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.     

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.     

Configurational entropy in the organic conductor quinolinium (tetracyanoquinodimethane)2

The thermoelectric power (TEP) measurements as a function of temperature and concentration of defects are reported on neutron irradiated organic quasi-one-dimensional conductor Qn(TCNQ)₂. The temperature dependence of the TEP suggests a freezing out of the entropy due to short range interactions, while the concentration dependence is shown to be due to the change in configurational entropy.     

Defect dependence of the dielectric permeability of Qn(TCNQ)2

The conductivity and 9.1 GHz dielectric constant of neutron irradiated Qn(TCNQ)₂ single crystals are strongly affected by defect concentrations less than 1 %. This indicates long range electronic coherence of the order of 100 lattice constants in the non-irradiated material for which the coherence length may still be impurity limited. It is suggested that a collective excitation of electrons is responsible for the temperature dependence of the large dielectric constant.     

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.     

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.     

Intrinsic and extrinsic magnetic susceptibilities of some TCNQ complexes

The magnetic susceptibilities (χ) of quinolinium·(TCNQ)₂, N-methyl phenazinium·TCNQ and Li·TCNQ were measured from 2 to 300 K and are discussed in connection with the low-temperature specific heats (C) measured by other authors, χ is decomposed into three parts: χ_d the temperature-independent part, χ_c, Curie-Weiss type paramagnetism, and χ_p, the remainder. Correspondingly, C is composed of three terms, γT, H/T² and αT³. The electronic state of these substances is discussed in terms of each type of susceptibility.The model, on which the above separation of χ and C is based, defines two types of electrons: localized electrons associated with a magnetic moment and band electrons. Though this model is useful phenomenologically, it is shown that the analysis of χ on the basis of this model indicates less band electrons and more localized electrons or stronger magnetic interactions than does that of C.     

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.     

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).     

Angular variation of ESR linewidth in organic ionic radical salt: 3,3′-diethyl-2,2′-selenacyanine-[TCNQ]2

Angular dependence of ESR linewidth of 3,3′-diethyl-2,2′-selenacyanine-[TCNQ]₂ single crystal reveals the two-dimensional magnetic nature of the system. The dependence is represented well by ΔH(θ)=α(3 cos²θ?1)²+β sin⁴θ where α and β are temperature-dependent parameters.     

ESR studies on ferromagnetic-spin dynamics in a nearly two-dimensional organic ionic radical salt: 1-methyl-3'-ethyl-2, 2'-quinoselenacy anine-[TCNQ]2

The angular dependence at 77 K of the ESR linewidth of 1-methyl-3'-ethyl-2, 2'-quinoselenacyanine-[TCNQ]₂ single crystal is well expressed by ΔH(θ) = 2.2(3 cos²θ-1)²Oe. The results reveal the two-dimensional ferromagnetic nature of the system.     

2-3-Benzacridinium (TCNQ)2: A small bandgap semiconductor

Single crystal conductivity and thermoelectric power experiments on 2-3-BAd(TCNQ)₂ are interpreted in terms of a highly anisotropic band semiconductor model. This behaviour is contrasted to that found in Ad(TCNQ)₂ and is claimed to be due to the difference in the dipole moments of the donors.     

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.     

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.     

Specific heat and magnetic susceptibility at low temperature of two conductive TCNQ salts

The specific heat measurements between 1.4 and 4.4 K of acridinium (TCNQ)₂ and quinolinium (TCNQ)₂ salts show up a linear component; moreover, in presence of a strong magnetic field (H = 40kG), an artificial Schottky anomaly is revealed. Magnetic susceptibility experiments confirm the simultaneous existence of charge carriers, in a partially filled energy band, and localized paramagnetic centers. A standard energy band model is proposed to interpret these two properties.     

Observation of quasi-universal magnetic behavior in a random exchange Heisenberg antiferromagnetic chain: Neutron irradiated quinolinium (TCNQ)2

Low field electron spin resonance measurements of the magnetic susceptibility (χ) and absorption linewidth over the temperature range 0.04 – 300 K are reported for quinolinium (TCNQ)₂ into which increased amounts of disorder have been introduced by fast neutron irradiation. It is found that below 20 K, χ = AT^-α; A increases linearly with the irradiation dose, but $\text{α (? 0.8)}$ is almost independent of it, in agreement with the quasi universal behavior predicted by recent renormalization calculations for a random exchange Heisenberg antiferromagnetic chain. Measurements of the g-shift at 4.2 K range indicate that all of χ is associated with TCNQ chains. These results are discussed in terms of the renormalization calculation of Soos and Bondeson.     

Angular variation of ESR linewidth in organic ionic radical salt: 1,3'-diethyl-2,2'-quinoselenacyanine-[TCNQ]2

Angular dependence of electron-spin-resonance (ESR) linewidth in 1,3'-diethyl-2,2'-quinoselenacyanine-[TCNQ]₂ single crystal reveals the two-dimensional magnetic nature of the system, where the nearly uniform distribution of magnetic-spins on the TCNQ layers is inferred.     

Infrared reflectivity of MTPA (TCNQ)2 single crystals

Polarized infrared reflectivity of single crystal MTPA (TCNQ)₂ was measured in the 40–4000 cm^?1 region and evaluated to obtain the dielectric function and conductivity. For E ⊥ b polarization, a very strong coupling between TCNQ intramolecular vibrations and electronic motion is observed. The bare electronic absorption is modelled by a sum of two classical oscillators.     

Self-induced generation of an off-axis frequency shifted radiation from atoms

The transient resonant linear response at wavelength λ_a of an N two-level atom vapor driven by a strong pulse with wavelength λ_f = λ_a - |Δλ| is shown to promote an emission of radiation peaked at wavelength λ_c = λ_a + |Δλ| in a conical shell around the propagation axis of the incident beam. In the limit of weak excitation, i.e. for an incident Rabi frequency much smaller than the detuning, the cone angle is found to be equal to 2λμ(2N/ch|Δλ|)^{$\text{1}\text{2}$} where μ is the transition dipole moment.     

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.