Resolution enhancement in a reflection mode near-field optical microscope by second-harmonic modulation signals

The motion of the probe tip in a near-field scanning optical microscope, dithered by vibration of a tuning fork, can modulate the reflection signal from the sample surface not only at the fundamental dithering frequency but also at its second harmonic. By lock-in amplification of these modulated signals, enhanced optical images are obtained, even with an uncoated fiber probe. In particular, accurate optical images with higher resolution are obtained when the second-harmonic signal is detected, which results from the parametric modulation of the tip-sample separation at the double frequency of the horizontal dithering motion of the tip. Using a DVD ROM with a track pitch of 0.74 mum as a test sample, we observed that the sharp edges around the pits are clearly resolved with the second-harmonic signals and obtained enhanced resolution of ~70 nm full width at half-maximum.     

Slow spin relaxation of Rb atoms confined in glass cells filled with dense 4He gas at 1.85 K

At temperatures below 2.1 K, long-lived gaseous Rb atoms in glass cells have been generated with a simple method: irradiating the cells, containing 4He gas and Rb metal, with a cw laser. The obtained atomic Rb density ( approximately 10(8) cm(-3)) decreases with a 1/e time constant of about 10 s at 1.85 K. We have performed optical pumping of the Rb atoms and measured the longitudinal electronic spin relaxation time at 1.85 K as well. For processes (such as Rb-He collisions) which do not remove the atomic Rb from the vapor, this relaxation time is found to be about 60+/-15 s.     

Thermodynamic phase diagram of the quantum hall skyrmion system

We numerically study the interacting quantum Hall Skyrmion system based on the Chern-Simons action. By noticing that the action is invariant under global spin rotations in the spin space with respect to the magnetic field direction, we obtain the low-energy effective action for a many-Skyrmion system. Performing extensive molecular dynamics simulations, we establish the thermodynamic phase diagram for a many-Skyrmion system.     

Origin of the high piezoelectric response in PbZr1-xTixO3

High resolution x-ray powder diffraction measurements on poled PbZr1-xTixO3 (PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the so-called morphotropic phase boundary) have shown that for both rhombohedral and tetragonal compositions the piezoelectric elongation of the unit cell does not occur along the polar directions but along those directions associated with the monoclinic distortion. This work provides the first direct evidence for the origin of the very high piezoelectricity in PZT.     

Magnetic response of microbially synthesized transition metal- and lanthanide-substituted nano-sized magnetites

The magnetic susceptibility (κ_RT) and saturation magnetization (M_S) of microbially synthesized magnetites were systematically examined. Transition metal (Cr, Mn, Co, Ni and Zn)- and lanthanide (Nd, Gd, Tb, Ho and Er)-substituted magnetites were microbially synthesized by the incubation of transition metal (TM)- and lanthanide (L)-mixed magnetite precursors with either thermophilic (TOR-39) or psychrotolerant (PV-4) metal-reducing bacteria (MRB). Zinc incorporated congruently into both the precursor and substituted magnetite, while Ni and Er predominantly did not. Microbially synthesized Mn- and Zn-substituted magnetites had higher κ_RT than pure biomagnetite depending on bacterial species and they exhibited a maximum κ_RT at 0.2 cationic mole fraction (CMF). Other TMs’ substitution linearly decreased the κ_RT with increasing substitution amount. Based on the M_S values of TM- and L-substituted magnetite at 0.1 and 0.02 CMF, respectively, Zn (90.7 emu/g for TOR-39 and 93.2 emu/g for PV-4)- and Mn (88.3 emu/g by PV-4)-substituted magnetite exhibited higher M_S than standard chemical magnetite (84.7 emu/g) or pure biomagnetite without metal substitution (76.6 emu/g for TOR-39 and 80.3 emu/g for PV-4). Lanthanides tended to decrease M_S, with Gd- and Ho-substituted magnetites having the highest magnetization. The higher magnetization of microbially synthesized TM-substituted magnetites by the psychrotroph, PV-4 may be explained by the magnetite formation taking place at low temperatures slowing mechanics, which may alter the magnetic properties compared to the thermophile, through suppression of the random distribution of substituted cations.     

