Search Results (15 total)

Ferroelectric transition has been detected in a ferrimagnetic spinel oxide of CoCr₂O₄ upon the transition to the conical spin order below 25 K. The direction [1̅ 10] of the spontaneous polarization is normal to both the magnetization easy axis [001] and to the propagation axis [110] of the transverse spiral component, in accord with the prediction based on the spin-current model. The reversal of the spontaneous magnetization by a small magnetic field (∼0.1  T) induces the reversal of the spontaneous polarization, indicating the clamping of the ferromagnetic and ferroelectric domain walls.

We investigate the statistical properties of cosmic baryon fluid in the nonlinear regime, which is crucial for understanding the large-scale structure formation of the Universe. With the hydrodynamic simulation sample of the Universe in the cold dark matter model with a cosmological constant, we show that the intermittency of the velocity field of cosmic baryon fluid at redshift z=0 in the scale range from the Jeans length to about 16  h^-1 Mpc can be extremely well described by She-Leveque’s universal scaling formula. The baryon fluid also possesses the following features: (1) for volume weight statistics, the dissipative structures are dominated by sheets, and (2) the relation between the intensities of fluctuations is hierarchical. These results imply that the evolution of highly evolved cosmic baryon fluid is similar to a fully developed turbulence.

We study the magnetic soliton dynamics of spinor Bose-Einstein condensates in an optical lattice which results in an effective Hamiltonian of anisotropic pseudospin chain. A modified Landau-Lifshitz equation is derived and exact magnetic soliton solutions are obtained analytically. Our results show that the time oscillation of the soliton size can be controlled in practical experiment by adjusting of the light-induced dipole-dipole interaction. Moreover, the elastic collision of two solitons is investigated.

Using in situ scanning tunneling microscopy, we observed that germanium deposited on the Ru(0001) surface near room temperature forms a two-dimensional wetting layer in the submonolayer regime, followed with growth of a segregated layer of Ge three-dimensional (3D) clusters of heights within about 1 nm. The growth of the first flat wetting layer can be understood in terms of optimal surface energy reduction by coating the Ru surface with a Ge layer which has a lower surface free energy. The nucleation and growth kinetics agrees with that derived from the conservative Ising model. Domains of a (√21×√21)R10.9° superstructure are observed on the wetting layer. Formation of a layer consisting of 1-nm-high clusters above the wetting layer indicates that the Ge wetting layer is extremely inert so that Ge adatoms can migrate large distances on the top of the wetting layer. The 3D Ge clusters seem to have a relatively narrow size distribution.

Ultraviolet photoemission spectra of Yb_2.75C₆₀ thin films are measured. The valence band is a wide hump centered at ∼0.8 eV below the Fermi level. The result also indicates the semiconducting property of Yb_2.75C₆₀ since no Fermi edge is observed. The hybridization between 6s states of Yb and the lowest-unoccupied-molecular-orbital (LUMO) band of C₆₀ is non-negligible although it should not be considered to be strong. More than 14% of Yb 6s electrons are estimated to be distributed in the covalent bonds between Yb and C₆₀. The spectra for submonolayer C₆₀ on Yb film reveal that Yb 6s electrons can easily transfer to C₆₀ and such demonstrate that the bonding in Yb_2.75C₆₀ is mainly ionic. The LUMO+1 orbital of the submonolayer C₆₀ is partially occupied, which is different from the case in Yb_2.75C₆₀. There is no evidence of trivalent Yb in the spectra.

Photoemission measurements for N,N^′-bis-(1-naphthyl)-N,N^′-diphenyl-1,1^′-biphenyl-4,4^′-diamine (NPB) overlayer on Ag(111) are reported. The growth of NPB overlayer from submonolayer on the single-crystal surface of Ag(111), ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS) measurements at each growth step allowed a better determination of band bending and interface dipole related change in work function. The XPS measurements show a weak interaction between NPB and Ag(111), and the C 1s core level shift provides a measurement of the band bending occurring with the NPB overlayer. UPS measurements show a decrease in work function with increasing thickness of NPB overlayer. The dipole contribution to the work function as a function of NPB thickness is deduced by subtracting the band bending from the change in work function. The dipole contribution has a minimum at a NPB thickness of about a monolayer. The decrease of work function is attributed to both the band bending occurring within NPB overlayer and dipole layer formed due to surface polarization. Additionally, the top of the highest occupied molecular orbital of NPB is located at about 1.7 eV below Fermi level.

