Search Results (125 total)

Submonolayer coverages of the molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) deposited on NaCl(001) surfaces were imaged with high resolution noncontact-atomic force microscopy. Two island types were observed: monolayer islands with a p3×3 epitaxy at low coverage and a mixture of these and bulklike crystallites at higher coverage. The transition between the pure monolayer islands and mixed islands occurs at ∼0.85  ML, corresponding to a complete p3×3 layer. Calculations show the p3×3 epitaxy to be incompatible with a multilayer crystal of PTCDA. Consequently, the growth of additional layers results in an adaptation of the interface structure forcing a dewetting transition.

The spin reorientation transition (SRT) and magnetic domain structure of Co ultrathin films on faceted Au(455) surfaces have been investigated as a function of thickness by using magneto-optic Kerr effect and x-ray photoemission electron microscopy in combination with x-ray magnetic circular dichroism. The magnetization easy axis of the Co films is found to rotate from an out-of-plane to an in-plane orientation perpendicular to the atomic steps of the substrate due to the dominant contribution of the step-induced magnetic anisotropy. Moreover, magnetic stripe domains are observed to replicate the underlying periodic facets of the Au surface. Depending on the facet, the SRT occurs at a well-defined critical thickness and proceeds via a different mechanism: a state of uniform canted magnetization is formed for (233) facets, while the SRT involves continuous magnetization rotation followed by the nucleation of small in-plane magnetic domains for (677) facets.

We present the calculations of the next-to-leading order (NLO) QCD corrections to the inclusive total cross sections for the associated production of the W^±H^∓ through bb̅ annihilation in the minimal supersymmetric standard model at the CERN Large Hadron Collider. The NLO QCD corrections can either enhance or reduce the total cross sections, but they generally efficiently reduce the dependence of the total cross sections on the renormalization/factorization scale. The magnitude of the NLO QCD corrections is about 10% in most of the parameter space and can reach 15% in some parameter regions. We also show the Monte Carlo simulation results for the 2j+τ_jet+p̸_T signature from the W^± and the H^∓ decays including the NLO QCD effects, and find an observable signal at a 5σ level in some parameter region of the minimal supergravity model.

Dipole-dipole interactions between excited Rb atoms at long range (∼300a₀–2150a₀) have been observed with molecular wave packets and a coherent nonlinear optical process. Fourier analysis of the parametric four-wave mixing (PFWM) signal wave intensity produced in femtosecond pump-probe experiments demonstrates the appearance of sidebands associated with the Rb 7s-5d_5/2 quantum beating frequency of ∼18.3  THz. Calculations show that the observed sideband splittings and Fourier domain profiles result from multiple atom, dipole-dipole interactions, and ensembles comprising five or fewer Rb (7s, 6p) atoms account for virtually all of the data.

One of the important applications of electromagnetically induced transparency (EIT) is to delay and store light in atomic ensembles. In this paper, the noise spectrum of the delayed quantum light throughout an EIT medium is investigated. With zero detection frequency, we can have minimum noise of delayed light in two-photon resonance of EIT, and the noise is larger than the minimum noise at off two-photon resonance due to the phase-to-amplitude noise conversion. It is shown that the noise for nonzero detection frequency can be suppressed by operating the system at off two-photon resonance, even when the unavoidable dephasing is included.

Chaos synchronization in coupled chaotic oscillator systems with diffusive and gradient couplings forced by only one local feedback injection signal (boundary pinning control) is studied. By using eigenvalue analysis, we obtain controllable regions directly in control parameter space for different types of coupling links (including diagonal coupling and nondiagonal couplings). The effects of both diffusive and gradient couplings on chaos synchronization become clear. Some relevant factors on control efficiency such as coupled system size, transient process, and feedback signal intensity are also studied.

