Low-dimensional Organic Conductors 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}.

Emergent atomistic Kondo resonances in strained carbon-benzene single chains

Using real-space generalized gradient approximation plus dynamical mean-field theory ($\mathrm{GGA}+\mathrm{DMFT}$) we explore the effect of uniaxial strain on the correlated electronic structure reconstruction of atomistic carbon patterned out in carbon-benzene single chains. Firstly, based on GGA we show how strain can trigger a valley electronic structure with similarities akin to narrow-band semiconductors. The correlated electronic structure we derive is promising in the sense that it enables the observation of emergent Kondo quasiparticle resonances for future valleytronics based on low-dimensional organic conductors. Our proposal is a key step to iteratively understand the intricate and interdependent changes in orbital and electronic degrees of freedom at the atomistic level.

Donor–anion interactions in quarter-filled low-dimensional organic conductors

In contrast to currently accepted ideas anions play an essential role in directing the structural and transport properties of many molecular conductors.

Optical investigations of the metal-insulator transition in a low-dimensional organic conductor (EDT-TTF)4[Hg3I8

The polarized reflectance spectra of single crystals of the low-dimensional organic conductor (EDT-TTF)4[Hg3I8] undergoing a metal-insulator phase transition at a temperature T < 35 K have been presented. The spectral region of the study is 700–6000 cm−1 (0.087–0.74 eV), and the temperature range is 300–9 K. It has been shown that the reflectance spectra are determined by a system of quasi-free electrons of the upper half-occupied molecular π-orbitals, which form a half-filled metallic band in the crystals. A high anisotropy of the spectra and their temperature dependences have been found. For two polarizations, the quantitative analysis of the spectra at 100 and 25 K has been performed in the framework of the phenomenological Drude model, the effective mass and the width of the initial metallic π-electron band have been deter-mined, and it has been found that the conducting system in the crystals has a quasi-one-dimensional character. As temperature decreases, the spectra demonstrate substantial changes indicating the formation of the energy gap (or pseudogap) in the spectrum of electronic states in the range of ∼1500–2500 cm−1. In the low-frequency region (700–1600 cm−1), a vibrational structure has been observed, and the most intense feature of the structure (ω = 1340 cm−1) is caused by the interaction of electrons with intramolecular vibrations of the C=C bonds of the EDT-TTF molecule. For temperatures of 15 and 9 K, the analysis of the spectra has been performed in the framework of the theoretical “phase phonon” model taking into account the interaction of electrons with the intramolecular vibrations. It has been concluded that the metal-insulator transition observed in the reflectance spectra of the crystals is similar to the Peierls dielectric transition that occurs in a system of electrons coupled with the intramolecular vibrations of the molecules forming the crystal.

Infrared evidence of a Slater metal-insulator transition in NaOsO3

The magnetically driven metal-insulator transition (MIT) was predicted by Slater in the fifties. Here a long-range antiferromagnetic (AF) order can open up a gap at the Brillouin electronic band boundary regardless of the Coulomb repulsion magnitude. However, while many low-dimensional organic conductors display evidence for an AF driven MIT, in three-dimensional (3D) systems the Slater MIT still remains elusive. We employ terahertz and infrared spectroscopy to investigate the MIT in the NaOsO3 3D antiferromagnet. From the optical conductivity analysis we find evidence for a continuous opening of the energy gap, whose temperature dependence can be well described in terms of a second order phase transition. The comparison between the experimental Drude spectral weight and the one calculated through Local Density Approximation (LDA) shows that electronic correlations play a limited role in the MIT. All the experimental evidence demonstrates that NaOsO3 is the first known 3D Slater insulator.

