Modulated phases, commensurate or incommensurate with the host crystal lattice, are ubiquitous in solids. The transition between such phases involves formation and rearrangement of domain walls and is generally slow. Using ultrafast electron diffraction, we directly record the photoinduced transformation between a nearly commensurate and an incommensurate charge-density-wave phase in 1T-TaS(2). The transformation takes place on the picosecond time scale, orders of magnitude faster than previously observed for commensurate-to-incommensurate transitions. The transition speed and mechanism can be linked to the peculiar nanoscale structure of the photoexcited nearly commensurate phase.

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.

Two new members of the family of bilayer compounds (CNB-EDT-TTF)4X, (CNB-EDT-TTF = 5-cyanobenzene-ethylenedithio-tetrathiafulvalene) with anions X = ReO4− and SbF6− are reported, their electron transport and optical properties investigated, and then compared to the ClO4− salt that was previously described. These compounds share the same structural type, i.e. bilayers of donors, which are packed in a β″-type pattern and then separated by layers of highly disordered anions. The absolute values of the electrical resistivity measured in single crystals within the layers were found in the range of 5 to 18 (Ωcm)−1, with a significantly sample dependence being ascribed to intrinsic disorder effects. The ClO4− and SbF6− salts exhibit metallic behavior with the resistivity decreasing upon cooling almost linearly with temperature until a broad minimum is reached between 15 and 80 K, depending on crystal quality; this is followed by an upturn of resistivity reaching values at T = 1.5 K that were comparable to those attained at room temperature. The electrical resistivity of the ReO4− salt follows a thermally activated behavior already at T = 300 K, although with a small activation energy in the range 16−18 meV. Thermoelectric power measurements yield large positive values (75–80 µV/K) at ambient temperature with a metallic behavior that is identical for all compounds. Temperature and polarization dependent infrared reflection measurements on single crystals of (CNB-EDT-TTF)4X salts, with X = ClO4−, ReO4−, and SbF6−, have been performed to obtain the optical conductivity and analyze the electronic and vibrational properties. For (CNB-EDT-TTF)4ClO4 the molecular vibrations exhibit a significant variation below T = 23 K, which suggests a charge localization phenomena.

In the all-organic paramagnetic correlated metal β ′′ -(BEDT-TTF)2SF5CH2CF2SO3 spin fluctuations can be ruled out as glue to superconductivity below Tc = 5.5 K. Instead, we demonstrate the coupling of charge degrees of freedom to the superconducting state, which occurs adjacent to a charge-ordered insulating phase. Comparison to isostructural β ′′ -(BEDT-TTF)2SF5RSO3 compounds reveals that superconductivity is suppressed when the system is tuned away from the insulator towards the metal; enhanced charge imbalance, on the other hand, leads to an increase in Tc suggesting an involvement of charge fluctuations. By locally probing the charge disproportionation through the superconducting transition, we discover slight modifications of the Fano coupling constant and intensity at Tc indicating the effect of the paired electrons on the molecular vibrations.

The electronic properties in the quasi-one-dimensional Fabre salts are strongly affected by electronic correlations along the molecular stacks, but also by the interactions with the anions located in a cage that is formed by the methyl end groups. We systematically compare the charge transport in deuterated and protonated (TMTTF)2X salts with the anions X = Br, PF6, SbF6, and ClO4, ranging from Mott and Efros–Shklovskii variable-range hopping to activated band transport with a temperature dependent energy gap. The strong dependence of charge localization and ordering on the anion size and deuteration confirms the subtle structural involvement of the anions in the charge transport along the TMTTF stack.