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5 - The polaron scenario for high-Tc superconductors
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- By J. Ranninger, Centre de Recherches sur les Très Basses Températures, laboratoiré associe à l'Université Joseph Fourier, C.N.R.S., BP 166, 38042 Grenoble-Càdex 9, France
- Edited by E. K. H. Salje, University of Cambridge, A. S. Alexandrov, University of Cambridge, W. Y. Liang, University of Cambridge
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- Book:
- Polarons and Bipolarons in High-Tc Superconductors and Related Materials
- Published online:
- 24 November 2009
- Print publication:
- 07 September 1995, pp 67-79
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Summary
Abstract
There is ample experimental evidence for localized polaronic charge carriers in high-Tc materials in the insulating phase as well as in the metallic phase at high temperatures. This would rule out a priori any condensation of bipolarons, since for that purpose they should be in free-particle-like states in the longwavelength limit. Yet, provided that the localized bipolarons hybridize with a band of itinerant electrons, such a mixture of Bosons (bipolarons) and Fermion pairs (pairs of conduction electrons) can undergo an instability towards a superconducting ground state in which at high temperatures the initially localized bipolarons become superfluid upon lowering of the temperature. The experimental situation leading up to such a picture and its physical consequences are discussed.
Introduction
The large values of the critical temperature Tc, the small number of charge carriers together with the short coherence length, the strong dependence of Tc on n/m (n being the carrier concentration and m their mass) and the large temperature regime near Tc (with a Ginzburg temperature of TG 0.1−0.01) controlled by X−Y universality strongly suggest that high-Tc superconductivity is more closely related to Bose–Einstein condensation of real-space pairs than to a BCS state of Cooper pairs. The polaronic nature of at least part of the charge carriers in these materials has been experimentally established in both the insulating and the metallic phase of these compounds. On theoretical grounds one expects small polarons to interact with each other over short distances and in a practically unretarded fashion.
16 - Bose–Einstein Condensation of Bipolarons in High-Tc Superconductors
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- By J. Ranninger, Centre de Recherches sur les Tres Basses Températures, CNRS, BP 166 38042 Grenoble-Cedex 9 France
- Edited by A. Griffin, University of Toronto, D. W. Snoke, University of Pittsburgh, S. Stringari, Università degli Studi di Trento, Italy
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- Book:
- Bose-Einstein Condensation
- Published online:
- 15 December 2009
- Print publication:
- 06 April 1995, pp 393-417
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Summary
Abstract
The short coherence lengths and the low carrier concentrations in high temperature superconductors (HTcSC) quite naturally favor a scenario where preformed bound electron pairs undergo a Bose–Einstein condensation (BEC). There are numerous experimental indications in the normal state properties (anomalous diamagnetic susceptibility, NMR spin-lattice relaxation, Knight shift and crystal electric field transitions, as well as entropy non-linear in T and strong local lattice deformations seen by EXAFS), all of which indicate a characteristic temperature Tpb associated with the breaking of such preformed electron pairs and the existence of a pseudogap above Tc. As the number of charge carriers is increased by chemical doping, Tpb decreases and the critical temperature Tc, where superconductivity sets in, in this so-called “underdoped regime”, increases. The maximum value for Tc is reached when Tc ≲ TPB. Upon further doping, going into the so-called “overdoped regime”, Tc decreases. We take this as an indication that the superconducting state depends on the existence of preformed electron pairs. The normal state properties of the “underdoped regime” seem to show significant differences from Fermi liquid behavior, while in the “overdoped regime” they seem to be typical of ordinary metals.
In order to capture this empirically established scenario in HTCSC, we discuss a phenomenological model based on a mixture of localized bosons (bound electron pairs such as bipolarons) and itinerant fermions (electrons), with a local exchange between bosons and fermion pairs.