Probing elastic anisotropy from defect dynamics in Langmuir monolayers

We study the dynamics of annihilation of point defects in Langmuir monolayers. The absence of hydrodynamic effects allows us to quantitatively relate the asymmetry in defect mobility to the elastic anisotropy of the material, which in turn can be varied through the control of the surface pressure applied to the monolayer. Using the proposed theoretical analysis, we are able to obtain rather elusive equilibrium properties out of relatively simple dynamical measurements. In particular, we measure the elastic constants and their pressure dependence.     

Monolayer pattern evolution via substrate strain-mediated spinodal decomposition

Investigations of octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are reported. Scanning tunneling microscopy data display the evolution of a approximately 6 nm scale pattern of interwoven features concomitant with ejection of surface Au atoms and relaxation of the Au(111) 23xsqrt[3] surface reconstruction. Numerical simulations suggest the surface dynamics are governed by a substrate strain-mediated spinodal decomposition mechanism, novel to organic monolayer formation. Collectively, the experimental and theoretical data indicate strain-inducing Si-Au bond interactions drive the pattern formation and the alkyl chains play a negligible role.     

Tuning of the Electron Spin Relaxation Anisotropy via Optical Gating in GaAs/AlGaAs Quantum Wells

The carrier-density-dependent spin relaxation dynamics for modulation-doped GaAs/Al_(0.3)Ga_(0.7)As quantum weiis is studied using the time-resolved magneto-Kerr rotation measurements.The electron spin relaxation time and its in-plane anisotropy are studied as a function of the optically injected electron density.Moreover,the relative strength of the Rashba and the Dresselhaus spin-orbit coupling fields,and thus the observed spin relaxation time anisotropy,is further tuned by the additional excitation of a 532 nm continuous wave laser,demonstrating an effective spin relaxation manipulation via an optical gating method.     

Femtosecond Dynamics of Optically Induced Anisotropy of Complex Molecules in the Gas Phase

The evolution of optically induced anisotropy in an equilibrium ensemble of free asymmetric tops under collisionless conditions has been investigated theoretically and experimentally. The dynamics of the orientation correlation functions has been studied, the results of femtosecond experiments on measurement of relaxation of the optically induced anisotropy for perylene in the gas phase are presented, and their interpretation within the framework of the theory developed is given. It is shown that in contrast to stationary anisotropy, its time kinetics has more complete characteristic information about the dynamics of vector correlations in a molecular ensemble.     

Nonequilibrium dynamics of anisotropic large spins in the kondo regime: time-dependent numerical renormalization group analysis

We investigate the time-dependent Kondo effect in a single-molecule magnet (SMM) strongly coupled to metallic electrodes. Describing the SMM by a Kondo model with large spin S>1/2, we analyze the underscreening of the local moment and the effect of anisotropy terms on the relaxation dynamics of the magnetization. Underscreening by single-channel Kondo processes leads to a logarithmically slow relaxation, while finite uniaxial anisotropy causes a saturation of the SMM's magnetization. Additional transverse anisotropy terms induce quantum spin tunneling and a pseudospin-1/2 Kondo effect sensitive to the spin parity.     

Dynamics of optically induced anisotropy in an ensemble of asymmetric top molecules in the gas phase

A complex theoretical and experimental investigation of the evolution of optically induced anisotropy in an ensemble of free asymmetric tops under collisionless conditions is performed. The dynamics of the orientational correlation functions is studied and the results of femtosecond measurements of the relaxation of the anisotropy of perylene fluorescence in the gas phase are reported. The results obtained are interpreted in terms of the theory developed. It is shown that, in contrast to steady-state anisotropy, the time kinetics of optically induced anisotropy yields more complete and typical data on the dynamics of vector correlations in an ensemble of molecules.     

Water-filled MCM-41 characterized by double-quantum-filtered2H NMR spectral analysis

The dynamics of water molecules confined in adsorbed layers of siliceous MCM-41 with a pore diameter of 2.8 nm is investigated at 230 K by deuteron nuclear magnetic resonance (NMR) relaxation studies including line shapes of theT ₁ process and double quantum filtered (DQF) spectral analyses.²H DQF NMR is a particularly sensitive tool for the determination of the adsorbate dynamics resulting from residual quadrupolar interaction due to the local order. The amount of monolayer water is determined. The monolayer water is composed of two different water components characterized by water, with isotropic reorientational motions, exchanging with water displaying a solid-like spectrum with 4 kHz edge splitting. One may expect that the latter water is situated on surface sites in MCM-41. The restricted wobbling motion of the D-O bond is used to describe its dynamics which is one order of magnitude slower than the isotropic reorientational motion. The order parameter, the motional correlation time, and the exchange rate thus determined provide useful information on the structure and the adsorptive properties of the mesoporous system.     

