This image shows the plate-shaped
hydroxyapatite produced by
bacteria. Hydroxyapatite is naturally
found in bones. Thus, it is very
biocompatible. Hydroxyapatite
coated titanium is widely used as
implants, such as the hip
replacement and some dental root
implants. The project we are
working on is to use bacteria to
produce hydroxyapatite, the main
components of bones, on titanium.
© Physics Department, courtesy of Anqi Wang
© Physics Department, courtesy of John Halpin
MOSFET’s are the building blocks of
the chips that power our electrical
devices. The picture shows
germanium MOSFET’s being tested
on a probing station. Germanium
MOSFET’s can give performance far
higher than that obtained from the
MOSFET’s using the current industry
standard of silicon. The NanoSilicon
group in the Department researches
the use of new materials and device
designs to push the limits of devices
for a new generation of applications.
This ultrahigh vacuum chamber has
less particles floating around inside
than deep space! A sample is
inserted and electrons fired at it. The
resulting electrons and x-rays that
come off the sample tell us about the
surface of the material, which we can
then relate to other physical
properties we are trying to improve.
© Physics Department, courtesy of Nathan Barrow
© Physics Department, courtesy of Ceri Lyn-Adams
The eye of a fruit fly at high
magnification! The ‘bumps’ are
individual lenses (ommatidia); each
fruit fly contains 700-800 of them
interspersed with bristles. Many
years of research has developed tools
allowing us to express (make)
proteins in the eyes of fruit-flies’. By
looking at the eye structure we can
gain an insight into the interactions
between the proteins, and hence how
diseases progress.
© Physics Department, courtesy of Dean Keeble
A major plane of reciprocal space of
Strontium Yttrium Manganate. The
"Bragg" spots represent a direction of
spatial periodicity in the crystal, and
can reveal the exact arrangement of
atoms that make up the structure.
This image was acquired by
illuminating a sub-millimetre piece
of crystal with X-rays, and collecting
the scattered photons. The faint lines
of intensity between the main spots
indicate that, on the nanometre scale,
the crystal is locally disordered.
© Physics Department, courtesy of David Pearmain
The image shows a palladium cluster
resting on a carbon surface. The
atomic columns are visible and the
brightness of each column
corresponds to how many atoms are
stacked up on top of each other. Each
atom diameter is 700,000 times
smaller than the width of a human
hair! The image was taken using an
instrument called a Scanning
Transmission Electron Microscope,
and used aberration correction to
enable single atoms to be imaged.
This is a picture of a multifractal electron
wave in a strongly disordered cubic
environment. The electron state shown
here is neither a metal (uniform-sized
boxes all around in the cube) nor an
insulator (isolated large boxes) but
contains characteristics of both. The
bigger the box size is the greater is the
chance of finding an electron at that site.
This complex structure persists even if
we enlarge or reduce the cube. Hence we
say that the electron is a multifractal.
© Physics Department, courtesy of Louella Vasquez
© Physics Department, courtesy of Richard Beanland
The symmetry of a crystal normally
determines its functional properties. This
is equally true on the nano-scale as it is at
the macro-scale. Whilst for bulk material
the structure and symmetry can
routinely be solved by X-ray diffraction,
there is no comparable technique for
nanostructured materials. Here, we
demonstrate that computer control of
beam tilt and image capture in a
conventional transmission electron
microscope can be used to overcome this
problem and quickly provide equally
rich diffraction datasets.
NR Wilson, AJ Marsden, M Saghir, CJ Bromley, R Schaub, G Costantini, TW White, C Partridge, A Barinov,
P Dudin, AM Sanchez, James J Mudd, M Walker, GR Bell
Abstract
Graphene growth by low-pressure chemical vapor deposition on low cost copper
foils shows great promise for large scale applications. It is known that the local
crystallography of the foil influences the graphene growth rate. Here we find an
epitaxial relationship between graphene and copper foil. Interfacial restructuring
between graphene and copper drives the formation of (n10) facets on what is
otherwise a mostly Cu(100) surface, and the facets in turn influence the graphene
orientations from the onset of growth. Angle resolved photoemission shows that
the electronic structure of the graphene is decoupled from the copper indicating a
weak interaction between them. Despite this, two preferred orientations of
graphene are found, ±8° from the Cu[010] direction, creating a non-uniform
distribution of graphene grain boundary misorientation angles. Comparison with
the model system of graphene growth on single crystal Cu(110) indicates that this
orientational alignment is due to mismatch epitaxy. Despite the differences in
symmetry the orientation of the graphene is defined by that of the copper. We
expect these observations to not only have importance for controlling and
understanding the growth process for graphene on copper, but also to have wider
implications for the growth of two-dimensional materials on low cost metal
substrates.
see Nano Research, February 2013, Volume 6, Issue 2, pp 99-112
Helen R. Thomas, Cristina Vallés, Robert J. Young, Ian A. Kinloch, Neil R. Wilson and Jonathan P. Rourke
Abstract
Graphene growth by low-pressure chemical vapor deposition on low cost copper
foils shows great promise for large scale applications. It is known that the local
crystallography of the foil influences the graphene growth rate. Here we find an
epitaxial relationship between graphene and copper foil. Interfacial restructuring
between graphene and copper drives the formation of (n10) facets on what is
otherwise a mostly Cu(100) surface, and the facets in turn influence the graphene
orientations from the onset of growth. Angle resolved photoemission shows that
the electronic structure of the graphene is decoupled from the copper indicating a
weak interaction between them. Despite this, two preferred orientations of
graphene are found, ±8° from the Cu[010] direction, creating a non-uniform
distribution of graphene grain boundary misorientation angles. Comparison with
the model system of graphene growth on single crystal Cu(110) indicates that this
orientational alignment is due to mismatch epitaxy. Despite the differences in
symmetry the orientation of the graphene is defined by that of the copper. We
expect these observations to not only have importance for controlling and
understanding the growth process for graphene on copper, but also to have wider
implications for the growth of two-dimensional materials on low cost metal
substrates.
