Bechgaard Salts Research Articles

Potential for exciton condensation in a highly conductive amorphous polymer

An outstanding challenge is to develop molecular materials that exhibit highly efficient energy transfer at ambient conditions. The authors of this study demonstrate the potential of an amorphous material to become an exciton condensate, in which energy can flow with minimal frictional loss. Unlike the Bechgaard salts that support superconductivity only at high pressures, the authors show that the amorphous polymer NiTTFtt, recently synthesized by Xie and Anderson, exhibits the computational signature of exciton condensation at atmospheric pressure and its synthetic geometry. This research supports the realistic possibility of harnessing chemical tunability to achieve ``strongly correlated'' phenomena at more ambient conditions, opening novel avenues for superefficient energy transfer in technologically relevant materials.

Old Donors for New Molecular Conductors: Combining TMTSF and BEDT-TTF with Anionic (TaF6)1−x/(PF6)x Alloys

Tetramethyl-tetraselenafulvalene (TMTSF) and bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) are flagship precursors in the field of molecular (super)conductors. The electrocrystallization of these donors in the presence of (n-Bu4N)TaF6 or mixtures of (n-Bu4N)TaF6 and (n-Bu4N)PF6 provided Bechgaard salts formulated as (TMTSF)2(TaF6)0.84(PF6)0.16, (TMTSF)2(TaF6)0.56(PF6)0.44, (TMTSF)2(TaF6)0.44(PF6)0.56 and (TMTSF)2(TaF6)0.12(PF6)0.88, together with the monoclinic and orthorhombic phases δm-(BEDT-TTF)2(TaF6)0.94(PF6)0.06 and δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57, respectively. The use of BEDT-TTF and a mixture of (n-Bu4N)TaF6/TaF5 afforded the 1:1 phase (BEDT-TTF)2(TaF6)2·CH2Cl2. The precise Ta/P ratio in the alloys has been determined by an accurate single crystal X-ray data analysis and was corroborated with solution 19F NMR measurements. In the previously unknown crystalline phase (BEDT-TTF)2(TaF6)2·CH2Cl2 the donors organize in dimers interacting laterally yet no organic-inorganic segregation is observed. Single crystal resistivity measurements on the TMTSF based materials show typical behavior of the Bechgaard phases with room temperature conductivity σ ≈ 100 S/cm and localization below 12 K indicative of a spin density wave transition. The orthorhombic phase δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57 is semiconducting with the room temperature conductivity estimated to be σ ≈ 0.16–0.5 S/cm while the compound (BEDT-TTF)2(TaF6)2·CH2Cl2 is also a semiconductor, yet with a much lower room temperature conductivity value of 0.001 to 0.0025 S/cm, in agreement with the +1 oxidation state and strong dimerization of the donors.

One-Dimensional Alternating Extended Hubbard Model at Quarter-Filling and Its Applications to Structural Instabilities of Organic Conductors

The one-dimensional extended Hubbard model with lattice dimerization and alternated site potentials is analyzed using the renormalization group method. The coupling of electrons to structural degrees of freedom such as the anion lattice and acoustic phonons is investigated to obtain the possible instabilities against the formation of lattice superstructures. Applications of the theory to anionic and spin-Peierls instabilities in the Fabre and Bechgaard salts series of organic conductors and ordered alloys are presented and discussed.

Fermi surface electron–hole instability of the (TMTSF)2PF6 Bechgaard salt revealed by the first-principles Lindhard response function

We report the first-principles DFT calculation of the electron–hole Lindhard response function of the (TMTSF)2PF6 Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2kF spin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2kF broad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron–hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2kF bond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron–hole Lindhard response of (TMTSF)2PF6 should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2kF SDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)2PF6 which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors.

