85 research outputs found
Integral field spectroscopy of nitrogen overabundant blue compact dwarf galaxies
We study the spatial distribution of the physical properties and of oxygen
and nitrogen abundances in three Blue Compact Dwarf Galaxiess (HS 0128+2832, HS
0837+4717 and Mrk 930) with a reported excess of N/O in order to investigate
the nature of this excess and, particularly, if it is associated with
Wolf-Rayet (WR) stars We have observed these BCDs by using PMAS integral field
spectroscopy in the optical spectral range (3700 - 6900 {\AA}), mapping their
physical-chemical properties, using both the direct method and appropriate
strong-line methods. We make a statistical analysis of the resulting
distributions and we compare them with the integrated properties of the
galaxies. Our results indicate that outer parts of the three galaxies are
placed on the "AGN-zone" of the [NII]/H{\alpha} vs. [OIII]/H{\beta} diagnostic
diagram most likely due to a high N/O combined with the excitation structure in
these regions. From the statistical analysis, it is assumed that a certain
property can be considered as spatially homogeneous (or uniform) if a normal
gaussian function fits its distribution in several regions of the galaxy.
Moreover, a disagreement between the integrated properties and the mean values
of the distribution usually appears when a gaussian does not fit the
corresponding distribution. We find that for Mrk 930, the uniformity is found
for all parameters, except for electron density and reddening. The rotation
curve together with the H{\alpha} map and UV images, reveal a perturbed
morphology and possible interacting processes. The N/O is found to be constant
in the three studied objects at spatial scales of the order of several kpc so
we conclude that the number of WR stars estimated from spectroscopy is not
sufficient to pollute the ISM and to produce the observed N/O excess in these
objectsComment: 17 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Blue compact dwarf galaxies with nitrogen overabundance: a view from integral field spectroscopy
This is an electronic version of the poster presented at the IX Scientific Meeting of the Spanish Astronomical Society (SEA), held on September 13-17, 2010, in Madrid.The summary of the poster appears in Zapatero Osorio, M.R. et al. (eds.). Highlights of Spanish Astrophysics VI. Proceedings of the IX Scientific Meeting of the Spanish Astronomical Society (SEA), held on September 13-17, 2010, in Madrid. Barcelona: Sociedad Española de Astronomía, 2011. 39
Basic obstacle for electrical spin-injection from a ferromagnetic metal into a diffusive semiconductor
We have calculated the spin-polarization effects of a current in a two
dimensional electron gas which is contacted by two ferromagnetic metals. In the
purely diffusive regime, the current may indeed be spin-polarized. However, for
a typical device geometry the degree of spin-polarization of the current is
limited to less than 0.1%, only. The change in device resistance for parallel
and antiparallel magnetization of the contacts is up to quadratically smaller,
and will thus be difficult to detect.Comment: Revtex, 4 pages, 3 figures (eps), Definition of spin pilarization
changed to standard definition in GMR, some straight forward algebra removed.
To appear as PRB Rap. Comm. August 15t
Rashba precession in quantum wires with interaction
Rashba precession of spins moving along a one-dimensional quantum channel is
calculated, accounting for Coulomb interactions. The Tomonaga--Luttinger model
is formulated in the presence of spin-orbit scattering and solved by
Bosonization. Increasing interaction strength at decreasing carrier density is
found to {\sl enhance} spin precession and the nominal Rashba parameter due to
the decreasing spin velocity compared with the Fermi velocity. This result can
elucidate the observed pronounced changes of the spin splitting on applied gate
voltages which are estimated to influence the interface electric field in
heterostructures only little.Comment: now replaced by published versio
Theory of electrical spin injection: Tunnel contacts as a solution of the conductivity mismatch problem
Theory of electrical spin injection from a ferromagnetic (FM) metal into a
normal (N) conductor is presented. We show that tunnel contacts (T) can
dramatically increase spin injection and solve the problem of the mismatch in
the conductivities of a FM metal and a semiconductor microstructure. We also
present explicit expressions for the spin-valve resistance of FM-T-N- and
FM-T-N-T-FM-junctions with tunnel contacts at the interfaces and show that the
resistance includes both positive and negative contributions (Kapitza
resistance and injection conductivity, respectively).Comment: 4 pages, to appear in Phys. Rev. B (rapid communications
Building safer robots: Safety driven control