see Journal of Materials Chemistry C, 2013, Volume 1, pp 338-342
M. G. Dowsett, R. J. H. Morris, A. Adriaens, N. R. Wilson
Figure 1. (a) AFM
image showing the
poly(CuPc) and
exposed Au surfaces,
(b) Height histogram
taken from the AFM
image and Gaussian
fits determining the
poly(CuPc) thickness.
(c) AFM image
showing the
Co(II)T(o-NH2)PP
and exposed Au
surfaces, (d) Height
histogram taken from
the AFM image and
Gaussian fits
determining the
poly(CuPc) thickness
Abstract
Using an O2+ beam at 90 eV, close to the sputtering threshold, we have
obtained depth profiles from superficial monolayers on gold. The samples
were polymeric copper phthalocyanine (poly(CuPc)) (C32H12N8Cu)n and
Co(II)T(o-NH2)PP (5,10,15,20-tetrakis-(2-aminophenyl) porphyrin-cobalt(II))
(C44H28CoN4) monomolecular layers, deposited on gold films on silicon. The
layer thicknesses, determined using atomic force microscopy, were ~4.5 nm for
the poly(CuPc) and ~1.8 nm for the Co(II)T(o-NH2)PP. The ion signals
monitored included: poly(CuPc) layer: 63Cu+, 65Cu+, 30ON+ and 12 C+; Co(II)T(oNH2)PP layer: 59Co+, 30ON+ and 12 C+. Comparisons between 90 eV and 235 eV
profiles from the layers and bare substrates showed that the monomolecular
layers are resolvable at 90 eV and distinct from any surface contamination
spike. Copyright © 2012 John Wiley & Sons, Ltd.
see Surface and Interface Analysis, January 2013, Volume 45, Issue 1, pp 324-328
M. Adam Dyson, Ana M. Sanchez, Joseph P. Patterson, Rachel K. O'Reilly, Jeremy Sloan and Neil R. Wilson
Abstract
Determining the structure of macromolecular samples is vital for
understanding and adapting their function. Transmission electron microscopy
(TEM) is widely used to achieve this, but, owing to the weak electron scattering
cross-section of carbon, TEM images of macromolecular samples are generally
low contrast and low resolution. Here we implement a fast and practically
simple routine to achieve high-contrast imaging of macromolecular samples
using exit wave reconstruction (EWR), revealing a new level of structural
detail. This is only possible using ultra-low contrast supports such as the
graphene oxide (GO) used here and as such represents a novel application of
these substrates. We apply EWR on GO membranes to study self-assembled
block copolymer structures, distinguishing not only the general morphology or
nanostructure, but also evidence for the substructure (i.e. the polymer chains)
which gives insight into their formation mechanisms and functional properties.
see Soft Matter, February 2013, Volume 9, pp 3741-3749
Abstract
P.A. Pandey, NR Wilson, J.A Covington
The detection of low levels of hydrogen is becoming of ever-greater importance due to its
potential use as a green energy. Current methods of hydrogen detection are predominantly
based around three core technologies, semi-conductive chemiresistive metal oxides,
electrochemical and chemical field-effect transistors. Of these techniques, metal-oxide and
electrochemical sensors suffer from cross-sensitivity to a range of interferrent gases, CO being
the most critical. Field-effect techniques are far more selective, but have a high cost of
manufacture due to their complex construct, thus there is a need for a low-cost and selective
sensing technology that could be applied to mass production. Here, we report on the
fabrication, electrical and chemical characterisation of a Pd-doped reduced graphene oxide
chemiresistors. Sensors were produced by the spin coating of graphene oxide onto a silicon
based gold electrode structure, the graphene oxide was reduced and then Pd sputter coated.
Initial investigation showed that the sensors have a clear response to hydrogen with
sensitivity down to 50 parts per million level. Cross-sensitivity tests indicated that these
sensors did not respond to CO, ethanol and toluene. Thermal characterisation showed that an
increase in temperature improves sensor response by a factor of three between 30 °C and 75
°C. The effect of humidity was also explored; here an increase in sensor response was
achieved at higher humidity concentrations, with sensor operation up to 75 °C. In addition,
the effect of Pd and reduced graphene oxide layers was investigated to evaluate its
significance as a function of sensor response. Here a thickness of 3 nm of Pd and 2 nm of
reduced graphene oxide were shown to offer the highest sensitivity. Finally, Pd doped
graphene sensors were tested for comparison purposes. Here the sensor response of these
sensors was found to be higher, though the manufacture process is far more complex. Doped
reduced graphene oxide chemisresistors offer considerable promise as a mass produced gas
sensor due to its ease of manufacture, good chemical performance and low cross-sensitivity.
