Organic Conductor TTF-TCNQ Research Articles

Effects of finite-range interactions on the one-electron spectral properties of TTF-TCNQ

The electronic dispersions of the quasi-one-dimensional organic conductor TTF-TCNQ are studied by angle-resolved photoelectron spectroscopy (ARPES) with higher angular resolution and accordingly smaller step width than in previous studies. Our experimental results suggest that a refinement of the single-band 1D Hubbard model that includes finite-range interactions is needed to explain these photoemission data. To account for the effects of these finite-range interactions we employ a mobile quantum impurity scheme that describes the scattering of fractionalized particles at energies above the standard Tomonaga-Luttinger liquid limit. Our theoretical predictions agree quantitatively with the location in the $(k,\omega)$ plane of the experimentally observed ARPES structures at these higher energies. The nonperturbative microscopic mechanisms that control the spectral properties are found to simplify in terms of the exotic scattering of the charge fractionalized particles. We find that the scattering occurs in the unitary limit of (minus) infinite scattering length, which limit occurs within neutron-neutron interactions in shells of neutron stars and in the scattering of ultracold atoms but not in perturbative electronic condensed-matter systems. Our results provide important physical information on the exotic processes involved in the finite-range electron interactions that control the high-energy spectral properties of TTF-TCNQ. Our results also apply to a wider class of 1D and quasi-1D materials and systems that are of theoretical and potential technological interest.

Temperature-Pressure Phase Diagram in TTF-TCNQ: Strong Suppression of Charge-Density-Wave State under Extremely High Pressure

We have measured the resistivity of the organic conductor TTF-TCNQ under high pressure up to 8.0 GPa using a cubic anvil apparatus. Below 3 GPa, the pressure dependence of the charge-density-wave (CDW) transition temperature, T CDW , is consistent with previous results, except for the observation of the “49 K-transition” at around 1 GPa. Above 3 GPa, T CDW decreases with increasing pressure but the CDW state survives even at 8.0 GPa. At 8.0 GPa, however, the resistivity shows metallic behavior below 25 K after the CDW transition with T CDW = 31 K. The origin of suppression is discussed in terms of the imperfect nesting originating from the increase in the interchain transfer integral, the significant deviation from the third-order commensurability, as well as the bandwidth broadening for both TTF and TCNQ stacks. The complete suppression of the CDW phase is expected near 9 GPa.

Anomalous temperature dependence of the single-particle spectrum in the organic conductor TTF-TCNQ

The angle-resolved photoemission spectrum of the organic conductor TTF-TCNQ exhibits an unusual transfer of spectral weight over a wide energy range for temperatures $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}<T<260\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In order to investigate the origin of this finding, here we report numerical results on the single-particle spectral weight $A(k,\ensuremath{\omega})$ for the one-dimensional (1D) Hubbard model and, in addition, for the 1D extended Hubbard and the 1D Hubbard-Holstein models. Comparisons with the photoemission data suggest that the 1D Hubbard model is not sufficient for explaining the unusual $T$ dependence, and the long-range part of the Coulomb repulsion also needs to be included.

Temperature dependence of spectral functions for the one-dimensional Hubbard model: Comparison with experiments

We study the temperature dependence of the single particle spectral function as well as of the dynamical spin and charge structure factors for the one-dimensional Hubbard model using the finite temperature auxiliary field quantum Monte Carlo algorithm. The parameters of our simulations are chosen so to at best describe the low temperature photoemission spectra of the organic conductor TTF-TCNQ. Defining a magnetic energy scale, T_J, which marks the onset of short ranged 2k_f magnetic fluctuations, we conclude that for temperatures T < T_J the ground state features of the single particle spectral function are apparent in the finite temperature data. Above T_J spectral weight transfer over a scale set by the hopping t is observed. In contrast, photoemission data points to a lower energy scale below which spectral weight transfer occurs. Discrepancies between Hubbard model calculations and experiments are discussed.

The study on the Peierls phase transition of TTF-TCNQ

According to Peierls phase transition theory on the basis of the electron-phonon interaction, the Peierls phase transition temperature of a one-dimensional organic conductor TTF-TCNQ is computed by using the deformation potential model with the data calculated by using a semi-empirical crystal orbital method. The result indicates that the Peierls phase transition temperature of TTF chains is lower than that of TCNQ chains, which demonstrate that the electron-phonon coupling interaction in TTF chains is weaker than that in TCNQ chains, and the metal-insulator phase transition in TTF-TCNQ at 54K mainly takes place in TCNQ chains.

