FENG, GEN; FENG, JIA-FENG; VAN, DIJKEN SEBASTIAN; COEY, JOHN MICHAEL DAVID

Abstract:

peer-reviewed Co40Fe40B20/MgO single and double barrier magnetic tunnel junctions (MTJs) were grown using target-facing-target sputtering for MgO barriers and conventional dc magnetron sputtering for Co40Fe40B20 ferromagnetic electrodes. Large tunnel magnetoresistance (TMR) ratios, 230% for single barrier MTJs and 120% for the double barrier MTJs, were obtained after postdeposition annealing in a field of 800 mT. The lower TMR ratio for double barrier MTJs can be attributed to the amorphous nature of the middle Co40Fe40B20 free layer, which could not be crystallized during postannealing. A highly asymmetric bias voltage dependence of the TMR can be observed for both single and double barrier MTJs in the as-deposited states and after field annealing at low temperature. The asymmetry decreases with increasing annealing temperature and the bias dependence becomes almost symmetric after annealing at 350??C. Maximum output voltages of 0.65 and 0.85 V were obtained for both single and...

peer-reviewed We present optically pumped emission data for ZnO, showing that high excitation effects and
stimulated emission/lasing are observed in nanocrystalline ZnO thin films at room temperature,
although such effects are not seen in bulk material of better optical quality. A simple model of
exciton density profiles is developed which explains our results and those of other authors.
Inhibition of exciton diffusion in nanocrystalline samples compared to bulk significantly increases
exciton densities in the former, leading?via the nonlinear dependence of emission in the exciton
bands on the pump intensity?to large increases in emission and to stimulated emission.

BLAU, WERNER; COLEMAN, JONATHAN NESBIT; DRURY, ANNA

Abstract:

Control of interchain separation enables the optical properties of polymer thin films to be altered.
We present a thin film preparation technique that increases the free volume fraction in spin coated
polymer thin films resulting in reduced interchain interaction. The polaron yield, measured using
photoinduced absorption, was significantly reduced as a direct result of the increased interchain
separation, leading to a higher value for the measured photoluminescence efficiency. Impedance
spectroscopy showed an increase in permittivity, probably due to greater polarizability. Increases
approaching one order of magnitude were observed for direct current hole conductivity and mobility
values. Space charge limited conduction analysis suggests a narrowing of the highest occupied
molecular orbital band tail on treatment resulting in reduced trapping. Single layer light emitting
devices prepared using this technique were found to be significantly brighter and to have longer
operating lifeti...

We present a theoretical description of the first-order scattering of interacting electrons and holes in
a double quantum dot. Assuming infinitely high walls, strong confinement, and a two-band
approximation, we derive general expressions for the two-particle matrix elements of the screened
Coulomb potential. We also determine the selection rules for different scattering channels and
consider special cases where the corresponding matrix elements can be represented by simple
analytical expressions. Numerical calculations of the matrix elements and an analysis of their
dependence on the geometrical and material parameters of the double quantum dot have also been
performed.

COFFEY, WILLIAM THOMAS; KALYMYKOV, YURI PETROVICH; TITOV, SERGEY

Abstract:

peer-reviewed The translational Brownian motion of a particle in a tilted washboard potential is considered. The dynamic structure factor and longest relaxation time are evaluated from the solution of the governing Langevin equation by using the matrix continued fraction method. The longest relaxation time is compared with the Kramers theory of the escape rate of a Brownian particle from a potential well as extended to the Kramers turnover region by Mel'nikov [Physics Reports 209, 1 (1991)]. It is shown that in the low temperature limit, the universal Mel'nikov expression for the escape rate provides a good estimate of the longest relaxation time for all values of dissipation including the very low damping (VLD), very high damping (VHD), and turnover regimes. For low barriers (where the Mel'nikov method is not applicable) and zero tilt, analytic equations for the relaxation times in the VLD and VHD limits are derived.

peer-reviewed The notion of asymmetry arises in many physical experiments, being a natural expression of the relative difference between two quantities. In this paper, we discuss the estimation of asymmetries between the rates of two event processes subject to a common background. Such asymmetries have many applications in solid-state physics - a measurement of the difference in arrival rates of elctrons with one of two possible spin orientations to a detector, for example. We describe three techniques for estimating asymmetries from observations subject to uncertainty: a naive method based on an unbiassed estimate and a Gaussian approximation, a bootstrap approach, and a Bayesian approach. Differences in the interpretation of the estimates are discussed and their performances are compared by means of a simulation study. The aim is both to review the methodology on estimation of asymmetries and to contribute to the more general discussion of the relative merits of frequentist an...