This may make such sensors ideal for future hydrogen detection applications.
see Sensors and Actuators B: Chemical, July 2013, Volume 183, pp 478-487
A. J. Marsden, M. Phillips and NR Wilson
Abstract
At a single atom thick, it is challenging to distinguish graphene from
its substrate using conventional techniques. In this paper we show
that friction force microscopy (FFM) is a simple and quick technique
for identifying graphene on a range of samples, from growth
substrates to rough insulators. We show that FFM is particularly
effective for characterizing graphene grown on copper where it can
correlate the graphene growth to the three-dimensional surface
topography. Atomic lattice stick–slip friction is readily resolved and
enables the crystallographic orientation of the graphene to be mapped
nondestructively, reproducibly and at high resolution. We expect FFM
to be similarly effective for studying graphene growth on other
metal/locally crystalline substrates, including SiC, and for studying
growth of other two-dimensional materials such as molybdenum
disulfide and hexagonal boron nitride.
see Nanotechnology, May 2013, Volume 24, pp 255704
Alexander J. Marsden, Maria-Carmen Asensio, José Avila, Pavel Dudin, Alexei Barinov, Paolo Moras,
Polina M. Sheverdyaeva, Thomas W. White, Ian Maskery, Giovanni Costantini, Neil R. Wilson, Gavin R.
Bell
Abstract
Angle-resolved photoemission spectroscopy (ARPES) and X-ray
photoemission spectroscopy have been used to characterise epitaxially
ordered graphene grown on copper foil by low-pressure chemical
vapour deposition. A short vacuum anneal to 200 °C allows
observation of ordered low energy electron diffraction patterns. High
quality Dirac cones are measured in ARPES with the Dirac point at the
Fermi level (undoped graphene). Annealing above 300 °C produces ntype doping in the graphene with up to 350 meV shift in Fermi level,
and opens a band gap of around 100 meV.
Dirac cone dispersion for graphene on Cu foil after vacuum anneals
(left: 200 °C, undoped; right: 500 °C, n-doped). Centre: low energy
electron diffraction from graphene on Cu foil after 200 °C anneal. Data
from Antares (SOLEIL).
see Physica Status Solidi RRL, September 2013, Volume 7, Issue 9 pp 643-646
Helen Rosemary Thomas , Stephen P. Day , William E. Woodruff, Cristina Valles, Robert J Young, Ian A.
Kinloch, Gavin W. Morley, John V Hanna, Neil Richard Wilson, and Jonathan Patrick Rourke
Abstract
We show that the two-component model of graphene oxide (GO), that
is, composed of highly oxidized carbonaceous debris complexed to
oxygen functionalized graphene sheets, is a generic feature of the
synthesis of GO, independent of oxidant or protocol used. The debris
present, roughly one-third by mass, can be removed by a base wash. A
number of techniques, including solid state NMR, demonstrate that
the properties of the base-washed material are independent of the
base used and that it contains similar functional groups to those
present in the debris but at a lower concentration. Removal of the
oxidation debris cleans the GO, revealing its true monolayer nature
and in the process increases the C/O ratio (i.e., a deoxygenation). By
contrast, treating GO with hydrazine both removes the debris and
reduces (both deoxygenations) the graphene sheets.
see Chemistry of Materials, May 2013, Volume 25, Issue 18, pp 3580-3588
Cristina Vallés, Ian A. Kinloch, Robert J. Young, Neil R. Wilson, Jonathan P. Rourke
Abstract
Graphene oxide (GO) prepared using the Hummers’ method is known to be
composed of functionalized graphene sheets decorated by strongly-bound oxidative
debris that can be removed by a simple base wash. The use of as-made GO and basewashed GO as reinforcing fillers in poly(methyl methacrylate) (PMMA)
nanocomposites has been compared through dynamic mechanical thermal analysis
and tensile testing. Nanocomposites with loadings from 0.5 to 10 wt.% were
produced by melt mixing using a twin screw extruder. Large shifts in the values of
Tg for the nanocomposites with respect to PMMA suggest the presence of
interactions between the GO and polymer. Thermogravimetric analysis also
revealed a significant increase in the decomposition temperatures upon the addition
of the GO. Optimal loadings of 1 wt.% were found for both fillers, up to which
substantial mechanical reinforcement was observed. Comparison with previous
nanotube systems, suggests that there was a good dispersion of both fillers below 1
wt.%, with aggregation and a deterioration of the mechanical properties occurring at
higher loadings. Stress-induced shifts of the Raman D band in the GO revealed the
existence of stress-transfer from the PMMA matrix to the fillers during deformation.
Overall the as-made GO gave nanocomposites with better properties than those
reinforced with based-washed material. Hence, it appears that the presence of the
oxidative debris in GO, which acts as a compatibilising surfactant, is beneficial in
producing nanocomposites with both a good dispersion and a strong interface
between GO and a polymer matrix.
see Composites Science and Technology, November 2013, Volume 88, pp 158-164
Cristina E. Giusca, Vlad Stolojan, Jeremy Sloan, Felix Börrnert, Hidetsugu Shiozawa, Kasim Sader, Mark H.
