Cavitation in Superfluid Helium Possibly Arising from Penning Ionization of Dimers

We have perforated a series of experiments to study cavitation in superfluid helium into which electrons are injected by field-emission from a sharp tip. The injected electrons force open small cavities in the liquid (“electron bubble”). These objects explode at a critical negative pressure P _c, and in previous experiments we have studied the cavitation that resugts from these explosions. In the present experiments we have detected cavitation events that occur before a negative pressure as large as P _c is reached. We suggest that these events may arise from a process in which two neutral helium dimers interact and an electron is injected into the liquid through Penning ionization.     

Magnetic Levitation of liquid helium

We report on the stable levitation of liquid helium drops of up to 2 cm diameter in a magnetic trap at temperatures down to 1.5 K in the earth's gravitational field. The production and properties of a magnetic trap for diamagnetic materials is discussed. The behavior of liquids in such a trap is analyzed, including the deformation of a liquid drop by the trap forces. We frequently observe two drops in the magnetic trap which are held in apparent contact for up to 3 minutes without coalescing. This non-coalescence effect was only seen above the superfluid transition temperature. We explain this effect in terms of the existence of a vapor layer between the drops caused by evaporation of the drops, much like the suspension of a liquid drop above a hot surface known as the Leidenfrost effect.     

Supersolidity and the Thermodynamics of Solid Helium

The specific heat of solid helium in the temperature range below about 1.2 K has been found to contain a term varying as T ⁷, in addition to the usual T ³ contribution always found in a crystalline dielectric solid. It has been proposed by Anderson, Brinkman and Huse, (Science 310, 1164 (2005)) that the existence of this T ⁷ term supports their theory of supersolidity. However, in this paper we show that corrections to the phonon specific heat arising from phonon dispersion are much larger than expected based on simple order of magnitude estimates and, as a consequence, it is very unlikely that the existence of this T ⁷ term can be considered as evidence for supersolidity.      

Directionality from anisotropic phonon production in solid state dark matter detection

We present results from computer simulations of the events immediately following the scattering of a dark matter particle off a nucleus in a crystal detector. Our simulations show that with NaF as the target, the recoil produces solitary waves that decay slowly, resulting in a narrow wake of phonons. The phonon wake allows a determination of direction of the nuclear recoil.     

The performance of foliage mass and crown radius models in forming the input of a forest reflectance model: A test on forest growth sample plots and Landsat 7 ETM+ images

Several published foliage mass and crown radius regression models were tested on the preparation of the input for the reflectance model of Kuusk and Nilson [Kuusk, A. and Nilson, T. (2000), A directional multispectral forest reflectance model. Remote Sensing of Environment, 72(2):244–252.] for 246 forest growth sample plots in Estonia. In each test, foliage mass and crown radius for trees in the sample plots were predicted with a particular pair of allometric regression models. The forest reflectance model was then run using the estimated foliage mass and crown radius values. Reflectance factors were simulated and compared with the reflectance values obtained from three atmospherically corrected Landsat 7 Enhanced Thematic Mapper (ETM+) scenes. The statistics of linear regression between the simulated and measured reflectance factors were used to assess the performance of foliage and crown radius models. The hypothesis was that the best allometric regression models should provide the best fit in reflectance. The strongest correlation between the simulated and measured reflectance factors was found in the short-wave infrared band (ETM + 5) for all the images. The highest R² = 0.71 was observed in Picea abies dominated stands. No excellent combination of foliage mass and crown radius functions was found, but the ranking based on determination coefficients showed that some linear crown radius models are not applicable to our data. Processing of raster images, reflectance measurement for small sample plots, usage of tree-species-specific fixed parameters (specific leaf area, etc.), and the ignored influence of phenology introduced additional variation into the relationships between simulated and measured reflectance factors. Further studies are needed, but these preliminary results demonstrate that the proposed method could serve as an effective way of testing the performance of foliage mass and canopy cover regressions.     

Study of cavitation in superfluid helium-4 at low temperatures

We report studies of the negative pressure required to lead to the nucleation of bubbles in superfluid helium-4 in the temperature range down to 0.8 K. At the lowest temperatures studied, the tensile strength is found to be 3 bar. The results are compared with nucleation theory and the possible importance of nucleation on vortices is discussed.     

Revisiting free riding and the Tit-for-Tat in BitTorrent: A measurement study

In this paper we report new results of our continuous effort on analyzing the impact of incentive mechanisms on user behavior in BitTorrent. In this second measurement and analysis study we find that free riders’ population has significantly increased comparing to our previous measurement study. We relate this increase to the advance in end-users’ connection speeds and to users’ increased knowledge in BitTorrent. We also categorize free riders based on the behavior they exhibit in multiple-torrent system into three types: cheaters, strategic and lucky peers. Furthermore, refuting the findings of other studies, we show that peers who exploit the system in BitTorrent are both high bandwidth capacity peers and low bandwidth capacity peers. Moreover, we argue that the Tit-for-Tat mechanism does not discriminate peers based on their bandwidth capacities and that it reacts successfully against inter-class bandwidth capacity strategic peers. Finally, we propose a memory-backoff approach to the optimistic unchoke policy that reduces the volume of free riding in BitTorrent.

Phonon physics and low temperature detectors of dark matter

We discuss a number of topics in phonon physics that are relevant to the design of low temperature phonon-based dark matter detectors. These topics include the generation of primæval phonons by the nuclear recoil, the processes by which these phonons decay into lower energy phonons, and the ballistic propagation of these excitations to the sensors on the crystal surface.