Xinsheng Sean Ling

Electron-beam-induced deformations of SiO2 nanostructures

The imaging beam of a transmission electron microscope can be used to fine tune critical dimensions in silicon oxide nanostructures. This technique is particularly useful for the fabrication of nanopores with single-nanometer precision, down to 2 nm. We report a detailed study on the effect of electron-beam irradiation on apertures with various geometries. We show that, on the same wafer, pores that are smaller than a certain critical size shrink and that larger ones expand. Our results are in agreement with the hypothesis that surface-tension effects drive the modifications. Additionally, we have determined the chemical composition in the pore region before and after modifications and found no significant changes. This result proves that contamination growth is not the underlying mechanism of pore closure.

Equilibrium Configurations and Energetics of Point Defects in Two-Dimensional Colloidal Crystals

We demonstrate a novel method of introducing point defects (mono- and divacancies) in a confined monolayer colloidal crystal by manipulating individual particles with optical tweezers. Digital video microscopy is used to study defect dynamics in real space and time. We verify the numerical predictions that the stable configurations of the defects have reduced symmetry compared to the triangular lattice and discover that in addition they are characterized by distinct topological arrangements of the particles in the defect core. Surprisingly, point defects are thermally excited into separated dislocations, from which we extract the dislocation pair potential.

On the distribution of DNA translocation times in solid-state nanopores: an analysis using Schrödinger’s first-passage-time theory

In this short paper, a correction is made to the recently proposed solution of Li and Talaga to a 1D biased diffusion model for linear DNA translocation, and a new analysis will be given to their data. It was pointed out by us recently that this 1D linear translocation model is equivalent to the one that was considered by Schrödinger for the Ehrenhaft–Millikan measurements on electron charge. Here, we apply Schrödinger’s first-passage-time distribution formula to the data set in Li and Talaga. It is found that Schrödinger’s formula can be used to describe the time distribution of DNA translocation in solid-state nanopores. These fittings yield two useful parameters: the drift velocity of DNA translocation and the diffusion constant of DNA inside the nanopore. The results suggest two regimes of DNA translocation: (I) at low voltages, there are clear deviations from Smoluchowski’s linear law of electrophoresis, which we attribute to the entropic barrier effects; (II) at high voltages, the translocation velocity is a linear function of the applied electric field. In regime II, the apparent diffusion constant exhibits a quadratic dependence on the applied electric field, suggesting a mechanism of Taylor-dispersion effect likely due the electro-osmotic flow field in the nanopore channel. This analysis yields a dispersion-free diffusion constant value of 11.2 nm2 µs-1 for the segment of DNA inside the nanopore, which is in quantitative agreement with the Stokes–Einstein theory. The implication of Schrödinger’s formula for DNA sequencing is discussed.

Video microscopy and micromechanics studies of one- and two-dimensional colloidal crystals

Colloidal suspension of monodispersed charged polystyrene microspheres provides an excellent experimental system for a study of condensed matter physics. Here we report a series of experiments using video microscopy to study the structure and dynamics of colloidal matter. In the first set of experiments, we give a clear demonstration of how thermal fluctuations destroy positional order in one dimension. The second set of experiments is related to the micromechanics of a colloidal solid.

Nature of phase transitions of superconducting wire networks in a magnetic field.

We study {ital I}{minus}{ital V} characteristics of periodic square Nb wire networks as a function of temperature in a transverse magnetic field, with a focus on three fillings 2/5, 1/2, and 0.618 that represent very different levels of incommensurability. For all three fillings, a scaling behavior of {ital I}{minus}{ital V} characteristics is found, suggesting a finite temperature continuous superconducting phase transition. The low-temperature {ital I}{minus}{ital V} characteristics are found to have an exponential form, indicative of the domain-wall excitations. {copyright} {ital 1996 The American Physical Society.}

Giant peak effect observed in an ultrapure YBa 2 Cu 3 O 6.993 crystal

A giant peak in the temperature dependence of the screening current is observed in the ac magnetic response of an ultra-pure YBa

Pinning and vortex lattice structure in NbTi alloy multilayers

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Multifilamentary NbTi with artificial pinning centers: the effect of alloy, pin material, and geometry on the superconducting properties

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Flux pinning in NbTi/Nb multilayers

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Experimental results on Nb 25 wt.% Ta 45 wt.% Ti superconducting wire

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