Rümmeli, Bernd Büchner, and S. Ravi P. Silva
Abstract
The demand for high-density memory in tandem with limitations imposed
by the minimum feature size of current storage devices has created a need
for new materials that can store information in smaller volumes than
currently possible. Successfully employed in commercial optical data storage
products, phase-change materials, that can reversibly and rapidly change
from an amorphous phase to a crystalline phase when subject to heating or
cooling have been identified for the development of the next generation
electronic memories. There are limitations to the miniaturization of these
devices due to current synthesis and theoretical considerations that place a
lower limit of 2 nm on the minimum bit size, below which the material does
not transform in the structural phase. We show here that by using carbon
nanotubes of less than 2 nm diameter as templates phase-change nanowires
confined to their smallest conceivable scale are obtained. Contrary to
previous experimental evidence and theoretical expectations, the nanowires
are found to crystallize at this scale and display amorphous-to-crystalline
phase changes, fulfilling an important prerequisite of a memory element. We
show evidence for the smallest phase-change material, extending thus the
size limit to explore phase-change memory devices at extreme scales.
see Nano Letterers, August 2013, Volume 13, Issue 9 pp 4020-4027
Adam J. Cooper, Neil R. Wilson, Ian A. Kinloch, Robert A.W. Dryfe
Abstract
We present a non-oxidative production route to few layer graphene via
the electrochemical intercalation of tetraalkylammonium cations into
pristine graphite. Two forms of graphite have been studied as the
source material with each yielding a slightly different result. Highly
orientated pyrolytic graphite (HOPG) offers greater advantages in
terms of the exfoliate size but the source electrode set up introduces
difficulties to the procedure and requires the use of sonication. Using
a graphite rod electrode, few layer graphene flakes (2 nm thickness)
are formed directly although the flake diameters from this source are
typically small (ca. 100–200 nm). Significantly, for a solvent based
route, the graphite rod does not require ultrasonication or any
secondary physical processing of the resulting dispersion. Flakes have
been characterized using Raman spectroscopy, atomic force
microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).
see Carbon, January 2014 (Currently online), Volume 66, pp 340-350
R. Beanland, P. J. Thomas, D. I. Woodward, P. A. Thomas and R. A. Roemer
Abstract
The advantages of convergent-beam electron diffraction for symmetry
determination at the scale of a few nm are well known. In practice, the
approach is often limited due to the restriction on the angular range of
the electron beam imposed by the small Bragg angle for high-energy
electron diffraction, i.e. a large convergence angle of the incident beam
results in overlapping information in the diffraction pattern.
Techniques have been generally available since the 1980s which
overcome this restriction for individual diffracted beams, by making a
compromise between illuminated area and beam convergence. Here a
simple technique is described which overcomes all of these problems
using computer control, giving electron diffraction data over a large
angular range for many diffracted beams from the volume given by a
focused electron beam (typically a few nm or less). The increase in the
amount of information significantly improves the ease of
interpretation and widens the applicability of the technique,
particularly for thin materials or those with larger lattice parameters
see Acta Crystallographica Section A, July 2013 , Volume 69, pp 427-434
Aurelia R. Honerkamp-Smith, Francis G. Woodhouse, Vasily Kantsler, and Raymond E. Goldstein
Abstract
The viscosity of lipid bilayer membranes plays an important role in
determining the diffusion constant of embedded proteins and the
dynamics of membrane deformations, yet it has historically proven
very difficult to measure. Here we introduce a new method based on
quantification of the large-scale circulation patterns induced inside
vesicles adhered to a solid surface and subjected to simple shear flow
in a microfluidic device. Particle image velocimetry based on spinning
disk confocal imaging of tracer particles inside and outside of the
vesicle and tracking of phase-separated membrane domains are used
to reconstruct the full three-dimensional flow pattern induced by the
shear. These measurements show excellent agreement with the
predictions of a recent theoretical analysis, and allow direct
determination of the membrane viscosity
see Physical Review Letters , July 2013 , Volume 111, pp 038103
Dmytro V. Dudenko, Jonathan R. Yates, Kenneth D. M. Harris and Steven P. Brown
Abstract
Density functional theory (DFT) calculations using the Perdew–Burke–Ernzerhof (PBE)
exchange-correlation functional are presented for a 1:1 cocrystal formed by indomethacin and
nicotinamide (IND-NIC) as well as for crystal structures of the individual components. DFTD approaches which correct the DFT energy for dispersion effects, specifically the Grimme
(G06) and Tkatchenko–Scheffler (TS) schemes, are investigated: for geometry optimisation
starting with crystal structures determined experimentally by diffraction and allowing the
atomic positions and the unit cell to vary, closest agreement with the experimental unit cell
parameters is achieved with the PBE-TS approach (calculated volumes are less than 4%
smaller than in experiment). Calculations of solid-state NMR chemical shifts using the
GIPAW (gauge including projector augmented wave) approach are presented. Closest
agreement between NMR chemical shifts calculated with variable and fixed (experimental)
unit cell parameters is also observed for the PBE-TS approach: the root mean squared
standard deviation difference is 0.15 ppm (1H) and 0.29 ppm (13C) for PBE-TS, as compared to
0.45 ppm (1H) and 0.68 ppm (13C) with standard PBE. Differences in 1H chemical shifts
calculated for the full periodic crystal structure and for isolated molecules extracted from the
geometry-optimised crystal structure are presented in conjunction with NICS (nucleus
independent chemical shift) maps, so as to separately quantify intermolecular hydrogen
bonding and π–π interactions. This analysis is complemented by total energy calculations,
including also at the B97D/6-311+G* level of theory with basis set superposition error
correction, in order to understand the interactions that drive cocrystallisation.
