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

Abstract

We utilize a glasslike structural transition in order to induce a Mott metal-insulator transition in the quasi-two-dimensional organic charge-transfer salt $\ensuremath{\kappa}\ensuremath{-}{\text{(BEDT-TTF)}}_{2}\mathrm{Cu}[\mathrm{N}{\text{(CN)}}_{2}\mathrm{Br}]$. 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 glasslike ordering transition at ${T}_{g}\ensuremath{\sim}75$ 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 $1000 \mathrm{K}/\mathrm{min}$. 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.