Optical properties of low-dimensional organic conductors with differently oriented conducting layers: (EDT-TTF)3Hg2Br6 and (EDT-TTF)3Hg(SCN)3I0.5(PhCl)0.5

Spectral optical investigations of two low-dimensional organic molecular conductors with differently oriented conducting layers of ethylenedithiotetrathiafulvalene (EDT-TTF) molecules, namely, the (EDT-TTF)3Hg2Br6 and (EDT-TTF)3Hg(SCN)3I0.5(PhCl)0.5 single crystals, have been carried out. The polarized reflectance spectra of the single crystals have been measured in the frequency range 700–6500 cm−1 (0.087–0.810 eV) at temperatures from 300 to 15 K. The optical conductivity spectra have been obtained using the Kramers-Kronig relations, and their quantitative analysis has been performed in terms of a theoretical model that takes into account electron-electron correlations in the approximation of the Hubbard Hamiltonian for trimerized stacks, the vibronic coupling, and the influence of the counterion on the electronic states in the trimer. A satisfactory agreement between the theoretical and experimental spectra for both crystals made it possible to estimate the parameters of the electronic structure of the crystals in the conducting plane: the integral t of the electron transfer between the EDT-TTF molecules in the trimer, the energy U of the Coulomb repulsion between two electrons (holes) in one EDT-TTF molecule, the electron transfer damping constant γe, the energy shift Δ of the molecular orbital under the influence of the anions and vibronic coupling, the vibronic coupling constant gn, and the binding energy Ep of the molecular polaron. It has been found that there are large differences in the anisotropies of the optical properties and the obtained Hubbard parameters of the electronic structure for the studied crystals.

Charge ordering in low dimensional organic conductors: Structural aspects

AbstractThe paper points out the importance of the coupling between anions and donors in order to achieve the 4kF charge localization observed in (TMTTF)2PF6, δ‐(EDT‐TTF‐CONMe2)2Br and (o‐DMTTF)2Cl/Br salts, the 2kF charge density wave (CDW) ground state of α‐(BEDT‐TTF)2KHg(SCN)4 and the metal to insulator transition of α‐(BEDT‐TTF)2I3. This coupling leads to a cooperative displacement wave of the anions accompanied by a modulation of the density of π holes on the donors. We distinguish two principal anion–donor coupling mechanisms: a direct mechanism via the Hartree anion potential on donor sites and an indirect mechanism via the polarization of σ bonds activated by the modification of the H bonds network. Both kinds of interaction are tuned by the relative displacement of the anions with respect to the donors.

Photochemical modification of magnetic properties in organic low-dimensional conductors

Magnetic properties of organic charge transfer salts Ag( DX) 2 ( DX=2,5-dihalogeno- N, N′-dicyanoquinonediimine; X=Cl, Br, I) were modified by UV irradiation from paramagnetism to diamagnetism in an irreversible way. The temperature dependence of susceptibility revealed that such change in magnetic behavior could be continuously controlled by the duration of irradiation. The observation with scanning electron microprobe revealed that the original appearance of samples, e.g. black well-defined needle-shaped shiny single crystals, remained after irradiation irrespective of the irradiation conditions and the duration. Thermochemical analysis and X-ray diffraction study demonstrated that the change in the physical properties were due to (partial) decomposition of Ag( DX) 2 to Ag X, which was incorporated in the original Ag( DX) 2 lattices. Because the physical properties of low-dimensional organic conductors are very sensitive to lattice defects, even a small amount of Ag X could effectively modify the electronic properties of Ag( DX) 2 without making the original crystalline appearance collapse.

Charge-transfer processes in radical ion molecular conductors κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x : The superconductor (x = 0.9) and the conductor with the metal-insulator transition (x = 0)

Optical spectral investigations of low-dimensional organic molecular conductors κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x with x = 0.9 (the superconductor with T c = 11.3 K) and x = 0 (the metal with the metal-insulator transition at T < 50 K) are performed in the range 50–6000 cm−1 (6 meV–0.74 eV) at temperatures from 300 to 20 K. The optical conductivity spectra are quantitatively analyzed in terms of the proposed model, according to which the charge transfer involves two types of charge carriers, i.e., electrons (holes) localized on clusters (dimers and tetramers formed by BEDT-TTF molecules) and quasi-free charge carriers, with the use of the tetramer “cluster“ model based on the Hubbard Hamiltonian for correlated electrons and the Drude model for quasi-free charge carriers. Physical parameters of the model, such as the energy of Coulomb repulsion between two electrons (holes) in one molecule, the transfer integrals between molecules inside the dimer and between dimers, and the electron-molecular vibration coupling constants, are determined. The anisotropy of the spectra in the conducting plane is explained. The inference is made that only electrons localized on clusters couple with intramolecular vibrations.