see CrystEngComm, August 2013 , Volume 15, pp 8797-8807
R O Dendy, S C Chapman and S Inagaki
Abstract
In some magnetically confined plasmas, an applied pulse of
rapid edge cooling can trigger either a positive or negative
excursion in the core electron temperature from its steady state
value. We present a new model which captures the time
evolution of the transient, non-diffusive local dynamics in the
core plasma. We show quantitative agreement between this
model and recent spatially localized measurements (Inagaki et
al 2010 Plasma Phys. Control. Fusion 52 075002) of the local timeevolving temperature pulse in cold pulse propagation
experiments in the Large Helical Device
see Plasma Physics and Controlled Fusion, October 2013 , Volume 55, Issue 11
W. N. Lai, S. C. Chapman and R. O. Dendy
Abstract
Suprathermal tails in the distributions of electron velocities parallel to the magnetic
field are found in many areas of plasma physics, from magnetic confinement fusion to
solar system plasmas. Parallel electron kinetic energy can be transferred into plasma
waves and perpendicular gyration energy of particles through the anomalous Doppler
instability (ADI), provided that energetic electrons with parallel velocities vjj ðx þ
XceÞ=kjj are present; here Xce denotes electron cyclotron frequency, x the wave angular
frequency, and kjj the component of wavenumber parallel to the magnetic field. This
phenomenon is widely observed in tokamak plasmas. Here, we present the first fully
self-consistent relativistic particle-in-cell simulations of the ADI, spanning the linear
and nonlinear regimes of the ADI. We test the robustness of the analytical theory in the
linear regime and follow the ADI through to the steady state. By directly evaluating the
parallel and perpendicular dynamical contributions to j E in the simulations, we follow
the energy transfer between the excited waves and the bulk and tail electron
populations for the first time. We find that the ratio Xce=ðxpe þ XceÞ of energy transfer
between parallel and perpendicular, obtained from linear analysis, does not apply
when damping is fully included, when we find it to be xpe=ðxpe þ XceÞ; here xpe
denotes the electron plasma frequency. We also find that the ADI can arise beyond the
previously expected range of plasma parameters, in particular when Xce > xpe. The
simulations also exhibit a spectral feature which may correspond to the observations of
suprathermal narrowband emission at xpe detected from low density tokamak
plasmas. VC 2013 AIP Publishing LLC.
see Physics of Plasmas, October 2013 , Volume 20, pp102122
C. E. Giusca, V. Stolojan, J. Sloan, F. Börrnert, H. Shiozawa, K. Sader, M. H. Rümmeli, B. Büchner, S. R. P. Silva
Abstract
The demand for high-density memory in tandem with limitations imposed by
the minimum feature size of current storage devices has created a need for new
materials that can store information in smaller volumes than currently possible.
Successfully employed in commercial optical data storage products, phasechange materials, that can reversibly and rapidly change from an amorphous
phase to a crystalline phase when subject to heating or cooling have been
identified for the development of the next generation electronic memories.
There are limitations to the miniaturization of these devices due to current
synthesis and theoretical considerations that place a lower limit of 2 nm on the
minimum bit size, below which the material does not transform in the
structural phase. We show here that by using carbon nanotubes of less than 2
nm diameter as templates phase-change nanowires confined to their smallest
conceivable scale are obtained. Contrary to previous experimental evidence and
theoretical expectations, the nanowires are found to crystallize at this scale and
display amorphous-to-crystalline phase changes, fulfilling an important
prerequisite of a memory element. We show evidence for the smallest phasechange material, extending thus the size limit to explore phase-change memory
devices at extreme scales.
see Nano Letters, August 2013 , Volume 13, Issue 9 pp4020-4027
L. Daniels, M. Weber, M. Lees, M. Guennou, R. Kashtiban, J. Sloan, J. Kreisel, R. I. Walton
Abstract
A new mixed rare-earth orthochromite series, LaxSm1–xCrO3, prepared through singlestep hydrothermal synthesis is reported. Solid solutions (x = 0, 0.25, 0.5, 0.625, 0.75,
0.875, and 1.0) were prepared by the hydrothermal treatment of amorphous mixedmetal hydroxides at 370 °C for 48 h. Transmission electron microscopy (TEM) reveals
the formation of highly crystalline particles with dendritic-like morphologies. Rietveld
refinements against high-resolution powder X-ray diffraction (PXRD) data show that
the distorted perovskite structures are described by the orthorhombic space group
Pnma over the full composition range. Unit cell volumes and Cr–O–Cr bond angles
decrease monotonically with increasing samarium content, consistent with the presence
of the smaller lanthanide in the structure. Raman spectroscopy confirms the formation
of solid solutions, the degree of their structural distortion. With the aid of shell-model
calculations the complex mixing of Raman modes below 250 cm–1 is clarified.
Magnetometry as a function of temperature reveals the onset of low-temperature
antiferromagnetic ordering of Cr3+ spins with weak ferromagnetic component at Néel
temperatures (TN) that scale linearly with unit cell volume and structural distortion.
