Spin-cloud model

Certainly, the spin-cloud model is based upon an oversimplified picture of spin glasses. It provides qualitative descriptions of several spin-glass properties, but thereby often unphysical fitting parameters are obtained. It is a correct, microscopic description of superparamagnetism, however, in spin glasses no evidence for independent domain structure is available. Let us give some examples to these statements. 

Finally, we refer to work by Alloul (1979a, b) on zero-field NMR on Cu in CuMn alloys (Mn concentrations from 0.4% to 4.7%) at low temperatures T 0.2 T(, who finds no evidence for independent domain structure. The enhancement factor 7] of the rf-field and NMR signal intensity, associated with the rotation of domain magnetization, is found roughly proportional to the remanent magnetization and is negligible when the sample is ZFC. These results contradict the independent spin-cloud model, for which 17 should be independent of M. A similar conclusion has also been reached from the square hysteresis loops observed well below Tf in CwMn (Monod et al. 1979) and is corroborated by computer simulations of CwMn alloys (Walker and Walstedt 1980). 

In summary, the theoretical model makes predictions for weakly doped cuprates for temperatures up to T where the trends are in remarkable agreement with experimentation. Our end conclusion is that high temperature superconductivity is primarily an electron coneladon effect possibly supplemented by longer range polaronic attraction of the type discussed by Mott and Alexandrov (see [8,9] for other references). The spin gap , which is a widely recognised property of cuprates, and which is resposible for much of the magnetic susceptibility above Tc is possibly due to removal of the antiferromagnetic spin cloud which dresses layer electrons but a detailed theory remains to be worked out... 

Blewitt, D. N., J. F. Yohn, R. R Koopman, and T. C. Brown. 1987a. Conduct of Anhydrous Hydrofluoric Acid Spin Experiments, in American Institute of Chemical Engineers, Proceedings, International Conference on Vapor Cloud Modeling, Boston, November 2-4. 

Figure 1-3. Electron pair cloud models of binary hydrogen compounds arranged according to the position of the kernel element in the periodic table. The inner sphere is the kernel with its net positive charge shown. The other spheres are the tetrahedral (sp ) clouds of spin-paired electrons. The dots represent the protons embedded within electron clouds. These protons are actually too small to put to scale. Features to note (1) The protons move farther from the nucleus as the kernel charge increases. The molecule becomes more acidic toward water. (2) The cloud size (from covalent radii) becomes smaller as the kernel charge increases, and larger as distance from the nucleus increases while kernel charge is held constant from HF to HI. (3) Two factors affect acidity, kernel charge, from CH4 to HF, and cloud size, from HF to HI. The compact cloud of HF can bind the proton better than the diffuse cloud of HI.

For a number of years this concept of spin clouds has been very popular for analyzing experimental data of spin glasses (Tholence and Tournier 1974, Wohl-farth 1977, Prejean 1978, von Lohneysen and Tholence 1979). As an example, fig. 30 reproduces a plot according to eq. 48 for data of (La ggCdo 02) 12 g ss (with Tq = 10 s), which also follows eq. 36 and eq. 37 (fig. 26). von Lohneysen and Tholence (1979) claim that fig. 30 gives support to the model of spin clouds . 

The quantum mechanics model is more modern and more mathematical. It describes a volume of space surrounding the nucleus of an atom where electrons reside, referred to earlier as the electron cloud. Similar to the Bohr model, the quantum mechanics model shows that electrons can be found in energy levels. Electrons do not, however, follow fixed paths around the nucleus. According to the quantum mechanics model, the exact location of an electron cannot be known, but there are areas in the electron cloud where there is a high probability that electrons can be found. These areas are the energy levels each energy level contains sublevels. The areas in which electrons are located in sublevels are called atomic orbitals. The exact location of the electrons in the clouds cannot be precisely predicted, but the unique speed, direction, spin, orientation, and distance from the nucleus of each electron in an atom can be considered. The quantum mechanics model is much more complicated, and accurate, than the Bohr model. 

It might be said in extenuation of the hydridic model that, according to the ideas of Kimball (19), the H atom should enter a more or less spherical electron cloud representing an unpaired electron associated with a metal atom. Thus, LiH is represented in Kimball s theory as a pair of tangent electron cloud spheres, or spheroids, each comprising two electrons of opposite spin centered about a +3 and a +1 nucleus, respectively. This picture is equally applicable to the hydrogen in CH4, HC1, or a metallic hydride—i.e., in all cases hydrogen is surrounded by a pair of electrons. 

Vaara and Pyykko presented a theory for the magnetic-field-dependent quadrupole splitting in the Xe NMR spectra in isotropic media and tested it by ab initio electronic structure calculations. Evidence exists only for even-power magnetic field dependence. The dominant mechanism is verified to be the electric field gradient caused by the diamagnetic distortion of the atomic electron cloud, quadratic in the magnetic field. NQCC for diatomic molecules were calculated by Bryce and Wasylishen. Turner et al performed a systematic computational study of the geometrical dependence of the deuteron quadrupole interaction parameters (DQCC and asymmetry parameter) for the water-formaldehyde model system. Bematowicz and Szymanski studied NMR spectra of a spin nucleus scalar coupled to two equivalent spin-1 nuclei... 

Fig. 8 Model of the collapse of thermoresponsive dendronized polymers as seen by EPR spin probes few individual hydrophobic and dynamic patches of 5 nm are sufficient to achieve a macroscopic collapse at the cloud point. Only at temperatures well above a static state (on EPR time scales) is reached...

ABSTRACT. Recent work on radiative processes and collisional excitation in molecular Hydrogen and its deuterated isotopic substitute and in molecular Carbon is reviewed. Particular attention is drawn to non-adiabatic coupling effects on the intensities of Lyman and Werner band systems of the vacuum ultraviolet spectrum of Hj and to the role of nuclear spin on ortho-para transitions in Hj due to collisions. The inter-relation between those processes and state to state chemistry is stressed out. We discuss the implications of these new data in a recent comprehensive model of diffuse interstellar clouds (Viala et al., 1987). 

The motion of a heavy particle when accompanied by a screening cloud of band electrons was first studied by Kondo (1984) and later by Kagan and Prokofev (1986) as a model for muon diffusion in metals. Liu (1987) and Kagan and Prokofev (1987) independently proposed that the same mechanism applies in heavy-fermion systems. The idea is that the f band is formed by the hopping of an f hole whose motion is accompanied by the screening cloud. Just like the band problem in the spin fluctuation resonance model, the hopping is the result of the hybridization interaction. Consequently, the dispersion of the f band is again solved from eq. (52) where Gf(to) is now calculated from the f hole spectrum in eq. (57) (Liu 1987, 1988) ... 

As outlined above, the physics associated with the Kondo model can be ascribed to the local moment inducing a compensating spin polarization cloud in the gas of conduction electrons, at low temperatures. The models considered in the next section are categorized as screening models, in which considerations of electrical charge neutrality in the unit cell are brought into play. 

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