Jahn-Teller polaron

Already in the seminal paper of Bednorz and Muller [1], the guide to look for systems with a high superconductive transition temperature (Tc), has been the presence of strong electron-phonon interactions. Such interaction has been known to exist in a wide class of perovskite type oxides. The authors mention [1] the vibronic Jahn-Teller polaron effect [2] in this context. They also emphasize the fact that the Cu2+-ion is a well-known Jahn-Teller system and this circumstance preserves significance in the physics of cuprate superconductors [3-7]. As a microscopic cause for ferroelectric ordering the interband vibronic hybridisation has been supposed [8-11] enlargening the view on perovskites as Jahn-Teller systems. 

Jahn-Teller Polarons, Bipolarons and Inhomogeneities. A Possible Scenario for Superconductivity in Cuprates... 

From Jahn-Teller Polarons to Mesoscopic Inhomogeneities... 

As X increases through x = 1, the Fermi energy drops from the bottom of the Fe3+ 2+ 3 (j6 bands to the top of the Mn " 3 d band, and the activation energy of the conductivity increases abruptly from 0.05 eV for small-polaron Fe ions to 0.3 eV for small-polaron Mn holes . The Mn ion is a strong Jahn-Teller ion (configuration Eg), so the holes in the Mn 3d band form more stable small polarons. 

The assumption that the carriers are small or intermediate polarons in no way militates against discussions of ths band structure of the ground state (see e.g. Camphausen et al. 1972, Cullen and Callen 1971,1973). The absence of Jahn-Teller distortion (Goodenough 1971) also, in our view, indicates not the absence of a polaron mass-enhancement but rather a value.of V0jB not too far from the critical value. These conclusions seem to be in agreement with the considerations of Sokoloff (1972), who used a description in terms of a degenerate band of small polarons. Samara (1968) showed that pressure lowers the temperature of the Verwey transition. If this depended only on e2/ ca then the opposite should be the case. But pressure will increase B, and push the substance nearer to the critical value for the metal-insulator transition. 

If the concept of spin polarons as described here is correct, the most important theoretical problem at the present time is to describe the way in which they are bound together to form bosons, of dimensions about 20 A and with binding energy of the order of the Neel temperature. Any binding mechanism will certainly distort the surrounding lattice, so the cohesion may well involve dielectric or dynamic Jahn-Teller terms, subject of course to the presence of only a very small (or zero) isotope effect. 

Kugel KI., and Khomskii D.L., (1980). Polaron effects and exchange interaction in magnetic insulators with Jahn-Teller ions. Zh. Eksp. Teor. Fiz. 79 987-1005 [Sov. Phys. JETP 52 501-515],... 

Abstract A review is made on the developments in the last two decades in the held of the Jahn-Teller effect on itinerant electrons in Jahn-Teller crystals. Special attention is paid to the current status of the researches on the fullerene superconductors and the manganite perovskites exhibiting the colossal magnetoresistance. Present knowledge about the polarons and bipolarons in the typical Jahn-Teller model systems is also summarized, together with some original results of our own. 

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