Coupling effects between Cr3+ and Sm3+ ions are examined with enhanced
susceptibilities below TN due to polarization of Sm3+ moments. At low temperatures the
Cr3+ sublattice is shown to undergo a second-order spin reorientation observed as a
rapid decrease of susceptibility.
see Inorganic Chemistry, October 2013 , Volume 52, Issue 20 pp12161-12169
LHCb collaboration inc T. Gershon
Abstract
First observations and measurements of the branching fractions of the
Bˉºs →D+D-, Bˉºs →Ds+D- and Bˉºs→DºDˉº decays are presented using
1.0 fb-1 of data collected by the LHCb experiment. These branching
fractions are normalized to those of Bˉºs →D+D, Bº→D-Ds+ and
Bˉ→DºDˉs, respectively. An excess of events consistent with the decay Bˉº
→DºDˉº is also seen, and its branching fraction is measured relative to
that of Bˉ→DºDˉs. Improved measurements of the branching fractions
B(Bˉºs→Ds+Ds-) and B(Bˉ→DºDs-) are reported, each relative to B(Bº→DDs+). The ratios of branching fractions are B(Bˉºs →D+D-)
B(Bˉºs→DºDˉº)=1.08±0.20±0.10, B(Bˉºs →D+D-) B(Bº→DDs+)=0.050±0.008±0.004, B( Bˉºs→DºDˉº)/B(Bˉ→DºDˉs)=0.019±0.003±0.003,
B( Bˉºs→DºDˉº)/B(Bˉ→DºDˉs)<0.0024 at 90% CL, B(Bˉºs →D+D-) B(Bº→DDs+)=0.56±0.03±0.04, B(Bˉ→DºDˉs)/B(Bº→D-Ds+)=1.22±0.02±0.07, where
the uncertainties are statistical and systematic, respectively.
see Physical Review D , May 2013, Volume 87, Issue 9
LHCb collaboration inc T. Gershon
Abstract
During 2011 the LHCb experiment at CERN collected 1.0
fb−1 of √s=7~TeV pp collisions. Due to the large heavy
quark production cross-sections, these data provide
unprecedented samples of heavy flavoured hadrons. The
first results from LHCb have made a significant impact
on the flavour physics landscape and have definitively
proved the concept of a dedicated experiment in the
forward region at a hadron collider. This document
discusses the implications of these first measurements on
classes of extensions to the Standard Model, bearing in
mind the interplay with the results of searches for onshell production of new particles at ATLAS and CMS.
The physics potential of an upgrade to the LHCb
detector, which would allow an order of magnitude more
data to be collected, is emphasised.
see the European Physical Journal C, April 2013, Volume 73, pp2373
F. Magnus, R. Moubah, A. H. Roos, A. Kruk, V. Kapaklis, T. Hase, B. Hjorvarsson and G. Andersson
Abstract
SmCo thin films have been grown by magnetron
sputtering at room temperature with a composition of
2–35 at.% Sm. Films with 5 at.% or higher Sm are
amorphous and smooth. A giant tunable uniaxial inplane magnetic anisotropy is induced in the films
which peaks in the composition range 11–22 at.% Sm.
This cross-over behavior is not due to changes in the
atomic moments but rather the local configuration
changes. The excellent layer perfection combined
with highly tunable magnetic properties make these
films important for spintronics applications. VC 2013
AIP Publishing LLC
see Applied Physics Letters, April 2013, Volume 102, pp162402
Christopher W. Burrows, Andrew Dobbie, Maksym Myronov, Thomas P. A. Hase, Stuart B. Wilkins,
Marc Walker, James J. Mudd, Ian Maskery, Martin R. Lees, Christopher F. McConville, David R. Leadley, and Gavin R. Bell
Abstract
Molecular beam epitaxial growth of ferromagnetic
MnSb(0001) has been achieved on high quality, fully
relaxed Ge(111)/Si(111) virtual substrates grown by
reduced pressure chemical vapor deposition. The
epilayers were characterized using reflection high energy
electron diffraction, synchrotron hard X-ray diffraction, Xray photoemission spectroscopy, and magnetometry. The
surface reconstructions, magnetic properties, crystalline
quality, and strain relaxation behavior of the MnSb films
are similar to those of MnSb grown on GaAs(111). In
contrast to GaAs substrates, segregation of substrate
atoms through the MnSb film does not occur, and
alternative polymorphs of MnSb are absent.
see Crystal Growth & Design, October 2013
C. S. Brady, C. P. Ridgers, T. D. Arber, A. R. Bell and J. G. Kirk
Abstract
A novel absorption mechanism for linearly polarized lasers
propagating in relativistically underdense solids in the ultrarelativistic
(a ! 100) regime is presented. The mechanism is based on strong
synchrotron emission from electrons reinjected into the laser by the
space charge field they generate at the front of the laser pulse. This
laser absorption, termed reinjected electron synchrotron emission, is
due to a coupling of conventional plasma physics processes to
quantum electrodynamic processes in low density solids at intensities
above 1022 W=cm2. Reinjected electron synchrotron emission is
identified in 2D QED-particlein-cell simulations and then explained in
terms of 1D QED-particle-in-cell simulations and simple analytical
theory. It is found that between 1% (at 1022 W=cm2) and 14% (at 8 "
1023 W=cm2) of the laser energy is converted into gamma ray photons,
potentially providing an ultraintense future gamma ray source.
see Physical Review Letter, December 2012, Volume 109, pp245006
K. I. Doig, F. Aguesse, A. K. Axelsson, N. M. Alford, S. Nawaz, V. R. Palkar, S. P. P. Jones, R. D. Johnson, R.
A. Synowicki, and J. Lloyd-Hughes
Abstract
Coherent magnons and acoustic phonons were impulsively
excited and probed in thin films of the room temperature
multiferroic Bi1−x−yDyxLayFeO3 using femtosecond laser
pulses. The elastic moduli of rhombohedral, tetragonal,
and rare-earth doped BiFeO3 were determined from
acoustic-mode frequencies in conjunction with
spectroscopic ellipsometry. A weak ferromagnetic order,
induced alternately by magnetization in the growth
direction or by tetragonality, created a magnon oscillation
at 75 GHz, indicative of a Dzyaloshinskii-Moriya
interaction energy of 0.31 meV.
see Physical Review B, September 2013, Volume 88, Issue 9
C.K. Yong, J. Wong-Leung, H.J. Joyce, J. Lloyd-Hughes, Q. Gao, H.H. Tan, C. Jagadish, M.B.