Anion ordering transition and Fermi surface electron–hole instabilities in the (TMTSF)2ClO4 and (TMTSF)2NO3 Bechgaard salts analyzed through the first-principles Lindhard response function

The first-principles electron–hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)2 X (X = ClO4 and NO3) Bechgaard salts undergoing different anion-ordering (AO) transitions. The calculation was carried out using the real triclinic low-temperature structures. The evolution of the electron–hole response with temperature for both relaxed and quenched salts is discussed. It is shown that the 2k F response of the quenched samples of both salts display a low temperature curved and tilted triangular continuum of maxima. This is not the case for the relaxed samples. (TMTSF)2ClO4 in the AO state exhibits a more quasi-1D response than in the non AO state and relaxed (TMTSF)2NO3 shows a sharp maximum. The curved triangular plateau of the quenched samples results from multiple nesting of the warped quasi-1D Fermi surface which implies the existence of a large q range of electron–hole fluctuations. This broad maxima region is around 1% of the Brillouin zone area for the X = ClO4 salt (and X = PF6) but only 0.1% for the X = NO3 salt. It is suggested that the strong reduction of associated SDW fluctuations could explain the non detection of the SDW-mediated superconductivity in (TMTSF)2NO3. The calculated maxima of the Lindhard response nicely account for the modulation wave vector experimentally determined by NMR in the SDW ground state of the two salts. The critical AO wave vector for both salts is located in regions where the Lindhard response is a minimum so that they are unrelated to any electron–hole instability. The present first-principles calculation reveals 3D effects in the Lindhard response of the two salts at low temperature which are considerably more difficult to model in analytical approaches.

Effects of surface structure and halogen substitution on electronic charge transfer in adlayers of (BETS)2-GaCl4 on silver surfaces

Results of DFT calculations are presented that focus on changes to charge transfer, energetics, and geometry in adlayers of (BETS)2-GaCl4 (an organic superconductor Bechgaard salt, ʻOSC’) as modulated by changes in surface and degree of substitution to its halogens. Specifically, results are presented for the charge transfer to/in adlayers of λ-(BETS)2-GaCl4 on the Ag(111) and Ag(100) surfaces; and on both surfaces including further the effects of multiple successive F substitutions for Cl onto the Ga center. Results show two interesting important phenomena: first, that the amount of charge transfer is indeed highly surface sensitive (by about 0.5 net electron in this case), and second, that substitution of different amounts of F atoms on the Ga center also has profound effects on the charge transfer to the Ga atom in the GaCl(4−x)Fx center, albeit in nonlinear and counterintuitive ways which essentially conserve total net charge in the overall adlayer.The results of this work, in summary, provide insight which should help understand the degree to which surface-mediated modulation of assembled adlayers of nanoelectronic materials and their electronic structure can be varied and further supplements recent work on superhalogenated OSC charge transfer materials.

Even- and Odd-Frequency Superconductivity in Q1D Organic Superconductors Under Magnetic Field

One of the quasi-one-dimensional organic superconductors $$(\hbox {TMTSF})_2X$$ , called Bechgaard salts, has attracted a lot of interest since its discovery in 1980. These compounds have high critical magnetic field $$H_{\mathrm {c}2}$$ in the direction parallel to the layers, and their pairing symmetries seem to be different between low-field phase and high-field phase. In addition, recently, the possibility of the realization of odd-frequency superconductivity was pointed out. Therefore, we study pairing symmetries of Bechgaard salts under magnetic field with frequency-dependent pairing interaction due to spin and charge fluctuation. The model is given by the extended Hubbard model on a quasi-one-dimensional square lattice, and the spin susceptibility and charge susceptibility for the effective pairing interactions are derived by random-phase approximation under magnetic field. The odd-frequency s-wave and even-frequency f-wave regions expand as the magnetic field is increased.

Inelastic neutron scattering investigation of magnetostructural excitations in the spin-Peierls organic system (TMTTF)2PF6

