C13NMR study on the charge-ordering saltα′−(

Abstract

Among quasi-two-dimensional organic conductors, ͑BEDT-TTF͒ 2 X, what form the two column structure of ␣ or is thought to show a charge-ordering insulating state. The ␣Ј modification is the other candidate for a charge-ordering state. To determine the existence of a charge-ordering state and electron magnetism in the ␣Ј phase, which has a symmetry that differs from that of the ␣ and phases, we utilized 13 C-NMR to examine ␣Ј-͑BEDT-TTF͒ 2 IBr 2 in which one side of the central carbon on the BEDT-TTF molecule is substituted with 13 C nuclei. We observed changes in its spectrum at 200 K. The angular dependence of the NMR shift at 15 K indicated that the charge-ordering pattern is intracolumn disproportionation and the ratio of charge disproportionation on the spin-poor site and spin-rich sites is almost 1:0. The angle dependence of NMR spectrum at 60 K suggests that the hyperfine coupling tensor in the paramagnetic charge-ordering state is determined not only by the charge density of on-site molecules but also by the contribution of off-site molecules. The change in hyperfine coupling tensor must be taken into account in NMR studies of the paramagnetic CO state.

Charge ordering and antiferromagnetism in (TMTTF)2SbF6

“…In earlier analysis, the spin disproportionation had been identified as the charge one. Recently, it was pointed out that the Knight shift had not only the contribution from on-site local spin susceptibility but also one from the neighboring molecules in the paramagnetic phase of α -(BEDT-TTF) 2 IBr 2 19 and there was the discrepancy between the disproportionation estimated by the Knight shift determined from the state at Fermi energy and one estimated by the chemical shift determined from all states below Fermi energy in the paramagnetic phase of α-(BEDT-TTF) 2 I 3 . 20 Therefore, to estimate the degree of charge disproportionation in the CO state, we should use the chemical shift.…”

“…Since Knight shift had not only the contribution from the on-site molecular spin susceptibility but also one from the neighboring molecule, 19 a i is expressed as a i = a 0 + j b i,j ; here a 0 is on-site and b i,j is off-site contribution, respectively. In the CO state, the NMR shift for i site can be expressed as…”

13C-NMR studies of the paramagnetic and charge-ordered states of the organic superconductorβ′′-(BEDT−Nagata1, Ogura2, Kawamoto3 et al. 2011Phys. Rev. BSelf Cite6230View full textAdd to dashboardCite

“…As a result, we obtain a gap of ∼ 482 K. On many CO salts, the spin susceptibility has been found to decrease with CO formation. 13,23 B. NMR shift under ambient pressure Figure 3 shows the temperature dependence of the 13 C-NMR spectrum from 295 K to 45 K at ambient pressure. With the exception of a signal from grease at ∼30 ppm, a four-peak spectrum was observed at 295 K; the ratio of the peak's intensities is estimated at approximately 1:2:2:1.…”

Charge Transport in Charge-Ordered States of Two-Dimensional Organic Conductors, α-(BEDT-TTF)2I3 and α'-(BEDT-TTF)2IBr2

“…22) Optical measurement shows a well-defined charge order below 200 K and a charge disorder above 200 K. 23) NMR measurement shows the CO pattern is not a simple horizontal or diagonal structure, but is more complicated. 24) The magnetic susceptibility shows no significant change at T CO , but has a broad maximum at about 70 K and then a sudden drop at 30 K. 13,25) The broad maximum is interpreted by the onedimensional alternating Heisenberg model. The sudden drop at 30 K, associated with hysteresis (first-order phase transition), demonstrates a nonmagnetic ground state.…”