The oxygen isotope effect in Pr, Ca, and Zn substituted YBa2Cu3O7−δ and EuBa2Cu3O7−δ

The Van Hove singularity (VHS) provides a paradigm for the study of the role of peaks in the density of states (dos) on electronic properties. More importantly, it appears to play a major role in the physics of the high-Tc superconductors, particularly since recent photoemission studies have found that the VHS is close to the Fermi level in most of the high-Tc cuprates near the composition of optimum Tc. This paper offers a comprehensive survey of the VHS model, describing both theoretical properties and experimental evidence for the picture. Special topics discussed include a survey of the Fermi surfaces of the cuprates and related compounds, and an analysis of the reliability of the slave boson approach to correlation effects. While many properties of the cuprates can be qualitatively understood by a simple rigid-band-filling model, this is inadequate for more quantitative results, since correlation effects tend to pin the Fermi level near the VHS over an extended doping range, and can lead to a nanoscale phase separation. Furthermore, the peaks in the dos lead to competition from other instabilities, both magnetic and structural (related to charge density waves). A novel form of dynamic structural instability, involving dynamic VHS-Jahn-Teller effects has been predicted. Scattered through the literature, there is considerable experimental evidence for both nanoscale phase separation of holes, and for local, possibly dynamic, structural disorder. This review attempts to gather these results into a comprehensive database, to sort the results, and to see how they fit into the Van Hove scenario. Recent experiments on underdoped cuprates are found to provide a strong confirmation that the pseudogap is driven by a splitting of the VHS degeneracy.

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“…Moreover, as a function of doping, it is found that α has a minimum at the VHS, Fig. 26, as found experimentally [244].…”

Section: Isotope Effectsupporting

confidence: 62%

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

Markiewicz

1997

Journal of Physics and Chemistry of Solids

371

280

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“…This is because the free-plasmon frequency is decreased in such a case following Eqs. (10), (11), (18). Quite recently such an effect also has been studied numerically [42].…”

Section: Review Articlementioning

confidence: 98%

“…Refs. [11,12]. Moreover, theories predicting charge inhomogeneities in the CuO plane in form of stripes [13][14][15][16] have attracted interest in the phonons because such charge distributions will inevitably couple to the lattice.…”

Section: Introductionmentioning

confidence: 99%

Phonons, electronic charge response and electron–phonon interaction in the high‐temperature superconductors

Falter¹

2004

Physica Status Solidi (b)

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We investigate the complete phonon dispersion, the phonon induced electronic charge response and the corresponding self-consistent change of the crystal potential an electron feels as a direct measure of the electron-phonon interaction in the high-temperature superconductors within a microscopic model in the framework of linear response theory. Moreover, dielectric and infrared properties are calculated. The experimentally observed strong renormalization of the in-plane oxygen bond-stretching modes which appears upon doping in the high-temperature superconductors is discussed. It is shown that the characteristic softening, indicating a strong nonlocal electron-phonon interaction, is most likely a generic effect of the CuO plane and is driven by a nonlocal coupling of the displaced ions to the localized charge-fluctuations at the Cu and the O xy ions. At hand of the oxygen bond-stretching modes it is illustrated how lattice-, charge-and spin-degrees of freedom may act synergetically for anisotropic pairing in the hightemperature superconductors. The different behaviour of these modes during the insulator-metal transition via the underdoped phase is calculated and from a comparison of these generic modes in the different phases conclusions about the electronic state are drawn. For the non-cuprate potassium doped hightemperature superconductor Ba-Bi-O also a very strong and anisotropic renormalization of the oxygen bond-stretching modes is predicted. In another investigation c-axis polarized infrared-and Raman-active modes of the HTSC's are calculated in terms of charge fluctuations and anisotropic dipole-fluctuations. Mode assignments discussed controversially in the literature are proposed. Finally, interlayer phonons propagating along the c-axis and their accompanying charge response are investigated. Depending on the strength of the interlayer coupling calculations are performed ranging from the static, adiabatic response regime to the non-adiabatic regime where dynamical screening of the bare Coulomb interaction and phonon-plasmon coupling becomes relevant within a certain region around the direction of the c-axis. A comparison with the experimental situation is given. Both, the oxygen bond-stretching modes calculated in adiabatic approximation and the non-adiabatic coupled c-axis phonon-plasmon modes are found to be important for pairing via lattice-and charge-degrees of freedom. Favouring aspects to achieve hightemperature superconductivity are also discussed.

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“…[7,8]. On the other hand a remarkable isotope effect on the zero temperature London penetration depth L (0) l has been observed both in the nearly optimal and in the underdoped regime [9 -11].…”

Section: Introductionmentioning

confidence: 99%

Nonadiabatic electron–phonon effects in low carrier density superconductors

Cappelluti

Pietronero

2004

Physica Status Solidi (b)

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Different 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-c T 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 MgB 2 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-c T materials.

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The oxygen isotope effect in Pr, Ca, and Zn substituted YBa2Cu3O7−δ and EuBa2Cu3O7−δ

Cited by 33 publications

References 8 publications

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

Phonons, electronic charge response and electron–phonon interaction in the high‐temperature superconductors

Nonadiabatic electron–phonon effects in low carrier density superconductors

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