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

Abstract:

peer-reviewed Double-barrier magnetic tunnel junctions with two MgO barriers and three CoFeB layers exhibiting tunneling magnetoresistance (TMR) values of more than 100% were fabricated. The bias voltage dependence of the TMR ratio is highly asymmetric after annealing at low temperatures, indicating dissimilar CoFeB/MgO interfaces. The TMR effect decays very slowly for positive bias and is only reduced to half of its maximum value at V1/2 = 1.88?V when the junctions are processed at 200??C. The largest output voltage, 0.62?V, is obtained after annealing at 300??C, a temperature that combines high TMR ratios with a considerable asymmetric bias dependence.

peer-reviewed A versatile technique for performing spin polarization measurements via point contact Andreev
reflection has been developed. This technique involves depositing a superconductor lead onto a
thin film of the material to be studied through a nanohole formed in a layer of photoresist, using an
atomic force microscope as a nanoindenter. Copper and nickel were used to demonstrate the method.
The polarizations of CrO2 and Co2MnSi were also measured, the former giving a value of 95%, as
expected, and the latter giving 20%, which was surprisingly low for a candidate half metal.

peer-reviewed A method for the controlled fabrication of a nickel perpendicular nanocontact with diameters less
than 5 nm is reported. The method involves milling pores through the 100 nm thick silicon nitride
membrane using a focused ion beam, and depositing thin Ni film on the both side of the silicon
nitride membrane. The shape of the resulting pore and nanocontact size is determined by the initial
hole size and redeposition of the sputtered material onto the sidewalls of the holes. A sub-5 nm Ni
nanocontact was prepared and the I-V and R-H characteristics measured.

peer-reviewed Nickel nanocontacts have been fabricated by focused ion-beam #1;FIB#2; milling of e-beam patterned
planar contacts, FIB milling of conical-shaped nanoperforations in a silicon nitride membrane, and
nanoimprinting using an atomic force microscope. Their sizes ranged from 1 to 30 nm.
Magnetoresistance of up to 3% is developed in a field of a few millitesla. This is interpreted in terms
of ballistic magnetoresistance across a wide domain wall whose structure is determined by dipolar
interactions at the contact.

We have studied the transport properties of mechanically stable Fe3O4 and La0.7Sr0.3MnO3
nanoconstrictions patterned by focused ion-beam milling. The magnetoresistance decreases with the
square of the applied voltage and scales with the resistance of the constriction, with values up to
8000% for magnetite and 100% for La0.7Sr0.3MnO3. These results are interpreted within a model for
domain-wall magnetoresistance. Some samples exhibit electrical hysteresis with discrete changes of
resistance that disappear in the presence of a magnetic field, indicating domain-wall displacement
driven by a spin-polarized current.

peer-reviewed Thin films of HfO2 produced by pulsed-laser deposition on sapphire, yttria-stabilized zirconia, or silicon substrates show ferromagnetic magnetization curves with little hysteresis and extrapolated Curie temperatures far in excess of 400 K. The moment does not scale with film thickness, but in terms of substrate area it is typically in the range 150?400 ?B nm?2. The magnetization exhibits a remarkable anisotropy, which depends on texture and substrate orientation. Pure HfO2 powder develops a weak magnetic moment on heating in vacuum, which is eliminated on annealing in oxygen. Lattice defects are the likely source of the magnetism.

peer-reviewed Thin films of Co(1?x?y)MnxSiy (x = 0.18?0.26 and y = 0.23?0.29) were deposited from elemental targets onto thermal-oxidized Si substrates at room temperature using dc magnetron cosputtering. The as-deposited films appear to be amorphous showing no distinguishable peaks in x-ray diffraction, and the magnetic moment at room temperature is nearly zero. Despite of its low magnetic moment (less than 0.001?B per formula), the disordered Heusler magnetic tunnel junctions show tunnel magnetoresistance ratios as large as 10% at room temperature. The dependence of the tunnel magnetoresistance ratio on compositions x and y is also reported.

peer-reviewed Contact between a Co/Pt multilayer and an IrMn film leads to perpendicular exchange bias. The exchange bias field does not depend on the degree of (111) film texture and for Co/Pt multilayers with IrMn at the bottom it can be enhanced by magnetic field annealing. The perpendicular exchange bias of the Co/Pt?IrMn system is limited by a misalignment between the Co spins and the film normal, which is due to a negative magnetic anisotropy contribution from the Co/IrMn interface (KSCo/IrMn = ?0.09?mJ/m2). The insertion of a 3?? thick Pt layer at the Co/IrMn interface maximizes the perpendicular exchange-bias field.

peer-reviewed Perpendicular exchange bias has been observed for IrMn/[Co/Pt]n and [Pt/Co]n/IrMn multilayers in the as-deposited state. The exchange bias field is largest when the IrMn film is grown on magnetically saturated Co/Pt multilayers (8.12?mT for n = 3), whereas it is considerably smaller when domain formation in the IrMn film occurs before Co/Pt deposition (3.30?mT for n = 3). After annealing at 220??C in an out-of-plane magnetic field the perpendicular exchange bias field and magnetic anisotropy are considerably larger for the Co/Pt multilayers with an IrMn film at the bottom. The apparent correlation between bias and anisotropy is explained by the dependence of the perpendicular exchange bias field on the orientation of the Co spins near the Co/IrMn interface.

peer-reviewed We report on field annealing effects in spin valves with an IrMn pinning layer and spin valves with
a synthetic antiferromagnet. The exchange bias field and magnetoresistance of spin valves with an
IrMn/CoFe bilayer at the bottom improve drastically upon annealing in large magnetic fields. The
evolution of the exchange bias field with annealing field strength shows a rapid increase up to an
applied field of 0.5 T, which is followed by a more gradual improvement up to an annealing field
of 5.5 T. The increase of the exchange bias field in large magnetic fields indicates that the interfacial
spin structure of the IrMn layer is directly influenced by the annealing field strength.

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.

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...