Eur. Phys. J. B (2012) 85: 178
https://doi.org/10.1140/epjb/e2012-30172-5

Regular Article

Cooperative Jahn-Teller effect in a 2D mesoscopic C₆₀^n- system with D_5d symmetry adsorbed on buffer layers using Ising EFT model

¹ Department of Physics, Texas A and M University at Qatar, Education City, P.O. Box 23874, Doha, Qatar
² Departamento de Física, Universidade Federal de Minas Gerais, 30123-970 Belo Horizonte, Brazil
³ School of Physics and Astronomy, University of Nottingham, NG7 2 RD Nottingham, UK
⁴ Laboratoire de Physique de L’État Condensé, UMR 6087, Université du Maine, 72085 Le Mans, France

^a e-mail: michel.aboughantous@qatar.tamu.edu

Received: 28 February 2012
Received in final form: 20 April 2012
Published online: 30 May 2012

Abstract

Fullerene molecules adsorbed on surfaces often show macroscopic average distortions. As charged ions C₆₀^n- are known to be Jahn-Teller (JT) active, it is suggested that these distortions could be a manifestation of cooperative JT effects (CJTE) due to interactions between neighbouring fullerene ions. In order to understand the distortion properties it is necessary to take correlations between different distortions into account. However, this can’t easily be done in the mean field approximation usually used to describe the CJTE. We therefore propose an alternative procedure to describe 2D mesoscopic islands of C₆₀ ions in which a pseudo vector spin is evoked to represent degenerate JT-distorted states when the quadratic JT coupling is considered. This approach is analogous to methods used for 2D magnetic systems. We then use the differential operator technique in effective field theory within the Ising approach. We include the effects of weak surface interactions and dynamic motion between equivalent distortions via terms equivalent to anisotropy and a transverse field in magnetism respectively. For distortions to D_5d symmetry, we determine single site correlations as a function of temperature, the macroscopic average distortion describing a structural phase transition, and the isothermal response function. Phase diagrams are presented for relevant cases of the system parameters.