Strongly correlated electrons

In the materials found in your laptop or mobile phone the interactions between the electrons are relatively small; this greatly simplifies the task of describing their behaviour. Technically, one says that the electrons in these materials are ‘weakly correlated’. Over the past few decades many new materials have been found which cannot be described within the weakly correlated paradigm. These materials, which are referred to as ‘strongly correlated’ and include cuprates [Lee], manganites [Milward] and organic charge transfer salts [Powell, Seo], have a wide variety of potential applications including catalysis [Shelef], energy applications [Fleming], and display technologies [Lo]. However, our lack of fundamental understanding of strongly correlated materials greatly hampers materials development for such applications.


Emergence and complexity in strongly correlated materials.

Qualitatively new behaviours emerge as one increases the complexity of a system [Anderson, Dagotto]. For example, when one grows crystals of a single atomic species one can observe insulating, semiconducting, metallic and superconducting states – none of these behaviours are seen in individual atoms. Strongly correlated materials show yet more novel effects such as colossal magnetoresistance in the manganites [Milward], high temperature superconductivity in the cuprates [Lee] and heavy fermion behaviour in many rare earth compounds [Monthoux]. If we further increase the complexity of the systems we reach organic chemistry and, eventually, biology. Organic charge transfer salts represent an important rung on this ladder of complexity as they sit between the most complicated inorganic crystalline materials, such as transition metal oxides, and the simplest biomolecular systems. Organic charge transfer salts display a fascinating range of behaviours, many of which are not yet well understood [Powell, Seo]. Thus, organic charge transfer salts are model systems for the study of strongly correlated electrons in chemically complex systems.

[Anderson] PW Anderson, Science 177, 4047 (1972).
[Dagotto] E Dagotto, Science 309, 257 (2005).
[Fleming] GR Fleming and MA Ratner, Phys. Today 61, 28 (2008).
[Lee] PA Lee, N Nagaosa and X-G Wen, Rev. Mod. Phys. 86, 17 (2006).
[Lo] S-C Lo and PL Burn, Chem. Rev., 107, 1097 (2007).
[Milward] GC Millward, MJ Calderón and P Littlewood, Nature 433, 607 (2005).
[Monthoux] P Monthoux, D Pines and GG Lonzarich, Nature 450, 1177 (2007) and references therein.
[Powell] BJ Powell and RH McKenzie, J. Phys.: Condens. Matter. 18, R827 (2006).
[Shelef] M Shelef and RW McCabe, Catal. Today 62, 35 (2000).
[Seo] H Seo, J Merino, H Yoshioka and M Ogata, J. Phys. Soc. Jpn. 75, 051009 (2006).