ORAL SESSION 2A3: HTS Theory III

Tuesday, Feb. 22, 3:30 p.m. – 6:00 p.m., Room 303A (GRB)

Chairs: P.A. Lee (MIT), J.P. Carbotte (McMaster U)

2A3.1 SO(5) theory of high T_c superconductivity

Shoucheng Zhang, Stanford University, Dept. of Physics, Stanford, CA 94305, USA

Presenting Author: S.C. Zhang

In this talk I shall review recent theoretical progress on understanding the relationship between antiferromagnetism and superconductivity in the high T_c cuprates. I shall also give a update on the comparison between the SO(5) theory and some key experiments.

2A3.3 Theory of high-T_c superconducting cuprates based on experimental evidence

Alexei A. Abrikosov, Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439

Presenting Author: A.A. Abrikosov

A model of superconductivity in layered high-temperature superconducting cuprates is proposed, based on the extended saddle point singularities in the electron spectrum, weak screening of the Coulomb interaction and phonon-mediated interaction between electrons plus a small short-range repulsion of Hund's, or spin-fluctuation, origin. This permits to explain the large values of T_c, features of the isotope effect on oxygen and copper, the existence of two types of the order parameter, the peak in the inelastic neutron scattering, the positive curvature of the upper critical field, as function of temperature etc.

2A3.4 Intrinsic Inhomogeneity and Dynamic Screening in Layered Superconductors; Anomalous Diamagnetism Above Resistive T_c.

Vladimir Z. Kresin 1, Andreas Bill 1, Stuart A. Wolf 2, Yurii N. Ovchinnikov 3, and Hans Morawitz 4. 1 Lawrence Berkeley Lab., Berkeley, CA 94720. 2 Naval Rersearch Laboratory, Washington, D.C. 20375. 3 Landau Institute of Theoretical Physics, Moscow, 11733V, Russia. 4 IBM Research Division, San Jose, CA 95120.

Presenting Author: V.Z. Kresin

The intrinsic inhomogeneity of superconducting system allows one to observe coexistence of an anomalous diamagnetic moment with normal resistive dissipation. Such a phenomenon occurs at T> T_cres and H>H_c2 (T_cres denotes a relatively sharp resistive transition). The doped cuprates possess such inhomogeneity, and in these compounds one can indeed observe a separation of the resistive and magnetic transitions. The relevant experimental data for overdoped and underdoped cuprates will be discussed.

The problem of the pseudogap will be also discussed. The proximity effect plays a very important role. Layered superconductors such as the high T_c oxides and organics are characterized by a peculiar dynamic screening which affects the value of the critical temperature.

2A3.5 Relaxation Processes and Pairing in High-T_c Systems

Evgenii G. Maksimov, Department of Theoretical Physics, P.N. Lebedev Physical Institute of Russian Academy of Sciences, Moscow 117924, Russia

Presenting Author: E.G. Maksimov

The origin of high T_c in cuprates continues to be mysterious till now. Moreover, the normal state behavior of these systems looks also very different from that of corresponding conventional metals. The most prominent feature of the normal state is the existence of strong relaxation processes.

Relaxation rates demonstrate quasilinear dependence on temperature and frequency. There is a large difference between the relaxation rates corresponding to one-particle excitations and transport processes. It will be shown using the conserving approximation that all these peculiarities of relaxation processes can be explained with strong electron-phonon interaction.

The results of first-principles calculations of the optical spectra of YBa₂Cu₃O₇ will be presented demonstrating an excellent agreement with experimental data. The Eliashberg function describing the normal state properties can give per se the value of critical temperature T_c» 90 K.

Nevertheless, the detailed analysis of the optical spectra and tunneling characteristics of optimally doped cuprates leads to the conclusion that the superconducting state of high-T_c systems, in contrast to the normal state, cannot be explained only in the framework of strong electron-phonon coupling or any other single boson mediated coupling. It means the existence of some additional mechanisms of superconductivity supplementary to the electron-phonon one.

2A3.6 Unified Theory of Colossal Magnetoresistance in Manganites and High-Temperature Superconductivity in Cuprates

Alexandre S. Alexandrov, Department of Physics, Loughborough University, Leics. LE11 3TU, United Kingdom

Presenting Author: A.S. Alexandrov

The Frohlich electron-phonon interaction in cuprates and manganites is shown to be almost one order of magnitude larger than any relevant magnetic interaction. Pairing of polaronic carriers into small bipolarons explains both the high-temperature superconductivity in cuprates [1] and ferromagnetism and colossal magnetoresistance in manganites [2]. I argue that by replacing the magnetic ions of Mn by nonmagnetic Cu one can turn a doped charge-transfer magnetic insulator into a high-temperature superconductor owing to the Bose-Einstein condensation of bipolarons. The bipolaron theory provides a parameter-free expression for the superconducting critical temperature [3], d-wave order parameter and a single-particle spectral function compatible with the tunnelling and photoemission in cuprates.

(Bi)polarons also account for the spectral weight transfer and normal state pseudogaps, and for the isotope effect on the superconducting, Neel and Curie critical temperature and on the (super)carrier mass.

[1] A.S. Alexandrov, in Models and Phenomenology for Conventional and High-temperature Superconductivity (Course CXXXVI of the Intenational School of Physics `Enrico Fermi'), eds. G. Iadonisi, J.R. Schrieffer and M.L. Chiofalo, (IOS Press, Amsterdam), p. 309 (1998).

[2] A.S. Alexandrov and A.M. Bratkovsky, Phys. Rev. Lett. 82, 141 (1999).

[3] A.S. Alexandrov and V.V. Kabanov, Phys. Rev. B 59, 13628 (1999).

2A3.7 The Electron-Phonon Interaction Renormalized by Strong Correlations: The Way to HTS

Miodrag L. Kulic, University of Bayreuth, Physics Departmnet, 95440 Bayreuth, Germany

Presenting Author: M.L. Kulic

All nonphononic mechanisms of pairing, as well as the standard phononic mechanism (with frequency dependent electron self-energy only) fail to explain basic experimental facts for the normal and superconducting state of HTS oxides.

However, the electron-phonon (EP) interaction (whose strength is dominated by the Madelung energy) which is renormalized by strong electronic correlations

(SEC) gives rise to a strong momentum and frequency dependent electronic self-energy, i.e. to the appearance of the forward scattering peak.

This fact explains: (i) the reduction of the transport EP coupling constant with respect to the pairing one; (ii) the presence of large d-wave EP coupling constant for some doping concentration, etc.

By including also moderate residual repulsive quasiparticle interaction - for instance via spin fluctuations, the EP theory explains in a natural way d-wave pairing in optimally doped HTS oxides. The same theory predicts the dominance of anisotropic s-wave pairing in overdoped materials.

The impurity scattering is also renormalized by SEC giving rise to suppression of residual resistivity and to robustness of d-wave pairing in these materials.