Unconventional critical behaviour in a quasi-two-dimensional organic conductor

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Changing the interactions between particles in an ensemble—by varying the temperature or pressure, for example—can lead to phase transitions whose critical behaviour depends on the collective nature of the many-body system. Despite the diversity of ingredients, which include atoms, molecules, electrons and their spins, the collective behaviour can be grouped into several families (called ‘universality classes’) represented by canonical spin models1. One kind of transition, the Mott transition2, occurs when the repulsive Coulomb interaction between electrons is increased, causing wave-like electrons to behave as particles. In two dimensions, the attractive behaviour responsible for the superconductivity in high-transition temperature copper oxide3,4 and organic5,6,7 compounds appears near the Mott transition, but the universality class to which two-dimensional, repulsive electronic systems belongs remains unknown. Here we present an observation of the critical phenomena at the pressure-induced Mott transition in a quasi-two-dimensional organic conductor using conductance measurements as a probe. We find that the Mott transition in two dimensions is not consistent with known universality classes, as the observed collective behaviour has previously not been seen. This peculiarity must be involved in any emergent behaviour near the Mott transition in two dimensions.

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Figure 1: Crystal structure and conductance profile around the critical endpoint of a Mott transition.
Figure 2: Critical exponents of the Mott transition in κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl.
Figure 3: Comparison of the critical exponents ( δ, β, γ ) of the present case with those of the known universality classes (mean-field, Ising model, XY model and Heisenberg model).
Figure 4: Scaling plot of the whole data set of GT(P ) in the metallic region.


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We thank M. Imada, T. Itou, S. Miyashita, N. Nagaosa, S. Onoda, Y. Shimizu and N. Todoroki for discussions.

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Correspondence to K. Kanoda.

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Supplementary information

Supplementary Figure S1

This figure shows the pressure dependence of conductance at the critical temperature. It also gives the critical conductance and the critical pressure needed to determine the critical exponents. (PDF 121 kb)

Supplementary Table S1

This table lists universality classes and the representative phase transitions belonging to them. (DOC 32 kb)

Supplementary Discussion

This provides the theoretical proof and the experimental demonstration for the validity of the present data analysis. This also includes Supplementary Figures S2 and S3. (PDF 842 kb)

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Kagawa, F., Miyagawa, K. & Kanoda, K. Unconventional critical behaviour in a quasi-two-dimensional organic conductor. Nature 436, 534–537 (2005) doi:10.1038/nature03806

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