Kondo resonances are a very precise measure of spin-polarized transport through magnetic impurities. However, the Kondo temperature, indicating the thermal range of stability of the magnetic properties, is very low. By contrast, we find for iron phthalocyanine a Kondo temperature in spectroscopic measurements which is well above room temperature. It is also shown that the signal of the resonance depends strongly on the adsorption site of the molecule on a gold surface. Experimental data are verified by extensive numerical simulations, which establish that the coupling between iron states and states of the substrate depends strongly on the adsorption configuration.

The electron transport through a three-terminal single-molecular transistor (SMT) is theoretically studied. We find that the differential conductance of the third and weakly coupled terminal versus its voltage matches well with the spectral function versus the energy when certain conditions are met. Particularly, this excellent matching is maintained even for the complicated structure of the phonon-assisted side peaks. Thus, this device offers an experimental approach to explore the shape of the phonon-assisted spectral function in detail. In addition, we discuss the conditions of a perfect matching. The results show that at low temperatures, the matching survives regardless of the bias and the energy levels of the SMT. However, at high temperatures, the matching is destroyed.

Axially phase-matched parametric six-wave mixing (PSWM) has been observed in Rb vapor by two-photon excitation of the atom (5s→→7s,5d) with ∼150  fs pulses, centered at λ∼769  nm and generated by a Ti:Al₂O₃ laser system. For a peak laser pulse intensity of 30  GW  cm⁻², PSWM is observed for Rb number densities ([Rb])>2.5×10¹⁵  cm⁻³ whereas, for [Rb]=2.2×10¹⁶  cm⁻³, the PSWM process is detectable for pump intensities above ∼3  GW  cm⁻². Upconversion of the 1.367  μm and 1.323  μm signal waves into the visible (λ∼494  nm) has been accomplished by sum frequency generation in LiIO₃, and quantum beating in Rb at 18.3  THz (∼608  cm⁻¹) has been observed by monitoring the upconverted signal wave intensity produced in pump-probe experiments. These results broaden the scope of the wave packet detection scheme reported by Tran [Opt. Lett. 23, 70 (1998)] in that a variety of coherent nonlinear optical processes (χ⁽³⁾,χ⁽⁵⁾,…) appear to be suitable for observing the amplitude and phase of atomic (or molecular) wave packets.

We report scanning-tunneling-microscopy observations and first-principles calculations for the formation and evolution of self-organized Ge nanostructures on Si(111)-(7×7) surfaces for Ge coverages up to 0.5 ML. We show that individual Ge atoms initially form a triangular lattice. At higher coverages, Ge nanoparticles 1 nm in diameter gradually form in both the faulted and unfaulted half unit cells with an initial preference in the faulted halves, ultimately driving ordered hexagonal arrays. The underlying 7×7 surface periodicity, the triangular single-Ge lattice, and the nanoparticle hexagonal superstructures coexist. Charge transfer from Si adatoms to Ge nanoparticles is shown to play a key role in the self-organization.

Transport properties of La_0.8Ca_0.2MnO₃ thin films 15 and 130 nm thick have been investigated and confronted with the properties of bulk single crystals of the same composition. It has been found that low-temperature resistivity of the films is sensitive to electric current and/or field treatment and thermal history of the sample. Thin films exhibit a variety of metastable resistive states and spontaneously evolve toward high-resistivity state in which the films exhibit highly nonlinear transport behavior at low temperatures. Nonlinear V-I characteristics are well described by indirect tunneling model. The memory of the resistivity can be, at least partly, erased by a heat treatment at temperatures above the memory erasing temperature. The memory erasing temperature for thin films, T=450 K, is significantly higher than that of single crystals. The results are interpreted in the context of strain driven phase separation. Coexistence of two ferromagnetic phases with different orbital orders and different conductivities is influenced by strains due to thermal cycling and current flow.

