Charge-localization-driven metal-insulator phase transition in layered molecular conductors

TL;DR

This study addresses the driving mechanism of metal-insulator transitions in Se-substituted α-(BEDT-TTF)2I3 salts by combining transport measurements, broadband infrared spectroscopy, vibrational analysis, and DFT calculations. It shows that α-(STF)2I3 and α-(BETS)2I3 undergo MITs without evidence of charge ordering, instead displaying low-energy absorption and carrier localization driven by strong electron–phonon coupling, with gaps that shrink under Se substitution. The work contrasts these results with the charge-order–driven MIT in the parent α-(ET)2I3 and proposes a phase diagram that highlights a transition from correlation-driven charge order to phonon-mediated charge localization under chemical pressure. Overall, the findings reveal a distinct MIT mechanism controlled by electron–phonon interactions in Se-substituted layered molecular conductors and suggest strategies to tune ground states via chemical substitution and strain.

Abstract

The organic conductor $α$-(BEDT-TTF)$_2$I$_3$ provides the prime example of a charge-order-driven metal-insulator transition. Restricted chemical substitution of S atoms by Se in the constituent molecules allows us to modify the electronic properties. This not only decreases the transition temperature but, in addition, alters the phase transition mechanism, resulting in the ground state deviating from the charge-ordered insulator state of the parent compound. Employing infrared optical spectroscopy, we investigate changes in the charge dynamics. Furthermore, we demonstrate the absence of charge ordering in the Se-substituted materials and suggest that the phase transition is instead driven by the localization of the itinerant charge carriers due to strong electron-phonon interactions.

Figures (9)

• Figure 1: Organic framework of BEDT-TTF, BEDT-STF and BEDT-TSF formed on substitution of S atoms by Se. The crystal structure (bottom) depicts the herringbone arrangement of these organic entities forming the $\alpha$-pattern of the conducting organic layer, sandwiched between insulating I$_{3}^{-}$ layers.
• Figure 2: (a) Temperature-dependent resistivity of $\alpha$-(ET)2I3, $\alpha$-(STF)2I3, and $\alpha$-(BETS)2I3, measured along the conducting $ab$-plane, illustrating the MIT transition by the drastic increase at 135 K for $\alpha$-(ET)2I3, 66 K for $\alpha$-(STF)2I3, and 50 K for $\alpha$-(BETS)2I3; (b-d) Fits by Arrhenius equation cannot capture the abrupt upturn in resistivity at MIT for (b) $\alpha$-(ET)2I3, (c) $\alpha$-(STF)2I3, and (d) $\alpha$-(BETS)2I3.
• Figure 3: (a) Out-of-plane ($E \parallel c$) optical response in the range of the infrared-active charge-sensitive vibrational mode $\nu_{27}$ (depicted in the inset) for the three compounds under scrutiny here. Already above $T_{\rm MIT}$, multiple peaks indicate charge disproportionation; compared to $\alpha$-(ET)2I3 it is lower for $\alpha$-(STF)2I3 and $\alpha$-(BETS)2I3. (b) Evolution of the spectra upon cooling from $T=300$ to 10 K. $\alpha$-(ET)2I3 exhibits an abrupt splitting, $\alpha$-(STF)2I3 demonstrates unusual broadening and eventual separation of multiple peaks, while in the case of $\alpha$-(BETS)2I3, the $\nu_{27}$ feature does not exhibit a prominent splitting but develops several small peaks and a shoulder peak near phase transition. (c-e) Temperature dependence of the different peak positions extracted from the spectra shown in panel (b). The data for $\alpha$-(ET)2I3 are reproduced from Ref. ivek2011electrodynamic.
• Figure 4: Temperature-dependent optical conductivity in the range of 10 to 300 K for two different in-plane polarization axes ($E \parallel a$ and $E \parallel b$) for (a,d) $\alpha$-(ET)2I3, (b,e) $\alpha$-(STF)2I3, and (c,f) $\alpha$-(BETS)2I3. The arrows indicate the center of the absorption peak in the low-energy region for $\alpha$-(STF)2I3 and $\alpha$-(BETS)2I3.
• Figure 5: Optical conductivity fitted by the Drude-Lorentz oscillator model with inclusion of Fano resonance peaks for 10 K (upper row) and 300 K (lower panels) for the polarization parallel to the $b$-axis: (a,d) $\alpha$-(ET)2I3, (b,e) $\alpha$-(STF)2I3 and (c,f) $\alpha$-(BETS)2I3. The most distinct feature at low temperatures for $\alpha$-(STF)2I3 and $\alpha$-(BETS)2I3 is the strong absorption peak (green) at low energies.