One-dimensional Organic Conductor Research Articles

Charge-Order Phase Transition in the Quasi One-Dimensional Organic Conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the {hbox {(TMTTF)}}_2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3. We found that the {hbox {(TMTTF)}}_2 {hbox {NO}}_3 compound develops a peculiar anisotropy with a doubled periodicity (ab'-plane) of the ESR linewidth below about T_{text {CO}}= ({250pm 10})~hbox {K}. This behavior is similar to observations in the related compounds {hbox {(TMTTF)}}_2X (X = {hbox {PF}}_6, {hbox {SbF}}_6 and {hbox {AsF}}_6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous {hbox {(TMTTF)}}_2 {hbox {ClO}}_4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below T_{text {CO}} approx 250~hbox {K}.

Decoupling anion-ordering and spin-Peierls transitions in a strongly one-dimensional organic conductor with a chessboard structure, (o-Me2TTF)2NO3.

A mixed-valence conducting cation radical salt of the unsymmetrically substituted o-Me2TTF donor molecule (TTF is tetrathiafulvalene) was obtained upon electrocrystallization in the presence of the non-centrosymmetric NO3- anion. It crystallizes at room temperature in the monoclinic P21/c space group, with the anion disordered on an inversion centre. The donor molecules are stacked along the a axis. A 90° rotation of the longest molecular axis of o-Me2TTF generates a chessboard-like structure, preventing lateral S⋯S contacts between stacks and providing a strongly one-dimensional electronic system, as confirmed by overlap interaction energies and band structure calculations. A strong dimerization within the stacks explains the semi-conducting behaviour of the salt, with σroom temp = 3-5 S cm-1 and Eactivated = 0.12-0.14 eV. An X-ray diffuse scattering survey of reciprocal space, combined with full structure resolutions at low temperatures (250, 85 and 20 K), evidenced the succession of two structural transitions: a ferroelastic one with an anion-ordering (AO) process and the establishment of a (0, ½, ½) superstructure below 124 (±3) K, also visible via resistivity thermal dependence, followed by a stack tetramerization with the establishment of a (½, ½, ½) superstructure below 90 (±5) K. The latter ground state is driven by a spin-Peierls (SP) instability, as demonstrated by the temperature dependence of the magnetic susceptibility. Surprisingly, these two kinds of instability appear to be fully decoupled here, at variance with other tetra-methyl-tetra-thia-fulvalene (TMTTF) or tetramethyl-tetra-selena-fulvalene (TMTSF) salts with such non-centrosymmetric counter-ions.

Magnetic resonance investigation for a possible antiferromagnetic subphase in (TMTTF)2Br

To understand the electronic states on the boundary region between the commensurate antiferromagnetic (II) phase and the incommensurate spin-density-wave phase in the generalized phase diagram for one-dimensional organic conductors, we performed antiferromagnetic resonance (AFMR) and nuclear magnetic resonance (NMR) measurements for ${(\mathrm{TMTTF})}_{2}\mathrm{Br}$. The angular dependence of the AFMR fields at 1.5 K is different from that at 4.8 K, and the temperature dependence of the two AFMR modes is enhanced below 5 K. Furthermore, $^{2}\mathrm{D}$(Deuterium)-NMR measurements were performed for deuterated ${(\mathrm{TMTTF}\text{\ensuremath{-}}{d}_{12})}_{2}\mathrm{Br}$ to investigate charge distribution by quadrupole splitting at low temperatures. We found that the $^{2}\mathrm{D}$-NMR spectrum changes at 4 K in the antiferromagnetic phase. Successive phase transition and a possible magnetic structure are discussed.

Peierls and Spin-Peierls Instabilities in the Per2[M(mnt)2] Series of One-Dimensional Organic Conductors; Experimental Realization of a 1D Kondo Lattice for M = Pd, Ni and Pt

