Charge order inβ″-phase BEDT-TTF salts

Abstract

Experimentally, we establish the quantitative phase diagram of 1/4-filled ${\ensuremath{\beta}}^{\ensuremath{''}}$-(BEDT-${\mathrm{TTF})}_{2}{\mathrm{SF}}_{5}$R${\mathrm{SO}}_{3}$ with a tendency toward charge order. Comprehensive optical, transport, and susceptibility measurements reveal the insulating nature and magnetic properties of the charge-ordered ground state. Going from $R=\mathrm{CHF}$ via ${\mathrm{CH}}_{2}{\mathrm{CF}}_{2}$ and ${\mathrm{CHFCF}}_{2}$ toward ${\mathrm{CH}}_{2}$, a finite charge disproportionation appears and grows up to $2\ensuremath{\delta}=0.5e$, as evidenced by charge-sensitive molecular vibrations. This is accompanied by an increase of electronic correlation strength, i.e., the intersite 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 $R={\mathrm{CH}}_{2}$ (checkerboard type) as compared to the other compounds (stripelike). 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.