Low-Dimensional organic conductors as thermoelectric materials

We have developed a system to measure electrical resistivity, thermopower and thermal conductivity of tiny fragile organic conductors simultaneously. Figure of merit Z has been successfully determined from these transport coefficients for a two-dimensional organic conductor τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+y, (y≤0.875), where EDO-S,S-DMEDT-TTF is ethylenedioxy-S,S-dimethylethylenedithio-tetrathiafulvalene, for the first time.

Ordering phenomena in quasi-one-dimensional organic conductors

Low-dimensional organic conductors could establish themselves as model systems for the investigation of the physics in reduced dimensions. In the metallic state of a one-dimensional solid, Fermi liquid theory breaks down and spin and charge degrees of freedom become separated. But the metallic phase is not stable in one dimension: as the temperature is reduced, the electronic charge and spin tend to arrange themselves in an ordered fashion due to strong correlations. The competition of the different interactions is responsible for which broken-symmetry ground state is eventually realized in a specific compound and which drives the system toward an insulating state. Here, we review the various ordering phenomena and how they can be identified by optic and magnetic measurements. While the final results might look very similar in the case of a charge density wave and a charge-ordered metal, for instance, the physical cause is completely different. When density waves form, a gap opens in the density of states at the Fermi energy due to nesting of the one-dimension Fermi surface sheets. When a one-dimensional metal becomes a charge-ordered Mott insulator, on the other hand, the short-range Coulomb repulsion localizes the charge on the lattice sites and even causes certain charge patterns. We try to point out the similarities and conceptional differences of these phenomena and give an example for each of them. Particular emphasis will be put on collective phenomena that are inherently present as soon as ordering breaks the symmetry of the system.

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)2FSO3 probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)2FSO3 along (E $\|a$ ) and perpendicular (E $\|b'$ ) to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. We also report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)2X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO3 anion is proposed, in order to explain the anomalous behavior of this new mode.

Transport and Optics in Quasi-One-Dimensional Organic Conductors

We review the transport and optical properties of an extended series of low dimensional organic Bechgaard conductors, which are hosting the first organic superconductor (TMTSF) 2 PF 6 . It is shown that all compounds of this family are one-dimensional Mott insulators with an activation energy for longitudinal transport depending on their chemical composition and decreasing under pressure. This activation energy in the transport results vanishes at a (chemical) pressure induced dimensionality cross-over, while a gap feature is still observable in the far-infrared spectrum. Particularly, according to the data of transverse transport, the signature of a gap in the quasi particle spectrum is observable even when the longitudinal transport becomes metal-like. In this survey, we address a variety of relevant problems and concepts associated with the physics of an interacting electron gas in low dimensions, such as non-Fermi liquid behavior, dimensionality crossover, and specifically the appearance of the Luttinger liquid state.

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The details of the magnetoresistance anisotropy of a low dimensional organic conductor (TMTSF)2ClO4 have been studied in the normal metallic phase. Clear fine structures are found superposed on the background angular dependence of magnetoresistance. They appear at the field angles whose tangent satisfies the rational condition tanθ = (p/q) (2b'/c∗), where b' and c∗ are the lattice constants. This new phenomenon is considered to result from the “field-induced commensurability effect”, that is, the resonances of two periodicities in an open orbit under titled magnetic fields, as predictedby Lebed and Bak.

Scaling behavior of the longitudinal and transverse transport in quasi-one-dimensional organic conductors