We compute the conformal anomaly in the free d = 6 superconformal (2,0) tensor
multiplet theory on generic curved background. Up to a trivial covariant totalderivative
term, it is given by the sum of the type A part proportional to the 6-d
Euler density, and the type B part containing three independent conformal invariants:
two CCC contractions of Weyl tensors and a C?2C + ... term. Multiplied by the
factor 4N3, the latter Weyl-invariant part of the anomaly reproduces exactly the corresponding
part of the conformal anomaly of large N multiple M5-brane (2,0) theory
as predicted (hep-th/9806087) by AdS7 supergravity on the basis of AdS/CFT correspondence.
The coefficients of the type A anomaly differ by the factor 4
7 ? 4N3, so
that the free tensor multiplet anomaly does not vanish on a Ricci-flat background. The
coefficient 4N3 is the same as found (hep-th/9703040) in the comparison of the tensor
multiplet theory and the d = 11 supergravity predictions for the absorption crosssection...

We show that a low-energy action for massless fluctuations around a tachyonic soliton
background representing a codimension one D-brane coincides with the Dirac-Born-Infeld
action. The scalar modes which describe transverse oscillations of the D-brane are translational
collective coordinates of the soliton. The appearance of the DBI action is a
universal feature independent of details of a tachyon effective action, provided it has the
structure implied by the open string sigma model partition function.

Gurrin, Cathal; Ruskin, Heather J.; Crane, Martin; LI, Na

Abstract:

Visual lifelogging is the process of automatically recording
images and other sensor data. Microsoft’s SenseCam is
lifelogging camera have mostly been used in medical applications. Experience shows that the SenseCam can be an effective memory aid device, as it helps users to improve recollecting an experience. Given the vast amount of images that are maintained in a visual lifelog, it is a significant challenge to deconstruct a sizeable collection of images into meaningful events for users. In this paper random matrix theory (RMT) is applied to a cross-correlation
matrix C, constructed using SenseCam lifelog data
streams to identify such events. The analysis reveals a number of eigenvalues that deviate from the spectrum suggested by RMT. The components of the deviating eigenvectors are found to correspond to “distinct significant events” in the visual lifelogs.
Finally, the cross-correlation matrix is cleaned by separating the noisy part from non-noisy part of cross-correlation mat...

Aarts, Gert; Allton, Chris; Kim, Seyong; Lombardo, M. P.; Oktay, M. B.; Ryan, S. M.; Sinclair, D. K.; Skullerud, Jon-Ivar

Abstract:

We study bottomonium spectral functions in the quark-gluon
plasma in the Υ
and
η
b
channels, using lattice QCD simulations with two flavours of
light quark on highly
anisotropic lattices. The bottom quark is treated with nonr
elativistic QCD (NRQCD). In
the temperature range we consider, 0
.
42
≤
T/T
c
≤
2
.
09, we find that the ground states
survive, whereas the excited states are suppressed as the te
mperature is increased. The
position and width of the ground states are compared to analy
tical effective field theory
(EFT) predictions. Systematic uncertainties of the maximu
m entropy method (MEM),
used to construct the spectral functions, are discussed in s
ome detail.

We calculate (
q
-deformed) Clebsch–Gordan and 6
j
-coefficients for rank 2
quantum groups. We explain in detail how such calculations are done,
which should allow the reader to perform similar calculations in other cases.
Moreover, we tabulate the
q
-Clebsch–Gordan and 6
j
-coefficients explicitly, as
well as some other topological data associated with theories corresponding to
rank 2 quantum groups. Finally, we collect some useful properties of the fusion
rules of particular conformal field theories.

Kells, G.; Kailasvuori, J.; Slingerland, J.K.; Vala, J.

