Pressure-dependent optical investigations of Fabre salts in the charge-ordered state

Pressure-Induced Superconductivity of the Quasi-One-Dimensional Organic Conductor (TMTTF)2TaF6

We investigated the superconductivity of (TMTTF)2TaF6 (TMTTF: tetramethyl-tetrathiafulvalene) by conducting resistivity measurements under high pressure up to 8 GPa. A cubic anvil cell (CAC) pressure generator, which can produce hydrostatic high-pressure, was used for this study. Since the generalized temperature-pressure (T-P) diagram of (TMTCF)2X (C = Se, S, X: monovalent anion) based on (TMTTF)2PF6 (TCO = 70 K and spin-Peierls: SP, TSP = 15 K) was proposed by Jérome, exploring superconductivity states using high-pressure measurement beyond 4 GPa has been required to confirm the universality of the electron-correlation variation under pressure in (TMTTF)2X (TMTTF)2TaF6, which has the largest octahedral-symmetry counter anion TaF6 in the (TMTTF)2X series, possesses the highest charge-ordering (CO) transition temperature (TCO = 175 K) in (TMTTF)2X and demonstrates an anti-ferromagnetic transition (TAF = 9 K) at ambient pressure. A superconducting state in (TMTTF)2TaF6 emerged after a metal-insulator transition was suppressed with increasing external pressure. We discovered a superconducting state in 5 ≤ P ≤ 6 GPa from Tc = 2.1 K to 2.8 K, whose pressure range is one-third narrower than that of X = SbF6 (5.4 ≤ P ≤ 9 GPa). In addition, when the pressures with maximum SC temperatures are compared between the PF6 and the TaF6 salts, we found that (TMTTF)2TaF6 has a 0.75 GPa on the negative pressure side in the T-P phase diagram of (TMTTF)2PF6.

Tuning Charge Order in (TMTTF)2X by Partial Anion Substitution

In the quasi-one-dimensional (TMTTF)2X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions and cations. Here, we investigate the charge-ordered state through partial substitution of the anions in (TMTTF)2[AsF6]1−x[SbF6]x with x≈0.3, determined from the intensity of infrared vibrations, which is sufficient to suppress the spin-Peierls state. Our dc transport experiments reveal a transition temperature TCO = 120 K and charge gap ΔCO=430 K between the values of the two parent compounds (TMTTF)2AsF6 and (TMTTF)2SbF6. Upon plotting the two parameters for different (TMTTF)2X, we find a universal relationship between TCO and ΔCO yielding that the energy gap vanishes for transition temperatures TCO≤60 K. While these quantities indicate that the macroscopic correlation strength is continuously tuned, our vibrational spectroscopy results probing the local charge disproportionation suggest that 2δ is modulated on a microscopic level.

Light-Induced Current Oscillations in the Charge-Ordered State of (TMTTF)2SbF6

Below T CO = 157 K the quasi-one-dimensional charge-transfer salt (TMTTF) 2 SbF 6 undergoes a pronounced phase transition to a charge-ordered ground state. We have explored the non-linear and photoconductive behavior as a function of applied voltage, laser pulse energy and temperature. Besides a decay of the photoconductive signal in a double exponential fashion in the millisecond range, we discover current oscillations in the kHz range induced by the application of short laser pulses. While the resonance frequencies do not depend on voltage or laser intensity and vary only slightly with temperature, the amplitude changes linearly with the laser intensity and voltage. We suggest that photo-induced fluctuations of the charge-ordered state alter the current flow of the single particles and hence, the photocurrent. The findings are discussed and compared to comparable phenomena in other low-dimensional electron systems.