Charge order inβ″-phase BEDT-TTF salts

Abstract

Experimentally we establish the quantitative phase diagram of 1/4-filled β ′′ -(BEDT-TTF)2SF5RSO3 with a tendency towards charge order. Comprehensive optical, transport and susceptibility measurements reveal the insulating nature and magnetic properties of the chargeordered ground state. Going from R =CHF via CH2CF2 and CHFCF2 towards CH2, a finite charge disproportionation appears and grows up to 2δ = 0.5e, as evidenced by charge-sensitive molecular vibrations. This is accompanied by an increase of electronic correlation strength, i.e. the inter-site Coulomb repulsion V becomes more pronounced in relation to the bandwidth W . The broadband electronic excitations and their anisotropy unveil a distinct charge pattern for R =CH2 (checkerboard-type) as compared to the other compounds (stripe-like). Our results validate theoretical predictions for 1/4-filled systems, substantiating the importance of charge fluctuations for unconventional superconductivity at the verge between metal and insulator.

Effects of electron correlations and chemical pressures on superconductivity of β′′ -type organic compounds

“…It is clearly found that the parameters of the two salts fall in the correlation derived from our results. Quite recently, optical measurements [14,15] point out that the superconductivity found in β ′′ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 also coexists with the charge order similar to the present case. These facts suggest that the β ′′ -type organics can be universally discussed in a generic phase diagram with the electron correlation like Fig.…”

“…As is shown in Fig. 1(a), the universal phase diagram of the β ′′ -type salts proposed in the earlier works [11][12][13][14][15] claims that the superconducting phase is adjacent to the charge ordered (CO) phase caused by intersite Coulomb repulsion V . It implies that the CO phase appears as an underlying phase for the emerging superconductive phase.…”

“…iron-based compounds, cage compound, etc. However, it is difficult to examine the relationship between the quantum degrees of freedom and superconductivity because the number of such exotic superconductors is rather small and the pairing states are often in complicated situations due to the coexistence/competition of some degrees of freedom.Recently, β ′′ -type organic charge-transfer salts consisting of BEDT-TTF molecules with counter anions and guest solvent molecules have drawn extensive attention because they are expected to have a novel Cooper pairing mechanism related to charge degrees of freedom [6][7][8][9][10][11][12][13][14][15]. The critical temperatures T c of some β ′′ -salts are relatively high around 7-8 K, which is almost com-parable to the T c of well-known higher-T c dimer-Mott organic superconductors, κ-(BEDT-TTF) 2 X.…”

Coexistence of charge order and superconductivity in β″−(BEDT−TTF)2SF5CH2<

“…To that end, we measured the out-of-plane (E c) optical conductivity in the low-temperature insulating state of β ′′ -I and β ′′ -MI and compare it in Fig. 2 to β ′′ -SC and β ′′ -M 14,17 . The ν 27 (B 1u ) vibration around 1460 cm −1 is known to be most sensitive to the charge on the molecule and is therefore commonly utilized as a local probe 22,23 .…”

“…(b) A generic phase diagram of 1/4-filled two-dimensional electron systems suggests 8 that the superconducting state is located right at the boundary of chargeordered insulator and Fermi-liquid metal. We show the position of β ′′ -SC and its fully-deuterated analogue β ′′ -d8SC, as well as β ′′ -(BEDT-TTF)2SF5RSO3 with slightly modified anions R = CH2, CHFCF2 and CHF, called β ′′ -I, β ′′ -MI and β ′′ -M, respectively 14 . The upper horizontal scale indicates the charge disproportionation 2δ determined in this work.…”