Knight shift contributions

As usual in the early stage of investigation different experiments gave inconclusive results on the question of the nature of gap anisotropy. The low temperature specific heat exhibits a Cs(T) r" power law in a rather reduced range between 0.65 K and 1.2 K which points to some kind of nodal state. In Sb-NQR experiments (Kotegawa et al., 2003) the nuclear spin lattice relaxation l/Ti rate was determined. It has an itinerant quasiparticle contribution that contains information on the SC nodal state below Tc and in addition a localised contribution from broadened CEF excitations which decreases exponentially for temperatures T A. There is no unique way to separate these contributions, this problem is similar to the two Knight shift contributions in the case of UPd2Al3 (sect. 4.2) with its isoelectronic 5f localised states. The NQR measurements did not show ai r evidence for a coherence peak below Tc which points to an unconventional SC state, for lower temperatures an exponential decay of r, , in conflict with the existence of gap nodes was reported. However, this result depends critically on the subtraction procedure of the localised contribution. 

MAS-NMR of 19 samples of GaN nanopowders also characterized by XRPD were prepared by 4 different synthesis methods, and some were subjected to various treatments [234]. It also provides a warning against the attempts to interpret measured chemical shift positions in nano-semiconductors without taking into account the possible contribution of Knight shifts. 

In solid-state NMR [1,51-64], the magnetic coupling between the fullerene anions has to be taken into accoimt. In the case of metal intercalated fullerides that have metallic properties a contribution from the conduction electrons must be added, a phenomenon called the Knight shift . Even if this additional shift affects the C-chemical resonance, the correspondence between extended and discrete systems of comparable Cjq oxidation state is quite close [1]. 

Xd is the electronic contribution to the paramagnetism, obtainable either from the ESR signal or by direct measurements with subtraction of the diamagnetic term RH is the Hall coefficient, the formula being due to Friedman (1971). K is the Knight shift, g is found to drop from 1 to about 0.3 in this range and a is then about 50 2 1 cm x. This then should mark the metal-insulator transition. A plot of g deduced from the various data is shown in Fig. 10.20. This we consider further evidence for the absence of quantum interference in liquids (Section 2). 

In organic metals, the nature of the molecular tt orbitals that form the conduction bands leads to a dipolar hyperfine interaction that may be nonnegligible when compared with the contact contribution discussed above [23]. The various terms in the dipolar interaction modify K and Tx 1 in different ways. The dipolar component [3] can be written as a sum of terms, some of which produce anisotropic Knight shifts (or line broadening in powder samples) and contribute to the spin-lattice relaxation rate. 

Many explanations have been given to account for the departures from the expected enhanced-Pauli Xs in the metallic regime [3]. First, we have to consider if the compound that we are studying has one or two magnetic stacks. If it has two, we should try to separate the contributions from each one. This has been done successfully in a number of cases. The decomposition can be done through the g factor or through the Knight shift. Second, if a chain is metallic, the susceptibility should, in principle, be Pauli-like. If there are localized spins, the behavior should be of Bonner-Fisher type. 

G/kHz). Both samples, however, showed considerable NMR intensity on both sides of the bulk position, contrary to what was found for Pt. Regarding Eqs. (15)-(17), the large difference between the NMR of the two metals is the influence of the orbital contributions. Whereas for they are relatively unimportant so that in the NMR layer model the orbital shift is taken as site independent, this is probably not the case for ° Rh. Indeed, it is well-knowai that the orbital shifts are large for metals in the center of the transition series, whereas the (pure) Knight shift is more important toward the end. 

Knight shifts. When the orbital contributions arc more important, the resulting resonance will occur to lower field, and when the spin value is larger the resonance shifts to higher field. In principle, this increased sensitivity to the orbital shift could be interesting in chemisorption studies. No such work has been published to date. 

If the symmetry of the site is lower than cubic the full tensor form of the electron-nucleus interaction needs to be used, so that in addition to an isotropic term there is an anisotropic contribution. If in the PAS of the Knight shift tensor the components of the tensor are Kx, Ky and Kz, then in the laboratory frame with its orientation in the frame defined by Bo described by the Euler angles 0 and [Pg.49]

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