Signatures of spin-charge separation in the 1D organic conductor TTF-TCNQ from photoelectron spectroscopy

We have studied the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by angle-resolved photoelectron spectroscopy (ARPES). The data reveal significant discrepancies to conventional band theory. Instead the experimental dispersions can be quantitatively reproduced by the one-dimensional (1D) Hubbard model, if one allows for a surface-enhancement of the hopping integral induced by a relaxation of the tomost molecular layer. The TCNQ-related conduction band is thus found to display spectroscopic signatures of spin-charge separation on the energy scale of the band width. In contrast, the TTF-derived band seems to be only weakly correlated, at variance with other findings. The important role of electronic correlations in this material is further corrborated by a peculiar temperature dependence of the spectra. While the 1D Hubbard model thus yields a good description at finite excitation energies, it fails concerning the low-energy spectral behavior, most likely due to the additional importance of strong electron-phonon interaction and interchain electronic hopping on small energy scales.

New method for evaluation of finite-energy few-electron spectral function expressions

We present a method for the calculation of few-electron spectral functions of the one-dimensional Hubbard model which relies on a pseudofermion description introduced recently in Ref. [6]. The spectral functions are expressed as a convolution of pseudofermion dynamical correlation functions. Our general method involves the direct evaluation of the matrix elements of pseudofermion operators between the ground state and the excited states. We briefly discuss the application of our general method to the study of the unusual finite-energy spectral properties observed in the quasi-one-dimensional organic conductor TTF-TCNQ.

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

We study the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by means of density-functional band theory, Hubbard model calculations, and angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant quantitative and qualitative discrepancies to band theory. We demonstrate that the dispersive behavior as well as the temperature-dependence of the spectra can be consistently explained by the finite-energy physics of the one-dimensional Hubbard model at metallic doping. The model description can even be made quantitative, if one accounts for an enhanced hopping integral at the surface, most likely caused by a relaxation of the topmost molecular layer. Within this interpretation the ARPES data provide spectroscopic evidence for the existence of spin-charge separation on an energy scale of the conduction band width. The failure of the one-dimensional Hubbard model for the {\it low-energy} spectral behavior is attributed to interchain coupling and the additional effect of electron-phonon interaction.

Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ.

The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as a function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction bandwidth.

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

We present an investigation of the electronic structure of a prototypical quasi-one-dimensional conductor, the organic compound TTF-TCNQ, by means of angle-resolved photoemission spectroscopy. By comparison with LDA-band structure calculations we identify significant deviations between experiment and theory. We discuss possible origins for these deviations in terms of electronic correlations and surface effects in this material.

Ab initiopseudopotential calculation for TTF-TCNQ and TSeF-TCNQ

We have investigated the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ [at room temperature (RT) and 100 K] and TSeF-TCNQ (RT), which have isomorphic crystal structure, by an ab initio plane-wave pseudopotential band calculation. To express the exchange and correlation energy for electrons, we used both the local density approximation and generalized gradient approximation for comparison. For each case, electronic band dispersions were calculated along several symmetric lines and tight-binding parameters were evaluated. The Fermi surface shape was also obtained. The six sets of results (for three structures and two approximations) were compared systematically.

Ab initio calculation of positron distribution, ACAR and lifetime in TTF-TCNQ

We have performed ab initio calculations of positron distribution, ACAR and lifetime in the quasi-one-dimensional organic conductor TTF-TCNQ. The electronic structure is obtained within the LDA, while the positron state is calculated either with the LDA or with the GGA. Except the positron lifetime, differences between the LDA and GGA results are rather small. The obtained results are compared with our previous experiments and calculations.

Photoemission study of organic and inorganic CDW-systems

We investigate by means of high resolution angular resolved photoemission spectroscopy the classical one-dimensional organic conductor TTF-TCNQ and the inorganic one-dimensional conductor (TaSe4)(2)I. We measure dispersive features (1D bands) in accordance with band structure calculations. We observe typical bulk specific properties like the band periodicities (i.e. the Fermi wavevector) and the CDW-gap, which are in agreement with other experimental techniques. Peculiar for one-dimensional systems, we do not observe a real Fermi level crossing of a quasi-particle state. Instead the spectral weight near the chemical potential is strongly suppressed, and we observe a pseudogap in the normal state in both systems. For the organic system our data suggest that electronic correlations are important. Our temperature dependent data further reveal that the pseudogap persists even above the mean field temperature, indicating that CDW-fluctuations cannot account for the pseudogap. For the inorganic compound CDW-fluctuations cannot be neglected in the measured temperature range. Nevertheless the observed energy scare of the pseudogap is much larger than expected from these fluctuations.

