Mass flow through solid 4He induced by the fountain effect

Using an apparatus that allows superfluid liquid 4He to be in contact with hcp solid \4he at pressures greater than the bulk melting pressure of the solid, we have performed experiments that show evidence for 4He mass flux through the solid and the likely presence of superfluid inside the solid. We present results that show that a thermomechanical equilibrium in quantitative agreement with the fountain effect exists between two liquid reservoirs connected to each other through two superfluid-filled Vycor rods in series with a chamber filled with solid 4He. We use the thermomechanical effect to induce flow through the solid and measure the flow rate. On cooling, mass flux appears near T = 600 mK and rises smoothly as the temperature is lowered. Near T = 75 mK a sharp drop in the flux is present. The flux increases as the temperature is reduced below 75 mK. We comment on possible causes of this flux minimum.Comment: 20 pages, 22 figures, 7 table

The overlap parameter across an inverse first order phase transition in a 3D spin-glass

We investigate the thermodynamic phase transition taking place in the Blume-Capel model in presence of quenched disorder in three dimensions (3D). In particular, performing Exchange Montecarlo simulations, we study the behavior of the order parameters accross the first order phase transition and its related coexistence region. This transition is an Inverse Freezing.Comment: 9 pages, 6 figures, Contribution to the XII International Workshop on Complex System

A Degenerate Bose-Fermi Mixture of Metastable Atoms

We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10^6 atoms of each isotope at temperatures below 1 micro-Kelvin, and achieve a fermionic degeneracy parameter of T/Tf=0.45. Due to their high internal energy, the detection of individual metastable atoms with sub-nanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.Comment: 4 pages, 3 figures submitted to PR

The random Blume-Capel model on cubic lattice: first order inverse freezing in a 3D spin-glass system

We present a numerical study of the Blume-Capel model with quenched disorder in 3D. The phase diagram is characterized by spin-glass/paramagnet phase transitions of both first and second order in the thermodynamic sense. Numerical simulations are performed using the Exchange-Monte Carlo algorithm, providing clear evidence for inverse freezing. The main features at criticality and in the phase coexistence region are investigated. The whole inverse freezing transition appears to be first order. The second order transition appears to be in the same universality class of the Edwards-Anderson model. The nature of the spin-glass phase is analyzed by means of the finite size scaling behavior of the overlap distribution functions and the four-spins real-space correlation functions. Evidence for a replica symmetry breaking-like organization of states is provided.Comment: 18 pages, 24 figures, 7 table

Focusing of Intense Subpicosecond Laser Pulses in Wedge Targets

Two dimensional particle-in-cell simulations characterizing the interaction of ultraintense short pulse lasers in the range 10^{18} \leq I \leq 10^{20} W/cm^{2} with converging target geometries are presented. Seeking to examine intensity amplification in high-power laser systems, where focal spots are typically non-diffraction limited, we describe key dynamical features as the injected laser intensity and convergence angle of the target are systematically varied. We find that laser pulses are focused down to a wavelength with the peak intensity amplified by an order of magnitude beyond its vacuum value, and develop a simple model for how the peak location moves back towards the injection plane over time. This performance is sustained over hundreds of femtoseconds and scales to laser intensities beyond 10^{20} W/cm^{2} at 1 \mu m wavelength.Comment: 5 pages, 6 figures, accepted for publication in Physics of Plasma

Ultrafast demagnetization of Co 25Ni 75/Pt multilayers with perpendicular anisotropy at elevated temperatures

Copyright © 2005 American Institute of PhysicsUltrafast demagnetization has been studied in Si/Pt(160 Å)/[Co25Ni75(x)/Pt(8 Å)]20 (x = 3, 4.5, and 6 Å) multilayers with perpendicular anisotropy by magneto-optical pump-probe measurements in the polar geometry. Time-resolved measurements made in the saturated state showed that maximum demagnetization was achieved within 300 fs. Hysteresis loops were measured at a time delay of 1.3 ps for temperatures from 20 to 300 °C. The Curie temperature was found to increase from 150 to 250 °C with increasing Co25Ni75 thickness. By comparing the loops obtained with and without pump excitation, the increase in electron temperature due to the pump was estimated to be about 60 K

The Coupled Magmatic and Hydrothermal Systems of the Restless Aluto Caldera, Ethiopia

Seismicity can be used to better understand interactions between magma bodies, hydrothermal systems and their host rocks—key factors influencing volcanic unrest. Here, we use earthquake data to image, for the first time, the seismic velocity structure beneath Aluto, a deforming volcano in the Main Ethiopian Rift. Traveltime tomography is used to jointly relocate seismicity and image 3D P- and S-wave velocity structures and the ratio between them (VP/VS). At depths of 4–9 km, the seismicity maps the top of a large low velocity zone with high VP/VS, which we interpret as a more ductile and melt-bearing region. A shallow (<3 km) hydrothermal system exhibits low seismic velocities and very low VP/VS (∼1.40), consistent with the presence of gases exsolved from a deeper melt-rich mush body. The Artu Jawe fault and fracture system provides the migration pathway that connects the deeper mush body with the shallow hydrothermal system. Together, these observations demonstrate that the interaction between magmatic and hydrothermal systems, driven by the exchange of fluids, is responsible for the restless behavior of Aluto

Correlation effects in Ni 3d states of LaNiPO

The electronic structure of the new superconducting material LaNiPO experimentally probed by soft X-ray spectroscopy and theoretically calculated by the combination of local density approximation with Dynamical Mean-Field Theory (LDA+DMFT) are compared herein. We have measured the Ni L2,3 X-ray emission (XES) and absorption (XAS) spectra which probe the occupied and unoccupied the Ni 3d states, respectively. In LaNiPO, the Ni 3d states are strongly renormalized by dynamical correlations and shifted about 1.5 eV lower in the valence band than the corresponding Fe 3d states in LaFeAsO. We further obtain a lower Hubbard band at -9 eV below the Fermi level in LaNiPO which bears striking resemblance to the lower Hubbard band in the correlated oxide NiO, while no such band is observed in LaFeAsO. These results are also supported by the intensity ratio between the transition metal L2 and L3 bands measured experimentally to be higher in LaNiPO than in LaFeAsO, indicating the presence of the stronger electron correlations in the Ni 3d states in LaNiPO in comparison with the Fe 3d states in LaFeAsO. These findings are in accordance with resonantly excited transition metal L3 X-ray emission spectra which probe occupied metal 3d-states and show the appearance of the lower Hubbard band in LaNiPO and NiO and its absence in LaFeAsO.Comment: 6 pages, 5 figure

Dialogue based interfaces for universal access.

Conversation provides an excellent means of communication for almost all people. Consequently, a conversational interface is an excellent mechanism for allowing people to interact with systems. Conversational systems are an active research area, but a wide range of systems can be developed with current technology. More sophisticated interfaces can take considerable effort, but simple interfaces can be developed quite rapidly. This paper gives an introduction to the current state of the art of conversational systems and interfaces. It describes a methodology for developing conversational interfaces and gives an example of an interface for a state benefits web site. The paper discusses how this interface could improve access for a wide range of people, and how further development of this interface would allow a larger range of people to use the system and give them more functionality