Micromagnetic Domain Structures in Cylindrical Nickel Dots

The magnetic domain structures of cylindrical nickel dots (diameters from 40 nm to 1700 nm) with anisotropy parallel to the cylinder axis is predicted by the ratio of the dot diameter to the stripe period of unpatterned films with the same perpendicular anisotropy. The dominant domain structure for a given ratio increases in complexity as the ratio increases. We present evidence for the full micromagnetic domain structure for the simplest cases

Jamming and unjamming of concentrated colloidal dispersions in channel flow

We investigated the pressure driven flow of concentrated colloidal dispersions in a converging channel geometry. Optical microscopy and image analysis were used to track tracer particles mixed into dispersions of sterically stabilized poly(methyl methacrylate) (PMMA) spheres. The dispersions were drawn into a round \unit[0.5]{mm} capillary at one of two pump speeds ($\equiv$ applied pressure): v_1=\unit[0.245]{ml\,\, min^{-1}} and v_2=\unit[0.612]{ml\,\, min^{-1}}. We observed that the dispersions at particle volume fractions $\phi\leqslant0.50$ followed Hagen-Poiseuille flow for a simple fluid; i.e. the mean flow rate $\langle V\rangle$ is approximately proportional to pressure drop (pump speed) and inversely proportional viscosity $\eta$. Above this concentration ($\phi\geqslant0.505$), the dispersions exhibit granular-like jamming behavior with $\langle V\rangle$ becoming independent of the pressure drop. However, at the highest applied pressure ($v_2$), the dispersions are able to unjam and switch from granular-like behaviour back to a simple hard-sphere liquid like system, due to the formation of rotating vortices in the spatial flow pattern. This mechanism is consistent with computer simulations of granular systems and supports for example proposed explanations of anomalously low friction in earthquake faults

Peak-ratio analysis method for enhancement of LOM protection using M class PMUs

A novel technique for loss of mains (LOM) detection, using Phasor Measurement Unit (PMU) data, is described in this paper. The technique, known as the Peak Ratio Analysis Method (PRAM), improves both sensitivity and stability of LOM protection when compared to prevailing techniques. The technique is based on a Rate of Change of Frequency (ROCOF) measurement from M-class PMUs, but the key novelty of the method lies in the fact that it employs a new “peak-ratio” analysis of the measured ROCOF waveform during any frequency disturbance to determine whether the potentially-islanded element of the network is grid connected or not. The proposed technique is described and several examples of its operation are compared with three competing LOM protection methods that have all been widely used by industry and/or reported in the literature: standard ROCOF, Phase Offset Relay (POR) and Phase Angle Difference (PAD) methods. It is shown that the PRAM technique exhibits comparable performance to the others, and in many cases improves upon their abilities, in particular for systems where the inertia of the main power system is reduced, which may arise in future systems with increased penetrations of renewable generation and HVDC infeed

Electronic structure of boron and aluminum δ\delta-doped layers in silicon

Recent work on atomic-precision dopant incorporation technologies has led to the creation of both boron and aluminum $\delta$-doped layers in silicon with densities above the solid solubility limit. We use density functional theory to predict the band structure and effective mass values of such $\delta$ layers, first modeling them as ordered supercells. Structural relaxation is found to have a significant impact on the impurity band energies and effective masses of the boron layers, but not the aluminum layers. However, disorder in the $\delta$ layers is found to lead to significant flattening of the bands in both cases. We calculate the local density of states and doping potential for these $\delta$-doped layers, demonstrating that their influence is highly localized with spatial extents at most 4 nm. We conclude that acceptor $\delta$-doped layers exhibit different electronic structure features dependent on both the dopant atom and spatial ordering. This suggests prospects for controlling the electronic properties of these layers if the local details of the incorporation chemistry can be fine tuned.Comment: Main text 8 pages, 6 figures + Appendices 3 pages, 2 figure

The initiator methionine tRNA drives cell migration and invasion leading to increased metastatic potential in melanoma