Johnston, L.M. Herz
Abstract
We have investigated the dynamics of hot charge carriers in InP
nanowire ensembles containing a range of densities of zinc-blende
inclusions along the otherwise wurtzite nanowires. From timedependent photoluminescence spectra, we extract the temperature of
the charge carriers as a function of time after nonresonant excitation.
We find that charge-carrier temperature initially decreases rapidly
with time in accordance with efficient heat transfer to lattice
vibrations. However, cooling rates are subsequently slowed and are
significantly lower for nanowires containing a higher density of
stacking faults. We conclude that the transfer of charges across the
type II interface is followed by release of additional energy to the
lattice, which raises the phonon bath temperature above equilibrium
and impedes the carrier cooling occurring through interaction with
such phonons. These results demonstrate that type II heterointerfaces
in semiconductor nanowires can sustain a hot charge-carrier
distribution over an extended time period. In photovoltaic
applications, such heterointerfaces may hence both reduce
recombination rates and limit energy losses by allowing hot-carrier
harvesting.
see Nano Letters, August 2013, Volume 13, Issue 1
Annemie Adriaens, Paul Quinn, Sergey Nikitenko, and Mark G. Dowsett
Abstract
In experiments preliminary to the design of an X-ray-excited optical
luminescence (XEOL)-based chemical mapping tool we have used X-ray micro
(4.5 × 5.2 μm) and macro (1 × 6 mm) beams with similar total fluxes to assess
the effects of a high flux density beam of X-rays at energies close to an
absorption edge on inorganic surfaces in air. The near surface composition of
corroded cupreous alloys was analyzed using parallel X-ray and optical
photoemission channels to collect X-ray absorption near-edge structure
(XANES) data at the Cu K edge. The X-ray fluorescence channel is
characteristic of the composition averages over several micrometers into the
surface, whereas the optical channel is surface specific to about 200 nm. While
the X-ray fluorescence data were mostly insensitiv to the X-ray dose, the
XEOL-XANES data from the microbeam showed significant dose-dependent
changes to the superficial region, including surface cleaning, changes in the
oxidation state of the copper, and destruction of surface compounds
responsible for pre-edge fluorescence or phosphorescence in the visible. In one
case, there was evidence that the lead phase in a bronze had melted.
Conversely, data from the macrobeam were stable over several hours. Apart
from localized heating effects, the microbeam damage is probably associated
with the O3 loading of the surface and increased reaction rate with
atmospheric water vapor.
see Analytical Chemistry, September 2013, Volume 85, Issue 20, pp9556-9563
A. Rodriguez, A. Chakrabarti and R. A. Römer
Abstract
We describe how to engineer wave-function delocalization in
disordered systems modeled by tight-binding Hamiltonians in d>1
dimensions. We show analytically that a simple product structure for
the random on-site potential energies, together with suitably chosen
hopping strengths, allows a resonant scattering process leading to
ballistic transport along one direction, and a controlled coexistence of
extended Bloch states and anisotropically localized states in the
spectrum. We demonstrate that these features persist in the
thermodynamic limit for a continuous range of the system parameters.
Numerical results support these findings and highlight the robustness
of the extended regime with respect to deviations from the exact
resonance condition for finite systems. The localization and transport
properties of the system can be engineered almost at will and
independently in each direction. This study gives rise to the possibility
of designing disordered potentials that work as switching devices and
band-pass filters for quantum waves, such as matter waves in optical
lattices.
see Physical Review Letters B, August 2012, Volume 86, 085119
C. Paez, P. Schulz, N. Wilson, R. A. Römer
Abstract
In this work, we numerically calculate the electric current through
three kinds of DNA sequences (telomeric, λ-DNA and p53-DNA)
described by different heuristic models. A bias voltage is applied
between two zigzag edged graphene contacts attached to the DNA
segments, while a gate terminal modulates the conductance of the
molecule. Calculation of the current is performed by integrating the
transmission function (calculated using the lattice Green's function)
over the range of energies allowed by the chemical potentials. We show
that a telomeric DNA sequence, when treated as a quantum wire in the
fully coherent low-temperature regime, works as an excellent
semiconductor. Clear steps are apparent in the current–voltage curves
of telomeric sequences and are present independent of length and
sequence initialization at the contacts. We also find that the molecule–
electrode coupling can drastically influence the magnitude of the
current. The difference between telomeric DNA and other DNAs, such
as λ-DNA and DNA for the tumour suppressor p53, is particularly
visible in the length dependence of the current.