We consider structural instabilities exhibited by the one-dimensional (1D) (arene)2X family of organic conductors in relation with their electronic and magnetic properties. With a charge transfer of one electron to each anion X, these salts exhibit a quarter-filled (hole) conduction band located on donor stacks. Compounds built with donors such as fluorenthene, perylene derivatives and anions X such as PF6 or AsF6 exhibit a high temperature (TP ~ 170 K) conventional Peierls transition that is preceded by a sizeable regime of 1D 2kF charge density wave fluctuations (kF is the Fermi wave vector of the 1D electron gas located on Per stack). Surprisingly, and probably because of the presence of a multi-sheet warped Fermi surface, the critical temperature of the Peierls transition is considerably reduced in the perylene series α-(Per)2[M(mnt)2] where X is the dithiolate molecule with M = Au, Cu, Co and Fe. Special attention will be devoted to physical properties of α-(Per)2[M(mnt)2] salts with M = Pt, Pd and Ni which incorporate segregated S = 1/2 1D antiferromagnetic (AF) dithiolate stacks coexisting with 1D metallic Per stacks. We analyze conjointly the structural and magnetic properties of these salts in relation with the 1D spin-Peierls (SP) instability located on the dithiolate stacks. We show that the SP instability of Pd and Ni derivatives occurs in the classical (adiabatic) limit while the SP instability of the Pt derivative occurs in the quantum (anti-adiabatic) limit. Furthermore, we show that in Pd and Ni derivatives 1st neighbor direct and frustrated 2nd neighbor indirect (through a fine tuning with the mediated 2kF RKKY coupling interaction on Per stacks) AF interactions add their contribution to the SP instability to stabilize a singlet-triplet gap. Our analysis of the data show unambiguously that magnetic α-(Per)2[M(mnt)2] salts exhibit the physics expected for a two chain Kondo lattice.

Critical dynamics and domain motion from permittivity of the electronic ferroelectric (TMTTF)2AsF6

The quasi one-dimensional organic conductor (TMTTF)2AsF6 shows the charge ordering transition at TCO=101K to a state of the ferroelectric Mott insulator which is still well conducting. We present and interpret the experimental data on the gigantic dielectric response in the vicinity of TCO, concentrating on the frequency dependence of the inverse 1/ε of the complex permittivity ε=ε′+iε′′. Surprisingly for a ferroelectric, we could closely approach the 2nd order phase transition and to deeply reach the critical dynamics of the polarization. We could analyze the critical slowing-down when approaching TCO from both sides and to extract the anomalous power law for the frequency dependence of the order parameter viscosity. Moreover, below TCO we could extract a sharp absorption feature coming from a motion of domain walls which shows up at a frequency well below the relaxation rate.

Re-entrant magnetic-field–induced charge and spin gaps in the coupled dual-chain quasi–one-dimensional organic conductor perylene2[Pt(mnt)2

An inductive method is used to follow the magnetic-field–dependent susceptibility of the coupled charge density wave (CDW) and spin-Peierls (SP) ordered state behavior in the dual-chain organic conductor perylene2[Pt(mnt)2]. In addition to the coexisting SP-CDW state phase below 8 K and 20 T, the measurements show that a second spin-gapped phase appears above 20 T that coincides with a field-induced insulating phase. The results support a strong coupling of the CDW and SP order parameters even in high magnetic fields, and provide new insight into the nature of the magnetic susceptibility of dual-chain spin and charge systems.

Evidence for sensory effects of a 1D organic conductor under gas exposure

This work describes the gas-sensitive properties of a one-dimensional organic conductor before and after exposure to carbon monoxide and human breath. A sensitive material, an anion-radical salt of tetracyanoquinodimethane, has been investigated by infrared spectroscopy and electrical resistivity measurements. Drastic spectral and electrical changes are found after gas exposure showing that the compound interacts strongly with human breath, carbon monoxide, and ammonia. Under breath action the resistance changes by more than three orders of magnitude while the adsorption of CO, one of the components of breath, results in a decrease in both IR absorption and electrical conductivity. The intensity of the IR absorption spectrum of the material in the CO medium decreases down to 30% in the 2180–2500 cm−1 range. This absorption varies by about 10% between 750 and 2500 cm−1 after breath action. Direct electrical measurements show that actions of donor or acceptor gas result in opposite changes of electric resistance. The electrical resistance of the sample can drop down to 0.4 MΩ due to the pulse action of ammonia at 4 ppm concentration, while it increases upon exposure to carbon monoxide media at concentrations of 6–25 ppm. The response signal of the investigated samples changes proportionally to the concentration of the acting gas. The results substantiate prominent gas sensitivity of the investigated material, which might find applications for breath analysis, in particular, for the development of noninvasive diagnosis of gastric diseases.

Infrared investigations of the one-dimensional organic conductors (perylene)2 M (mnt)2, M = Au, Pt

The optical properties of the one-dimensional organic conductor (perylene)2Pt(mnt)2 and its Au analogue have been investigated at ambient and low temperatures. The spectra along the stacks of the molecules show a simple Drude behavior with no detectable influence of electron-electron interactions. Basing on the spectra for different polarizations an assignment of the vibrational modes is made. Perpendicular to the stacks, a wide band observed in the mid-infrared range evidences a charge transfer between perylene and anion stacks. The electronic properties are discussed in comparison with other quasi-one-dimensional synthetic metals.

