Interchain Transfer Integrals Research Articles

Low-Symmetry Gap Functions of Organic Superconductors

Superconducting gap functions of various low-symmetry organic superconductors are investigated starting from the tight-binding energy band and the random phase approximation by numerically solving Eliashberg’s equation. The obtained singlet gap function is approximately represented by an asymmetrical \(d_{x^{2} - y^{2}}\) form, where two cosine functions are mixed in an appropriate ratio. This is usually called d + s wave, where the ratio of the two cosine functions varies from \(1:1\) in the two-dimensional limit to \(1:0\) in the one-dimensional limit. A single cosine function does not make a superconducting gap in an ideal one-dimensional conductor, but works as a relevant gap function in quasi-one-dimensional conductors with slight interchain transfer integrals. Even when the Fermi surface is composed of small pockets, the gap function is obtained supposing a globally connected elliptical Fermi surface. In such a case, we have to connect the second energy band in the second Brillouin zone. The periodi...

Tuning the Magnetic Dimensionality by Charge Ordering in the Molecular TMTTF Salts

We theoretically investigate the interplay between charge ordering and magnetic states in quasi-one-dimensional molecular conductors TMTTF(2)X, motivated by the observation of a complex variation of competing and/or coexisting phases. We show that the ferroelectric-type charge order increases two-dimensional antiferromagnetic spin correlation, whereas in the one-dimensional regime two different spin-Peierls states are stabilized. By using first-principles band calculations for the estimation for the transfer integrals and comparing our results with the experiments, we identify the controlling parameters in the experimental phase diagram to be not only the interchain transfer integrals but also the amplitude of the charge order.

Deconfinement transition and dimensional crossover in the Bechgaard-Fabre salts: Pressure- and temperature-dependent optical investigations

The infrared response of the organic conductor (TMTSF)$_2$PF$_6$ and the Mott insulator (TMTTF)$_2$PF$_6$ are investigated as a function of temperature and pressure and for the polarization parallel and perpendicular to the molecular stacks. By applying external pressure on (TMTTF)$_2$PF$_6$, the Mott gap rapidly diminishes until the deconfinement transition occurs when the gap energy is approximately twice the interchain transfer integral. In its deconfined state (TMTTF)$_2$PF$_6$ exhibits a crossover from a quasi-one-dimensional to a higher-dimensional metal upon reducing the temperature. For (TMTSF)$_2$PF$_6$ this dimensional crossover is observed either with increase in external pressure or with decrease in temperature. We quantitatively determine the dimensional crossover line in the pressure-temperature diagram based on the degree of coherence in the optical response perpendicular to the molecular stacks.

Computational Study of the Structure and Charge-Transfer Parameters in Low-Molecular-Mass P3HT

Using classical molecular dynamics simulations and quantum chemical calculations, the structure and charge-transfer parameters in crystalline poly(3-hexylthiophene) (P3HT) are investigated. The changes in polymer structure with temperature are studied and, by performing DFT calculations on configurations found from MD, the changes in the charge-transfer characteristics are investigated. The system is found to adopt a structure consistent with X-ray diffraction experiments on the so-called type-II polymorph of the poly(3-alkylthiophenes). Upon increasing temperature, a conformational change in the polymer side chains occurs, which is found to lead to increased disorder in the interring torsions, which modulates the charge transfer along the polymer backbone. The intrachain transfer integrals are found to decrease slightly with temperature, while their distribution broadens considerably due to increased thermal motion of the rings. The interchain transfer integrals are found to be appreciable for both nearest and next-nearest neighbor rings. This, taken with the fact that the positions of rings on neighboring chains are strongly correlated, has consequences for the development of more accurate phenomenological charge-transport models, such as variable range hopping models.

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.

Effect of an external electric field on the charge transport parameters in organic molecular semiconductors

Internal reorganization energies and interchain transfer integrals are two key parameters governing the charge-transport properties of organic semiconducting materials. Here, in order to model some aspects of device operation in field-effect transistors based on conjugated oligomers, we investigate via semiempirical quantum-chemical calculations the way these two parameters are modified when a static electric field in the range 106–108 V/cm is applied along the long axis of pentacene and sexithienyl molecules. For the highest fields, a pronounced redistribution of the charges along the oligomer chains occurs, which is accompanied by significant geometric distortions. However, these charge redistribution effects are found not to impact significantly the transport parameters.

Correlation-induced dimensional crossovers of charge-transfer excitations in quasi-one-dimensional organic conductors

Applying the finite-temperature density-matrix renormalization-group method to the spinless fermion model on a two-leg ladder, we have calculated the dynamical structure factors for the local charge transfer processes along and across the chains. The intra-chain excitation spectra are sensitive to the inter-chain transfer integral and the chemical potential, while the inter-chain excitation spectra are sensitive to the intra-chain electron correlation. These dynamical properties are due to the collective motions of fermions along the chains and the energy-dependent confinement of fermions in the chains. At low but finite energies, the effects of the increasing inter-chain transfer integral on the local charge-transfer spectra are similar to those of the chemical potential deviating from zero.

