6,503 research outputs found
Topological Superconductor from the Quantum Hall Phase: Effective Field Theory Description
We derive low-energy effective field theories for the quantum anomalous Hall
and topological superconducting phases. The quantum Hall phase is realized in
terms of free fermions with nonrelativistic dispersion relation, possessing a
global symmetry. We couple this symmetry with a background gauge field
and compute the effective action by integrating out the gapped fermions. In
spite of the fact that the corresponding Dirac operator governing the dynamics
of the original fermions is nonrelativistic, the leading contribution in the
effective action is a usual Abelian Chern-Simons term. The proximity to
a conventional superconductor induces a pairing potential in the quantum Hall
state, favoring the formation of Cooper pairs. When the pairing is strong
enough, it drives the system to a topological superconducting phase, hosting
Majorana fermions. Even though the continuum symmetry is broken down to
a one, we can forge fictitious symmetries that enable us
to derive the effective action for the topological superconducting phase, also
given by a Chern-Simons theory. To eliminate spurious states coming from the
artificial symmetry enlargement, we demand that the fields in the effective
action are instead of gauge fields. In the case we have to
sum over the bundles in the partition function, which projects
out the states that are not invariants. The corresponding edge
theory is the orbifold, which contains Majorana fermions in
its operator content.Comment: 40 pages, 5 figures, journal versio
Mass, Angular Momentum and Thermodynamics in Four-Dimensional Kerr-AdS Black Holes
In this paper, the connection between the Lorentz-covariant counterterms that
regularize the four-dimensional AdS gravity action and topological invariants
is explored. It is shown that demanding the spacetime to have a negative
constant curvature in the asymptotic region permits the explicit construction
of such series of boundary terms. The orthonormal frame is adapted to
appropriately describe the boundary geometry and, as a result, the boundary
term can be expressed as a functional of the boundary metric, extrinsic
curvature and intrinsic curvature. This choice also allows to write down the
background-independent Noether charges associated to asymptotic symmetries in
standard tensorial formalism. The absence of the Gibbons-Hawking term is a
consequence of an action principle based on a boundary condition different than
Dirichlet on the metric. This argument makes plausible the idea of regarding
this approach as an alternative regularization scheme for AdS gravity in all
even dimensions, different than the standard counterterms prescription. As an
illustration of the finiteness of the charges and the Euclidean action in this
framework, the conserved quantities and black hole entropy for four-dimensional
Kerr-AdS are computed.Comment: 15 pages,no figures,few references added,JHEP forma
The effects of different concentrations of the α2-Adrenoceptor Agonist Medetomidine on basal excitatory synaptic transmission and synaptic plasticity in hippocampal slices of adult mice
α2-Adrenoceptor agonists are used frequently in human and veterinary anesthesia as sedative/analgesic drugs. However, they can impair cognition. Little is known about the concentration-dependent effects of α2-adrenoceptor agonists on synaptic plasticity, the neurophysiological basis of learning and memory. Therefore, we investigated the effects of different concentrations of medetomidine, an α2-adrenoceptor agonist, on basal excitatory synaptic transmission and on 2 forms of synaptic plasticity: paired-pulse facilitation (PPF) and long-term potentiation (LTP).Funding: This work was supported by FCT (Lisbon, Portugal) and cofunded by COMPETE: 01-0124-FEDER-009497 (Lisbon, Portugal), through the project grants PTDC/CVT/099022/2008 and PTDC/SAU-NSC/122254/2010 and through a personal PhD grant (SFRH /BD/48883/2008) to PatrĂcia do CĂ©u Oliveira Ribeiro and by QREN (09-68-ESR-FP-010).info:eu-repo/semantics/publishedVersio
Thin-layer nanofiltration membranes using engineered biopolymers for seawater desalination pre-treatment processes
Nowadays water demand already exceeds supply and water scarcity is a global problem. So it is
necessary to develop novel technologies to be able to use poorer quality source waters for drinking
water production. Once considered as an expensive, ultimate solution for water supply, desalination is
becoming affordable. The two most commonly used seawater desalination methods are Multi-stage
Flash Distillation (MSF) and Seawater Reverse Osmosis (SWRO). SWRO is less energy demanding
compared to MSF, which makes it economically attractive. However there is no backpulsing of the
expensive and delicate reverse osmosis (RO) membranes with air or water, so they are susceptible to
fouling, causing the loss of their performance. Therefore cleaning the feed water to the highest level
possible by nanofiltration, before it reaches the RO membranes would highly increase the efficiency of
the process.
Nanofiltration (NF) as a feed pre-treatment step is a pressure driven membrane separation process that
takes place on a selective layer formed by a semipermeable membrane with properties between RO
and ultrafiltration. The objective of this project is the developement of highly efficient thin-film composite
(TFC) membranes for SWRO pre-treatment processes based on low-fouling cyanobacterial extracellular
polymeric substances (EPS). TFC membranes combine high flux and mechanical strenght, and they are
expected to be the key components of any water purification technology in the future.
Cyanobacterial EPS are complex heteropolysaccharides with putative antimicrobial and antiviral
properties and a particular affinity to bind metal ions [1,2].Within this work, the unicellular N2-fixing
marine cyanobacterium Cyanothece sp. CCY 0110 was chosen for RPS production, since it is among
the most efficient released polysaccharide (RPS) producers and the polymer has been previously
extensively characterised [3]. RPS was produced by growing Cyanothece CCY 0110 in 10L bioreactors,
in conditions previously defined and the polymer was isolated following the standard methodology [3].
A polyvinyl alcohol (PVA) / cyanobacterial EPS blend nanofibrous membranes were fabricated by
electrospinning using polyvinylidene fluoride (PVDF) as a basal membrane, in order to obtain thin-layer
composite nanofiltration membranes. The production of the nanofibers using EPS and PVA as
plasticizer in different ratios was produced in a NF-103 MECC Nanon electrospinning equipment with an
applied electric field between 15 and 25 kV and a flow of 0,2 mL/h.
Morphological, mechanical, chemical and thermal characterization of the electrospun fibers deposited
on the basal membranes, were evaluated by atomic force microscopy (AFM), scanning electron
microscopy (SEM) and energy dispersive spectroscopy (EDS), dynamical and mechanical analysis
(DMA), thermogravimetry (TGA) and differential scanning calorimetry (DSC).
The AFM and SEM results show the presence of fibers with dimensions between 54 and 121 nm with
low bead formation. In the EDS analysis presence of sulfur elements was observed confirming the
inclusion of EPS in the nanofibers. The morphology and diameter of the nanofibers were mainly affected
by the concentration of the blend solution and the weight ratio of the blend, respectively. The best
PVA/EPS nanofibers were achieved in a ratio of 12 % PVA and 0.4 % EPS. The solution conductivity
was ranging 1500 to 3500 ÎĽS/cm with a viscosity of about 100 to 500 cP. The DMA results confirmed
the miscibility of PVA/EPS blends. The elastic modulus of the nanocomposite mats increased
significantly as a consequence of the reinforcing effect of EPS. Thermal and mechanical analysis
demonstrated that there were strong intermolecular hydrogen bonds between the molecules EPS-PVA
in the blends. The heat-treated electrospun blended membranes showed better tensile mechanical
properties when compared with PVA alone, and resisted more against disintegration. A lab-scale
nanofiltration was performed in a bench stainless steel Sterlitech tangential flow stirred cell (200 mL)
connected to an air pressure system that allow pressure driven filtration up to 10 BAR.
Bactericidal activity and biofilm formation were tested using Escherichia coli and Sthaphylococcus
aureus as pathogenic microorganisms
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