Detailed experimental results of staging two laser-driven, relativistic electron accelerators are presented. During the experiment called STELLA (staged electron laser acceleration), an inverse free-electron laser (IFEL) is used to modulate the electron energy, thereby, causing ∼3 fs microbunches to form separated by the laser wavelength at 10.6 μm (equivalent to a 35 fs period). A second IFEL accelerates the electrons depending upon the phase of the microbunches entering the second IFEL with respect to the laser beam driving the second IFEL. The data presented includes electron energy spectra as a function of the phase delay and laser power driving the first IFEL. Also shown is a comparison with the computer model, which includes space charge and misalignment effects.

Staging of two laser-driven, relativistic electron accelerators has been demonstrated for the first time in a proof-of-principle experiment, whereby two distinct and serial laser accelerators acted on an electron beam in a coherently cumulative manner. Output from a CO₂ laser was split into two beams to drive two inverse free electron lasers (IFEL) separated by 2.3 m. The first IFEL served to bunch the electrons into ∼3 fs microbunches, which were rephased with the laser wave in the second IFEL. This represents a crucial step towards the development of practical laser-driven electron accelerators.

Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at sqrt[s_NN]  =  130  GeV using the STAR Time Projection Chamber at the Relativistic Heavy Ion Collider. The elliptic flow signal, v₂, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.

7.6×10⁶ x-ray photons per 3.5 ps pulse are detected within a 1.8–2.3 Å spectral window during a proof-of-principle laser synchrotron source experiment. A 600 MW CO₂ laser interacted in a head-on collision with a 60 MeV, 140 A, 3.5 ps electron beam. Both beams were focused to a σ  =  32 μm spot. Our next plan is to demonstrate 10¹⁰ x-ray photons per pulse using a CO₂ laser of ∼1 TW peak power.

K₃C₆₀ single-crystal film was prepared on the cleaved (111) surface of C₆₀ single crystal. Synchrotron radiation angle-resolved photoemission spectra were measured at normal emission with sample temperature at ∼150 K. Up to four subpeaks of LUMO-derived bands were observed. These subpeaks exhibit distinct energy dispersions which resemble, in general, the theoretical ones calculated for K₃C₆₀ at low temperature with the so-called one-dimensional disordered structure. But there is a large deviation of experimental subband intervals from the theoretical values. By considering the influence of the Coulomb energy of K⁺ ion and the valence electron, the deviation, at least for the order of magnitude, is interpreted. This result is meaningful for the studies of the physical properties of alkali-doped C₆₀ solids, e.g., the mechanism for superconductivity. Better interpretations for the experimental band dispersions demand further research work, some possible directions of which are suggested.

An investigation of potassium-promoted oxidation of β-SiC has been performed by means of x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and the work-function change measured from the shift of the slow secondary-electron cutoff in the ultraviolet photoemission, for different potassium coverages and substrate temperatures. It is shown that there exist saturations for both K- and Si-bonded oxygen after sufficient oxygen doses at room temperature no matter what the coverage of the alkali metal, while there exists a saturation only for the K-bonded oxygen even after a much higher exposure at the elevated temperature of 500 K than that required for the saturation at RT; the oxygen bonded to the catalyst is in the state of O₂^2- at room temperature, and it converts to O^2- ion at 500 K; the existence of silicon suboxides SiO_x (x<2) is proved for substrate temperatures lower than 500 K, and they convert to SiO₂ at 700 K in an oxygen diffusion-controlled way.

A densely packed 1×1-O overlayer with an oxygen coverage of 1.0 (relative to the number of substrate surface atoms) was prepared on Ru(0001) by sequential dissociation of O₂ and NO₂ and studied by means of high-resolution electron-energy-loss spectroscopy. Two dipole-active modes were observed at 81 and 7.3 meV due to a ν_⊥(Ru-O) stretch mode and a new surface mode, respectively. The value of ν_⊥(Ru-O) is shifted to higher energy compared to that for the known 2×2-O and 2×1-O adlayers at smaller oxygen coverages. These measurements indicate that the O₂ dissociation is kinetically hindered for oxygen coverages larger than 0.5.

We report a high-resolution electron-energy-loss spectroscopy study of the coverage dependence of the Cs-Ru stretch vibration at 300 K. Surprisingly, the frequency of Cs-Ru stretch vibration shifts upward by about 30% with Cs coverage for 0<~θ_Cs<~0.19. Only a quarter of this shift can be explained through lateral dipole-dipole interaction. We discuss two further mechanisms that may cause the other part of the shift. A slight decrease of the stretch frequency for θ_Cs>0.19 is associated with the metallization of the alkali-metal layer including a structural change.