Band Mapping and Quasiparticle Suppression in the One-Dimensional Organic Conductor TTF-TCNQ

Dispersing 1D bands have been observed for the first time in an organic conductor by high resolution photoemission experiments on TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane). Their properties are extremely unusual: the bandwidth is much larger than traditional estimates, and the quasiparticle states are strongly renormalized, with no weight at the chemical potential. A deep pseudogap around the Fermi energy persists, and even increases, up to room temperature. We also report a direct determination of ${k}_{F}$ in this material, and the observation of the opening of a Peierls gap in the low-temperature charge density wave phase.

Surface morphology of grown thin films of the quasi one-dimensional organic conductor TTF-TCNQ studied by Atomic Force Microscopy

Thin films of the quasi one-dimensional organic conductor TTF-TCNQ grown on KCl (001) substrates by Chemical Vapor Deposition has been analyzed with Atomic Force Microscopy. The films are polycrystalline, composed of microcrystals with rectangular shape with the c∗ crystallographic axis perpendicular to the substrate. The stepped surface morphology of the microcrystals has been studied. The growth of the films is strongly dominated by an oriented nucleation and the one-dimensional nature of the compound.

Scanning tunneling microscopy study of the one-dimensional organic conductor TTF-TCNQ

Scanning tunneling microscopy investigations of the organic material TTF-TCNQ revealed two different types of atomic structure. The first type of image is in good agreement with previously existing computer simulations. The other type is explained in terms of a novel phenomenon wherein the adsorption of a TCNQ molecule from the sample on the tip changes drastically the observed surface topography. The removal of TCNQ molecules from the surface shows possibilities for nano-indentation on a molecular level.

The evolution of nonlinear electrical transport effects in TTF-TCNQ as driven through CDW commensurability

The b-axis conductivity of the one-dimensional two-chain organic conductor TTF-TCNQ was measured as a function of electric field E for temperatures T ≲ 30 K and pressures P ≳ 24 kbars. At P = 1 bar, TTF-TCNQ undergoes three successive phase transitions : the high temperature T H (incommensurate Peierls) transition involving the electron-like TCNQ l stacks, the lower two transitions (T M,T L) involving the hole-like TTF stacks. We have found at P = 1 bar deviations from Ohm's law, with a threshold field E T, in the neighbourhood and below T H. E T reaches a minimum just below T H before rapidly increasing below T M. At 4 and 9 kbars T M = T L , and hence, the range of T where there is a single incommensurate CDW is large. In this P range, nonlinear effects are observed and the T dependence of E T is less weak below its minimum value than at 1 bar. As the CDW is driven through third-order commensurability at ∼ 19 kbars, we find at constant T/T H a peak in E T and a minimum in the excess current at 30 V/cm. These results are discussed in terms of the depinning of the CDW by the electric field, taking into account the interactions between the two types of chains and the possibility of discommensuration transport near commensurability.

Observation of nonlinear electrical transport at the onset of a Peierls transition in an organic conductor.

The $b$-axis conductivity was measured as a function of the electric field $E$ for temperatures $T&gt;34$ K in the one-dimensional organic conductor TTF-TCNQ (tetrathiafulvalene-tetracyano-quinodimethane). Deviations from Ohm's law, with a threshold field ${E}_{T}$, have been observed for $T&lt;60$ K. ${E}_{T}$ reaches a minimum of 0.25 V/cm just below the onset of the 54-K Peierls transition before increasing to 6.4 V/cm at 34 K. The results are discussed in terms of the depinning of the charge-density-wave condensate at high fields; the interaction between the two types of chains in TTF-TCNQ is included.

NMR-Analysis of Magnetic Properties of TTF-TCNQ

13C-NMR Knight shift data for the organic conductor TTF-TCNQ are used to decompose the total spin susceptibility into contributions coming from individual TTF and TCNQ bands. The difference in elec...

13C NMR studies of TTF (13C)-TCNQ

The 13C NMR relaxation time T1 and Knight shift K have been measured in the organic conductor TTF-TCNQ with 13C-enriched TTF molecules. A clear anomaly in T1 and K shows that the gap in the conducting band of the TTF stack opens only below 38K. No T1 or K anomalies have been observed either at 54 or at 49K. The authors propose a new decomposition of the total spin susceptibility into contributions coming from individual TTF and TCNQ bands, which is based on 13C NMR Knight shift data. The salient new information provided by this work is that the ratio chi Q/ chi F is larger than unity above 60K. The present results have been used to reanalyse the 1H NMR data in the literature. They have reached the conclusion that the TTF band is broader ( approximately 0.65 eV) than the TCNQ band ( approximately 0.40 eV). The observed large enhancement of the Korringa relation suggests that the Coulomb correlation energy U is of the order of the band width for both stacks.