A Path to the Direct Detection of sub-GeV Dark Matter Using Calorimetric Readout of a Superfluid 4^4He Target

A promising technology concept for sub-GeV dark matter detection is described, in which low-temperature microcalorimeters serve as the sensors and superfluid $^4$He serves as the target material. A superfluid helium target has several advantageous properties, including a light nuclear mass for better kinematic matching with light dark matter particles, copious production of scintillation light, extremely good intrinsic radiopurity, a high impedance to external vibration noise, and a unique mechanism for observing phonon-like modes via liberation of $^4$He atoms into a vacuum (`quantum evaporation'). In this concept, both scintillation photons and triplet excimers are detected using calorimeters, including calorimeters immersed in the superfluid. Kinetic excitations of the superfluid medium (rotons and phonons) are detected using quantum evaporation and subsequent atomic adsorption onto a microcalorimeter suspended in vacuum above the target helium. The energy of adsorption amplifies the phonon/roton signal before calorimetric sensing, producing a gain mechanism that can reduce the techonology's recoil energy threshold below the calorimeter energy threshold. We describe signal production and signal sensing probabilities, and estimate electron recoil discrimination. We then simulate radioactive backgrounds from gamma rays and neutrons. Dark matter - nucleon elastic scattering cross-section sensitivities are projected, demonstrating that even very small (sub-kg) target masses can probe wide regions of as-yet untested dark matter parameter space

CLIMATE CHANGE, AGRICULTURE AND POVERTY

Even though much has been written about climate change and poverty as distinct and complex problems, the link between them has received little attention. Understanding this link is vital for the formulation of effective policy responses to climate change. In this article, we focus on agriculture as a primary means by which the impacts of climate change are transmitted to the poor, and as a sector at the forefront of climate change mitigation efforts in developing countries. In so doing, we offer some important insights that may help shape future policies as well as ongoing research in this area.Agricultural and Food Policy, Environmental Economics and Policy, Food Security and Poverty, Resource /Energy Economics and Policy,

Collisionless energy absorption in the short-pulse intense laser-cluster interaction

In a previous Letter [Phys. Rev. Lett. 96, 123401 (2006)] we have shown by means of three-dimensional particle-in-cell simulations and a simple rigid-sphere model that nonlinear resonance absorption is the dominant collisionless absorption mechanism in the intense, short-pulse laser cluster interaction. In this paper we present a more detailed account of the matter. In particular we show that the absorption efficiency is almost independent of the laser polarization. In the rigid-sphere model, the absorbed energy increases by many orders of magnitude at a certain threshold laser intensity. The particle-in-cell results display maximum fractional absorption around the same intensity. We calculate the threshold intensity and show that it is underestimated by the common over-barrier ionization estimate.Comment: 12 pages, 13 figures, RevTeX

Excited State Relaxation in Vacuum Deposited and Solution Processed Films of Merocyanine/Fulerene Blends

Exciton dynamics in merocyanine/fulerene blend films made by vacuum deposition and solution processing techniques were investigated by means of steady-state and time resolved fluorescence and absorption spectroscopy. Intermolecular charge transfer states are formed during several ps in neat merocianine films, which determine their fluorescence properties. Fullerene additives cause formation of new heterogeneous charge transfer states. Even a small fullerene concentration significantly influences the exciton dynamics by quenching inherent merocianine fluorescent states and causing appearance of new fluorescence bands caused by the charge transfer states between merocyanine and fullerene molecules. All fluorescence bands are quenched in films with high fulerence concentration due to the charge carrier generation, and the quenching effect is stronger in vacuum deposited films. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3529

Domain wall mobility in nanowires: transverse versus vortex walls

The motion of domain walls in ferromagnetic, cylindrical nanowires is investigated numerically by solving the Landau-Lifshitz-Gilbert equation for a classical spin model in which energy contributions from exchange, crystalline anisotropy, dipole-dipole interaction, and a driving magnetic field are considered. Depending on the diameter, either transverse domain walls or vortex walls are found. The transverse domain wall is observed for diameters smaller than the exchange length of the given material. Here, the system behaves effectively one-dimensional and the domain wall mobility agrees with a result derived for a one-dimensional wall by Slonczewski. For low damping the domain wall mobility decreases with decreasing damping constant. With increasing diameter, a crossover to a vortex wall sets in which enhances the domain wall mobility drastically. For a vortex wall the domain wall mobility is described by the Walker-formula, with a domain wall width depending on the diameter of the wire. The main difference is the dependence on damping: for a vortex wall the domain wall mobility can be drastically increased for small values of the damping constant up to a factor of $1/\alpha^2$.Comment: 5 pages, 6 figure

Angular-dependence of magnetization switching for a multi-domain dot: experiment and simulation

We have measured the in-plane angular variation of nucleation and annihilation fields of a multi-domain magnetic single dot with a microsquid. The dots are Fe/Mo(110) self-assembled in UHV, with sub-micron size and a hexagonal shape. The angular variations were quantitatively reproduced by micromagnetic simulations. Discontinuities in the variations are observed, and shown to result from bifurcations related to the interplay of the non-uniform magnetization state with the shape of the dot.Comment: 4 pages, 4 figures, for submission as a regular articl

Negative Specific Heat in Astronomy, Physics and Chemistry

Starting from Antonov's discovery that there is no maximum to the entropy of a gravitating system of point particles at fixed energy in a spherical box if the density contrast between centre and edge exceeds 709, we review progress in the understanding of gravitational thermodynamics. We pinpoint the error in the proof that all systems have positive specific heat and say when it can occur. We discuss the development of the thermal runaway in both the gravothermal catastrophe and its inverse. The energy range over which microcanonical ensembles have negative heat capacity is replaced by a first order phase transition in the corresponding canonical ensembles. We conjecture that all first order phase transitions may be viewed as caused by negative heat capacities of units within them. We find such units in the theory of ionisation, chemical dissociation and in the Van der Waals gas so these concepts are applicable outside the realm of stars, star clusters and black holes.Comment: 17 pages, LaTeX with 4 encapsulated postscript figures included. To appear in Proceedings of XXth IUPAP International Conference on Statistical Physics, Paris, July 20-24, 199

Calibration of a two-phase xenon time projection chamber with a 37^{37}Ar source

We calibrate a two-phase xenon detector at 0.27 keV in the charge channel and at 2.8 keV in both the light and charge channels using a $^{37}$Ar source that is directly released into the detector. We map the light and charge yields as a function of electric drift field. For the 2.8 keV peak, we calculate the Thomas-Imel box parameter for recombination and determine its dependence on drift field. For the same peak, we achieve an energy resolution, $E_{\sigma}/E_{mean}$, between 9.8% and 10.8% for 0.1 kV/cm to 2 kV/cm electric drift fields.Comment: 12 pages, 7 figure

Quantum Dynamics of Spin Wave Propagation Through Domain Walls

Through numerical solution of the time-dependent Schrodinger equation, we demonstrate that magnetic chains with uniaxial anisotropy support stable structures, separating ferromagnetic domains of opposite magnetization. These structures, domain walls in a quantum system, are shown to remain stable if they interact with a spin wave. We find that a domain wall transmits the longitudinal component of the spin excitations only. Our results suggests that continuous, classical spin models described by LLG equation cannot be used to describe spin wave-domain wall interaction in microscopic magnetic systems