Direct and simultaneous measurements of the normal and lateral forces encountered by a nanosize spherical silicon tip approaching a solid surface in purified water are reported. For tip-surface distances, 0±0.03 nm<d<2 nm, experiments and grand canonical molecular-dynamics simulations find oscillatory solvation forces for hydrophilic surfaces, mica and glass, and less pronounced oscillations for a hydrophobic surface, graphite. The simulations reveal layering of the confined water density and the development of hexagonal order in layers proximal to a quartz surface. For subnanometer hydrophilic confinement, the lateral force measurements show orders of magnitude increase of the viscosity with respect to bulk water, agreeing with a simulated sharp decrease in the diffusion constant. No viscosity increase is observed for hydrophobic surfaces.

Scanning tunneling microscopy observations and first-principles quantum mechanical calculations were employed to investigate the nanoscale structures formed on Si(111) surfaces upon germanium deposition at a coverage of ∼0.3 monolayer. At room temperature, Ge atoms form nanoclusters with sizes of 1.5–6 nanometers in width. After annealing, the nanoclusters become two-dimensional islands with typical size of ∼10 nanometers in width. We propose that the annealing or high-T deposition results in a partial transformation of (7×7) reconstructed unit cells to unreconstructed Si(111) configurations on which the Ge adatoms reside at the T₄ sites and form a (sqrt[3]×sqrt[3])R30° reconstruction.

It is well known that in the single-spiral-stable parameter regimes of the two-dimensional complex Ginzburg-Landau equation, spiral-domain patterns spontaneously appear. These patterns are disordered cells of frozen spiral waves well separated by thin walls (shocks), and to a good approximation, the walls are segments of hyperbolas. In this paper, we take a closer look at the global structure of spiral-domain patterns by using rigorous mathematical analysis and considering the unusual effect of the chirality (handedness) of spiral wave. An equation that determines the slope of the shock line is derived. We generalize this analytical method to study the interaction of a pair of spirals with different rotation frequencies, and obtain the geometrical structures of the shock line and the wave front of the invasion wave in transient processing.

Time series from complex systems with interacting nonlinear and stochastic subsystems and hierarchical regulations are often multiscaled. In devising measures characterizing such complex time series, it is most desirable to incorporate explicitly the concept of scale in the measures. While excellent scale-dependent measures such as ϵ entropy and the finite size Lyapunov exponent (FSLE) have been proposed, simple algorithms have not been developed to reliably compute them from short noisy time series. To promote widespread application of these concepts, we propose an efficient algorithm to compute a variant of the FSLE, the scale-dependent Lyapunov exponent (SDLE). We show that with our algorithm, the SDLE can be accurately computed from short noisy time series and readily classify various types of motions, including truly low-dimensional chaos, noisy chaos, noise-induced chaos, random 1∕f^α and α-stable Levy processes, stochastic oscillations, and complex motions with chaotic behavior on small scales but diffusive behavior on large scales. To our knowledge, no other measures are able to accurately characterize all these different types of motions. Based on the distinctive behaviors of the SDLE for different types of motions, we propose a scheme to distinguish chaos from noise.

The wealth of quantum coherence effects depending on the orientation of external magnetic field, the polarization of coupling and probe lights, and the Rabi frequency of the coupling beam are studied in transition F_e=2↔F_g=3 of Cs D₂ line. The split of electromagnetically induced transparency (EIT) resonances on two or three resonances determined by the different combination of the polarization of interaction lights and the direction of applied magnetic fields is obtained. The shifting and widening of the EIT resonances with the strength of the magnetic field (i.e., Zeeman splitting in the upper and lower levels) and Rabi frequency of the coupling beam increasing are also discussed. It may develop into the potential application for tunable multichannel optical information storage. On the other hand, an explanation of observed asymmetry of spectra by laser frequency offset from the optical resonance is given with theoretical calculation, which is in good agreement with the experimental results.

