Conduction electron spin

The issue of defects in nanotubes is very important in interpreting the observed properties of nanotubes. For instance, electronic and magnetic properties will be significantly altered as is already clear from observation of the conduction electron spin resonance]20,23]. 

Some metals are diamagnetic because the conduction electron spin susceptibility is smaller than the induced diamagnetic susceptibility component. On the other hand, various rare earth metals display very strong paramagnetism because of unpaired / electrons that remain associated with individual atoms rather than entering into energy bands. 

Reactions of KI with Cl2 and of NaCl and KC1 with F2 have been studied [92, 93, 103, 104] and a variety of properties have been measured as a function of the extent of reaction in the conversion of CuCl to CuCl2 by reaction with Cl2 [89, 105, 106]. A consistent feature of all these studies is that, together with or sometimes preceding nucleation and growth of new solid phases, the solid reactant is damaged by electron withdrawal, leading to the formation of various types of electronic defect of the trapped-hole or V-centre type. Studies of electrical conductivity, electron spin resonance spectra and UV absorption are useful in elucidating the behaviour of these defects. 

EPR measurements were first performed on wurtzite GaN in 1993 by Carlos and co-workers [2-4] and on cubic GaN by Fanciulli and co-workers at about the same time [5], The primary resonance in the wurtzite films is slightly anisotropic (gy = 1.9510 and gi = 1.9483) with a width 0.5 mT at 4.2 K and generally acknowledged to be due to a band of delocalised effective mass (EM) donor electrons. The average g value is consistent with the expectations of a 5-band k.p analysis and is also similar to that obtained by Fanciulli [5] for a much broader line (—10 mT) in their conduction electron spin resonance experiments on zincblende films. With this exception all of the work discussed in this Datareview is on the wurtzite phase. 

The direct synthesis by anodic oxidation of a new series of electrically conducting poljnners is described.. Our polymers derive from sulfur and/or nitrogen containing hetero-cycles such as 2-(2-thienyl)pyrrole, thiazole, indole, and phthalazine. The anodic oxidation of these monomers is carried out in acetonitrile solutions containing tetrabu-tylammonium salts (TBA X ) ith X = BF, tetraethylammonium salt, TEA H C-C H -S0. Characterization of the materials by electrical conductivity, electron spin resonance, uv-visible spectroscopy, and cyclic voltammetry is discussed. 

The magnetic interaction between the ions in the magnetic metals for example, can then be considered as carried by the conduction electrons in the well known Rudermann-Kittel-Kasuya-Yoshida (1 ) interaction. The physical origin of this interaction is a point like polarization of the conduction electrons (CE), at the atomic sites, by the magnetic moments of the f electrons, resulting in an oscillation of the spin density of the CE. The point like approximation is useful because the maxima of the f wave functions are found well inside the atomic core, in radii smaller than 0.7 atomic units. This polarization is carried from ion to ion by the generated polarization oscillation of the conduction electron spins, which has a wave length = 27r/e (C =... 

The effective magnetic field, from the orbital part onto the conduction electrons, can be approximated from the spin orbit splitting but it should also be remembered that the effective field could be larger because the forces between the f-electron spin and the conduction electron spins, will add to the dipole field of the orbital moment. To illustrate the effect of this dynamic effective mass m+ we present the change in the f-band width of Ce in YCe as a function of the effective mass. Figure 2. Other calculations supporting this analysis will be published elsewhere. 

Since K depends on the wavefunction density at the nucleus, the effect is dominated by s-electrons which is certainly true in metals with unpaired s-electrons. If the Pauli susceptibility and electron density can be independently measured then the Knight shift will give an independent measure of the s-component of the conduction electron spin density. These shifts are positive and are much larger than chemical shift effects, some typical values being Li — 0.025%, Ag — 0.52% and Hg — 2.5%. In other metals the situation is more complicated when the s-electrons are paired but there are other electrons (e.g. p but especially d). As only s-electrons have significant density at the nucleus the effects of these other electrons are much smaller. The hyperfine fields of these electrons induce polarisation in the s-electrons that subsequently produce a shift, termed core polarisation. 