Theoretical study on structure of boron nitride fullerenes

We propose the parameters of the Stillinger-Weber potential for hexagonal boron nitride (BN) structures. For the reliability of these parameters, the structural property of BN fullerenes is investigated. The stability of BN fullerenes increases with increasing the number of atoms, due to the reduction of the curvature effect of BN fullerenes. The structures of the relative stable fullerenes are B₁₆N₁₆, B₁₈N₁₈, B₂₂N₂₂, B₂₅N₂₅, and B₂₈N₂₈.     

Interfacial kinetic enhancement of metal ion adsorption on binary mixed self-assembled monolayers

The adsorption of metal ions, a type of surface reaction on binary mixed self-assembled monolayers (SAMs) on a gold surface composed of 1,6-hexanedithiol (HDT) with 11-mercaptoundecanoic acid (MUA), was monitored by in situ surface plasmon resonance (SPR) measurements. The differential SPR reflectance (ΔR) enables the kinetics of adsorption of Pt²⁺ on the mixed SAMs to be investigated. Unlike single HDT SAM, kinetic analyses of the mixed SAMs showed that the rate of adsorption of Pt²⁺ was enhanced and that it was highly dependent on the fraction of MUA present. These SPR measurements suggest that the adsorption rate of metal ions can be readily manipulated simply by using mixed SAMs.     

Surface status and size influences of nickel nanoparticles on sulfur compound adsorption

Metallic nickel nanoparticles were incorporated on mesoporous silica to remove sulfur compounds in diesel selectively. In the first method, nickel nanoparticles were formed on mesoporous silica SBA-15 by impregnation and subsequent reduction of nickel nitrate. The sulfur adsorption capacity was strongly dependent on the nickel loading and the average nickel particle size. In the second method, nickel nanoparticles were synthesized in solution in the presence of a capping agent and then incorporated in mesoporous silica MCF by sonication. Although these particles maintain their sizes on the MCF surface after heat treatment, capping agent remaining on the Ni particle surface might interfere the adsorption of sulfur compounds.     

Influence of pretreatment methods on adsorption and catalytic characteristics of toluene over heterogeneous palladium based catalysts

The adsorption and catalytic characteristics of heterogeneous palladium based catalyst and its modified catalysts with gases (air and hydrogen) and acidic aqueous solution (HCl) were studied for evaluating the influence of pretreatment methods for toluene. The structural and energetic adsorption properties of the parent and pretreated catalysts were analyzed by means of nitrogen adsorption isotherms and gravimetric methods. The light-off curve and the XPS investigation were used for analyzing the catalytic activity and the surface state of palladium. It was clearly shown from the experimental results that hydrogen pretreated catalysts having metallic surface state exhibited the highest adsorption capacity and catalytic activity compared to that of parent and modified catalysts. The adsorption equilibrium data for toluene were obtained at three different temperatures and correlated successfully with the two-site Langmuir molecular isotherm model (L2m). It was also found that the palladium phase has more adsorption affinity for toluene molecules than the alumina support. The isosteric heat of adsorption calculated by using the Clausius-Clapeyron equation significantly changed with the coverage and the lateral interactions between the adsorbate-adsorbate molecules control the system. Furthermore, comparative analysis of the adsorption energy distribution revealed that the parent and its modified catalysts have different types of surface energetic heterogeneities.     

Dynamic stability of rotor-bearing systems subjected to random axial forces

This paper investigates the lateral vibration of a spinning disk-shaft system supported by a pair of ball bearings and subjected to a pair of random axial forces at both ends. The axial forces are assumed as the sum of a static force and a random process with a zero mean. Due to the random axial forces, the rotor-bearing system may experience parametric random instability under certain situations. In this work, the finite element method is applied to yield a set of discretized system equations first. The set of discretized system equations is partially uncoupled by the modal analysis procedure suitable for gyroscopic systems. The stochastic averaging method is then adopted to obtain Ito's equations for the response amplitudes of the system. Finally the first- and second-moment stability criteria are utilized to determine the stability boundaries of the system. Numerical results show that the rotor-bearing system is always stable in the sense of the first-moment stability, and the effects of the average axial compressive force and the disk mass, which will lower all frequencies of the system, tend to destabilize the second-moment stability of the system.