Mesoscopic Objects Research Articles

Monte Carlo dynamics below the Au-GaAs interface for ballistic-electron-emission microscopy

Monte Carlo simulations of hot electron transport in the {Gamma}, {ital L}, and {ital X} valleys of GaAs were performed to study the transport dynamics for ballistic-electron-emission microscopy (BEEM) of mesoscopic objects beneath the Au-GaAs interface. For the starting electron flux distribution at the interface, the distribution for the strong scattering limit of Smith-Lee-Narayanamurti theory [D. L. Smith, E. Y. Lee, and V. Narayanamurti, Phys. Rev. Lett. {bold 80}, 2433 (1998)] was used. For the subsequent electron transport within GaAs, phonon and intervalley scatterings were simulated. With increasing object depth beneath the Au-GaAs interface, the simulations predict (1) significant cooling of hot electrons, on the order of {approximately}3 meV/nm at 1.5 V, and (2) significant redistribution of electrons among the conduction valleys. These effects should be taken into account for proper interpretation of BEEM experiments, especially for large biases and deep objects. {copyright} {ital 1999} {ital The American Physical Society}

Light scattering from mesoscopic objects in diffusive media

The diffuse intensity propagating in turbid media is sensitive to the presence of any kind of object embedded in the medium, e.g. obstacles or defects. The long-ranged effects of isolated objects can be described by a stationary diffusion equation, the effect of any single object being parametrized in terms of a multipole expansion. An absorbing object is chiefly characterized by a negative charge, while the leading effect of a non-absorbing object is due to its dipole moment. The associated intrinsic characteristics of the object (capacitance $Q$ or effective radius $R_{\rm eff}$, polarizability $P$) can be evaluated within the diffusion approximation for large enough objects. The situation of mesoscopic objects, with a size comparable to the mean free path, requires a more careful treatment, for which the appropriate framework is radiative transfer theory. This formalism is worked out in detail for spheres and cylinders of the following kinds: totally absorbing (black), transparent, and totally reflecting.

Josephson array of mesoscopic objects. Modulation of system properties through the chemical potential

The phase diagram of a two-dimensional Josephson array of mesoscopic objects is examined. Quantum fluctuations in both the modulus and phase of the superconducting order parameter are taken into account within a lattice boson Hubbard model. Modulating the average occupation number $n_0$ of the sites in the system leads to changes in the state of the array, and the character of these changes depends significantly on the region of the phase diagram being examined. In the region where there are large quantum fluctuations in the phase of the superconducting order parameter, variation of the chemical potential causes oscillations with alternating superconducting (superfluid) and normal states of the array. On the other hand, in the region where the bosons interact weakly, the properties of the system depend monotonically on $n_0$. Lowering the temperature and increasing the particle interaction force lead to a reduction in the width of the region of variation in $n_0$ within which the system properties depend weakly on the average occupation number. The phase diagram of the array is obtained by mapping this quantum system onto a classical two-dimensional XY model with a renormalized Josephson coupling constant and is consistent with our quantum Path-Integral Monte Carlo calculations.

Lateral Separation of Macromolecules and Polyelectrolytes in Microlithographic Arrays

A new approach to separation of a variety of microscopic and mesoscopic objects in dilute solution is presented. The approach takes advantage of unique properties of a specially designed separation device (sieve), which can be built using already developed microlithographic techniques. Because of the broken reflection symmetry in its design, the direction of motion of an object in the sieve varies as a function of its self-diffusion constant, causing separation transverse to its direction of motion. This gives the device some significant and unique advantages over existing fractionation methods based on centrifugation and electrophoresis.

Mechanism of porous-silicon luminescence

We discuss the discrete spectrum induced by bulges on threadlike mesoscopic objects, using two models, a continuous hard-wall waveguide and a discrete tight-binding model with two sorts of atomic orbitals. We show that elongated bulges induce numerous quasibound states. In the discrete model we also evaluate the probability of transition between the localized states and extended ones of the ``valence'' band. We suggest this as a mechanism governing the porous-silicon luminescence. In addition, the model reproduces the dominance of nonradiative transitions, blueshift for finer textures, and luminiscence suppression at low temperatures.

New model for systems of mesoscopic Josephson junctions

Quantum fluctuations of the phases of the order parameter in 2D arrays of mesoscopic Josephson junctions and their effect on the destruction of superconductivity in the system are investigated by means of a quantum-cosine model that is free of the incorrect application of the phase operator. The proposed model employs trigonometric phase operators and makes it possible to study arrays of small superconducting granules, pores filled with superfluid helium, or Josephson junctions in which the average number of particles $n_0$ (effective bosons, He atoms, and so on) is small, and the standard approach employing the phase operator and the particle number operator as conjugate ones is inapplicable. There is a large difference in the phase diagrams between arrays of macroscopic and mesoscopic objects for $n_0 < 5$ and $U<J$ ($U$ is the characteristic interaction energy of the particle per granule and $J$ is the Josephson coupling constant). Reentrant superconductivity phenomena are discussed.

