2,242 research outputs found
Evidence for a small hole pocket in the Fermi surface of underdoped YBa2Cu3Oy
The Fermi surface of a metal is the fundamental basis from which its
properties can be understood. In underdoped cuprate superconductors, the Fermi
surface undergoes a reconstruction that produces a small electron pocket, but
whether there is another, as yet undetected portion to the Fermi surface is
unknown. Establishing the complete topology of the Fermi surface is key to
identifying the mechanism responsible for its reconstruction. Here we report
the discovery of a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a
small quantum oscillation frequency in the thermoelectric response and in the
c-axis resistance. The field-angle dependence of the frequency demonstrates
that it is a distinct Fermi surface and the normal-state thermopower requires
it to be a hole pocket. A Fermi surface consisting of one electron pocket and
two hole pockets with the measured areas and masses is consistent with a
Fermi-surface reconstruction caused by the charge-density-wave order observed
in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are
removed by a separate mechanism, possibly the pseudogap.Comment: 23 pages, 5 figure
Anisotropy of the Optimally-Doped Iron Pnictide Superconductor Ba(Fe0.926Co0.074)2As2
Anisotropies of electrical resistivity, upper critical field, London
penetration depth and critical currents have been measured in single crystals
of the optimally doped iron pnictide superconductor
Ba(FeCo)As, =0.074 and 23 K. The normal state
resistivity anisotropy was obtained by employing both the Montgomery technique
and direct measurements on samples cut along principal crystallographic
directions. The ratio is about 41 just
above and becomes half of that at room temperature. The anisotropy of the
upper critical field, , as determined from
specific heat measurements close to , is in the range of 2.1 to 2.6,
depending on the criterion used. A comparable low anisotropy of the London
penetration depth, , was recorded
from TDR measurements and found to persist deep into the superconducting state.
An anisotropy of comparable magnitude was also found in the critical currents,
, as determined from both direct transport
measurements (1.5) and from the analysis of the magnetization data
(3). Overall, our results show that iron pnictide superconductors
manifest anisotropies consistent with essentially three-dimensional
intermetallic compound and bear little resemblance to cuprates
1D TiO2 nanostructures probed by 2D transmission electron microscopy
Hybrid solar cells based on nanoparticulate TiO2, dye and poly(3-hexylthiophene) are a common benchmark in the field of solid-state dye-sensitized solar cells. One-dimensionally nanostructured titanium dioxide is expected to enhance power-conversion efficiency (PCE) due to a high surface area combined with a direct path for electrons from the active interface to the back electrode. However, current devices do not meet those expectations and cannot surpass their mesoporous counterparts. This work approaches
the problem by detailed investigation of diverse nanostructures on a nanoscale by advanced transmission electron microscopy (TEM). Anodized TiO2 nanotubes are analyzed concerning their crystallinity. An unexpectedly large grain size is found, and its implication is shown by corresponding solar cell characteristics which feature an above-average fill factor. Quasi-single crystalline rutile nanowires are grown hydrothermally, and a peculiar defect structure consisting of free internal surfaces is revealed. A growth model based on Coulombic repulsion and steric hindrance is developed to explain the resulting V-shaped defect cascade. The influence of the defects on solar cell performance is investigated and interpreted by a combination of TEM, electronic device characterization and photoluminescence spectroscopy, including lifetime measurements. A specific annealing treatment is proposed to counter the defects, suppressing several loss mechanisms and resulting in an improvement of PCEs by 35 %. Simultaneously, a process is developed to streamline electron-tomographic reconstruction of complex nanoparticles. Its suitability is
demonstrated by the reconstruction of a gold nanostar and a number of iron-based particles distributed on few-layered graphene
High Speed Human Action Recognition using a Photonic Reservoir Computer
The recognition of human actions in videos is one of the most active research
fields in computer vision. The canonical approach consists in a more or less
complex preprocessing stages of the raw video data, followed by a relatively
simple classification algorithm. Here we address recognition of human actions
using the reservoir computing algorithm, which allows us to focus on the
classifier stage. We introduce a new training method for the reservoir
computer, based on "Timesteps Of Interest", which combines in a simple way
short and long time scales. We study the performance of this algorithm using
both numerical simulations and a photonic implementation based on a single