Cantilever stress measurements of ferromagnetic monolayers

This overview summarizes important aspects of the cantilever stress measurement technique and its application to measurements of adsorbate-induced surface stress changes and magnetization-induced magnetoelastic stress (also called magnetostrictive stress) of ferromagnetic monolayers. The application of the cantilever technique as a torque magnetometer with monolayer sensitivity is demonstrated. The stress measurements indicate a correlation between surface stress changes and surface reconstruction, and they also identify the often decisive role of the magnetoelastic anisotropy for the non-bulklike magnetic anisotropy of ferromagnetic monolayers. PACS 68.35.Gy; 68.35.Bs; 75.70.Ak; 75.80.+q     

A comparison of ion scattering spectra and molecular dynamics simulations at the surface of silicate glasses

Molecular dynamics simulations are correlated with experimental ion scattering spectra to elucidate the surface structure and composition of fused silica and potassium trisilicate glass. The ion scattering spectra and molecular dynamics simulations both show that the oxygen atoms dominate the surface monolayer of fused silica. The ion scattering spectra of fracture surfaces of potassium trisilicate glass show a large potassium signal with little scattering signal from the oxygen or silicon atoms indicating a predominance of potassium in the surface monolayer. This local enrichment of potassium in the surface monolayer is due to their shielding of the charged silicate tetrahedra at the surface. This is also consistent with the simulations.     

Analysis of mechanically induced processes in the Langmuir film

The understanding of processes in the monolayer at the air-water interface induced by mechanical compression is important as a part of basic research of the system with reduced dimensionality as well as for the investigation of processes during the Langmuir-Blodgett deposition. The Maxwell displacement current technique provides a substantial contribution for the study of structural and electrical properties. Analysis based on imperfect gas approximation with semi-empirical intermolecular potentials is used. Detail theoretical study of molecular tilt in a continual lateral compression and dielectric relaxation phenomena (step-compression) is presented. Obtained results are confronted with standard surface pressure analysis and surface potential measurement.     

High-temperature slow relaxation of the magnetization in Ni10 magnetic molecules

We investigate a family of molecular crystals containing noninteracting Ni10 magnetic molecules. We find slow relaxation of the magnetization below a temperature as high as 17 K and we show that this behavior is not associated with an anisotropy energy barrier. Ni10 has a characteristic magnetic energy spectrum structured in dense bands, the lowest of which makes the crystal opaque to phonons of energy below about 1 meV. We ascribe the nonequilibrium behavior to the resulting resonant trapping of these low-energy phonons. Trapping breaks up spin relaxation paths leading to a novel kind of slow magnetic dynamics which occurs in the lack of anisotropy, magnetic interactions and quenched disorder.     

Multiscale modelling of molecular monolayers adsorbed on silicon

We used a combination of ab-initio force relaxation and empirical molecular dynamics simulations to study the atomic structure of a dense monolayer of benzene- or anthracene-terminated alkyl chains chemisorbed onto a (100) Si surface. We find a transition to a free-rotor phase at about 400 K, with the chains rising to a nearly perpendicular orientation to the surface. At the same temperature, other internal degrees of freedom of the molecules are activated. Above the transition an increase in free-surface roughness is observed. We compare our findings to experimental results of the synthesis of self-assembled monolayers as molecular current rectifiers in silicon-integrated nanoscale electronics. PACS 02.70.Ns; 68.47.Pe; 81.16.Rf; 64.70.Nd     

Diffraction peak profiles of surface relaxed spherical nanocrystals

Abstract

A model is proposed for surface relaxation of spherical nanocrystals. Besides reproducing the primary effect of changing the average unit cell parameter, the model accounts for the inhomogeneous atomic displacement caused by surface relaxation and its effect on the diffraction line profiles. Based on three parameters with clear physical meanings - extension of the sub-coordination effect, maximum radial displacement due to sub-coordination, and effective hydrostatic pressure - the model also considers elastic anisotropy and provides parametric expressions of the diffraction line profiles directly applicable in data analysis. The model was tested on spherical nanocrystals of several fcc metals, matching atomic positions with those provided by Molecular Dynamics (MD) simulations based on embedded atom potentials. Agreement was also verified between powder diffraction patterns generated by the Debye scattering equation, using atomic positions from MD and the proposed model.     

