Mott metal-insulator transition induced by utilizing a glass-like structural ordering in low-dimensional molecular conductors

Abstract

We utilize a glass-like structural transition in order to induce a Mott metal-insulator transition in the quasi-two-dimensional organic charge-transfer salt $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br. In this material, the terminal ethylene groups of the BEDT-TTF molecules can adopt two different structural orientations within the crystal structure, namely eclipsed (E) and staggered (S) with the relative orientation of the outer C$-$C bonds being parallel and canted, respectively. These two conformations are thermally disordered at room temperature and undergo a glass-like ordering transition at $T_g\sim 75$K. When cooling through $T_g$, a small fraction that depends on the cooling rate remains frozen in the S configuration, which is of slightly higher energy, corresponding to a controllable degree of structural disorder. We demonstrate that, when thermally coupled to a low-temperature heat bath, a pulsed heating current through the sample causes a very fast relaxation with cooling rates at $T_g$ of the order of several 1000K/min. The freezing of the structural degrees of freedom causes a decrease of the electronic bandwidth $W$ with increasing cooling rate, and hence a Mott metal-insulator transition as the system crosses the critical ratio $(W/U{)}_c$ of bandwidth to on-site Coulomb repulsion $U$. Due to the glassy character of the transition, the effect is persistent below $T_g$ and can be reversibly repeated by melting the frozen configuration upon warming above $T_g$. Both by exploiting the characteristics of slowly-changing relaxation times close to this temperature and by controlling the heating power, the materials can be fine-tuned across the Mott transition. A simple model allows for an estimate of the energy difference between the E and S state as well as the accompanying degree of frozen disorder in the population of the two orientations.

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Cited by in corpus (5)

• Critical Slowing Down of the Charge Carrier Dynamics at the Mott Metal-Insulator Transition
• Effects of Disorder on the Pressure-Induced Mott Transition in $\kappa$-BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Cl
• Ingredients for Generalized Models of $\kappa$-Phase Organic Charge-Transfer Salts: A Review
• Partially-ordered vortex lattices in the high-field, low-temperature mixed state of quasi two-dimensional organic superconductors
• Comparison of the charge-crystal and charge-glass state in geometrically frustrated organic conductors studied by fluctuation spectroscopy