One-dimensional (1D) conductors such as Bechgaard and Fabre salts are a prototypal example of correlated systems where the phase diagram is controlled by sizable electron-electron repulsions. In deuterated ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$, where this interaction achieves charge localization at ambient pressure on donor stacks, magnetostructural coupling plays a decisive role to stabilize a spin-Peierls (SPs) ground state at ${T}_{\mathrm{SP}}=13\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In this paper, we present the first inelastic neutron scattering investigation of SP magnetic excitations in organics. Our paper reveals the presence above ${T}_{\mathrm{SP}}$ of sizable critical fluctuations leading to the formation of a pseudogap in the 1D antiferromagnetic (AF) $S=1/2$ magnetic excitation spectrum of the donor stack, concomitant with the local formation of singlet of paired spins into dimers below ${T}_{\mathrm{SP}}^{\mathrm{MF}}\ensuremath{\approx}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition, the inelastic neutron scattering investigation allows us also to probe the SP critical lattice dynamics and to show that at ambient pressure these dynamics are of relaxation or order-disorder type. Below ${T}_{\mathrm{SP}}$, our paper reveals the emergence of a two gap SP magnetic excitation spectrum towards a well-defined $S=1$ magnon mode and a continuum of two excitations, as theoretically predicted. Our measurements allow us to locate the ambient pressure SP phase of ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$ in the classical (adiabatic) limit close to the classical/quantum crossover line. Then we provide arguments suggesting that pressurized ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$ shifts to the quantum (antiadiabatic) SP gapped phase, which ends in a quantum critical point allowing the stabilization of an AF phase that competes with superconductivity at higher pressure. Finally, we propose that the magnetostructural coupling mechanism in the Fabre salts is caused by dimer charge/spin fluctuations driven by the coupling of donors with anions.

Bipolar Electrochemistry with Organic Single Crystals for Wireless Synthesis of Metal–Organic Janus Objects and Asymmetric Photovoltage Generation

Bipolar electrochemistry has recently emerged as a very unique method to address conducting particles in a wireless manner. The technique is often applied to the fabrication of Janus particles; however the chemical nature of the bipolar electrode has been essentially limited to carbon- or metal-based materials. Here, we report for the first time the use of conducting organic single crystals as bipolar electrodes for the preparation of a new generation of Janus objects. Fabre and Bechgaard salts involving respectively tetrathia- and tetraselenafulvalene were selected for proof-of-concept experiments. Such an approach allows to preserve the integrity of these fragile substrates because it necessitates neither electronic wiring nor mechanical contact. The site-selective electrodeposition of copper is successfully achieved, leading thus to a new metal–organic Janus structure. Subsequently, asymmetric generation of photovoltage under illumination is achieved due to the anisotropic presence of copper, making th...

Superconducting and density-wave instabilities of low-dimensional conductors with a Zeeman coupling to a magnetic field

In the framework of the weak coupling renormalization group technique we examine the possible instabilities of the extended quasi-one-dimensional electron gas model with both intrachain and interchain electron-electron interactions, including the influence of umklapp scattering and the coupling of spins to a magnetic field. In the limit of purely repulsive intrachain interactions, we confirm the passage from singlet d-wave like superconductivity to an inhomogeneous FFLO state under magnetic field. The passage is accompanied by an anomalous increase of the upper critical field that scales with the antinesting distance from the quantum critical point joining superconductivity to antiferromagnetism in the phase diagram, as well as the strength of interactions. Adding weak repulsive interchain interactions promotes the passage from singlet to triplet $f$-wave superconductivity which is expanded under field by the development of a triplet FFLO state with zero angular momentum projection for the Cooper pairs. The connection between theory and experiments on the anomalous upper critical field in the Bechgaard salts is discussed.

Application of superhalogens in the design of organic superconductors

This study shows that the acceptors of super-electrons in organic superconductors belong to the class of superhalogens and proposes that a new series of salts, (TMTSF)2X, can be realized where X is a superhalogen, which possess similar properties to those of the existing Bechgaard salts. Thus, the concept of superhalogens can be useful in designing potential candidates for organic superconductors.

NMR Evidences of the Coupling between Conduction Electrons and Molecular Degrees of Freedom in the Exotic Member of the Bechgaard Salt (TMTSF)2FSO3

We performed Se and F-NMR measurements on single crystals of (TMTSF)2FSO3 to characterize the electronic structures of different phases in the Temperature-Pressure phase diagram, determined by precise transport measurements [Jo et al., Phys. Rev. B67, 014516 (2003)]. We claim that such varieties of electronic states in the refined phase diagram are caused by strong couplings of the conduction electrons with FSO3 anions, especially with the permanent electric dipoles on the anions. We suggest that as temperature decreases, the FSO3 anions form orientational ordering through two steps; first only the tetrahedrons form an orientational order leaving the orientations of the electronic dipoles in random (transition I); then the dipoles form a perfect orientational order at a lower temperature (transition II). In the intermediate temperature range between transitions I and II, we found an appreciable enhancement of homogeneous and inhomogeneous widths of 77Se-NMR spectrum. From the analysis of the angular dependence of the linewidth, we attributed these anomalies to the intramolecular charge disproportionation or imbalance and its slow dynamics caused by the coupling with the permanent electric dipole of anion. Results of 19F-NMR relaxation and lineshape measurements support this picture very well. Electronic structures at higher pressures up to 1.25 GPa are discussed on the basis of the results of the 77Se and 19F-NMR measurements.

