Defects and glassy dynamics in solid He-4: Perspectives and current status

We review the anomalous behavior of solid He-4 at low temperatures with particular attention to the role of structural defects present in solid. The discussion centers around the possible role of two level systems and structural glassy components for inducing the observed anomalies. We propose that the origin of glassy behavior is due to the dynamics of defects like dislocations formed in He-4. Within the developed framework of glassy components in a solid, we give a summary of the results and predictions for the effects that cover the mechanical, thermodynamic, viscoelastic, and electro-elastic contributions of the glassy response of solid He-4. Our proposed glass model for solid He-4 has several implications: (1) The anomalous properties of He-4 can be accounted for by allowing defects to freeze out at lowest temperatures. The dynamics of solid He-4 is governed by glasslike (glassy) relaxation processes and the distribution of relaxation times varies significantly between different torsional oscillator, shear modulus, and dielectric function experiments. (2) Any defect freeze-out will be accompanied by thermodynamic signatures consistent with entropy contributions from defects. It follows that such entropy contribution is much smaller than the required superfluid fraction, yet it is sufficient to account for excess entropy at lowest temperatures. (3) We predict a Cole-Cole type relation between the real and imaginary part of the response functions for rotational and planar shear that is occurring due to the dynamics of defects. Similar results apply for other response functions. (4) Using the framework of glassy dynamics, we predict low-frequency yet to be measured electro-elastic features in defect rich He-4 crystals. These predictions allow one to directly test the ideas and very presence of glassy contributions in He-4.Comment: 33 pages, 13 figure

Energetic particle influence on the Earth's atmosphere

This manuscript gives an up-to-date and comprehensive overview of the effects of energetic particle precipitation (EPP) onto the whole atmosphere, from the lower thermosphere/mesosphere through the stratosphere and troposphere, to the surface. The paper summarizes the different sources and energies of particles, principally galactic cosmic rays (GCRs), solar energetic particles (SEPs) and energetic electron precipitation (EEP). All the proposed mechanisms by which EPP can affect the atmosphere are discussed, including chemical changes in the upper atmosphere and lower thermosphere, chemistry-dynamics feedbacks, the global electric circuit and cloud formation. The role of energetic particles in Earth’s atmosphere is a multi-disciplinary problem that requires expertise from a range of scientific backgrounds. To assist with this synergy, summary tables are provided, which are intended to evaluate the level of current knowledge of the effects of energetic particles on processes in the entire atmosphere

Modeling, Design, and Testing of a Microchannel Split-System Air Conditioner

A steady-state microchannel split-system simulation model has been developed based on previous research at the ACRC. This model was utilized as a design tool to optimize a microchannel split system with the goal of minimizing TEWI, or total equivalent warming impact. The system components were then selected and the optimized microchannel heat exchangers were fabricated. Next, the entire system was assembled and extensive tests were run at steady state conditions over a wide range of outdoor ambient conditions in a calorimeter test facility. The experimental results have been compared to the simulations for the purpose of model refinement and its eventual validation. The full system model overpredicts the total capacity of the system with a minimum error of 0.2%, a mean error of 5%, and a maximum error of 11 %. The evaporator submodel overpredicts the total capacity as well, with a minimum error of 0.7%, a mean error of 7%, and a maximum error of 11%. The condenser submodel also overpredicts with a minimum error of 0.5%, a mean error of 2%, and a maximum error of 6%. A major reason for the lower accuracy with the evaporator is because of the refrigerant maldisribution observed in the experiments. The model assumes perfect distribution, hence one reason for the overprediction of the system's capacity.Air Conditioning and Refrigeration Project 6