Low temperature EPR study of the “impurity” resonances in (TCNQ)- and (TCNQ)-2 salts

The so-called “impurity” resonances in polycrystalline Li⁺ (TCNQ)^-, Cu⁺ (TCNQ)^- and Cu⁺ (TCNQ)^-₂ have been studied at liquid helium-temperatures. The Li⁺ (TCNQ)^- resonance is partially resolved at low microwave powers with g_⊥ = 2.0025 ± 0.0005 and g_∥ = 2.008 ± 0.001. Itis found that the (TCNQ) and (TCNQ)^-₂ complexes give characteristics spectra. The possible origin of these resonances is discussed.     

Spectroscopic investigation of pressure-induced phase transitions in TCNQ complex salts

In most of the TCNQ complex salts, conduction electrons are localized on specific TCNQ sites, so that these salts show nonmetalic behavior. The caesium salt, Cs₂(TCNQ)₃, is one of the 2:3 complex salts. In the crystal, TCNQ molecules form trimeric units, which consist of two TCNQ radical anion sandwiching a neutral TCNQ along the column. The rubidium salt, Rb₂(TCNQ)₃, also has a similar crystal structure to Cs₂(TCNQ)₃. We measured infrared absorption (IR) and Raman spectra for these salts under high pressure by using a diamond anvil cell. In the case of IR spectra, Cs₂(TCNQ)₃ showed a spectral change probably due to a pressure-induced phase transition. Similar feature was not clearly observed in the Rb₂(TCNQ)₃. On the other hand, the Raman spectra, Cs₂(TCNQ)₃ showed two phase transition at 2.5 and 4.1 GPa in the compression stage. The change from localization phase to delocalization phase occurred at latter transition with large hysteresis. Similar phase transition occurred at 3.2 GPa in the Rb₂(TCNQ)₃. The reason for the difference in transition pressure is that the ion radius of Rb⁺ is smaller than that of Cs⁺, because a small ion radius of the counter ion probably favors the charge localization-delocalization transition of the TCNQ column.     

Temperature and frequency dependence of the nuclear relaxation rate in Qn(TCNQ)2

The magnitude, frequency dependence and temperature dependence of the proton relaxation rates in Qn(TCNQ)₂ are explained in terms of a theory which treats the short wavelength response as coherent and one-dimensional, while the long wavelength response is dominated by anisotropic (intrachain vs interchain) diffusion. The diffusive character of the response near q = 0 is consistent with recent attempts to understand the electronic properties of Qn(TCNQ)₂ in terms of weak localization of states in one-dimension. Analysis of the data leads to the conclusion that the effective screened Coulomb interaction is relatively weak.     

Determination of electron affinity of electron accepting molecules

The unoccupied π ^* states of the solid film of electron accepting organic molecules, 7,7,8,8-tetracyanoquinodimethane (TCNQ), fluorinated TCNQ derivatives, 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TNAP), C₆₀, and 6,6-phenyl-C₆₁-butyric acid methyl ester (PCBM) have been studied by inverse photoemission spectroscopy. The assignment of the π ^* affinity levels of these typical electron accepting molecules provides the basic information for the organic electronics and the new electronic functional molecular design. The comparison with density functional theory calculations enables understanding how the electron affinity evolves in terms of molecular orbitals. The correlation between the film morphology and the irradiation damage on the TCNQ derivative samples by electron impact during the inverse photoemission measurements is also discussed.     

Stabilization variation of organic conductor surfaces induced by π—π stacking interactions

The structures and stabilization of three crystal surfaces of TCNQ-based charge transfer complexes (CTCs) including PrQ(TCNQ)₂, MPM(TCNQ)₂, and MEM(TCNQ)₂, have been investigated by scanning tunneling microscopy (STM). The three bulk-truncated surfaces are all ac-surface, which are terminated with TCNQ molecular arrays. On the ac-surface of PrQ(TCNQ)₂, the TCNQ molecules form a tetramer structure with a wavelike row behavior and a γ angle of about 18° between adjacent molecules. Moreover, the dimer structures are resolved on both ac-surfaces of MPM(TCNQ)₂ and MEM(TCNQ)₂. In addition, the tetramer structure is the most stable structure, while the dimer structures are unstable and easily subject to the STM tip disturbance, which results in changeable unit cells. The main reasons for the surface stabilization variation among the three ac-surfaces are provided by using the 'π-atom model'.     

