Abstract
We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals to X-ray irradiation. The crystals were then fine-tuned across the Mott transition by the application of continuously controllable He-gas pressure at low temperatures. Measurements of the thermal expansion and resistance show that the first-order character of the Mott transition prevails for low irradiation doses achieved by irradiation times up to 100 h. For these crystals with a moderate degree of disorder, we find a first-order transition line which ends in a second-order critical endpoint, akin to the pristine crystals. Compared to the latter, however, we observe a significant reduction of both, the critical pressure p c and the critical temperature T c . This result is consistent with the theoretically-predicted formation of a soft Coulomb gap in the presence of strong correlations and small disorder. Furthermore, we demonstrate, similar to the observation for the pristine sample, that the Mott transition after 50 h of irradiation is accompanied by sizable lattice effects, the critical behavior of which can be well described by mean-field theory. Our results demonstrate that the character of the Mott transition remains essentially unchanged at a low disorder level. However, after an irradiation time of 150 h, no clear signatures of a discontinuous metal-insulator transition could be revealed anymore. These results suggest that, above a certain disorder level, the metal-insulator transition becomes a smeared first-order transition with some residual hysteresis.
Highlights
Organic charge-transfer salts of type D2 A, consisting of donor molecules D and acceptor molecules A, have been established as model systems to explore the physics of strong electron correlations [1,2]
Mott transition and its critical endpoint in the T − p phase diagram to higher disorder levels, i.e., irradiation doses up to 150 h, we present in Sections 3.4 and 3.5 the results of electrical resistance measurements on one single crystal at various disorder levels obtained after 50 h, 100 h and 150 h irradiation time
This is consistent with previous reports of the decreased magnetic ordering temperature in irradiated samples of κ-Cl at ambient pressure [65]. To investigate this ordering at higher irradiation, where disorder causes a change of the character of the Mott transition, and its interplay with the metal-insulator transition, we suggest to study the magnetic properties of strongly irradiated κ-Cl under pressure
Summary
Organic charge-transfer salts of type D2 A, consisting of donor molecules D and acceptor molecules A, have been established as model systems to explore the physics of strong electron correlations [1,2]. The role of Coulomb correlations is enhanced in this material class due to weak intermolecular interactions, giving rise to a small bandwidth W, in combination with a low charge-carrier concentration and an electronic structure with reduced dimensionality. The latter two aspects render the screening of the long-range part of the Coulomb interaction less effective. Salts of the κ-(BEDT-TTF) X family, with
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