953 research outputs found
Extended massive gravity in three dimensions
Using a first order Chern-Simons-like formulation of gravity we
systematically construct higher-derivative extensions of general relativity in
three dimensions. The construction ensures that the resulting higher-derivative
gravity theories are free of scalar ghosts. We canonically analyze these
theories and construct the gauge generators and the boundary central charges.
The models we construct are all consistent with a holographic c-theorem which,
however, does not imply that they are unitary. We find that Born-Infeld gravity
in three dimensions is contained within these models as a subclass.Comment: 35p, v2; minor changes, references adde
Logarithmic AdS Waves and Zwei-Dreibein Gravity
We show that the parameter space of Zwei-Dreibein Gravity (ZDG) in AdS3
exhibits critical points, where massive graviton modes coincide with pure gauge
modes and new `logarithmic' modes appear, similar to what happens in New
Massive Gravity. The existence of critical points is shown both at the
linearized level, as well as by finding AdS wave solutions of the full
non-linear theory, that behave as logarithmic modes towards the AdS boundary.
In order to find these solutions explicitly, we give a reformulation of ZDG in
terms of a single Dreibein, that involves an infinite number of derivatives. At
the critical points, ZDG can be conjectured to be dual to a logarithmic
conformal field theory with zero central charges, characterized by new
anomalies whose conjectured values are calculated.Comment: 20 page
Three-Dimensional Tricritical Gravity
We consider a class of parity even, six-derivative gravity theories in three
dimensions. After linearizing around anti-de Sitter space, the theories have
one massless and two massive graviton solutions for generic values of the
parameters. At a special, so-called tricritical, point in parameter space the
two massive graviton solutions become massless and they are replaced by two
solutions with logarithmic and logarithmic-squared boundary behavior. The
theory at this point is conjectured to be dual to a rank-3 Logarithmic
Conformal Field Theory (LCFT) whose boundary stress tensor, central charges and
new anomaly we calculate. We also calculate the conserved Abbott-Deser-Tekin
charges. At the tricritical point, these vanish for excitations that obey
Brown-Henneaux and logarithmic boundary conditions, while they are generically
non-zero for excitations that show logarithmic-squared boundary behavior. This
suggests that a truncation of the tricritical gravity theory and its
corresponding dual LCFT can be realized either via boundary conditions on the
allowed gravitational excitations, or via restriction to a zero charge
sub-sector. We comment on the structure of the truncated theory.Comment: 37 pages, 2 figures; v2: added reference; published versio
Gold free ohmic contacts for III-V MOSFET devices
Over the past forty years the development of CMOS has been able to follow Moore’s law using planar silicon technology. However, this technology is reaching its limits as the
density of transistors has a significant impact on the power dissipation in an integrated circuit. Alternative channel materials and device architectures will then be required in the future to reduce the power consumption of transistors. The development of CMOS technology with high mobility channel materials, specifically Ge for pMOS and III-V materials for nMOS, was the aim of the European Union FP7 funded Duallogic consortium, of which this project was part. The experimental work at the University of Glasgow was the III-V compound semiconductor MOSFET, in particular the study of Si processing compatible source/drain
contacts to III-V MOSFET devices with InxGa1-xAs channel materials, which was an important aspect of this thesis. Another area investigated in this thesis is the impact of
current crowding effects on source/drain contact resistance by aggressive scaling of devices.
During this thesis, optimisation of a PdGe-based ohmic contact to buried channel device material with a In0.75GaAs channel led to a contact resistance of 0.15Ohm.mm compared to 1Ohm.mm in previous work by R. Hill. The PdGe-based contact also proved to be scalable in both vertical and lateral dimensions. This scaled structure was then integrated in a surface channel MOSFET device with 1μm access regions and gate lengths varying from 100nm to 20μm. The performance of the devices with 20μm gate lengths was then compared to devices with a NiGeAu based ohmic contact. An increase in RC, 1.82Ohm.mm vs. 0.94Ohm.mm, and Ron, 11.1Ohm.mm vs. 8.55Ohm.mm, was observed in the PdGe-based contact, which resulted in a decrease in gm, 92.3mS/mm vs. 103mS/mm, and Id,sat, 103mA/mm vs. 122mA/mm. However, further optimisation of the PdGe-based ohmic contact showed promising results with a contact resistance of 0.45Ohm.mm.
