Bounday Condition histograms for modulated phases

Boundary conditions strongly affect the results of numerical computations for finite size inhomogeneous or incommensurate structures. We present a method which allows to deal with this problem, both for ground state and for critical properties: it combines fluctuating boundary conditions and specific histogram techniques. Our approach concerns classical systems possessing a continuous symmetry as well as quantum systems. In particular, current-current correlation functions, which probe large scale coherence of the states, can be accurately evaluated. We illustrate our method on a frustrated two dimensional XY model.Comment: 31 pages, 8 figure

Bounds for the Superfluid Fraction from Exact Quantum Monte Carlo Local Densities

For solid 4He and solid p-H2, using the flow-energy-minimizing one-body phase function and exact T=0 K Monte Carlo calculations of the local density, we have calculated the phase function, the velocity profile and upper bounds for the superfluid fraction f_s. At the melting pressure for solid 4He we find that f_s < 0.20-0.21, about ten times what is observed. This strongly indicates that the theory for the calculation of these upper bounds needs substantial improvements.Comment: to be published in Phys. Rev. B (Brief Reports

Phase Diagram for Magnon Condensate in Yttrium Iron Garnet Film

Recently, magnons, which are quasiparticles describing the collective motion of spins, were found to undergo Bose-Einstein condensation (BEC) at room temperature in films of Yttrium Iron Garnet (YIG). Unlike other quasiparticle BEC systems, this system has a spectrum with two degenerate minima, which makes it possible for the system to have two condensates in momentum space. Recent Brillouin Light scattering studies for a microwave-pumped YIG film of thickness d=5 $\mu$m and field H=1 kOe find a low-contrast interference pattern at the characteristic wavevector $Q$ of the magnon energy minimum. In this report, we show that this modulation pattern can be quantitatively explained as due to non-symmetric but coherent Bose-Einstein condensation of magnons into the two energy minima. Our theory predicts a transition from a high-contrast symmetric phase to a low-contrast non-symmetric phase on varying the $d$ and $H$, and a new type of collective oscillations.Comment: 6 figures. Accepted by Nature Scientific Report

Catalogue of lunar craters cross sections. I - Craters with peaks Research report no. 16

Lunar craters with centrally located peaks - tables and profile graph

Facile incorporation of technetium into magnetite, magnesioferrite, and hematite by formation of ferrous nitrate in situ: precursors to iron oxide nuclear waste forms.

The fission product, 99Tc, presents significant challenges to the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate (TcO4-), the stable Tc species in aerobic environments. Migration of 99Tc from disposal sites can potentially be prevented by incorporating it into durable waste forms based on environmentally stable minerals. Since Tc(iv) and Fe(iii) have the same ionic radius, Tc(iv) can replace Fe(iii) in iron oxides. Environmentally durable iron oxides include goethite (α-FeOOH), hematite (α-Fe2O3), and magnesioferrite (MgFe2O4). The incorporation of Tc into two of these, hematite and magnesioferrite, as well as magnetite (Fe3O4) by means of simple, aqueous chemistry is presented starting from TcO4- in 5 M nitric acid. A combination of X-ray diffraction and X-ray absorption fine structure spectroscopy reveals that Tc(iv) replaces Fe(iii) within the iron oxide structures. Following incorporation, Tc doped samples were suspended in deionized water under aerobic conditions, and the release rates of Tc were determined. The results of this work show that Tc leaches more quickly from Fe3O4 than from α-Fe2O3 or MgFe2O4. Modeling the leach rates and comparison with the leach rate of Tc from TiO2 indicate that release of Tc is controlled by solid state diffusion

Resonant switching using spin valves

Using micromagnetics we demonstrate that the r.f. field produced by a spin valve can be used to reverse the magnetization in a magnetic nanoparticle. The r.f. field is generated using a current that specifically excites a uniform spin wave in the spin valve. This current is swept such that the chirped-frequency generated by the valve matches the angular dependent resonant frequency of the anisotropy-dominated magnetic nanoparticle, as a result of which the magnetization reversal occurs. The switching is fast, requires currents similar to those used in recent experiments with spin valves, and is stable with respect to small perturbations. This phenomenon can potentially be employed in magnetic information storage devices or recently discussed magnetic computing schemes