Constraints on the warm dark matter model from gravitational lensing

Formation of sub-galactic halos is suppressed in warm dark matter (WDM) model due to thermal motion of WDM particles. This may provide a natural resolution to some puzzles in standard cold dark matter (CDM) theory such as the cusped density profiles of virialized dark halos and the overabundance of low mass satellites. One of the observational tests of the WDM model is to measure the gravitationally lensed images of distant quasars below sub-arcsecond scales. In this Letter, we report a comparison of the lensing probabilities of multiple images between CDM and WDM models using a singular isothermal sphere model for the mass density profiles of dark halos and the Press-Schechter mass function for their distribution and cosmic evolution. It is shown that the differential probability of multiple images with small angular separations down to 10 milliarcseconds should allow one to set useful constraints on the WDM particle mass. We discuss briefly the feasibility and uncertainties of this method in future radio surveys (e.g. VLBI) for gravitational lensing.Comment: 3 pages, 1 figure, accepted for publication in ApJ Let

Electron interferometry in quantum Hall regime: Aharonov-Bohm effect of interacting electrons

An apparent h/fe Aharonov-Bohm flux period, where f is an integer, has been reported in coherent quantum Hall devices. Such sub-period is not expected for non-interacting electrons and thus is thought to result from interelectron Coulomb interaction. Here we report experiments in a Fabry-Perot interferometer comprised of two wide constrictions enclosing an electron island. By carefully tuning the constriction front gates, we find a regime where interference oscillations with period h/2e persist throughout the transition between the integer quantum Hall plateaus 2 and 3, including half-filling. In a large quantum Hall sample, a transition between integer plateaus occurs near half-filling, where the bulk of the sample becomes delocalized and thus dissipative bulk current flows between the counterpropagating edges ("backscattering"). In a quantum Hall constriction, where conductance is due to electron tunneling, a transition between forward- and back-scattering is expected near the half-filling. In our experiment, neither period nor amplitude of the oscillations show a discontinuity at half-filling, indicating that only one interference path exists throughout the transition. We also present experiments and an analysis of the front-gate dependence of the phase of the oscillations. The results point to a single physical mechanism of the observed conductance oscillations: Aharonov-Bohm interference of interacting electrons in quantum Hall regime.Comment: 10 pages, 4 Fig

Correlation Between the Halo Concentration (c) and the Virial Mass (Mvir) Determined from X-ray Clusters

Numerical simulations of structure formation have suggested that there exists a good correlation between the halo concentration c (or the characteristic density delta_c) and the virial mass Mvir for any virialized dark halo described by the Navarro, Frenk & White (1995) density profile. In this Letter, we present an observational determination of the c-Mvir (or delta_c-Mvir) relation in the mass range of 10^14< Mvir <10^16 (solar mass) using a sample of 63 X-ray luminous clusters. The best-fit power law relation, which is roughly independent of the values of Omega_M and Lambda, is c propto Mvir^(-0.5) or delta_c propto Mvir^(-1.2), indicating n=-0.7 for a scale-free power spectrum of the primordial density fluctuations. We discuss the possible reasons for the conflict with the predictions by typical CDM models such as SCDM, LCDM and OCDM.Comment: 13 pages, 1 figure, two tables. Accepted for publication in ApJ

Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory

Magnetic skyrmions are promising for building next-generation magnetic memories and spintronic devices due to their stability, small size and the extremely low currents needed to move them. In particular, skyrmion-based racetrack memory is attractive for information technology, where skyrmions are used to store information as data bits instead of traditional domain walls. Here we numerically demonstrate the impacts of skyrmion-skyrmion and skyrmion-edge repulsions on the feasibility of skyrmion-based racetrack memory. The reliable and practicable spacing between consecutive skyrmionic bits on the racetrack as well as the ability to adjust it are investigated. Clogging of skyrmionic bits is found at the end of the racetrack, leading to the reduction of skyrmion size. Further, we demonstrate an effective and simple method to avoid the clogging of skyrmionic bits, which ensures the elimination of skyrmionic bits beyond the reading element. Our results give guidance for the design and development of future skyrmion-based racetrack memory.Comment: 15 pages, 6 figure

Quantum molecular dynamics simulations for the nonmetal-metal transition in shocked methane

We have performed quantum molecular-dynamics simulations for methane under shock compressions up to 80 GPa. We obtain good agreement with available experimental data for the principal Hugoniot, derived from the equation of state. A systematic study of the optical conductivity spectra, one-particle density of states, and the distributions of the electronic charge over supercell at Hugoniot points shows that the transition of shocked methane to a metallic state takes place close to the density at which methane dissociates significantly into molecular hydrogen and some long alkane chains. Through analyzing the pair correlation function, we predict the chemical picture of the shocked methane. In contrast to usual assumptions used for high pressure modeling of methane, we find that no diamond-like configurations occurs for the whole density-temperature range studied.Comment: Some revisions have been given in response to referees' sugestion

Exploring the magnetic properties of the largest single molecule magnets

The giant {Mn₇₀} and {Mn₈₄} wheels are the largest nuclearity single-molecule magnets synthesized to date, and understanding their magnetic properties poses a challenge to theory. Starting from first-principles calculations, we explore the magnetic properties and excitations in these wheels using effective spin Hamiltonians. We find that the unusual geometry of the superexchange pathways leads to weakly coupled {Mn₇} subunits carrying an effective S = 2 spin. The spectrum exhibits a hierarchy of energy scales and massive degeneracies, with the lowest-energy excitations arising from Heisenberg-ring-like excitations of the {Mn₇} subunits around the wheel. We further describe how weak longer-range couplings can select the precise spin ground-state of the Mn wheels out of the nearly degenerate ground-state band

Dynamical study of the light scalar mesons below 1 GeV in a flux-tube model

The light scalar mesons below 1 GeV as tetraquark states are studied in the framework of the flux-tube model, the multi-body confinement instead of the additive two-body confinement is used. From the calculated results, we find that the light scalar mesons, $\sigma$, $\kappa$ could be well accommodated in the diquark-antidiquark tetraquark picture in the flux-tube model and they could be color confinement resonances. The mass of the first radial excited state of $[ud][\bar{u}\bar{d}]$ is 1019 MeV, which is close to the mass of $f_0(980)$. Whereas $a_0(980)$ can not be fitted in this interpretation.Comment: 11 pages, 1 figur