non-linear node and a delay line on the well known KTH dataset. We solve the
task with high accuracy and speed, to the point of allowing for processing
multiple video streams in real time. The present work is thus an important step
towards developing efficient dedicated hardware for video processing
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Graphene Microelectrode Arrays to Combine Electrophysiology with Fluorescence Imaging of Amyloid Proteins
Alzheimer's disease (AD) and Parkinson's diseases (PD) are neurodegenerative diseases that affect 60\,million people worldwide. Both diseases are linked to the misfolding of proteins from their native conformational state into -sheeted amyloid fibrils. In AD the implicated proteins are amyloid- and tau, and for PD the implicated protein is -synuclein (aSyn). The motivation for this work is to develop and use physical techniques to better understand the role of amyloid proteins in neurodegenerative diseases. Two techniques used in amyloid research are fluorescence microscopy, to map the protein location and aggregation state, and electrophysiology, to examine the effect of the proteins on neurons. To enable these techniques to be combined, a transparent graphene microelectrode array (MEA) was designed, fabricated and characterised. The active electrode site was graphene since it is electrically conductive, optically transparent and biocompatible. The graphene MEA was characterised using Raman spectroscopy to check the graphene quality, and electrochemical impedance spectroscopy (EIS) to probe the electrode-electrolyte interface. The graphene MEAs enabled voltage trace recordings from cultured neurons to be combined with widefield, confocal fluorescence and fluorescence lifetime imaging microscopy (FLIM). Combining fluorescence imaging and electrophysiology will allow amyloid aggregation to be correlated with neuronal firing patterns. Another physical technique used was Fourier transform infrared spectroscopy (FTIR). A script was written to estimate the protein secondary structure content, and used to investigate polymorphism in the monomeric amyloid protein aSyn.Engineering and Physical Sciences Research Council, Wellcome Trust, Medical Research Counci
Interfacial charge transfer in nanoscale polymer transistors
Interfacial charge transfer plays an essential role in establishing the
relative alignment of the metal Fermi level and the energy bands of organic
semiconductors. While the details remain elusive in many systems, this charge
transfer has been inferred in a number of photoemission experiments. We present
electronic transport measurements in very short channel ( nm)
transistors made from poly(3-hexylthiophene) (P3HT). As channel length is
reduced, the evolution of the contact resistance and the zero-gate-voltage
conductance are consistent with such charge transfer. Short channel conduction
in devices with Pt contacts is greatly enhanced compared to analogous devices
with Au contacts, consistent with charge transfer expectations. Alternating
current scanning tunneling microscopy (ACSTM) provides further evidence that
holes are transferred from Pt into P3HT, while much less charge transfer takes
place at the Au/P3HT interface.Comment: 19 preprint pages, 6 figure
Event Control through Motion Detection
Computer Vision is the study of machines that extract information from an image and perform some processing on the captured images to extract necessary data
to solve some task. As a scientic discipline, the study of computer vision is concerned with the theories behind articial systems that extract information from images. The image data could in several dierent forms and formats, such as video sequences, views from multiple cameras, or multi-dimensional data acquired from a medical scanner. As a technological discipline, computer vision intends to
apply its theories and models to the construction and design of computer vision systems
The role of computer vison in robots is providing detailed information about the environment. A robust vision system should be able to detect and identify objects reliably and provide an accurate representation of the environment to
higher level processes. The vision system should also be highly ecient, allowing a resource limited agent to respond quickly to a changing environment. Each frame acquired by a digital camera must be processed in a small, usually xed,
amount of time. Algorithmic complexity is therefore constrained, introducing a tradeo between processing time and the quality of the information acquired. In most robotic applications, the vision system is the main perception device and autonomous robots must be capable of using it in order to self-localize and locate the objects that they have to manipulate.
The objective of the project was to build a computer controlled bot which could collect and deposit balls rolling down a ramp with the help of overhead/onboard
camera.The ojective was achieved with the use of Motion History Image(MHI) based image processing algortihms and microcontroller based controling of motors
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