Dynamics of the Heisenberg Ferromagnet at Low Temperatures

Dyson calculated the effect of spin-wave interactions on the static (thermodynamic) properties of the Heisenberg ferromagnet. Within the same approximation, that of including only the contributions of lowest-order (two-magnon) scattering processes and neglecting the kinematic interaction, we have calculated the dynamic properties of this system and find results consistent with Dyson\u27s in the zero-wave-vector limit. In the short-wavelength limit where perturbation theory diverges, we discuss nonperturbatively via the t matrix the influence of the two-spin-wave bound states and the two-spin-wave resonant scattering states on the single-particle spectrum as characterized by the transverse spectral weight function Ak(ω). We find that although the total cross section of the bound states is too small for them to be observed directly, the anomalous effect of the bound states and resonant scattering states on the renormalization of the spin-wave energy is observable under favorable conditions. In general, we find the quasiparticle picture to be valid; however, at the highest temperature considered the resonant scattering states cause an extra resonance in the susceptibility. Most of the results for Ak(ω) are given numerically and have been checked against the sum rules, although the energy shift and energy width as deduced from Σk(εk) are given analytically by rather simple expressions. We have obtained for the first time a Green\u27s function that is capable of yielding correctly at low temperatures both the static and dynamic properties for arbitrary spin

Effect of Bound States on the Excitation Spectrum of a Heisenberg Ferromagnet at Low Temperature

A compact expression for the energy shift and inverse lifetime (energy width) of spin waves in a Heisenberg ferromagnet at low temperatures is given. The two-particle bound states are observable via the resonance they cause in the self-energy of spin waves

Research on microwave joining of SiC

Results: identification of optimum joining temperature range for reaction bonded Si carbide at 1420-1500 C; demonstration that specimens joined within this range have fracture roughness greater than as-received material; and demonstration of ability to use SiC formed in situ from the decomposition of polycarbosilane as a joining aid for sintered Si carbide. In the latter case, the interlayer material was also shown to fill any pores in the joining specimens near the interlayer. Together with the demonstration of leaktight joints between tube sections of reaction bonded and sintered SiC under the previous contract, these results provide the foundation for scaleup to joining of the larger and longer tubes needed for radiant burner and heat exchanger tube assemblies. The formation of SiC in situ is important because maintaining roundness of these large tubes is a technical challenge for the tube manufacturer, so that formation of a leaktight joint may require some degree of gap filling

Microwave joining of SiC ceramics and composites

Potential applications of SiC include components for advanced turbine engines, tube assemblies for radiant burners and petrochemical processing and heat exchangers for high efficiency electric power generation systems. Reliable methods for joining SiC are required in order to cost-effectively fabricate components for these applications from commercially available shapes and sizes. This manuscript reports the results of microwave joining experiments performed using two different types of SiC materials. The first were on reaction bonded SiC, and produced joints with fracture toughness equal to or greater than that of the base material over an extended range of joining temperatures. The second were on continuous fiber-reinforced SiC/SiC composite materials, which were successfully joined with a commercial active brazing alloy, as well as by using a polymer precursor