Temperature dependent microwave dielectric constant of TTF-TCNQ and related one-dimensional conductors

The results of an experimental study of the low temperature (4.2–40 K) complex dielectric constant of the organic conductors (TTF)(TCNQ), (TMTTF)(TCNQ), (DSeDTF)(TCNQ), (TSeF)(TCNQ), and the alloy (TTF)_0.97(TSeF)_0.03(TCNQ) are described. The similar features observed in these different systems suggest that a common mechanism is involved.     

Irreversible quantum mechanics in the neutralK-system

The neutral kaon system is used to test the quantum theory of resonance scattering and decay phenomena. The two dimensional Lee-Oehme-Yang theory with complex Hamiltonian is obtained by truncating the complex basis vector expansion of the exact theory in rigged Hilbert space. This can be done forK ₁ andK ₂ as well as forK _S andK _L , depending upon whether one chooses the (self-adjoint, semibounded) Hamiltonian as commuting or noncommuting withCP. As an unexpected curiosity, one can show that the exact theory (without truncation) predicts long-time 2π decays of the neutral kaon system even if the Hamiltonian conservesCP.     

Spin susceptibility of DMM (TCNQ)2

DMM(TCNQ)₂ crystallizes into both monoclinic and triclinic crystal structures. The TCNQ stacks in the monoclinic structure tetramerized below 260K and the temperature dependence of the susceptibility can be fit to Bulaevskii's theory with an exchange energy of ≈260K. The susceptibility of the triclinic structure is Curie-Weiss from room temperature to 35K where it increases much less rapidly becoming independent of temperature below 6K. This is interpreted in terms of a linear one dimensional antiferromagnetic chain.     

Triplet excitons in (TEA) (TCNQ)2

We report the results of EPR studies on the ionic-radical salt (TEA)⁺ (TCNQ)₂^- composed of an oganic free radical anion and a diamagnetic cation. Between about 40 and 80 K this crystal exhibits the triplet exciton EPR spectrum characteristic of an alternating chain of spins. The triplet spin Hamiltonian parameters are |D| = 44 ± 2 G and |E| = 5.5 ± 1 G. The directions of the zero field splitting principal axes are determined through single crystal rotation studies at 55 K and related to the crystal structure.     

Surface magnetism in organic charge transfer salts

We have investigated the magnetic properties of the charge transfer salts Qn(TCNQ)₂, Cs₂(TCNQ)₃ and TTF—TCNQ in the form of single crystals and after strong pressing or grinding to a fine powder, which introduces lattice defects and increases the surface area. It is found that for the former two compounds pressing or grinding leads to a non-linear, saturating component in the magnetisation field curves, whereas the effect is absent for TTF—TCNQ. It is suggested that this behaviour could arise from strongly coupled localised spins at the surface, i.e. surface magnetism in these materials.     

Spin resonance of tetramethyltetrathiafulvalene tetracyanoquinodimethane: a comparison with tetrathiafulvalene tetracyanoquinodimethane

Electron spin resonance spectra and linewidth studies of two different phases of tetramethyltetrathiafulvalene (TMTTF) tetracyanoquinodimethane (TCNO) are presented. The linewidth of the solution grown 1:1 phase (TMTTF)(TCNQ), which is the structural analog to tetrathiafulvalene- tetracyanoquinodimethane (TTF) (TCNQ), is shown to have a similar temperature dependence to that of (TTF) (TCNQ) in contrast with earlier reports. The vapor grown phase, identified by X-ray studies as (TMTTF)_1.66 (TCNQ)₂, is shown to have different magnetic and electronic properties. These results are discussed and compared with earlier spin resonance reports on (TMTTF) (TCNQ).     