An underlying question in many of the ESR measurements of the lineshape of the N-donor and its temperature dependence is the possible existence of an additional broad line at a similar g-value to the three-line spectrum. Most authors agree that the three lines in the isolated N-donor spectrum should have symmetric lineshapes and, since there is a slight asymmetry to the full spectrum, this leads to the conclusion that there is another relatively broad ESR line shifted slightly from the three-line spectrum. This could be due to a second donor, a conduction electron spin resonance or a structural defect. A weak signal, possibly due to a Si vacancy, is also observed at higher temperatures (T > 50 K) in some samples. 

The Fe-Mn alloys (2-8-6-7 at. % Mn) have many similarities to the Fe-Al system [36], with the exception that the conduction-electron spin-density oscillations show a phase variation with concentration corresponding to a change in the Fermi radius and band structure (see Figs. 11.5 and 11.6) [32]. In this case the Mn atoms are contributing to the.magnetic structure. 

Particular interest has been expressed in the Co/copper system because of the existence of a bound state formed between the localised magnetic impurity moments and the conduction-electron spins below a critical temperature [88]. The impurity moment is significantly quenched by spin compensation from the conduction electrons below the critical temperature, but this bound state can be perturbed by application of large external magnetic fields. 

The frequency dependence of NMR spin-lattice relaxation is also a powerful tool in the study of spin dynamics in tlie conducting polymers, to reveal the microscopic dynamics of the charge carrier, polaron and/or conduction electron spins. However, one has to pay attention to one important point the localized paramagnetic spins and molecular motion which substantially contribute to the NMR 77]]. Since the relaxation caused by the spins diffusing is suppressed by a factor of D, the contribution due to the localized spins and the molecular motions could become dominant [6]. Therefore, interpretation of the NMR relaxation rate is generally useful but difficult. 

Recently, both polycrystalline and microcrystalline ( 200 A crystallite size) Si films have been investigated by ESR (Hasegawa ft a/., 1981b, 1983b). A dangling bond resonance in microcrystalline silicon films is observed at g = 2.0049. The spin density of this resonance is typically between 10 and 10 spins cm. In P-doped samples a conduction-electron spin-resonance signal is observed at = 1.997. 

Reduction does not stop at the ionic cluster level, but goes all the way to metallic particles. The latter is characterized by an isotropic conduction electron spin... 

Table 6. Conduction electron spin resonance signals in zeolite X and Y ...

The Peierls instability and the high degree of one-dimensionality are observable in a whole series of different experiments. These include the dc conductivity in low applied fields (see Sect 9.6.1 and Fig. 1.13), the diffuse reflections of the Ikp superlattice in X-ray scattering (Sect 9.6.2), the reflection spectra from the FIR up to the UV spectral ranges (Sect 9.6.3), the magnetic susceptibihty (Sect 9.6.4), the conduction electron spin resonance, and nuclear resonances (Sect 9.6.5), as well as the nonlinear electrical conductivity at high apphed electric fields or at high frequencies (Sect. 9.6.6). Most of these methods are also employed for the study of the other radical-ion salts, e.g. TTF-TCNQ or the DCNQl salts. They will therefore be treated as examples in this Sect 9.6. 

Fig. 9.22 The conduction-electron spin-echo amplitude A(r) from a (Fa)2Pp6 crystal in a constant magnetic field Bq with a constant magnetic-field gradient G Bo at room temperature. For the orientation Bq T a, one observes a mono-exponential decay for Bq a, the spin diffusion along the stacking axis a causes an additional decay as in Eq. (9.26). From [34].

As is evident from a comparison of eqs. (lid) and (lie), there is also a direct empirical access to the conduction electron spin polarization, if magnetic ordering temperatures and transferred hyperfine fields at the R site are compared. Both quantities contain the spin polarization at the R site, induced by all the other R electronic spins... 

The second type of bistability is intrinsic to the conduction electron spins. Under saturation conditions, Overhauser effect is the primary condition for the observation of bistability. Under such a condition, the effective magnetic field, and hence the nuclear field takes two stable steady-state values for a given value of the external magnetic... 

---