Theory of magnetic deflagration

Theory of magnetic deflagration (avalanches) in crystals of molecular magnets has been developed. The phenomenon resembles the burning of a chemical substance, with the Zeeman energy playing the role of the chemical energy. Non-destructive reversible character of magnetic deflagration, as well as the possibility to continuously tune the flammability of the crystal by changing the magnetic field, makes molecular magnets an attractive toy system for a detailed study of the burning process. Besides simplicity, new features, as compared to the chemical burning, include possibility of quantum decay of metastable spin states and strong temperature dependence of the heat capacity and thermal conductivity. We obtain analytical and numerical solutions for criteria of the ignition of magnetic deflagration, and compute the ignition rate and the speed of the developed deflagration front.Comment: 17 Pages, 17 Figure caption

Exploring the Structure of Distant Galaxies with Adaptive Optics on the Keck-II Telescope

We report on the first observation of cosmologically distant field galaxies with an high order Adaptive Optics (AO) system on an 8-10 meter class telescope. Two galaxies were observed at 1.6 microns at an angular resolution as high as 50 milliarcsec using the AO system on the Keck-II telescope. Radial profiles of both objects are consistent with those of local spiral galaxies and are decomposed into a classic exponential disk and a central bulge. A star-forming cluster or companion galaxy as well as a compact core are detected in one of the galaxies at a redshift of 0.37+/-0.05. We discuss possible explanations for the core including a small bulge, a nuclear starburst, or an active nucleus. The same galaxy shows a peak disk surface brightness that is brighter than local disks of comparable size. These observations demonstrate the power of AO to reveal details of the morphology of distant faint galaxies and to explore galaxy evolution.Comment: 5 pages, Latex, 3 figures. Accepted for publication in P.A.S.

Do Our Airmen Value Their CCAF Degree?

A recent article, “CCAF Continues to Provide Value to Air Force, Enlisted Members,” posted in the Community College of the Air Force (CCAF) alumni group on LinkedIn generated over 100 comments from CCAF graduates regarding the value of that college’s degree.1 Their perceptions of the worth of the CCAF degree ranged from no value at all to its having a tremendous impact on careers and goals.2The foregoing served as the catalyst for this two-phased research. Only by comparing both sides of the problem will we have truly answered the question regarding the value of the degree. Phase one consisted of the current research project, focused on the collection and analysis of CCAF graduates’ perceptions regarding the value of their degree. Phase two will involve the collection of data collected from hiring managers from various fields of industry regarding their perception of the CCAF degree and their estimation of it during a review of an applicant’s credentials

Quantum Magnetic Deflagration in Mn12 Acetate

We report controlled ignition of magnetization reversal avalanches by surface acoustic waves in a single crystal of Mn12 acetate. Our data show that the speed of the avalanche exhibits maxima on the magnetic field at the tunneling resonances of Mn12. Combined with the evidence of magnetic deflagration in Mn12 acetate (Suzuki et al., cond-mat/0506569) this suggests a novel physical phenomenon: deflagration assisted by quantum tunneling.Comment: 4 figure

Classical generalized constant coupling model for geometrically frustrated antiferromagnets

A generalized constant coupling approximation for classical geometrically frustrated antiferromagnets is presented. Starting from a frustrated unit we introduce the interactions with the surrounding units in terms of an internal effective field which is fixed by a self consistency condition. Results for the magnetic susceptibility and specific heat are compared with Monte Carlo data for the classical Heisenberg model for the pyrochlore and kagome lattices. The predictions for the susceptibility are found to be essentially exact, and the corresponding predictions for the specific heat are found to be in very good agreement with the Monte Carlo results.Comment: 4 pages, 3 figures, 2 columns. Discussion about the zero T value of the pyrochlore specific heat correcte

Galaxies in Southern Bright Star Fields I. Near-infrared imaging

As a prerequisite for cosmological studies using adaptive optics techniques, we have begun to identify and characterize faint sources in the vicinity of bright stars at high Galactic latitudes. The initial phase of this work has been a program of K_s imaging conducted with SOFI at the ESO NTT. From observations of 42 southern fields evenly divided between the spring and autumn skies, we have identified 391 additional stars and 1589 galaxies lying at separations 60" from candidate guide stars in the magnitude range 9.0 R 12.4. When analyzed as a "discrete deep field" with 131 arcmin^2 area, our dataset gives galaxy number counts that agree with those derived previously over the range 16 K_s 20.5. This consistency indicates that in the aggregate, our fields should be suitable for future statistical studies. We provide our source catalogue as a resource for users of large telescopes in the southern hemisphere.Comment: 10 pages, 7 figures, accepted by A&A; Table 3 is available at http://www.rzg.mpg.de/~ajb/data.html pending upload to CD

Anisotropic magnetic deflagration in single crystals of Gd5Ge4

Experimental evidence of the anisotropy of the magnetic deflagration associated with the low-temperature first order antiferromagnetic (AFM) --> ferromagnetic (FM) phase-transition in single crystals of Gd5Ge4 is reported. The deflagrations have been induced by controlled pulses of surface acoustic waves (SAW) allowing us to explore both the magnetic field and temperature dependencies on the characteristic times of the phenomenon. The study was done using samples with different geometries and configurations between the SAW pulses and the direction of the applied magnetic field with respect to the three main crystallographic directions of the samples. The effect of temperature is nearly negligible, whereas observed strong magnetic field dependence correlates with the magnetic anisotropy of the sample. Finally, the role of the SAW pulses in both the ignition and formation of the deflagration front was also studied, and we show that the thermal diffusivity of Gd5Ge4 must be anisotropic, following \kappaa>\kappab>\kappac.Comment: 10 pages, 4 figure