In recent experiments, attempts were made to use carbon nanotubes to replace the normal metal tips in the scanning tunneling microscope (STM), and stable atomic images were observed. However, does the one-dimensional characteristic band structure of the carbon nanotube (CNT) affect the tunneling? We present a theoretical analysis of the one-dimensional resonance tunneling model using the nonequilibrium Green's function method. The results clearly imply that the Van Hove singularities of the CNT probe play an important role in the tunneling process. The resonance curve is quite different from the one with a metallic tip; new peaks and peak splittings are induced. So these characteristics must be considered seriously if one uses a nanoprobe as the STM tip. We also notice that a sharp peak will appear near the first Van Hove singularity, which resembles the Kondo peak.

We have investigated the electron impact single ionization of the hydrogen molecule, with fully determined kinematics. The experimental and theoretical results are compared with He ionization under the same conditions. The results indicate that the ejected electron angular distribution for H₂ is modified due to Young-type interference between ionization amplitudes for scattering from the two centers in the hydrogen molecule. The observable result is a suppression of the backward scattering (recoil) peak compared with the binary peak.

We use low energy electron diffraction, scanning tunneling microscopy, first-principles density-functional theory, and molecular mechanics calculations to analyze the adsorption and growth of quinacridone derivatives (QA) with alkyl chains of 4 and 16 carbon atoms on a Ag(110) substrate. Surprisingly, we find that the alkyl chains determine the orientation of the molecular overlayers. While the interaction of QA and the Ag substrate is primarily due to chemical bonding of oxygen to the silver substrate, determining the molecular orientation and preferred adsorption site, the intermolecular arrangement can be adjusted via the length of alkyl chains. We are thus able to fabricate uniform QA films with very well controlled physical properties.

An antenna model is proposed for long (L≫λ) lasers with subwavelength cross sections (wire lasers). It is shown that the far-field pattern of the wire lasers is determined by the ratio of the wavelength to the length. The radiation of the wire laser is predicted to be concentrated in a narrow beam Θ≃sqrt[2λ/L] for laser modes where the longitudinal phase velocity is in synchronism with the velocity of light in air. Experimental results obtained using a terahertz quantum cascade wire laser are in agreement with the model.

We propose an asymmetric quantum cloning scheme. Based on the proposal and experiment by Andersen [Phys. Rev. Lett. 94, 240503 (2005)], we generalize it to two asymmetric cases: quantum cloning with asymmetry between output clones and between quadrature variables. These optical implementations also employ linear elements and homodyne detection only. Finally, we also compare the utility of symmetric and asymmetric cloning in an analysis of a squeezed-state quantum key distribution protocol and find that the asymmetric one is more advantageous.

The atomic structure of the thin SiO₂ film on a Mo(112) substrate has been determined based on a combination of density functional theory calculations and high-quality experimental data obtained from scanning tunneling microscopy, infrared reflection absorption spectroscopy, and x-ray photoelectron spectroscopy. The film consists of a honeycomblike, two-dimensional network of corner-sharing [SiO₄] tetrahedra. One oxygen atom of each tetrahedron binds to the Mo(112) substrate and is located in a bridge position between Mo atoms located in rows protruding from the metal surface. The other three oxygen atoms form Si-O-Si bonds with the neighboring tetrahedra.

Selective analysis of molecular states in scanning tunneling microscopy (STM) has so far been achieved in a few cases by tuning the bias range of the STM in high-resolution measurements. Correspondingly, perylene adsorbed in a close-packed monolayer on Ag(110) is imaged mainly through the π states of the molecule. By contrast, functionalizing the STM tip with a perylene molecule leads to a mismatch between the energy levels of the STM tip and the molecule adsorbates and, instead, images only the metal states of the underlying silver surface. The observation opens a route for better energy selectivity in electron transport measurements through organic interfaces.

Čerenkov radiation from cavities has been analyzed by quantum electrodynamic theory. Analytical expressions of basic radiation properties such as the Einstein’s A and B coefficients are derived and shown to be directly modified by the cavities. The analysis leads to the conclusion that the coherent radiation from the Čerenkov radiation devices is due to super radiance of spontaneous emission instead of stimulated emission. Coherent and incoherent radiations are analyzed in the THz radiation range.