51ÁÔÆæÈë¿Ú

Abstract:

An electron paramagnetic resonance spectroscopic study of the reaction of ground-state A1 atoms with NH3 in adamantane at 77 K on a rotating cryostat has demonstrated the formation of three mononuclear Al-NH3 complexes, Al(NH3)4 (A), HA1NH2 (B), and Al(NH3)2 (C). These species have the following magnetic parameters: aAl = 786.3 ± 0.22 MHz, = 23 ± 0.08 MHz, and g = 1.9997 ± 0.0001 for A1(NH3)4, aAl = 922.5 ± 0.41 MHz, an = 228.7 ± 2.00 MHz, = 27 ± 2.0 MHz, aH(1) = 27 ± 2.0 MHz, and g = 2.0003 ± 0.0002 for HAINH2, and a||(Al) = 131.5 MHz, a⊥(1) = ca. 20 MHz, and g||= g⊥ = 1.999 for Al(NH3)2. HAIOH (D) is also formed in this system from adventitious H20 and has the parameters aAl= 911.3 ± 1.06 MHz, aH= 286.4 ± 3.7 MHz, and g = 1.9998 ± 0.0004. © 1992, American Chemical Society. All rights reserved.

Abstract:

We consider a model system of three weakly coupled, weakly anharmonic oscillators, containing two near 2:1 Fermi resonances. This system is initially reduced via a resonance approximation to one of two coupled pendula, each pendulum representing one of the interoscillator resonances. This leads to a natural description of the system in terms of analytically predictable resonance zones and their intersection in action space. A full numerical analysis of the classical dynamics of the two-pendulum system is given, with respect to both the appearance of stochasticity and the development of periodic orbits in the resulting two-dimensional area-preserving map. The success of the resonance approximation in describing adequately the behavior of the three-oscillator model is shown by comparison with a representative study of the classical dynamics of the full three-oscillator model. © 1992 American Institute of Physics.

Abstract:

Using a self-consistent mean-field approach to a disordered Hubbard model the authors show that, regardless of the role of correlation in the half-filled band for which the conventional phenomenology is recovered, correlation effects can be strongly enhanced by disorder in the low-filling limit, leading to a well-defined quasi-atomic domain of strong local moments on an inhomogeneous scale. They argue that the theory provides a natural origin for the observed behaviour of the Cs-rich liquid alloys Cu-Au close to stoichiometry.

Abstract:

We consider a disordered Hubbard model for a system characterized by quenched liquid-like disorder, with correlation treated at the generalized Hartree-Fock level and the possibility of local magnetic moments introduced from the outset. A simple theory based on averaged Green functions is used to describe the properties of the system in the local moment domains in particular, and their evolution with number density and both structural and electronic parameters of relevance. A probabilistically based mean-field theory is then developed to ' address the localization characteristics of the HF pseudoparticle states, and the consequent disorder-induced metal-insulator transition. Three principal density domains of interest are identified: a low density insulator with local magnetic moments, a metallic phase with local moments at intermediate densities, and a higher density nonmagnetic metallic state. The theory is used to provide an interpretation of bulk experiments on expanded fluid alkali elements, with particular emphasis on the insulating and "dirty" metallic domains. © 1991 American Institute of Physics.