Inhomogeneous site charges at the boundary between the insulating, superconducting, and metallic phases ofβ″-type bis-ethylenedithio-tetrathiafulvalene molecular charge-transfer salts

Abstract

We have examined time-averaged charges of the molecular donors around the phase boundary between the insulating, superconducting, and metal phases for ␤Љ-type ET ͓ET= bis-ethelenedithio-tetrathiafulvalene͔ salts with one ͑two͒ hole͑s͒ per two ͑three͒ molecules by using vibrational spectroscopy. Around the phase boundary, the site charges are neither those expected for a well-developed charge-ordered ͑CO͒ state nor a homogeneous distribution. The molecular charges exhibit an inhomogeneous distribution just above the insulatorsuperconductor transition temperature. We have analyzed the distribution of the site charges from the viewpoint of the alternation of intersite Coulomb interactions along the stacking direction. The closeness in the energy of several CO patterns mostly contributes to the inhomogeneous distribution, whereas a large difference ͑no difference͒ in the energy levels contributes to the CO ͑homogeneous͒ state. Our observations indicate that the instability due to the closeness in the energy of several distributions correlates with the insulatorsuperconductor transition in nondimerized or weakly dimerized molecular conductors.

C13NMR study of the charge-ordered state near the superconducting transition in the organic superconductorβ′′−(BEDT-T

“…From the angle dependence of the NMR spectra in the charge-ordered state, we found that below 8.5 K the site charge deviates strongly from the formal average value. This deviation is much larger than that observed below 100 K from Raman spectroscopy [18]. The NMR intensity ratio of charge-rich to charge-poor sites leads us to suggest a threefold charge pattern, which can be stabilized by the competition between off-site and on-site Coulomb interactions.…”

“…As σ is directly related to ρ, different σ values between R and P sites confirm the modulation in the site charges. We see that in the low-temperature charge-ordered state the deviation from the formal charge is much larger than that observed between 10 and 100 K by Raman spectroscopy (ρ R − ρ P = 0.13e [18]). The NMR peak splitting due to this small charge disproportionation at high temperatures is probably concealed in the linewidth of our spectra (for the external field applied along the b axis).…”

“…In the BEDT-TTF salt with β -type molecular packing, β -(BEDT-TTF) 4 [(H 3 O)Ga(C 2 O 4 ) 3 ]·C 6 H 5 NO 2 (β -Ga), a charge instability was observed below 8.5 K, very close to the SC transition temperature T c = 7 K [15,16]. The β -Ga salt may therefore prove to be an exceptionally important model system for understanding the relationship between CO and SC states.Besides the charge instability at 8.5 K, a CO with a small charge modulation was already observed in β -Ga salt at 100 K with Raman spectroscopy and resistivity measurements [17,18]. At 100 K, the deviation of site charge from the formal value (0.5e) was estimated as ρ ∼ 0.1e.…”

“…Besides the charge instability at 8.5 K, a CO with a small charge modulation was already observed in β -Ga salt at 100 K with Raman spectroscopy and resistivity measurements [17,18]. At 100 K, the deviation of site charge from the formal value (0.5e) was estimated as ρ ∼ 0.1e.…”

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

“…Their insulating phase is likely to be the CO state. [7][8][9][10][11][12][13] Some of these salts exhibit superconductivity under applied or ambient pressure. 14,15 It has been suggested that the mechanism of superconductivity in these salts is intermediated not by antiferromagnetic fluctuations but by charge fluctuations.…”