW CARRIER DENSITY SUPERCONDUCTORS
No full textDifferent families of unconventional superconductors present a low charge carrier density as a common trait, suggesting that the low charge density can be at the basis of a unifying picture for different superconductors. In the past years we have suggested that the electron–phonon interaction can be responsible for a high-Tc superconducting pairing in a nonadiabatic regime, where nonadiabatic effects are triggered on by the small electronic Fermi energy associated with the low charge density character. A coherent picture of such a framework requires however reconciling the low charge density and the small Fermi energy with a finite metallic character (sizable density of states and large Fermi surfaces). In this paper we investigate the peculiar conditions which are needed to be encountered in order to fulfill these requirements. We discuss the specific case of fullerenes, cuprates and MgB2 alloys by analyzing their specific structural and electronic properties The comparison between these materials and simple instructive models permits to underline the different routes to reconcile these characteristics in different compounds. In cuprates and fullerenes the interplay between small Fermi energies and large Fermi surface is strictly connected with strong electronic correlation effects. A comprehensive understanding of these issues can be useful to the future search for new nonadiabatic high-Tc materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Transport properties in correlated systems: An analytical model
No full textSeveral studies have so far investigated transport properties of strongly correlated systems. Interesting features of these materials are the lack of resistivity saturation well beyond the Mott-Ioffe-Regel limit and the scaling of the resistivity with the hole density in underdoped cuprates. Due to the strongly correlated nature of these materials, mainly numerical techniques have been employed. A key role in this regard is thought to be played by the continuous transfer of spectral weight from coherent to incoherent states. In this paper we employ a simple analytical expression for the electronic Green's function to evaluate both quasiparticle and transport properties in correlated systems. Our analytical approach permits us to enlighten the specific role of the spectral transfer due to the correlation on different features. In particular we investigate the dependence of both quasiparticle and transport scattering rate on the correlation degree and the criterion for resistivity saturation
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