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Crystals 2018, 8(1), 38;

Effects of Disorder on the Pressure-Induced Mott Transition in κ-(BEDT-TTF)2Cu[N(CN)2]Cl

Institute of Physics, SFB/TR49, Goethe University Frankfurt, Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany
Institute for Theoretical Physics, Technical University Dresden, Zellescher Weg 17, 01062 Dresden, Germany
Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
Author to whom correspondence should be addressed.
Received: 14 December 2017 / Revised: 9 January 2018 / Accepted: 11 January 2018 / Published: 16 January 2018
(This article belongs to the Special Issue Advances in Organic Conductors and Superconductors)
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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. View Full-Text
Keywords: organic conductor; Mott transition; pressure; disorder; X-ray irradiation organic conductor; Mott transition; pressure; disorder; X-ray irradiation

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Gati, E.; Tutsch, U.; Naji, A.; Garst, M.; Köhler, S.; Schubert, H.; Sasaki, T.; Lang, M. Effects of Disorder on the Pressure-Induced Mott Transition in κ-(BEDT-TTF)2Cu[N(CN)2]Cl. Crystals 2018, 8, 38.

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