Crossover from hydrodynamic to acoustic drag on quartz tuning forks in normal and superfluid 4He

We present measurements of the drag forces on quartz tuning forks oscillating at low velocities in normal and superfluid 4He. We have investigated the dissipative drag over a wide range of frequencies, from 6.5 to 600 kHz, by using arrays of forks with varying prong lengths and by exciting the forks in their fundamental and first overtone modes. At low frequencies the behavior is dominated by laminar hydrodynamic drag, governed by the fluid viscosity. At higher frequencies acoustic drag is dominant and is described well by a three-dimensional model of sound emission

Observation of quantum turbulence in superfluid 3He-B using reflection and transmission of ballistic thermal excitations

We report measurements of quantum turbulence generated by a vibrating grid in superfluid $^3$He-B at zero pressure in the zero temperature limit. Superfluid flow around individual vortex lines Andreev-reflects incoming thermal ballistic quasiparticle excitations, and allows non-invasive detection of quantum vortices in $^3$He-B. We have compared two Andreev reflection-based techniques traditionally used to detect quantum turbulence in the ballistic regime: quasiparticle transmission through and reflection from ballistic vortex rings and a turbulent tangle. We have shown that the two methods are in very good agreement and thus complement each other. Our measurements reveal that vortex rings and a tangle generated by a vibrating grid have a much larger spatial extent than previously realised. Furthermore, we find that a vortex tangle can either pass through an obstacle made from a mesh or diffuse around it. The measured dependence of vortex signal as a function of the distance from the vibrating grid is consistent with a power-law behaviour in contrast to turbulence generated by a vibrating wire which is described by an exponential function

Licit and illicit drug policies: a typology

To foster comparison of policy interventions across the various categories of licit and illicit drugs, we develop a typology of policies intended to address drug abuse problems. The principal dimensions of the typology are policy type and intervention channel. While the typology has important limitations, as a mechanism to organize information and stimulate thought it holds the potential to improve understanding of commonalities and distinctions among policies applying to widely discrepant drug problems, both within and across cultures. As such, it could contribute to the development of more effective approaches to grappling with a diverse set of drug policy issues.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73129/1/j.1360-0443.1990.tb03081.x.pd

Competition between Magnetic and Structural Transition in CrN

CrN is observed to undergo a paramagnetic to antiferromagnetic transition accompanied by a shear distortion from cubic NaCl-type to orthorhombic structure. Our first-principle plane wave and ultrasoft pseudopotential calculations confirm that the distorted antiferromagnetic phase with spin configuration arranged in double ferromagnetic sheets along [110] is the most stable. Antiferromagnetic ordering leads to a large depletion of states around Fermi level, but it does not open a gap. Simultaneous occurence of structural distortion and antiferromagnetic order is analyzed.Comment: 10 pages, 10 figure

The Uptake of HO₂ on Meteoric Smoke Analogues

The kinetics of heterogeneous HO2 uptake onto Meteoric Smoke Particles (MSPs) has been studied in the laboratory using analogues of MSP aerosol entrained into a flow tube. The uptake coefficient, y, was determined on synthetic amorphous olivine (MgFeSiO4) to be (6.9 ± 1.2) × 10‐2 at a relative humidity (RH) of 10%. On forsterite (Mg2SiO4), y = (4.3 ± 0.4) × 10‐3 at RH = 11.6%, and (7.3 ± 0.4) × 10‐2 at RH = 9.9% on fayalite (Fe2SiO4). These results indicate that Fe plays a more important mechanistic role than Mg in the removal of HO2 from the gas phase. Electronic structure calculations show that Fe atoms exposed at the particle surface provide a catalytic site where HO2 is converted to H2O2 via an Eley‐Rideal mechanism, but this does not occur on exposed surface Mg atoms. The impact of this heterogeneous process in the middle atmosphere was then investigated using a whole atmosphere chemistry‐climate model which incorporates a microphysical treatment of MSPs. Using a global MSP production rate from meteoric ablation of 44 tons per day, heterogeneous uptake (with y = 0.2) on MSPs significantly alters the HOx budget in the night‐time polar vortex. This impact is highly latitude dependent and thus could not be confirmed using currently available satellite measurements of HO2, which are largely unavailable at latitudes greater than 70°

Nanoscale Real-Time Detection of Quantum Vortices at Millikelvin Temperatures

Since we still lack a theory of classical turbulence, attention has focused on the conceptually simpler turbulence in quantum fluids. Reaching a better understanding of the quantum case may provide additional insight into the classical counterpart. That said, we have hitherto lacked detectors capable of the real-time, non-invasive probing of the wide range of length scales involved in quantum turbulence. Here we demonstrate the real-time detection of quantum vortices by a nanoscale resonant beam in superfluid 4He at 10mK. Essentially, we trap a single vortex along the length of a nanobeam and observe the transitions as a vortex is either trapped or released, detected through the shift in the beam resonant frequency. By exciting a tuning fork, we control the ambient vortex density and follow its influence on the vortex capture and release rates demonstrating that these devices are capable of probing turbulence on the micron scale

Acoustic Damping of Quartz Tuning Forks in Normal and Superfluid 3^3He

We investigate the damping experienced by quartz tuning fork resonators in normal and superfluid 3He as a function of their resonance frequency from 22 kHz to 250 kHz and contrast it with the behavior of the forks in 4He. For our set of tuning forks the low frequency damping in both fluids is well described by the existing hydrodynamic models. We find that the acoustic emission becomes the dominating dissipation mechanism at resonator frequencies exceeding approximately 100 kHz. Our results show that the acoustic emission model used in 4He fluid also describes acoustic damping in superfluid 3He and normal 3He at low temperatures using same geometrical prefactor. The high temperature acoustic damping in normal 3He does not exceed prediction of this model and thus the acoustic damping of moderate frequency devices measured in 4He should be similar or smaller in 3He liquid