Thermoelectric power of ETPP (TCNQ)2 and DEPE (TCNQ)4

Results of measurements of thermoelectric power of two complexes of TCNQ, namely, ethyltriphenylphosphonium (TCNQ)₂ and 1,2 Di (N-ethyl-4-pyridinium) ethylene (TCNQ)₄ in the temperature range 100–370 K are presented. Over a certain temperature range, thermoelectric power remains independent of temperature suggesting that the most likely mechanism of charge transfer is hopping.     

Effect of pressure on the spin-Peierls temperature

Experiments have shown that a hydrostatic pressure increases the spin-Peierls transition temperature (T_c) in MEM-(TCNQ)₂ while a lowering is observed in TTF-CuBDT. A mean field theory, involving an anharmonic coupling between the distortion and the strain and a possibility for both structural and spin-Peierls instabilities is presented. From this theory, we conclude that, depending on the sign of the coupling energy between the dimerization soft mode and the strain, T_c can increase or decrease with pressure. Without this anharmonic coupling, T_c decreases with pressure.     

Model for conduction in TCNQ salts

We have extended our model for conductivity, σ, and its temperature, T, dependence to a group of molecular conductors including (Qn) (TCNQ)₂, (Adz) (TCNQ)₂ and (Adn) (TCNQ)₂. We have parametrically fit and then quantitatively calculated σ(T) for each of these materials as a product of an activated carrier concentration (600K, 450K, and 350K respectively) and a strongly T-dependent mobility determined by known electron-phonon coupling to the molecular vibrations of TCNQ.     

Nuclear relaxation in a one-dimensional triplet-exciton system

The proton spin-lattice relaxation time has been measured in (?₃AsCH₃)⁺ (TCNQ)^-₂ at 145 K as a function of the frequency. The results are interpreted in terms of interrupted one-dimensional random walk of the excitons along the TCNQ chains. The interruption is discussed in terms of the exciton-exciton collisions.     

Static dielectric constant of disordered organic quasi one-dimensional conductors: Localization by defects

We have measured the low temperature dielectric constant ? of two similar quasi one-dimensional organic conductors, N-Me-iso Qn(TCNQ)₂ and Qn(TCNQ)₂. For N-Me-iso Qn(TCNQ)₂ below 10 K, ? is independent of temperature and is frequency independent in the range 5 × 10⁵ Hz to 9 × 10⁹ Hz, within the 50% experimental uncertainty. Thus we believe the low temperature microwave dielectric constant to be a good approximation of the static value in this salt. For Qn(TCNQ)₂ at low temperatures, the relation ? ∝ (c+c₀)^-2 holds, where c is the defect concentration and c₀ is an effective defect concentration of the nominally pure material. This relation is predicted by the model of interrupted metallic strands with energy spacings larger than kT, and it indicates that electrons are strongly localized by defects along the conducting chains.     

Phase transition in MEM(TCNQ)2 single crystals by infrared reflection spectroscopy

Between 280 and 320 K the MEM⁺ ions in the salt MEM(TCNQ)₂ show an order-disorder phase transition. Moreover MEM(TCNQ)₂ undergoes the phase transition at 335 K. The influence of these two transitions on the i.r. reflectivity spectrum is studied.     

Dielectric permeability of the quasi-one-dimensional charge transfer salts: Qn(TCNQ)2 and NMeAd(TCNQ)2

The dielectric constant and conductivity of the high conductivity organic charge transfer comples salts Qn(TCNQ)₂ and NMeAd(TCNQ)₂ are measured at 9.1 GHz in the temperature range 4–320 K. The large increase with temperature of the dielectric constant is at variance with theoretical models describing the systems as metals or semiconductors at high temperatures. A model of localised states requires unreasonably large dipole moments in the excited state to fit with observations.     

Phase phonons and intramolecular electron-phonon coupling in the organic linear chain semiconductor TEA(TCNQ)2

The vibrational excitations responsible for the remarkable series of infrared absorption bands observed in the organic linear chain semiconductor TEA(TCNQ)₂ are identified to be the phase phonons which result from the coupling of the conduction electron molecular orbital to the totally symmetric vibrations of the TCNQ molecule.