Optical magnetic near-field intensities around nanometer-scale surface structures

Recently, local probes used in optical experiments added a new dimension to the study of the optical properties of small particles lying on a surface. Until now, several theoretical frameworks, developed to understand the interaction of optical fields with mesoscopic and nanoscopic objects, emphasized mainly the prediction of the electric near-field distributions generated by these structures. This paper demonstrates how such subwavelength dielectric surface structures also produce a particular confinement of the optical magnetic near field when the sample is illuminated by a surface wave.

Fuzzy Transitions from Quantum to Classical Mechanics and New Phenomena of Mesoscopic Objects

Abstract A new "phase invariant" equation of motion for both microscopic and macroscopic objects is proposed. It reduces to the probabilistic wave equation for small masses and the deterministic classical equation for large masses. The motions of mesoscopic objects and fuzzy transitions between quantum and classical mechanics are discussed on the basis of the generalized equation. Experimental tests of new predictions are discussed.

Computing the optical near-field distributions around complex subwavelength surface structures: A comparative study of different methods.

Some 15 years ago, optical topographic signals with subwavelength resolution were obtained independently by several experimental teams. Since this exploratory period, a growing number of experimental configurations have been proposed and continuously developed. Simultaneously, this research field was supported by different theoretical works, aimed at developing our understanding of the interaction of optical fields with mesoscopic objects. Over the past three years, several theoretical frameworks have been proposed (Green's functions, field susceptibility, boundary conditions methods, multiple multipoles expansions, etc.). In this paper, an attempt at a careful comparison between two classes of numerical models is presented. Using the same test object, we discuss and compare the numerical solutions issued from a reciprocal-space perturbative method (Rayleigh approximation) and the solution originating from a direct-space integral approach (Green's function or field susceptibility). The discussion is given for different values of the relevant experimental parameters. The convergence of both approaches is investigated. \textcopyright{} 1996 The American Physical Society.

Manifestation of quasiparticle branch imbalance in resistive measurements of mesoscopic superconductors.

A model of the resistive state for mesoscopic superconductors has been proposed. It is considered that the frequency of thermally induced discrete phase-slip events is equal to the rate of thermally driven fluctuations, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{T}}$. After each phase slippage the nonequilibrium distribution of chemical potentials for pairs ${\mathrm{\ensuremath{\mu}}}_{\mathit{p}}$ and quasiparticles ${\mathrm{\ensuremath{\mu}}}_{\mathit{q}}$ relaxes on a time scale ${\mathrm{\ensuremath{\tau}}}_{{\mathit{Q}}^{\mathrm{*}}}$. The measured time-averaged voltage across the mesoscopic sample with dimensions compared to a single phase-slip center is found to be proportional to the spatial difference of the corresponding chemical potential (${\mathrm{\ensuremath{\mu}}}_{\mathit{p}}$ for superconducting probes and ${\mathrm{\ensuremath{\mu}}}_{\mathit{q}}$ for normal probes) and should be multiplied by the time averaging weight \ensuremath{\sim}${\mathrm{\ensuremath{\tau}}}_{{\mathit{Q}}^{\mathrm{*}}}$${\mathrm{\ensuremath{\Gamma}}}_{\mathit{T}}$. The resulting effective resistance ratio 〈R(T)〉/${\mathit{R}}_{\mathrm{normal}}$ for mesoscopic objects may be noticeably greater than unity sufficiently close to the critical temperature; it displays a strong nonlinear dependence on the bias current and is greatly suppressed by an external magnetic field. \textcopyright{} 1996 The American Physical Society.

Atom-surface scattering from palladium growth clusters

The recent experimental results of the Marseille group concerning the scattering of 4He atoms from Pd growing, in the form of truncated pyramids, on a MgO substrate, are interpreted in the framework of the eikonal approximation, taking into account both hard-wall, geometric effects and the van der Waals interactions. The total cross sections obtained for the attenuation of the MgO diffraction peak agree with experiments and show the importance of van der Waals attraction for scattering from mesoscopic objects.

Mechanism for diffusion of nanostructures and mesoscopic objects on surfaces

We have studied the diffusion of mesoscopic objects created on surfaces by applying time rescaling Monte Carlo algorithms. It is shown that under the effect of very small forces (0.01–0.03 eV/lattice constant), these nanostructures reach large velocities and long displacements are observable. The main mechanism involved in this displacement is the fast movement of the particles at the cluster perimeter, i.e. at an adequate temperature, for instance Au at room temperature, Au atoms move very fast along the cluster periphery and the application of a small driving force gives rise to the displacement of the object as a whole. Atomic metallic ion emission (AMIE) experiments are presented and explained in terms of our MC results.

Movement of scattering centers in a point contact induced by a scanning tunneling microscope

A new method is introduced to study transport in a mesoscopic sample. The electric field from an STM-tip is used to locally influence a mesoscopic object. We detect abrupt changes in the mesoscopic signal both as a function of the tip position and tip-sample voltage. They could be interpreted as due to spatial shifts or changes in the activity of scattering centers in the sample.