Join the Hard Condensed Matter group in a seminar titled "Electron localization in the organic quantum spin liquid κ-(BEDT-TTF)2Cu2(CN)3” with Matija Čulo (Institute of Physics, Bijenička cesta 46, 10 000 Zagreb, Croatia).
Date: 28 October 2022, 2:00PM - 21 October 2022, 3:00PM
Live location: G44 Frank Lecture Theatre
Online: Zoom meeting (for information on how to join the meeting please contact jonathan.buhot@bristol.ac.uk)
Abstract:
κ-(BEDT-TTF)2Cu2(CN)3 belongs to the family of compounds based on the organic molecule bis- (ethylenedithio)-tethrathiafulvalene (BEDT-TTF) that has attracted a lot of attention because of plethora of intriguing phenomena such as unconventional superconductivity, Dirac fermions, ferroelectricity, Mott transition etc. κ-(BEDT-TTF)2Cu2(CN)3 is of particular interest because despite a strong antiferromagnetic superexchange interaction J ≈ 250 K, no indication of long range magnetic order has been observed down to temperatures as low as 32 mK. It is therefore considered to be one of the first realizations of the quantum spin liquid, theoretically proposed by Anderson almost 50 years ago. The ratio of the on-site Coulomb repulsion U and the bandwidth W, U/W ≈ 1.8, indicates that the material is also a Mott insulator. However, no clear-cut optical gap expected for a Mott insulator has been detected in infrared spectroscopy measurements, implying an unusual nature of the insulating state. Here I present our recent results of dc resistivity measurements which indicate that there is no clear transport gap either. In fact, our detailed analysis shows that the charge transport takes place via two-dimensional (2D) variable range hopping (VRH) which implies that the insulating properties of κ-(BEDT-TTF)2Cu2(CN)3 stem from disorder-induced localization of conducting electrons, rather than by opening of an energy gap. The presence of 2D VRH was additionaly confirmed by our Hall effect and magnetoresistance measurements and implies that κ-(BEDT-TTF)2Cu2(CN)3 is closer to Anderson, than to Mott insulators. The comparison with the two sister compounds κ-(BEDT-TTF)2Ag2(CN)3 and κ-(BEDT-TTF)2B(CN)4 however, shows that in sharp contrast to the Anderson localization scenario, the most disordered κ-(BEDTTTF)2Cu2(CN)3 has lowest resistivity and largest effective carrier density, i.e. it is closest to the metallic state, while the most ordered κ-(BEDT-TTF)2B(CN)4 has highest resistivity and smallest effective carrier density, i.e. it is furthest of the metallic state. We interpreted such counterintuitive behavior within the more complex theories of Mott-Anderson localization, where localization of conducting electrons stems from both, disorder and strong correlation effects