Abstract:

Exactly solvable lattice models for spins and non-interacting
fermions provide fascinating examples of topological phases, some of them
exhibiting the localized Majorana fermions that feature in proposals for
topological quantum computing. The Chern invariant
ν
is an important
characterization of such phases. Here we look at the square–octagon variant
of Kitaev’s honeycomb model. It maps to spinful paired fermions and enjoys
a rich phase diagram featuring distinct Abelian and non-Abelian phases with
ν
=
0
,
±
1
,
±
2
,
±
3 and
±
4. The
ν
= ±
1 and
ν
= ±
3 phases all support localized
Majorana modes and are examples of Ising and SU
(
2
)
2
anyon theories,
respectively

Burnell, F.J.; Simon, Steven H.; Slingerland, J.K.

Abstract:

We study transitions between phases of matter with topological
order. By studying these transitions in exactly solvable lattice models we show
how universality classes may be identified and critical properties described. As
a familiar example to elucidate our results concretely, we describe in detail a
transition between a fully gapped achiral 2D
p
-wave superconductor (
p
+i
p
for pseudo-spin up/
p
−
i
p
for pseudo-spin down) to an s-wave superconductor.
We show in particular that this transition is of the 2D transverse field Ising
universality class.

We examine how best to design qubits for use in topological
quantum computation. These qubits are topological Hilbert spaces associated
with small groups of anyons. Operations are performed on these by exchanging
the anyons. One might argue that in order to have as many simple single-qubit
operations as possible, the number of anyons per group should be maximized.
However, we show that there is a maximal number of particles per qubit,
namely 4, and more generally a maximal number of particles for qudits of
dimension
d
. We also look at the possibility of having topological qubits for
which one can perform two-qubit gates without leakage into non-computational
states. It turns out that the requirement that all two-qubit gates are leakage free
is very restrictive and this property can only be realized for two-qubit systems
related to Ising-like anyon models, which do not allow for universal quantum
computation by braiding. Our results follow directly from the representation
theory of ...

Aarts, G.; Allton, C.; Kelly, A.; Skullerud, J.-I.; Kim, S.; Harris, T.; Ryan, S. M.; Lombardo, M. P.; Oktay, M. B.; Sinclair, D. K.

Abstract:

We study the temperature dependence of bottomonium for
temperatures in the range 0.4Tc < T < 2.1Tc, using non-relativistic
dynamics for the bottom quark and full relativistic lattice QCD simulations
for Nf = 2 light flavors. We consider the behaviour of the correlators in
Euclidean space, we analyze the associated spectral functions and we study
the dependence on the momentum. Our results are amenable to a successful
comparison with effective field theories. They help build a coherent picture
of the behaviour of bottomonium in the plasma, consistent which the current
LHC results.

Huijse, L.; Mehta, D.; Moran, N.; Schoutens, K.; Vala, J.

Abstract:

We study a model for itinerant, strongly interacting fermions where
a judicious tuning of the interactions leads to a supersymmetric Hamiltonian.
On the triangular lattice this model is known to exhibit a property called
superfrustration, which is characterized by an extensive ground state entropy.
Using a combination of numerical and analytical methods we study various
ladder geometries obtained by imposing doubly periodic boundary conditions
on the triangular lattice. We compare our results to various bounds on the
ground state degeneracy obtained in the literature. For all systems we find that
the number of ground states grows exponentially with system size. For two
of the models that we study we obtain the exact number of ground states by
solving the cohomology problem. For one of these, we find that via a sequence
of mappings the entire spectrum can be understood. It exhibits a gapped phase at
1/4 filling and a gapless phase at 1/6 filling and phase separation at intermediate
f...

Graham, Leigh; van der Burgt, Peter J.M.; Alexander, John; Hunniford, C. Adam; Haughey, Sean; Field, Tom A.; McCullough, Robert W.

Abstract:

Relative cross sections for the production of negatively charged fragments have been determined as a
function of ion impact energy in low-energy (0.5 - 5.5 keV) collisions of H– and O– with acetonitrile molecules.
The most abundantly produced negative ions from fragmentation by H– and O– impact are CH3CN–, CH2CN– and
CN–. Notably, the parent negative ion CH3CN– is produced abundantly.

