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University of Queensland, Brisbane, Australia

Organic charge transfer salts, such as kappa-(BEDT-TTF)₂Cu(CN)₃, exhibit a subtle competition between bad metal, Fermi liquid, superconducting, Mott insulator, spin liquid, and antiferromagnetic phases [1]. They are a playground to investigate different states of quantum matter.

In the metallic phase, with increasing temperature there is a crossover from a Fermi liquid to a bad metal, at a temperature T_coh ~ 30 K. The signatures of the bad metal are a resistivity of order the Mott-Ioffe-Regel limit, thermopower of order k_B/e, and the absence of a Drude peak in the
optical conductivity [2].

The simplest possible effective Hamiltonian for these materials is a one-band Hubbard model on an anisotropic triangular lattice at half-filling [1]. At zero-temperature the model exhibits a transition from a Mott insulator to a metal as the interaction (U/t) is reduced or the frustration (t'/t) is varied. A Dynamical Mean-Field Theory treatment of the model captures the Fermi liquid-bad metal crossover [2], including the observed frequency and temperature dependence of the optical conductivity [3]. A recent study of this Hubbard model, using the finite-temperature Lanczos method (FTLM) [4], showed that near the Mott insulator T_coh ~ t/10, consistent with experiment. The bad metal is characterised by a small charge compressibility, a large spin susceptibility, and fluctuating local magnetic moments.

[1] B.J. Powell and R.H. McKenzie, Rep. Prog. Phys. 74, 056501 (2011).
[2] J. Merino and R.H. McKenzie, Phys. Rev. B 61, 7996 (2000).
[3] J. Merino et al., Phys. Rev. Lett. 100, 086404 (2008).
[4] J. Kokalj and R.H. McKenzie, Phys. Rev. Lett. 110, 206402 (2013).