Search Results (8 total)

The expansion of the Universe appears to be accelerating, and the mysterious antigravity agent of this acceleration has been called “dark energy.” To measure the dynamics of dark energy, baryon acoustic oscillations (BAO) can be used. Previous discussions of the BAO dark energy test have focused on direct measurements of redshifts of as many as 10⁹ individual galaxies, by observing the 21 cm line or by detecting optical emission. Here we show how the study of acoustic oscillation in the 21 cm brightness can be accomplished by economical three-dimensional intensity mapping. If our estimates gain acceptance they may be the starting point for a new class of dark energy experiments dedicated to large angular scale mapping of the radio sky, shedding light on dark energy.

In this Reply, we reexamine the beating Shubnikov–de Haas oscillations by a nonlinear curve-fitting technique. The results do not support the arguments of Tang [Phys. Rev. B 73, 037301 (2006)], and it is unlikely that the beating Shubnikov–de Haas oscillations we observed in Al_xGa_1−xN∕GaN heterostructures originate from magnetointersubband scattering.

We have studied the electronic properties of Al_xGa_1-xN/GaN heterostructures by using Shubnikov–de Haas (SdH) measurement. Two SdH oscillations were detected on the samples of x=0.35 and 0.31, due to the population of the first two subbands with the energy separations of 128 and 109 meV, respectively. For the sample of x=0.25, two SdH oscillations beat each other, probably due to a finite zero-field spin splitting. The spin-splitting energy is equal to 9.0 meV. The samples also showed a persistent photoconductivity effect after illuminating by blue light-emitting diode.

A model of surfactant-mediated epitaxy is investigated using the kinetic Monte Carlo method. This model assumes that (1) adatom-adatom interaction on the surfactant layer is weakly repulsive for dimers, and (2) a concerted atomic exchange of adatoms with surfactant atoms occurs when adatom clusters above the surfactant layer reach a threshold size. All essential features observed in a recent study of Ge/Pb/Si(111) reported by Hwang, Chang, and Tsong can be satisfactorily explained with this model.

We report a discovery that the nucleation and growth of two-dimensional (2D) Ge islands at a Pb layer covered Si(111) surface are reaction limited. Using scanning tunneling microscopy, a compact-to-fractal island shape transition is observed as the deposition flux is lowered, the temperature is raised, or at a low Ge coverage. This behavior is completely opposite to what was predicted from those theories based on diffusion-limited aggregation and previous experimental observations. Energy barriers are found to exist for the nucleation and growth of Ge islands, indicating that their growth behavior is exchange-reaction rate limited.

The initial stage of nucleation and growth of two-dimensional (2D) Ge islands on Pb covered Si(111) surfaces at room temperature is studied using scanning tunneling microscopy. The Pb overlayer is found to greatly enhance surface diffusion of deposited Ge adatoms. There is a critical deposition time above which the observed island density increases sharply while the average island size decreases. We believe nucleation as well as growth is initially hindered by the high energy barriers for Ge clusters to exchange with Pb atoms until Ge clusters reach a critical size. A mechanism is proposed to explain the observed phenomena.

The Al₆₅Cu₂₀Ru₁₅ icosahedral alloy was studied by ²⁷Al nuclear magnetic resonance from 150 to 1110 K. The Knight shift of the unresolved resonance line was observed to significantly increase above 500 K. This uncommon temperature dependence of the Knight shift is interpreted in terms of the presence of a pseudogap at the Fermi level. The spin-lattice relaxation rate deviates from the linear temperature dependence of Korringa relaxation below 500 K, and above 500 K it is dominated by a thermally activated process with a small activation energy of 0.48 eV. This energy is distinctly different from the activation energy observed in simple metallic alloys.

We have measured high-resolution electron decay-in-flight spectra of highly excited carbon ions, produced by 2-MeV C⁺ beams emerging from carbon foils. An almost complete identification for the resolved spectral lines is suggested for the prompt and time-delayed electron emission spectra. To assign the observed structures excitation energies for various core-excited states have been calculated using the generalized Brillouin theorem-multiconfiguration method of Schwarz and Chang. Experimental term energies for Coulomb and metastable autoionizing states in two-, three-, and four-electron carbon ions are tabulated and compared to calculated transition energies. Because of line blending most of the experimentally deduced term energies are uncertain to ±0.4%. Some spectral features in the high-energy portion of the prompt spectrum can be attributed to decays of doubly excited He-like carbon ions.