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

Abstract

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 models. One kind of transition, the Mott transition, 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 oxide and organic 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.

Thermodynamic Properties of κ-(BEDT-TTF)2X Salts: Electron Correlations and Superconductivity

“…In the intermediate region of U ≈ W, a first-order phase boundary, known as a Mott boundary, appears. The conceptual pressure versus temperature phase diagram in which the antiferromagnetic phase is adjacent to the superconducting phase was established by Kanoda et al [4,7,8], which was confirmed precisely by Kagawa et al by gas-pressure controlled experiments [9]. According to the phase diagram, the insulating and metallic/superconducting states are tunable by external or chemical pressures.…”

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compoundκ-(BEDT-TTF)2Cu2(CN)Elsässer1, Wu2, Dressel3 et al. 2012Phys. Rev. B

“…50 This argument, however, does not hold for κ-CN where no magnetic order is present down to lowest temperatures. Pressure dependent investigations on κ-Cl showed that the bandwidth-controlled Mott criticality involves critical fluctuations in charge, 58 spin, 59 and lattice. 60,61 Nothing like that is known for the spin-liquid compound κ-CN for the intermediate temperature range.…”

Antiferromagnetic spin fluctuations in the metallic phase of quasi-two-dimensional organic superconductors

“…In particular one needs to take care to account for the possible coexistence of metallic and insulating phases; this is expected as the Mott transition is first order in the organic charge-transfer salts. 62,63 The existence of a pseudogap has also been suggested in -͑BEDT-TSF͒ 2 GaCl 4 ͑Ref. 64͒ from microwave conductivity measurements.…”