arXiv subjects

D. Allen

Publications and source records attributed to D. Allen.

7 recordsLinked to original sources

arXiv2018

Point island dynamics under fixed rate deposition

In this paper we consider the dynamics of point islands during submonolayer deposition, in which the fragmentation of subcritical size islands is allowed. To understand asymptotics of solutions, we use methods of centre manifold theory, and for globalisation, we employ results from the theories of compartmental systems and of asymptotically autonomous dynamical systems. We also compare our results with those obtained by making the quasi-steady state assumption.

math.CA↗

arXiv2012

Periodicity and perfect state transfer in quantum walks on variants of cycles

We systematically investigated perfect state transfer between antipodal nodes of discrete time quantum walks on variants of the cycles C_4, C_6 and C_8 for three choices of coin operator. Perfect state transfer was found, in general, to be very rare, only being preserved for a very small number of ways of modifying the cycles. We observed that some of our useful modifications of C_4 could be generalised to an arbitrary number of nodes, and present three families of graphs which admit quantum walks with interesting dynamics either in the continuous time walk, or in the discrete time walk for appropriate selections of coin and initial conditions. These dynamics are either periodicity, perfect state transfer, or very high fidelity state transfer. These families are modifications of families known not to exhibit periodicity or perfect state transfer in general. The robustness of the dynamics is tested by varying the initial state, interpolating between structures and by adding decoherence.

quant-ph↗

arXiv2012

Three Dimensionial Surface Modelling: A Novel Analysis Technique for Non-Destructive X-Ray Diffraction Imaging of Semiconductor Die Warpage & Strain in Fully Encapsulated Integrated Circuits

Future complementary metal oxide semiconductor (CMOS) scaling for advanced integrated circuit (IC) technologies may well depend on "More than Moore" (MtM) approaches using heterogeneous integration of semiconductor-based devices. In order to realise this, advanced packaging technologies including System in Package (SiP), System on Chip (SoC) and 3D Integrated Circuits (3D ICs) are key enabling technologies. However, these advanced packages are plagued by reliability problems and to date there is no proven or accepted non-destructive metrology which can simultaneously probe materials properties such as strain, warpage, dislocation generation, etc. in these systems from bare silicon die through to a fully encapsulated packaged system. We report herein on the development of a novel, x-ray diffraction amd analysis technique, which can address this major metrology gap, and we demonstrate the non-destructive production of X-Y spatial maps of deformations and strain fields in Si die inside fully encapsulated integrated circuit packages. The technique, which we call 3-dimensional surface modelling (3DSM), is used to obtain high resolution (~3 {\mu}m) strain/warpage maps, and quantitative information on the nature and extent of warpage in a demonstration quad no flat lead (QFN) advanced package running from early stage silicon die bonding through to the end of the manufacturing process, i.e. a fully encapsulated and production ready chip.

cond-mat.mtrl-sci↗

arXiv2008

Processing and Characterization of Precision Microparts from Nickel-based Materials

The objective of this research was to study the influence of electroplating parameters on electrodeposit characteristics for the production of nickel (Ni) and nickel-iron (Ni-Fe) microparts by photoelectroforming. The research focused on the most relevant parameter for industry, which is the current density, because it determines the process time and the consumed energy. The results of the Ni and Ni-Fe characterisations can be divided into two aspects closely linked with each other ; the morphology and the hardness.

cs.OH↗

arXiv2007

The Effects of Additives on the Physical Properties of Electroformed Nickel and on the Stretch of Photoelectroformed Nickel Components

The process of nickel electroforming is becoming increasingly important in the manufacture of MST products, as it has the potential to replicate complex geometries with extremely high fidelity. Electroforming of nickel uses multi-component electrolyte formulations in order to maximise desirable product properties. In addition to nickel sulphamate (the major electrolyte component), formulation additives can also comprise nickel chloride (to increase nickel anode dissolution), sulphamic acid (to control pH), boric acid (to act as a pH buffer), hardening/levelling agents (to increase deposit hardness and lustre) and wetting agents (to aid surface wetting and thus prevent gas bubbles and void formation). This paper investigates the effects of some of these variables on internal stress and stretch as a function of applied current density.

cs.OH↗

arXiv1999

The spin-1 ladder : A bosonization study

We construct a field-theoretic description of two coupled spin-1 Heisenberg chains, starting with the known representation of a single spin-1 chain in terms of Majorana fermions (or Ising models). After reexamining the bosonization rules for two Ising models, taking particular care of order and disorder operators, we obtain a bosonic description of the spin-1 ladder. From renormalization-group and mean-field arguments, we conclude that, for a small interchain coupling, the spin-1 ladder is approximately described by three decoupled, two-frequency sine-Gordon models. We then predict that, starting with decoupled chains, the spin gap decreases linearly with interchain coupling, both in the ferromagnetic and antiferromagnetic directions. Finally, we discuss the possibility of an incommensurate phase in the spin-1 zigzag chain.

cond-mat.str-el↗

arXiv1994

Semi-classical description of the frustrated antiferromagnetic chain

The antiferromagnetic Heisenberg model on a chain with nearest and next nearest neighbor couplings is mapped onto the $SO(3)$ nonlinear sigma model in the continuum limit. In one spatial dimension this model is always in its disordered phase and a gap opens to excited states. The latter form a doubly degenerate spin-1 branch at all orders in $1/N$. We argue that this feature should be present in the spin-1 Heisenberg model itself. Exact diagonalizations are used to support this claim. The inapplicability of this model to half-integer spin chains is discussed.

cond-mat↗