Medina, Julieta; Huet, Idrish; O'Connor, Denjoe; Dolan, Brian P.

Abstract:

We present a manifestly Spin(5) invariant construction of s
quashed fuzzy CP3 as a fuzzy S2 bundle over fuzzy
S4 . We develop the necessary projectors and exhibit the
squashing in terms of the radii of the S2 and S4 . Our analysis allows us give both scalar
and spinor fuzzy action functionals whose low lying modes are truncated versions of those
of a commutative S4
.

We study the temperature flow of conductivities in a gated GaAs two-dimensional electron gas
(2DEG) containing self-assembled InAs dots and compare the results with recent theoretical
predictions. By changing the gate voltage, we are able to tune the 2DEG density and thus vary
disorder and spin-splitting. Data for both the spin-resolved and spin-degenerate phase
transitions are presented, the former collapsing to the latter with decreasing gate voltage
and/or decreasing spin-splitting. The experimental results support a recent theory, based on
modular symmetry, which predicts how the critical Hall conductivity varies with spin-splitting.

Sabol, D; Gleeson, M. R.; Liu, S.; Sheridan, J. T.

Abstract:

A deeper understanding of the processes, which occur during free radical photopolymerization, is
necessary in order to develop a fully comprehensive model, which represents their behavior during
exposure. One of these processes is photoinitiation, whereby a photon is absorbed by a
photosensitizer producing free radicals, which can initiate polymerization. These free radicals can
also participate in polymer chain termination primary termination, and it is therefore necessary to
understand their generation in order to predict the temporally varying kinetic effects present during
holographic grating formation. In this paper, a study of the photoinitiation mechanisms of Irgacure
784 photosensitizer, in an epoxy resin matrix, is presented. We report our experimental results and
present a theoretical model to predict the physically observed behavior.

Dolan, Brian P.; Huet, Idrish; Murray, Sean; O'Connor, Denjoe

Abstract:

We generalise the construction of fuzzy CPN in a manner that allows us to access
all noncommutative equivariant complex vector bundles over this space. We
give a simplified construction of polarization tensors on S2 that generalizes to complex
projective space, identify Laplacians and natural noncommutative covariant
derivative operators that map between the modules that describe noncommutative
sections. In the process we find a natural generalization of the Schwinger-Jordan
construction to su(n) and identify composite oscillators that obey a Heisenberg
algebra on an appropriate Fock space.

It is argued that there are strong similarities between the infra-red
physics of N=2 supersymmetric Yang-Mills and that of the quantum
Hall effect, both systems exhibit a hierarchy of vacua with a sub-group
of the modular group mapping between them. The coupling flow for
pure SU(2) N = 2 supersymmetric Yang-Mills in 4-dimensions is reexamined
and an earlier suggestion in the literature, that was singular
at strong coupling, is modified to a form that is well behaved at both
weak and strong coupling and describes the crossover in an analytic
fashion. Similarities between the phase diagram and the flow of SUSY
Yang-Mills and that of the quantum Hall effect are then described,
with the Hall conductivity in the latter playing the role of the θ-
parameter in the former. Hall plateaux, with odd denominator filling
fractions, are analogous to fixed points at strong coupling in N=2
SUSY Yang-Mills, where the massless degrees of freedom carry an
odd monopole charge.

We study the edge excitations of the Chern Simons matrix theory, describing the
Laughlin fluids for filling fraction n = 1k, with k an integer. Based on the semiclassical
solutions of the theory, we are able to identify the bulk and edge degrees of freedom.
In this way we can freeze the bulk of the theory, to the semiclassical values, obtaining
an effective theory governing the boundary excitations of the Chern Simons matrix
theory. Finally, we show that this effective theory is equal to the chiral boson theory
on the circle.

Howard, J.; van der Burgt, Peter J.M.; Manil, B.; Rousseau, P.; Huber, B.A.

Abstract:

We report on two collision experiments performed on nucleobases, (i) using electron impact on nucleobases in the gas phase, and (ii) using low-energy ion impact of small nucleobase clusters. In both experiments mass-resolved positive ions were detected using time-of-flight mass spectrometers, and neutral metastable fragments were detected, also using a time-of-flight technique.