We report on dc and microwave experiments of the low-dimensional organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$ClO$_4$ along the $a$, $b^{\prime}$, and $c^*$ directions. In the normal state of (TMTSF)$_2$PF$_6$ below T=70 K, the dc resistivity follows a power-law with $\rho_a$ and $\rho_{b^{\prime}}$ proportional to $T^2$ while $\rho_{c^*}\propto T$. Above $T = 100$ K the exponents extracted from the data for the $a$ and $c^*$ axes are consiste1nt with what is to be expected for a system of coupled one-dimensional chains (Luttinger liquid) and a dimensional crossover at a temperature of about 100 K. The $b^\prime$ axis shows anomalous exponents that could be attributed to a large crossover between these two regimes. The contactless microwave measurements of single crystals along the $b^{\prime}$-axis reveal an anomaly between 25 and 55 K which is not understood yet. The organic superconductor (TMTSF)$_2$ClO$_4$ is more a two-dimensional metal with an anisotropy $\rho_a/\rho_{b^{\prime}}$ of approximately 2 at all temperatures. Such a low anisotropy is unexpected in view of the transfer integrals. Slight indications to one-dimensionality are found in the temperature dependent transport only above 200 K. Even along the least conducting $c^*$ direction no region with semiconducting behavior is revealed up to room temperature.

Low-temperature behavior of disordered magnetic impurities: Distribution of effective Kondo temperatures

Several rare-earth compounds, low-dimensional organic conductors, and spin chains exhibit low-temperature divergences of their magnetic susceptibility and specific heat (non-Fermi-liquid behavior). Such divergences are often related to disordered ensembles of magnetic impurities in those systems. In this work the distribution function of the effective characteristic of a single magnetic impurity, the Kondo temperature, is derived. We calculate how the distributions of Kondo temperatures depend on the effective dimensionality of the problem and on the concentration of impurities.

Multistability in a BEDT-TTF based molecular conductor.

The low dimensional organic conductor (BEDT-TTF)(2)Br(1.3)I(1.1)Cl(0.6) [BEDT-TTF = bis(ethylenedithio)tertathiafulvalene] is shown to be a unique molecular solid that exists in three crystalline polymorphic forms (alpha'-, alpha' "-, beta' '-phase) and, surprisingly, is able to adopt the same metal-like beta' '-phase at both low (T < 185 K) and high (T > 395 K) temperatures. Several crystals of the alpha'- and alpha' "-phases have been studied using three different techniques: dc-conductivity measurements, ESR spectroscopy, and X-ray diffraction analysis. All these techniques show the existence of the reversible semiconductor <--> metal (alpha' " <--> beta' ') phase transition at both high and low temperatures as well as the alpha' <--> alpha' " phase transition at high temperatures. The phase transitions of these polymorphs are characterized by huge hysteresis and dramatic changes in the transport and magnetic properties. Based on ab initio calculations, it is suggested that dipole-dipole interactions can play a key role in the rich polymorphism of this molecular solid.

Some results and open problems in research on low-dimensional organic conductors

This paper broadly reviews some of the fundamental results of research on a family of quasi-one-dimensional organic conductors called the Bechgaard salts. To a lesser extent, the quasi-two-dimensional salts will also be discussed. Apart from a selection of known results, drawn from the literature and the author's research experience, some open problems will also be presented.

(TM) 2X organic superconductors: prototypes of one-dimensional conductors

Organic superconductivity has been discovered in 1979 in the one-dimensional organic conductor (TMTSF) 2PF 6. This discovery has to be considered in the framework of a long quest for new superconductors which started 15 years earlier with the suggestion made by W. Little for a new pairing mechanism in organic conductors. The extensive studies carried out over the last 20 years have revealed the existence of a vast series of isostructural compounds, (TM) 2 X, exhibiting signatures of a non-Fermi behavior at high temperature and a variety of ground states at low temperature. Besides superconductivity, low dimensional organic conductors have brought a wealth of new concepts in physics.

Electric field effect on magnetoresistance angular effects in low-dimensional organic conductors

The change of magnetoresistance angular effects induced by strong electric fields has been studied in quasi-one-dimensional (Q1D) organic conductors. Numerical and experimental results on the electric field effect are presented. For each sheet-like Fermi surface, the interlayer electric field works as the effective magnetic field giving the same force, so that the phenomena depending on magnetic field orientations must show shift generally. We show that the Lebed resonances, the most basic angular effect in the Q1D system, show shift (or split) under electric fields, and demonstrate that the Fermi velocity can be estimated from these experiments. By combining the electric field effect, the magnetoresistance angular effects could be the more useful tool to study not only the Fermi surface topology but also the band dispersion in Q1D conductors.