The novel test structure is the first test structure, which makes direct contact to III-V material, with critical dimensions below the transfer length. This structure is able to experimentally observe the current crowding effects and allows for the extraction of the sheet resistance underneath the contact and a more accurate extraction of the specific contact resistivity. This offers a significant insight into the impact of the sheet resistance underneath the contact and the role it plays
Performance evaluation of the dynamic trajectory design for an unmanned aerial base station in a single frequency network
Using an Unmanned Aerial Base Station (UABS) i.e., a base station carried by a UAV (Unmanned Aerial Vehicle) or drone, is a promising approach to offer coverage and capacity to those users that are not being served by the base stations of the terrestrial network. In this paper, we propose an approach to the design of the drone's trajectory to account for the quickly varying user traffic and pattern. This approach is based on the identification of clusters made of nearby users to be served. The decision on which cluster to visit next by the UABS depends on a cost-function considering the distance to the next cluster, the user density and spread in the cluster, and the direction compared to the previously visited cluster. Furthermore, we propose a radio resource assignment algorithm to minimize the interference from the UABS to the terrestrial network when both are operating in the same frequency band. The potential improvements in terms of network capacity (sum throughput) and user satisfaction are estimated in this study
Statistics of multipath component clustering in an office environment
In this paper, directional MIMO measurements in an indoor office environment are presented. A 5-D ESPRIT estimation algorithm is used to extract parameters associated with discrete propagation paths, such as their azimuth of arrival, azimuth of departure, delay, and power. The estimated path parameters are grouped into clusters using the statistical K-power-means algorithm. Statistical distributions are determined for the path parameters within individual clusters and for their change between clusters. To validate the distributional choices, the goodness-of-fit to the proposed distributions is verified using statistical hypothesis tests with sufficient power
Perturbation of indole-3-butyric acid homeostasis by the UDP-glucosyltransferase UGT74E2 modulates Arabidopsis architecture and water stress tolerance
Reactive oxygen species and redox signaling undergo synergistic and antagonistic interactions with phytohormones to regulate protective responses of plants against biotic and abiotic stresses. However, molecular insight into the nature of this crosstalk remains scarce. We demonstrate that the hydrogen peroxide–responsive UDP-glucosyltransferase UGT74E2 of Arabidopsis thaliana is involved in the modulation of plant architecture and water stress response through its activity toward the auxin indole-3-butyric acid (IBA). Biochemical characterization of recombinant UGT74E2 demonstrated that it strongly favors IBA as a substrate. Assessment of indole-3-acetic acid (IAA), IBA, and their conjugates in transgenic plants ectopically expressing UGT74E2 indicated that the catalytic specificity was maintained in planta. In these transgenic plants, not only were IBA-Glc concentrations increased, but also free IBA levels were elevated and the conjugated IAA pattern was modified. This perturbed IBA and IAA homeostasis was associated with architectural changes, including increased shoot branching and altered rosette shape, and resulted in significantly improved survival during drought and salt stress treatments. Hence, our results reveal that IBA and IBA-Glc are important regulators of morphological and physiological stress adaptation mechanisms and provide molecular evidence for the interplay between hydrogen peroxide and auxin homeostasis through the action of an IBA UGT
HIV-1 disease progression is associated with bile-salt stimulated lipase (BSSL) gene polymorphism
Paleoclimate of the subtropical Andes during the latest Miocene, Lauca T Basin, Chile
Uplift of the Andean Cordillera during the Miocene and Pliocene produced large-scale changes in regional atmospheric circulation that impacted local ecosystems. The Lauca Basin (northern Chilean Altiplano) contains variably fluvial and lacustrine sedimentary sequences spanning the interval from 8.7 to 2.3 Ma. Field samples were collected from paleo-lacustrine sediments in the basin. Sediments were dated using detrital zircon geochronology on volcanic tuffs, yielding an age range between ~5.57 and 5.44 Ma. These new age constraints provided an opportunity to evaluate changes in the Lauca Basin ecosystem across this dynamic Miocene-Pliocene transition. We employed multiple proxies (lithofacies analysis, diatoms, pollen, and oxygen stable isotopes of authigenic carbonates) to interpret ancient lacustrine and terrestrial paleoenvironments. Alternations among mudstone, carbonate, and evaporitic facies indicate lake-level variability through time. The diatom assemblage is characterized by meso- to hypersaline and alkaline-tolerant taxa typical of shallow lakes. The δ18O values ranged from −8.96 to −2.22‰ indicating fluctuations in water balance. Pollen taxa in the outcrop are typical of a transitional stage between seasonal cloud forest and open grassland. Together, these proxies indicate that the Lauca paleolake sediments were deposited under a wetter-than-modern climate with high temporal variability. Our results refine previous studies in the Lauca Basin and are consistent with other regional studies suggesting that the South American summer monsoon at the Miocene-Pliocene transition was more intense than it is at present
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Function of monocytes and monocyte-derived macrophages in α1-antitrypsin deficiency.
α1-antitrypsin deficiency is the most widely recognised genetic disorder causing chronic obstructive pulmonary disease (COPD). Mutant Z α1-antitrypsin expression has previously been linked to intracellular accumulation and polymerisation of this proteinase inhibitor. Subsequently, this has been described to underlie an exaggerated endoplasmic reticulum stress response and enhanced nuclear factor-κB signalling. However, whether monocyte-derived macrophages display the same features remains unknown. Monocytes from homozygous PiZZ α1-antitrypsin deficiency patients and PiMM controls were cultured for 6 days in the presence of granulocyte-macrophage or macrophage colony-stimulating factor to obtain pro- and anti-inflammatory macrophages (mφ-1 and mφ-2, respectively). We first showed that, in contrast to monocytes, pre-stressed mφ-1 and mφ-2 from healthy blood donors display an enhanced endoplasmic reticulum stress response upon a lipopolysaccharide trigger (XBP1 splicing, CHOP, GADD34 and GRP78 mRNA). However, this endoplasmic reticulum stress response did not differ between monocyte-derived macrophages and monocytes from ZZ patients compared to MM controls. Furthermore, these ZZ cells do not secrete higher cytokine levels, and α1-antitrypsin polymers were not detectable by ELISA. These data suggest that monocyte-derived macrophages are not the local source of Z α1-antitrypsin polymers found in the lung and that endoplasmic reticulum stress and pro-inflammatory cytokine release is not altered.This study was supported by a grant from the Netherlands Asthma Foundation (grant no. 3.2.08.0032). E.F.A. van't Wout is an European Alpha-1-Antitrypsin Laurell’s Training Awardee (sponsored by Grifols, Barcelona, Spain). D.A. Lomas is supported by the Medical Research Council (London, UK) and the NIHR UCLH Biomedical Research Centre (London). S.J. Marciniak is a Medical Research Council Senior Clinical Research Fellow (grant no. G1002610)
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