see New Journal of Physics, September 2012, Volume 14, 093049
K. Hashimoto, T. Champel, S. Florens, C. Sohrmann, J. Wiebe, Y. Hirayama, R. A. Römer, R.
Wiesendanger, M. Morgenstern
Abstract
Scanning tunneling spectroscopy is used to study the real-space
local density of states of a two-dimensional electron system in a
magnetic field, in particular within higher Landau levels. By
Fourier transforming the local density of states, we find a set of n
radial minima at fixed momenta for the nth Landau levels. The
momenta of the minima depend only on the inverse magnetic
length. By comparison with analytical theory and numerical
simulations, we attribute the minima to the nodes of the quantum
cyclotron orbits, which decouple in a Fourier representation from
the random guiding center motion due to disorder. Adequate
Fourier filtering reveals the nodal structure in real space in some
areas of the sample with relatively smooth potential disorder.
see Physical Review Letters, September 2012, Volume 109, Issue 11
Kyriacos C. Leptos, Kirsty Y. Wan, Marco Polin, Idan Tuval, Adriana I. Pesci, and Raymond
E. Goldstein
Abstract
Groups of beating flagella or cilia often synchronize so that
neighboring filaments have identical frequencies and phases. A
prime example is provided by the unicellular biflagellate
Chlamydomonas reinhardtii, which typically displays synchronous
in-phase beating in a low-Reynolds number version of
breaststroke swimming. We report the discovery that ptx1, a
flagellar-dominance mutant of C. reinhardtii, can exhibit
synchronization in precise antiphase, as in the freestyle
swimming stroke. High-speed imaging shows that ptx1 flagella
switch stochastically between in-phase and antiphase states, and
that the latter has a distinct waveform and significantly higher
frequency, both of which are strikingly similar to those found
during phase slips that stochastically interrupt in-phase beating
of the wild-type. Possible mechanisms underlying these
observations are discussed.
see Physical Review Letters, October 2013, Volume 111, 158101
Marc Warner, Salahud Din, Igor S Tupitsyn, Gavin W Morley, A Marshall Stoneham, Jules A Gardener,
Zhenlin Wu, Andrew J Fisher, Sandrine Heutz, Christopher W M Kay & Gabriel Aeppli
Abstract
Organic semiconductors are studied intensively for applications in electronics
and optics1, and even spin-based information technology, or spintronics2.
Fundamental quantities in spintronics are the population relaxation time (T1)
and the phase memory time (T2): T1 measures the lifetime of a classical bit, in
this case embodied by a spin oriented either parallel or antiparallel to an
external magnetic field, and T2 measures the corresponding lifetime of a
quantum bit, encoded in the phase of the quantum state. Here we establish
that these times are surprisingly long for a common, low-cost and chemically
modifiable organic semiconductor, the blue pigment copper phthalocyanine 3,
in easily processed thin-film form of the type used for device fabrication. At
5 K, a temperature reachable using inexpensive closed-cycle refrigerators, T1
and T2 are respectively 59 ms and 2.6 μs, and at 80 K, which is just above the
boiling point of liquid nitrogen, they are respectively 10 μs and 1 μs,
demonstrating that the performance of thin-film copper phthalocyanine is
superior to that of single-molecule magnets over the same temperature range4.
T2 is more than two orders of magnitude greater than the duration of the spin
manipulation pulses, which suggests that copper phthalocyanine holds
promise for quantum information processing, and the long T1 indicates
possibilities for medium-term storage of classical bits in all-organic devices on
plastic substrates.
see Nature, October 2013 Highlighted in popular press including New York Times
Physics Research 2013
M Scala, M S Kim, G W Morley, P F Barker and S Bose
Abstract
We show how the interference between spatially separated states of the
center of mass (c.m.) of a mesoscopic harmonic oscillator can be
evidenced by coupling it to a spin and performing solely spin
manipulations and measurements (Ramsey interferometry). We
propose to use an optically levitated diamond bead containing a
nitrogen-vacancy center spin. The nanoscale size of the bead makes the
motional decoherence due to levitation negligible. The form of the spinmotion coupling ensures that the scheme works for thermal states so
that moderate feedback cooling suffices. No separate control or
observation of the c.m. state is required and thereby one dispenses with
cavities, spatially resolved detection, and low-mass-dispersion
ensembles. The controllable relative phase in the Ramsey
interferometry stems from a gravitational potential difference so that it
uniquely evidences coherence between states which involve the whole
nanocrystal being in spatially distinct locations.
see Physical Review Letters October 2013, Volume 111, 180403
Physics Research 2013
G W Morley, Petra Lueders, M. Hamed Mohammady, Setrak J. Balian, Gabriel Aeppli, Christopher W. M.
Kay, Wayne M. Witzel, Gunnar Jeschke & Tania S. Monteiro
Abstract
Pulsed magnetic resonance allows the quantum state of electronic and nuclear
spins to be controlled on the timescale of nanoseconds and microseconds
respectively. The time required to flip dilute spins is orders of magnitude shorter
than their coherence times, leading to several schemes for quantum information
processing with spin qubits. Instead, we investigate ‘hybrid nuclear–electronic’
qubits consisting of near 50:50 superpositions of the electronic and nuclear spin
states. Using bismuth-doped silicon, we demonstrate quantum control over
these states in 32 ns, which is orders of magnitude faster than previous
experiments using pure nuclear states. The coherence times of up to 4 ms are
five orders of magnitude longer than the manipulation times, and are limited
only by naturally occurring 29Si nuclear spin impurities. We find a quantitative
agreement between our experiments and an analytical theory for the resonance
positions, as well as their relative intensities and Rabi oscillation frequencies.
These results bring spins in a solid material a step closer to research on ion-trap
qubits.
see Nature Materials, December 2012, Volume 12 , pp103-107
Physics Research 2013