Spin fluctuator

M. Uhl and J. Kiibler, Phys. Rev. Lett., 77, 334 (1996). Exchanged-Coupled Spin-Fluctuation... 

Spin-spin fluctuations can compete with spin-lattice effects an energy hwic can be supplied by a phonon as well as by a spin fluctuation in the dipolar field. A simple thermodynamic view is shown in Fig. 8. For convenience only two distinct carbon... 

In organic solids the determination of rotating frame relaxation is severely complicated by the presence of the strongly interacting proton spin system. Spin-spin fluctuations compete with spin-lattice fluctuations to produce an effective relaxation time large rf field amplitudes are mandated to discriminate against the spin-spin event. The burden of proof lies with the experimenter to establish that a rotating frame relaxation rate actually reflects a motional effect seen by the carbon nuclei. 

Ito H, IshUiara T, Tanaka H, Kuroda S, Suzuki T, Onari S, Tanaka Y, Yamada J, Kikuchi K (2008) Roles of spin fluctuation and frustration in the superconductivity of 8-(BDA-TTP)2X (X = SbEg, AsEg) under uniaxial compression. Phys Rev B 78 172506/1-4... 

For solids in which IN([Xf) is very near to 1, often, although no magnetic order occurs, long-range fluctuations of coupled spins may take place, giving particular form to properties such as the (Stoner enhanced) magnetic susceptibility x, the electrical resistivity, and the specific heat of the solid. Spin fluctuations have been observed in actinides, and will be discussed in more detail in Chap. D. 

However, the intra-atomic Coulomb interaction Uf.f affects the dynamics of f spin and f charge in different ways while the spin fluctuation propagator x(q, co) is enhanced by a factor (1 - U fX°(q, co)) which may exhibit a phase transition as Uy is increased, the charge fluctuation propagator C(q, co) is depressed by a factor (1 -H UffC°(q, co)) In the case of light actinide materials no evidence of charge fluctuation has been found. Most of the theoretical effort for the concentrated case (by opposition to the dilute one-impurity limit) has been done within the Fermi hquid theory Main practical results are a T term in electrical resistivity, scaled to order T/T f where T f is the characteristic spin fluctuation temperature (which is of the order - Tp/S where S is the Stoner enhancement factor (S = 1/1 — IN((iF)) and Tp A/ks is the Fermi temperature of the narrow band). 

Many light actinide alloys which are not magnetic have a T dependence of the resistivity at low temperature as well as a large electronic specific heat coefficient y (Table 4). However, the archetype of a spin fluctuation system is UAI2. The electrical resistivity is proportional to T with a very large coefficient a = 0.15 qQcm/K up to 5... 

Table 4. Actinide intermetallics showing spin fluctuations behaviour...

Resistivity has proved to be a very sensitive probe to the possibilities of magnetism contrary to the susceptibility which samples the spin fluctuation propagator (see Sect. Ill) only for q = 0, the electrical resistivity integrates over all q values. 

For spin fluctuation systems, the best example is again UAl2 In fact as shown in Fig. 3 it is the unique example where T InT dependence at low temperature, predicted by spin fluctuation theory, has been in practice observed (very recently, such behaviour... 

However, no fingerprint such as a final state multiplet structure, as expected for fully localized 5 f electron and found in Am metal, is observed. The localization of 5 f-states is only weak (band-narrowing and widthdrawal from hybridization) in 6-Pu. This in fact is also consistent with the absence of a magnetic moment formation in 6-Pu (rather, a spin-fluctuation regime is observed ). 

In UPts, a compound for which spin-fluctuations are known to exist, superconduction has recently been reported There is an apparent contradiction with the classical Bar-deen-Cooper model for superconductivity, in which superconduction is hindered by the onset of ordered magnetic phenomena, and is usually not found in very narrow bands. UBei3 which has a y of 1000 mg/mol K, exhibits similar behaviour. 

If the material is ferromagnetic then the entropy, susceptibility and resistance at temperatures just above the Curie point are to be calculated in much the same way (i.e. in terms of spin fluctuations). A treatment of this problem starting from the Stoner-Wohlfarth model is due to Moriya and Kawabata (1973). 

Our model for the density of states is thus as in Fig. 4.7. The total density of states is mainly due to spin fluctuations, and has a maximum for n=1, where n is the number of electrons per atom. The curve for current carriers needs to be used for calculating thermopower and resistance the experimental evidence discussed in the following chapters suggests, however, that the Hall coefficient RH is given by the classical formula 1 jnec. 

These questions were resolved with the use of the same relatively simple epoxy system. All C-13 nuclei in contact with the proton bath were counted when moderate spinning rates were used and in spin-lock cross polarization in rf fields not close to any Tle minimum. The molecular motion determines the relaxation rate, under the Hartmann-Hahn condition when T, = T2. The spin-spin effects determine relaxation when Tle does not equal T2 under the same conditions 62). The spin-spin fluctuations are in competition with the spin-lattice fluctuations in producing an effective relaxation time. To discriminate against the spin-spin fluctuations large rf fields are mandatory. It was pointed out that, with great care, C-13 NMR spectra can reflect molecular motion. 

In the second, some intermonomer spin fluctuation is introduced by combining two n.n. triplets into a singlet the corresponding operator is... 

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