The cell's repertoire of transfer RNAs (tRNAs) has been linked to cancer. Recently, levels of the initiator methionine tRNA (tRNAiMet) in stromal fibroblasts have been shown to influence extracellular matrix (ECM) secretion to drive tumour growth and angiogenesis. Here we show that increased tRNAiMet within cancer cells does not influence tumour growth, but drives cell migration and invasion via a mechanism that is independent from ECM synthesis and dependent on α5β1 integrin and levels of the translation initiation ternary complex. In vivo and ex vivo migration (but not proliferation) of melanoblasts is significantly enhanced in transgenic mice which express additional copies of the tRNAiMet gene. We show that increased tRNAiMet in melanoma drives migratory, invasive behaviour and metastatic potential without affecting cell proliferation and primary tumour growth, and that expression of RNA polymerase III-associated genes (which drive tRNA expression) are elevated in metastases by comparison with primary tumours. Thus specific alterations to the cancer cell tRNA repertoire drive a migration/invasion programme that may lead to metastasis

Quantum-Inspired Tempering for Ground State Approximation using Artificial Neural Networks

A large body of work has demonstrated that parameterized artificial neural networks (ANNs) can efficiently describe ground states of numerous interesting quantum many-body Hamiltonians. However, the standard variational algorithms used to update or train the ANN parameters can get trapped in local minima, especially for frustrated systems and even if the representation is sufficiently expressive. We propose a parallel tempering method that facilitates escape from such local minima. This methods involves training multiple ANNs independently, with each simulation governed by a Hamiltonian with a different "driver" strength, in analogy to quantum parallel tempering, and it incorporates an update step into the training that allows for the exchange of neighboring ANN configurations. We study instances from two classes of Hamiltonians to demonstrate the utility of our approach using Restricted Boltzmann Machines as our parameterized ANN. The first instance is based on a permutation-invariant Hamiltonian whose landscape stymies the standard training algorithm by drawing it increasingly to a false local minimum. The second instance is four hydrogen atoms arranged in a rectangle, which is an instance of the second quantized electronic structure Hamiltonian discretized using Gaussian basis functions. We study this problem in a minimal basis set, which exhibits false minima that can trap the standard variational algorithm despite the problem's small size. We show that augmenting the training with quantum parallel tempering becomes useful to finding good approximations to the ground states of these problem instances.Comment: 13 pages, 6 figures. v2: updated Figs. 3, 4, and 5. v3. included two new QPT schemes. Updated format to SciPost format. 30 pages, 11 figures. Submission to SciPos

Bright Source of Cold Ions for Surface-Electrode Traps

We produce large numbers of low-energy ions by photoionization of laser-cooled atoms inside a surface-electrode-based Paul trap. The isotope-selective trap loading rate of $4\times10^{5}$ Yb$^{+}$ ions/s exceeds that attained by photoionization (electron impact ionization) of an atomic beam by four (six) orders of magnitude. Traps as shallow as 0.13 eV are easily loaded with this technique. The ions are confined in the same spatial region as the laser-cooled atoms, which will allow the experimental investigation of interactions between cold ions and cold atoms or Bose-Einstein condensates.Comment: Paper submitted to PRL for review on 2/1/0

Dynamical Arrest in Attractive Colloids: The Effect of Long-Range Repulsion

We study gelation in suspensions of model colloidal particles with short-ranged attractive and long-ranged repulsive interactions by means of three-dimensional fluorescence confocal microscopy. At low packing fractions, particles form stable equilibrium clusters. Upon increasing the packing fraction the clusters grow in size and become increasingly anisotropic until finally associating into a fully connected network at gelation. We find a surprising order in the gel structure. Analysis of spatial and orientational correlations reveals that the gel is composed of dense chains of particles constructed from face-sharing tetrahedral clusters. Our findings imply that dynamical arrest occurs via cluster growth and association.Comment: Final version: Phys. Rev. Lett. 94, 208301 (2005

An Evaluation of the Load-Displacement Behavior and Load Test Interpretation of Micropiles in Rock

This paper summarizes a series of never-before reported axial compression load tests conducted on single micropiles that are embedded in or constructed on rock. These data are augmented by load tests on similar micropiles that have been reported by others. The observed displacements at the maximum test load (QMAX) and reported unfactored design load (QDL) are summarized. In addition, the small-strain load-displacement behavior of these foundations is evaluated by comparing the initial tangent slope (IS) to the theoretical elastic slope (ES), which is calculated by modeling the micropile as a free-standing column exhibiting fully -composite behavior. The data demonstrate that the ES/IS ratio has a strong dependence on the slenderness ratio D[depth]/B[diameter]. The observed results for micropiles in rock are discussed in the context of the micropile load test acceptance criteria proposed by the Deep Foundations Institute (DFI, 2001). In addition, recommendations are proposed for the maximum acceptable vertical displacement under the unfactored design load for such micropiles