Polarization-dependent carrier dynamics in a passive InAs/InP quantum dot waveguide

We investigate carrier dynamics in a passive InAs/InP quantum dot (QD) waveguide using 255 fs optical pulses at a central wavelength of 1568 nm. We observe strong anisotropy of absorption saturation for different polarizations. Pump-probe measurements indicate the presence of carrier relaxation dynamics on a timescale in the order of tens of picoseconds due to cascaded relaxation of carriers generated by two-photon absorption (TPA) from the bulk region to the QDs via the wetting layer. These relaxation timescales are much longer than in QD amplifiers. Our observations are supported by a rate-equation model which includes TPA, showing good agreement with the pump-probe measurements.     

Modeling dewetting of ultra-thin solid films

We review some models for the dynamics of dewetting of ultra-thin solid films. We discuss the similarities and the differences between faceted and non-faceted systems. The faceting of the dewetting rim leads to corrections in the velocity of dewetting fronts both in flat and axisymmetric geometries. The faceting of the edge of the dewetting rim leads to a strong anisotropy of the dewetting instability. Faceting also induces novel dewetting regimes such as layer-by-layer dewetting, and monolayer dewetting.     

Magnetic properties of Fe and Co nanoclusters embedded in the first Cu(100) surface layer

The magnetic anisotropy energies, as well as spin and orbital magnetic moments of the atoms involved in the simplest nanostructures formed due to the self-organization within the first Cu(100) surface layer, are calculated in the framework of the density functional theory. The critical role of the surface relaxation, which leads to the rotation of the easy magnetization axis in iron nanoclusters, is demonstrated in the calculations of magnetic anisotropy.     

Yielding of hard-sphere glasses during start-up shear

Concentrated hard-sphere suspensions and glasses are investigated with rheometry, confocal microscopy, and Brownian dynamics simulations during start-up shear, providing a link between microstructure, dynamics, and rheology. The microstructural anisotropy is manifested in the extension axis where the maximum of the pair-distribution function exhibits a minimum at the stress overshoot. The interplay between Brownian relaxation and shear advection as well as the available free volume determine the structural anisotropy and the magnitude of the stress overshoot. Shear-induced cage deformation induces local constriction, reducing in-cage diffusion. Finally, a superdiffusive response at the steady state, with a minimum of the time-dependent effective diffusivity, reflects a continuous cage breakup and reformation.     

Temperature dependence of Pt(111) surface relaxation

A careful investigation of the Pt(111) surface has been carried out, using the MeV heliumscattering technique. The anomalously large surface relaxation effect reported previously has been identified as an experimental artifact resulting from an unexpectedly large surface damage effect at low temperature. Optimum conditions have been established for minimizing the main experimental sources of error: background subtraction, radiation damage, and deviations from the Rutherford scattering law. Using these optimum conditions, a series of scattering measurements has been made over the temperature range 40–300 K. At all temperatures, we observe a significant anisotropy in the 〈111〉 angular scans, indicating an outward relaxation of the Pt(111) surface plane. By comparing this observed anisotropy with a set of Monte Carlo simulations, a value of 0.03 ? 0.01 Å (i.e. 1.3 ? 0.4%) is obtained for the surface relaxation. The temperature dependence of the surface peak also indicates that the enhanced vibrational amplitude of the surface atoms is not nearly as large as had been derived previously from high-temperature LEED studies.     

Dynamics of water in and around proteins characterized by 1H-spin-lattice relaxometry

Nuclear magnetic spin-lattice relaxation rate constants measured as a function of the magnetic field strength over wide ranges of Larmor frequency map the noise spectrum that drives spin relaxation. For water in and around protein systems, the spin relaxation reports on the average local translational mobility at the interface which is reduced by approximately factor of three from the bulk and there is anisotropy induced in the motions caused by the excluded volume created by the presence of the protein. Water also penetrates the protein and relatively few bound water sites provide a strong coupling between the protein dynamics and the water-proton-spin relaxation.     

Temperature-pressure scaling for air-fluidized grains near jamming

We present experiments on a monolayer of air-fluidized beads in which a jamming transition is approached by increasing pressure, increasing packing fraction, and decreasing kinetic energy. This is accomplished, along with a noninvasive measurement of pressure, by tilting the system and examining behavior versus depth. We construct an equation of state and analyze relaxation time versus effective temperature. By making time and effective temperature dimensionless using factors of pressure, bead size, and bead mass, we obtain a good collapse of the data but to a functional form that differs from that of thermal hard-sphere systems. The relaxation time appears to diverge only as the effective temperature to pressure ratio goes to zero.