Ultrafast Potential Energy Surface Softening of One-Dimensional Organic Conductors Revealed by Picosecond Time-Resolved Laue Crystallography

Time-resolved Laue crystallography has been employed to study the structural dynamics of a one-dimensional organic conductor (tetrathiafulvalene-p-chloranil) during photoexcitation in the regime of the neutral to ionic phase transition. Exciting this crystalline system with 800 nm 100 fs long optical pulses leads to ultrafast population of a structural intermediate as early as 50 ps after excitation with a lifetime of at least 10 ns. Starting from the neutral phase, this intermediate has been assigned as a precursor state toward the photoinduced population of the ionic phase. The observed intensity changes are significantly different from comparable equilibrium structures. The interpretation of this structural data is that the potential of this intermediate is being softened during its population in a dynamical process. The depopulation proceeds through thermal processes.

13C NMR investigation of low-temperature states in one-dimensional organic cation radical salt, (TMTTF)2SbF6, under high pressures

13C nuclear magnetic resonance (NMR) measurements under the application of hydrostatic pressure were carried out on the one-dimensional organic conductor, (TMTTF)2SbF6, to investigate the competed antiferromagnetic and spin-singlet ground states. The charge-ordering (CO) transition temperature, TCO, (155 K at ambient pressure), decreased to 100 K under a pressure of 5 kbar, and was suppressed above 8 kbar. Under pressures between 5 kbar and 14 kbar, the low-pressure side antiferromagnetic state (AF-I) was suppressed. At the same time, a spin-gap phase was stabilized. Above 17 kbar, another antiferromagnetic phase appeared below approximately 15 ~ 20 K. A possible reentrant antiferromagnetic phase diagram is discussed from a microscopic point-of-view.

Anisotropic pressure effects on the charge order transition of (TMTTF) 2X

We measured the conductivity along the a-direction σ a ( T ) of the quasi one-dimensional organic conductor (TMTTF) 2SbF 6 under several anisotropic pressures of uniaxial strains which are parallel and perpendicular to the a-direction. The uniaxial strains were generated by so-called “ Frozen oil method ”. It was found that the tendencies of the uniaxial strain dependence of the conductivity and the charge order (CO) transition temperature T CO are different. According to T CO , it decreases strongly with increasing uniaxial strain parallel to the a-direction; however, it is almost constant for that perpendicular to the a-direction. We discuss the effects of compressive uniaxial strains on the conductivity and the CO transition of (TMTTF) 2SbF 6.

13C NMR investigation of re-entrant antiferromagnetic states of (TMTTF)2SbF6

13C nuclear magnetic resonance (NMR) investigations were performed on the one-dimensional organic conductor (TMTTF)2SbF6 to clarify its electronic properties in the proximity of the ground states. An abrupt broadening of 13C NMR absorption lines below 8 K (TN = 8 K), confirmed a long-range antiferromagnetic phase transition. Below TN, the absorption lines are composed of four distinct broad lines, indicating a commensurate magnetic structure. The amplitude of staggered magnetization, ρ, is comparable to (TMTTF)2Br, and on the order of 0.1 μB according to the splitting of 13C NMR lines at 3 K.

Anomalously Wide Superconducting Phase of One-Dimensional Organic Conductor (TMTTF)2SbF6

We measured the temperature dependence of resistivity of (TMTTF) 2 SbF 6 under high pressures up to 10 GPa. We observed superconductivity in the title compound for the first time when the high pressure exceeded 5 GPa. The local maximum of the superconducting transition temperatures was T c = 2.8 K at 6 and 9 GPa. The superconducting phase of (TMTTF) 2 SbF 6 was observed in an anomalously wide pressure range of 5.4 < P ≤9 GPa. We discuss this particular superconducting phase and the electron correlation phase of (TMTTF) 2 SbF 6 under high pressure.

Redistribution of Charge in the Proximity of the Spin-Peierls Transition: 13C NMR Investigation of (TMTTF)2PF6

13 C nuclear magnetic resonance (NMR) investigations were carried out for a one-dimensional organic conductor, (TMTTF) 2 PF 6 , to determine the charge dynamics and configuration in the proximity of the spin-Peierls phase-transition temperature, T sP . Although no obvious NMR line splitting associated with the charge-ordering transitions in the intermediate paramagnetic states was observed, the charge separation amplitude was estimated to be ρ CO ∼0.14 using the 13 C NMR spin–lattice relaxation rates for the distinct lines. The abrupt decrease in the NMR linewidth below 25 K and the small anisotropy of the NMR linewidth in the spin-Peierls phase indicate the possible redistribution of the charge separation in the proximity of T sP . The low-temperature electronic states of (TMTTF) 2 PF 6 are discussed from a microscopic point of view.

