Micro-Hall Magnetometry Studies of Thermally Assisted and Pure Quantum Tunneling in Single Molecule Magnet Mn12-Acetate

We have studied the crossover between thermally assisted and pure quantum tunneling in single crystals of high spin (S=10) uniaxial single molecule magnet Mn12-acetate using micro-Hall effect magnetometry. Magnetic hysteresis experiments have been used toinvestigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow (~0.1 K) or broad (~1 K) temperature interval depending on the magnitude and direction of the applied field. For low external fields applied parallel to the easy axis, the energy levels that dominate the tunneling shift abruptly with temperature. In the presence of a transverse field and/or large longitudinal field these energy levels change with temperature more gradually. A comparison of our experimental results with model calculations of this crossover suggest that there are additional mechanisms that enhance the tunneling rate of low lying energy levels and broaden the crossover for small transverse fields.Comment: 5 pages, 5 figure

"Investigation of the Flowfield of a Scramjet Combustor with Parallel H2-Injection through a Strut by Particle Image Displacement Velocimetry."

The flow field in a supersonic combustor with H2-injection through a strut is investigated by particle image displacement velocimetry. The flowfield is characterized by shock trains which originate at the leading edge of the strut, an expansion originating at the interaction of the fuel and the air flows. PIV recordings are taken without and with ignition of the fuel/oxidizer mixture. Velocities up to 900 m/s were measured. The measured velocity fields are compared with the flow structures in the shadow photographs

Principal soils of Iowa

Many changes have taken place in agricultural land use in recent years. Technology (such as more efficient fertilizer use; improved chemical, weed and insect control; large capacity machinery; improved tillage practices and more effective erosion control methods) has introduced more land use alternatives. In addition, our growing population has increased the demand for land for residential, industrial, highway, recreational and other nonagricultural uses.</p

Thermodynamic Interpretation of Cryogenic Injection Experiments

This paper discusses a thermodynamic rather than mechanic discussion and interpretation of cryogenic injection of nitrogen in the vicinity of the critical point. There is no concensus in the literature on how to properly interpret and treat injection phenomena at supercritical pressures. While it is clear that the supercritical fluid loses many distinct liquid properties, such as heat of vaporization and surface tension, flows are being treated like they were liquid. Liquid core lengths are being determined in experiments, distinct droplets are tracked in computational fluid dynamic studies. And in fact, these approaches prove to be very successful. Nevertheless, a more appropriate treatment is desireable, taking into account the specifics of supercritical fluids. A contribution is attempted in this paper. The concept of pseudo-boiling, a maximum in heat capacity associated with a strong increase in specific volume, is discussed. It will be shown that the ensemble of supercritical maximum heat capacity states is in fact an extension to the saturation curve. A novel interpretation of the Clapeyron equation of thermodynamics in the limit of the critical point and beyond will be given. It will be shown that this generalization is able to characterize the pseudo-boiling line. Furthermore it will be shown that the slope (d log p/dT) is constant for supercritical conditions and equals the value at the critical point. The pseudo-boiling approach is then applied to characterize injection experiments. It can be shown that the power needed to reach the pseudo-boiling state correlates with the structure of the axial density distribution