Antiferromagnetic ordering and pseudogap in a model of quasi-1D organic superconductor electronic subsystem

ABSTRACTModel of electronic subsystem of Fabre salts (TMTTF)2X and Bechgaard salts (TMTSF)2X has been studied theoretically within Green function equation of motion approach with special attention to microscopical mechanism for antiferromagnetic order stabilization and pseudogap formation in the electronic spectrum. Conditions for pseudogap opening in the spectrum and anomalies in dispersion of current carriers are determined. Next-nearest-neighbor hopping is shown to break the perfect nesting property and destabilize antiferromagnetic state. On the basis of these results, experimental phase diagrams for quasi-one-dimensional organic superconductors (TMTSF)2X and (TMTTF)2X are interpreted.

Current-dependent formation of Bechgaard salt nanocrystals using nanoscale electrocrystallization for the fabrication of organic nanodevices

Herein, the nanocrystal formation of tetramethyltetraselenafulvalene (TMTSF)-based nanocrystals based on nanoscale electrocrystallization is reported. Whereas direct current electrolysis produced nanocrystals over the entire anode surface, nanocrystals were only formed in the gap between the two electrodes using alternating current (AC) electrolysis. Consequently, a two-terminal device where nanocrystals bridge the source and drain electrodes was simply prepared via this one-step electrolysis process. The prepared nanocrystal had the same structure as the bulk crystal of (TMTSF)2·ClO4, which is known as a superconducting Bechgaard salt. Although the nanocrystal exhibited a metallic nature, weak gate-voltage-dependent characteristics were observed in the current-voltage characteristics measurements. Facile fabrication of organic nanodevices was thus enabled using this AC electrolysis method.

Novel superconducting phenomena in quasi-one-dimensional Bechgaard salts

It is the saturation of the transition temperature Tc in the range of 24 K for known materials in the late sixties which triggered the search for additional materials offering new coupling mechanisms leading in turn to higher Tc's. As a result of this stimulation, superconductivity in organic matter was discovered in tetramethyl-tetraselenafulvalene-hexafluorophosphate, (TMTSF)2PF6, in 1979, in the laboratory founded at Orsay by Professor Friedel and his colleagues in 1962. Although this conductor is a prototype example for low-dimensional physics, we mostly focus in this article on the superconducting phase of the ambient-pressure superconductor (TMTSF)2ClO4, in which the superconducting phase has been studied most intensively among the TMTSF salts. We shall present a series of experimental results supporting nodal d-wave symmetry for the superconducting gap in these prototypical quasi-one-dimensional conductors.

Electrical transport near quantum criticality in low-dimensional organic superconductors

We propose a theory of longitudinal resistivity in the normal phase of quasi-one-dimensional organic superconductors near the quantum critical point where antiferromagnetism borders with superconductivity under pressure. The linearized semi-classical Boltzmann equation is solved numerically, fed in by the half-filling electronic umklapp scattering vertex as derived from one-loop renormalization group calculations for the quasi-one-dimensional electron gas model. The momentum and temperature dependence of umklapp scattering has an important impact on the behaviour of longitudinal resistivity in the the normal phase. Resistivity is found to be linear in temperature around the quantum critical point at which spin-density-wave order joins superconductivity along the antinesting axis, to gradually evolve towards the Fermi liquid behaviour in the limit of weak superconductivity. A comparison is made between theory and experiments performed on the (TMTSF)$_2$PF$_6$ member of the Bechgaard salt series under pressure.