Hall Effect in Quasi One-Dimensional Organic Conductors

We study the Hall effect in a system of weakly coupled Luttinger Liquid chains, using a Memory function approach to compute the Hall constant in the presence of umklapp scattering along the chains. In this approximation, the Hall constant decomposes into two terms: a high-frequency term and a Memory function term. For the case of zero umklapp scattering, where the Memory function vanishes, the Hall constant is simply the band value, in agreement with former results in a similar model with no dissipation along the chains. With umklapp scattering along the chains, we find a power-law temperature dependance of the Hall constant. We discuss the applications to quasi 1D organic conductors at high temperatures.

Effect of Electron Correlation on Angular Dependence of Magnetoconductivity

An effect of electron correlation is studied in an angular dependence of magnetoconductivity of quasi one-dimensional organic conductors. We investigate the effect of both quasi-particle’s lifetime and velocity due to the electron correlation near SDW in the magnetoconductivity. We found that the momentum dependence of lifetime strongly suppresses the one-dimensional axis magnetoconductivity at a magic angle θ=45°. On the other hand, the change of the momentum dependence of the velocity originating from the electron correlation enhances the interchain magnetoconductivity at θ=0°. These effects are explained from the momentum dependence of lifetime and velocity over the electron’s commensurate orbital on Fermi surface at the magic angles.

Redistribution of Electronic Charge in (TMTTF)2ReO4: 13C NMR Investigation

13C NMR measurements were performed for one-dimensional organic conductors, (TMTTF)2ReO4. An intermediate charge-ordering (CO) phase has been found firstly for a TMTTF salt with a Td-symmetry counter anion: The NMR parameters indicate two inequivalent molecules with unequal electron densities below 225 K. Moreover, the spin-singlet transition associated with ReO4 anion ordering was confirmed at around 158 K by 13C NMR measurements. A drastic change of NMR parameters below 158 K also indicates a redistribution of the electronic charge at the anion ordering temperature.

Optical study of the metal–insulator phase transition in β-(EDT-TTF-I 2) 2(Pb 5/6□ 1/6I 2) 3 and β-(EDT-TTF-I 2) 2(Pb 2/3+ xAg 1/3−2 x□ xI 2) 3 one-dimensional organic conductors

We report the polarized infrared reflectance and absorption spectra of the two radical cation salts β-(EDT-TTF-I 2) 2(Pb 5/6□ 1/6I 2) 3 and β-(EDT-TTF-I 2) 2(Pb 2/3+ x Ag 1/3−2 x □ x I 2) 3, where x = 0.05 and □ – vacancy, from room temperature down to 10 K. Both materials display metallic response characteristic for quasi-one-dimensional organic conductors. The single crystal polarized absorption spectra display a number of vibrational features related to intramolecular modes of the EDT-TTF-I 2 molecule. The 70 K metal–insulator phase transition has a modest influence on the infrared spectra of both materials. The temperature dependence of the electron-molecular vibration (EMV)-activated stretching C C mode of EDT-TTF-I 2 suggests, that the charge density wave (CDW) fluctuations appear in both materials above the phase transition temperature. The temperature evolution of the reflectance spectra is analyzed using Drude model.

Spin susceptibility of one-dimensional extended Hubbard model at quarter filling

The ground-state property of one-dimensional extended Hubbard model is investigated at quarter filling. By using spin-charge factorized wave function in the U → ∞ limit, the effective exchange coupling J eff and spin susceptibility χ are calculated as a function of nearest-neighbor Coulomb interaction V. It is found that J eff is suppressed by the effect of V through charge fluctuation. The results indicate these quantities have no singularities at the transition point because of the Berezinskii–Kosterlitz–Thouless-type transition of charge ordering. We also confirm the results by exact diagonalization method in the realistic parameter range. The relevance to the experiments on quasi one-dimensional organic conductor ( DI-DCNQI ) 2 Ag is discussed.

13C NMR Analyses of Successive Charge Ordering in (TMTTF)2ReO4

13 C NMR measurements were performed for a one-dimensional organic conductor, (TMTTF) 2 ReO 4 . The existence of an intermediate charge ordering (CO) phase was clarified for a TMTTF salt with a T d...