Collective excitations and confinement in the excitation spectra of the spinless fermion model on a ladder

Intrachain and interchain local charge-transfer excitation spectra and the single-particle density of states are calculated in the spinless fermion model on a ladder with varying intrachain nearest-neighbor repulsion and interchain transfer integral at and near half filling by using the finite-temperature density-matrix renormalization-group method. Collective excitations are found to govern the low-energy intrachain spectra, while only individual local excitations are present in the interchain spectra. For strong intrachain repulsion, the low-energy motion of fermions is confined within a chain. The interchain motion of fermions is not bandlike but incoherent. As a consequence, the low-energy intrachain spectra are sensitive at half filling to the interchain transfer integral that weakens the density-density correlation along the chains. Similarly, the low-energy intrachain spectra are sensitive near half filling to the chemical potential that reduces the effect of the umklapp process. Similarities are pointed out between these findings and the experimentally observed, optical conductivity spectra in the quasi-one-dimensional organic conductors $(\mathrm{TMTSF}{)}_{2}X.$

Pseudogaps in nearly one-dimensional metals

The linear chain conductors called the Bechgaard salts are materials where the electronic band is half filled, electron–electron interactions are moderate and the interchain transfer integral is varied from salt to salt. Spectroscopic evidences point to a transition from a (1D) insulating, Mott–Hubbard (MH) state to a conducting state when the interchain transfer integral exceeds the MH pseudogap. The conducting state is that of a correlation induced semimetal with the majority of charge excitations gapped, and with charge–spin separation. Spectroscopic evidences also point to a Tomonaga–Luttinger liquid at energies exceeding the interchain transfer integral.

Photoexcitations in polythiophene at high pressure.

We report optical-absorption, photoluminescence (PL), and picosecond photoinduced absorption (PA) decay in films of poly-3-hexyl-thiophene at pressures up to 80 kbar. The spectral bands redshift nonlinearly with pressure and the PL intensity decreases markedly. Thermochromic transitions are completely inhibited at pressures as low as 14 kbar. The picosecond recovery of the PA decay at high pressure is similar to that of unpressed polythiophene, but has a power-law exponent consistent with more ordered chains at high pressure. These effects suggest changes with pressure in the chain conformation and in the electronic polarizability; no changes in the interchain transfer integral are observed.

Electrical conduction in polyaniline compressed pellets doped with alkylbenzenesulfonic acids

Anion dependent processes of the electrical conduction in chemically prepared polyaniline compressed pellets doped with alkylbenzenesulfonate anion of alkyl chain length 1, 8, and 12 have been investigated by the measurement of the temperature dependence of the conductivity. The conductivity of the samples changes with the temperature as T−1/2, which shows the conduction followed a one dimensional variable range hopping process. With the increase of the alkyl chain length of the dopant anion, the conductivity of the heavily doped 50 mol % sample decreases in whole temperature range, but an exponential factor of the variable range hopping remains unchanged. The results concerning the alkyl chain length of the dopant anion have been discussed from the ‘‘quasi-one dimensional’’ variable range hopping mechanism and demonstrated the decrease of the transverse localization length and the interchain transfer integral of electron.

Influence of temperature on the band structures of (TMTSF) 2X and (TMTTF) 2X (X − = Tetrahedral anion).

The crystallographic structures under constraints of representatives salts of the title families lead to unexpected variations of the corresponding band structures and Fermi surfaces. We discuss in this study the role of the anion size for X −=ClO 4 −,ReO 4 − on the transfer integrals magnitude at different temperatures. For (TMTSF) 2ReO 4 at 125K, the two inequivalent dimers resulting from the anion ordering along a lead to a calculated site energy difference of 72 meV. For the same salt, it is found that in the 125K averaged structure, the interchain transfer integrals are very close to the (TMTSF) 2ClO 4 ones. Thus, it would be mainly the a -axis anion ordering which prevents at low T and ambient pressure the appearance of SC in the ReO 4 − salt.

Spin-orbit interaction effects in quasi-one-dimensional conductors

The author studies the effects of the spin-orbit interaction on the competing pairing states of a model quasi-one-dimensional conductor with hopping-type interchain coupling. Within a ladder approximation scheme use of the renormalisation-group method allows him to account explicitly for anisotropy effects involving the intrachain and interchain transfer integrals. Parametrising the spin-orbit interaction by a backward, g1n<0, and forward, g2n<0, scattering coupling constant, he concludes that its main effect is to stabilize the triplet-pairing states of spin density wave and triplet superconductivity over the corresponding singlet-pairing states of charge density wave and singlet superconductivity. In particular the boundary line between the density-wave-type states and the superconducting states is determined as: g1+g1n=2g2, where g1 and g2 are the usual spin-independent backward- and forward-scattering coupling constants. The boundary line between the triplet- and singlet-pairing states is determined as: 2g1=g1n+g2n- mod g1n-g2n mod . The direction of the triplet-pairing polarization axis m with respect to the spin-orbit-anisotropy axis n is shown to be dependent only upon the sign of the difference: g1n-g2n. The implications of these results of the (TMTSF)2X and (TMTTF)2X families of organic conductors is discussed. Finally, it is shown that the spin-dependent exchange interaction introduced recently by Abrikosov (1983) can be reduced to a simple definition of the spin-independent couplings g1 and g2. He concludes that a ferromagnetic type of exchange also favours the triplet-pairing states over the singlet-pairing states.

Pressure dependence of the photoabsorption of polyacety-lene

Abstract The optical absorption spectra of pure cis and trans-polyacetylene at various hydrostatic pressures, in the region 0–9 Kbar, are reported. We find a “red shift” of the absorption edge indicating a reduction of the energy gap as a function of pressure. At 9 Kbar, E is reduced by about 0.1 eV. In addition a decrease ing the slope of α (ω) is observed at high pressures. These changes result from an increase in the interchain transfer integrals, t1. Comparison of the data with the results of tight binding calculations lead to estimates of t1≃ 0.025 − 0.05 eV (ambient) and t1/t1≃ 10−2.