Spin density wave and superconducting properties of nanoparticle organic conductor assemblies

The magnetic susceptibilities of nanoparticle assemblies of two Bechgaard salts ${(\mathrm{TMTSF})}_{2}{\text{PF}}_{6}$ and ${(\mathrm{TMTSF})}_{2}{\text{ClO}}_{4}$, have been studied vs temperature and magnetic field. In the bulk these materials exhibit a spin density wave formation $({T}_{\text{SDW}}\phantom{\rule{4pt}{0ex}}=12\phantom{\rule{4pt}{0ex}}\mathrm{K})$ and superconductivity $({T}_{\text{c}}=1.2\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, respectively. We show from inductive (susceptibility) measurements that the nanoparticle assemblies exhibit ground-state phase transitions similar to those of randomly oriented polycrystalline samples of the parent materials. Resistivity and diamagnetic shielding measurements yield additional information on the functional nanoparticle structure in terms of stoichiometric and nonstoichiometric composition.

Interplay between electronic and structural degrees of freedom in quarter-filled low dimensional conductors

We review the basic aspects of the charge density wave (CDW) and bond order wave (BOW) instabilities observed in one dimension (1D) organic conductors at either the 2kF and/or 4kF critical wave vectors. We start by recalling the main features of the coupled structural/electronic Peierls instabilities observed in donor–acceptor (D–A) charge transfer (CT) salts. Then we consider the specific case of 2:1 salts D2X where X is a monovalent anion. We show that the incipient CDW/BOW instabilities of the Bechgaard and Fabre salts are those of the parent quarter-filled CT salts TMTSF-DMTCNQ and TMTTF-DMTCNQ respectively. We also consider more specifically the influence of specific features of D2X salts such as the stack dimerization, the Fermi surface warping and the coupling to the anions. Then we discuss more generally the role of the anions in the Bechgaard and Fabre salts by pointing out the influence of polarization and charge displacement induced by the anion shift. Finally we show that some of these features are also relevant to understand the subtle interplay between structural and electronic degrees of freedom in 2D quarter-filled organic salts such as the (BEDT-TTF)2X series.

Role of electron-phonon interaction in a magnetically driven mechanism for superconductivity

We use the renormalization group method to examine the effect of phonon mediated interaction on d-wave superconductivity, as driven by spin fluctuations in a quasi-one-dimensional electron system. The influence of a tight-binding electron-phonon interaction on the spin-density-wave and d-wave superconducting instability lines is calculated for arbitrary temperature, phonon frequency and antinesting of the Fermi surface.The domain of electron-phonon coupling strength where spin-density-wave order becomes unstable against the formation of a bond-order-wave or Peierls state is determined at weak antinesting. We show the existence of a positive isotope effect for spin-density-wave and d-wave superconducting critical temperatures which scales with the antinesting distance from quantum critical point where the two instabilities merge. We single out a low phonon frequency zone where the bond-oder-wave ordering gives rise to triplet f-wave superconductivity under nesting alteration, with both orderings displaying a negative isotope effect. We also study the electron-phonon strengthening of spin fluctuations at the origin of extended quantum criticality in the metallic phase above superconductivity. The impact of our results on quasi-one-dimensional organic conductors like the Bechgaard salts where a Peierls distortion is absent and superconductivity emerges near a spin-density-wave state under pressure is emphasized.

Four Molecular Superconductors Isolated as Nanoparticles

Abstract(TMTSF)2PF6 and (TMTSF)2ClO4 Bechgaard salts, (BEDT‐TTF)2I3, and TTF[Ni(dmit)2]2 [TMTSF = tetramethyltetraselenafulvalene; BEDT‐TTF = bis(ethylenedithio)tetrathiafulvalene; TTF = tetrathiafulvalene; dmit = 1,3‐dithiole‐2‐thione‐4,5‐dithiolate] are among the most popular molecular superconductors. They are grown as nanoparticles that exhibit properties in agreement with those of the bulk. The shape, size, and homogeneity of particles depend on the stabilizing agent and synthesis conditions. We report on the more recent conditions investigated (i) to produce nanoparticles of 35 nm for β‐(BEDT‐TTF)2I3, (ii) to reduce the particle size to 10–15 nm for TTF[Ni(dmit)2]2 and 3–5 nm for (TMTSF)2ClO4, and (iii) to improve the growth duration from days to one hour for (TMTSF)2PF6. Finally, we report evidence of superconductivity in (TMTSF)2ClO4 particles.