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Molecular Design of Organic Conductors
Published in Jean-Pierre Farges, Organic Conductors, 2022
Vladimir Khodorkovsky, James Y. Becker
The results from research on donor-acceptor systems accumulated to date show that the existence of partial charge transfer is a prerequisite for conducting CT complexes and ion-radical salts. In general, weak π-π complexes with a degree of charge transfer less than 0.4 exhibit semiconducting and photo-semiconducting properties. Ion-radical salts with a degree of charge transfer more than 0.7 are usually insulators. The degree of charge transfer can be determined experimentally by several methods, such as dipole moment measurements, nuclear quadrupole resonance, x-ray electronic spectroscopy, IR spectroscopy, and so on. Such relevant data and methodology have been reviewed and discussed in the literature (e.g., Ref. 92). The charge-transfer complex of TTF with tetracyanoquinodimethane (TCNQ) was historically the first organic metal-like conductor obtained [1,2] and the degree of charge transfer in this complex was found to be 0.59 [93]. Selected examples of the degree of charge transfer in tetrathiafulvalene (TTF) complexes with different acceptors are listed in Table 11.
Nonequilibrium Properties: Comparison of Experimental Results With Predictions of the BCS Theory
Published in R. D. Parks, Superconductivity, 2018
To make the measurements in the superconductor, a field-cycling method was used. An external field He > Hc(T) was applied to drive the sample normal during initial saturation of the nuclear quadrupole resonance transition. He was then quickly reduced to zero, and the quadrupole system was allowed to relax in the superconducting state for a variable time t, during which a portion of the signal would recover. The signal amplitude was then measured in the normal state after reapplication of He. Spin-echo techniques were used to observe the signal because of appreciably inhomogeneous broadening of the nuclear quadrupole line. The observed relaxation rate was found to be independent of He in the normal state and was independent of He in the superconducting state for He > Hc(T).
Electron states and bands in the cuprates
Published in J. R. Waldram, Superconductivity of Metals and Cuprates, 2017
The magnetic fluctuations revealed by the neutron scattering interact also with nuclear spins, and in nmr and nqr (nuclear quadrupole resonance) are responsible for the decay rate 1/T1 of the nuclear spin component in the direction of the effective static field. For the fluctuations in an ordinary Fermi-liquid metal this relaxation rate is proportional to temperature, so that 1/T1T should be a constant (the Korringa relation). Figure 13.9 shows values of this quantity for ybco and for La/Sr cuprate at various dopings. The Korringa relation is not obeyed. It is clear that we have an extra scattering rate, presumably due to the additional antiferromagnetic fluctuations, which as expected decreases when we move away from the afm state and the range of magnetic coherence decreases. It also decreases when the temperature rises, showing that thermal excitations have the effect of reducing the antiferromagnetic fluctuations.
Spin–spin relaxation of nuclear quadrupole resonance coherences and the important role of degenerate energy levels
Published in Molecular Physics, 2020
Christian Gösweiner, Per-Olof Westlund, Hermann Scharfetter
Another, less popular method to access nuclear spin relaxation is nuclear quadrupole resonance (NQR) spectroscopy [9,10]. In contrast to NMR, where mostly protons are in the centre of interest, in NQR nuclei with spin number in solids are addressed directly and the application of an external magnetic field is not necessary. Instead, an electric interaction between the quadrupole moment of the nucleus and an electric field gradient (EFG), produced from the surrounding atoms and molecules, is the origin of discrete spin states. Though the measurement procedure is closely related to NMR where the application of a sequence of radio frequency pulses redistributes the occupation of spin states. High spin nuclei exhibit a richer and also more complex energy level system which requires a very careful treatment when applying quantum mechanical models as, e.g. Redfield theory [11]. As NQR is much less extensively used than NMR techniques, there are no standard procedures available as, e.g. the intensively used Solomon–Bloembergen–Morgan (SBM) equation for the field-dependent R relaxation of protons in paramagnetic systems [12–14]. Several works treat temperature dependence of R relaxation of pure NQR transitions [15–18], but less studies were performed on the coherence decay (R) or the lineshape of NQR spectra [19]. Though, the total experimental lineshape carries information of quite practical nature, as e.g. crystal homogeneity, or nanoparticle size, which is connected with the EFG distribution [20–23]. However, one can only address this issue trustfully when the portion of line broadening due to dephasing and finite lifetime of spin states is exactly known.