In recent years there has been a concerted effort to address many of the safety issues associated with physical human-robot interaction (pHRI). However, a number of challenges remain. For personal robots, and those intended to operate in unstructured environments, the problem of safety is compounded. In this paper we argue that traditional system design techniques fail to capture the complexities associated with dynamic environments. We present an overview of our safety-driven control system and its implementation methodology. The methodology builds on traditional functional hazard analysis, with the addition of processes aimed at improving the safety of autonomous personal robots. This will be achieved with the use of a safety system developed during the hazard analysis stage. This safety system, called the safety protection system, will initially be used to verify that safety constraints, identified during hazard analysis, have been implemented appropriately. Subsequently it will serve as a high-level safety enforcer, by governing the actions of the robot and preventing the control layer from performing unsafe operations. To demonstrate the effectiveness of the design, a series of experiments have been conducted using a MobileRobots PeopleBot. Finally, results are presented demonstrating how faults injected into a controller can be consistently identified and handled by the safety protection system. © The Author(s) 2012
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
Spin-polarized Zener tunneling in (Ga,Mn)As
We investigate spin-polarized inter-band tunneling through measurement of
(Ga,Mn)As based Zener tunnel diode. By placing the diode under reverse bias,
electron spin polarization is transferred from the valence band of p-type
(Ga,Mn)As to the conduction band of an adjacent n-GaAs layer. The resulting
current is monitored by injection into a quantum well light emitting diode
whose electroluminescence polarization is found to track the magnetization of
the (Ga,Mn)As layer as a function of both temperature and magnetic field.Comment: 11 pages, 4 figures. Submitted, Physical Review B15 Rapid
Communication
Abnormal Frontostriatal Activity During Unexpected Reward Receipt in Depression and Schizophrenia: Relationship to Anhedonia.
Alterations in reward processes may underlie motivational and anhedonic symptoms in depression and schizophrenia. However it remains unclear whether these alterations are disorder-specific or shared, and whether they clearly relate to symptom generation or not. We studied brain responses to unexpected rewards during a simulated slot-machine game in 24 patients with depression, 21 patients with schizophrenia, and 21 healthy controls using functional magnetic resonance imaging. We investigated relationships between brain activation, task-related motivation, and questionnaire rated anhedonia. There was reduced activation in the orbitofrontal cortex, ventral striatum, inferior temporal gyrus, and occipital cortex in both depression and schizophrenia in comparison with healthy participants during receipt of unexpected reward. In the medial prefrontal cortex both patient groups showed reduced activation, with activation significantly more abnormal in schizophrenia than depression. Anterior cingulate and medial frontal cortical activation predicted task-related motivation, which in turn predicted anhedonia severity in schizophrenia. Our findings provide evidence for overlapping hypofunction in ventral striatal and orbitofrontal regions in depression and schizophrenia during unexpected reward receipt, and for a relationship between unexpected reward processing in the medial prefrontal cortex and the generation of motivational states.Supported by a MRC Clinician Scientist award (G0701911), a Brain and Behaviour Research Foundation Young Investigator, and an Isaac Newton Trust award to Dr Murray; an award to Dr Segarra from the Secretary for Universities and Research of the Ministry of Economy and Knowledge of the Government of Catalonia and the European Union; by the University of Cambridge Behavioural and Clinical Neuroscience Institute, funded by a joint award from the Medical Research Council and Wellcome Trust (G1000183 and 093875/Z/10Z respectively); by awards from the Wellcome Trust (095692) and the Bernard Wolfe Health Neuroscience Fund to Professor Fletcher, and by awards from the Wellcome Trust Institutional Strategic Support Fund (097814/Z/11) and Cambridge NIHR Biomedical Research Centre. The authors are grateful for the help of clinical staff in CAMEO, in the Cambridge Rehabilitation and Recovery service and Pathways, and in the Cambridge IAPT service, for help with participant recruitment.This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/npp.2015.37
Deficiency of Huntingtin Has Pleiotropic Effects in the Social Amoeba Dictyostelium discoideum
Huntingtin is a large HEAT repeat protein first identified in humans, where a polyglutamine tract expansion near the amino terminus causes a gain-of-function mechanism that leads to selective neuronal loss in Huntington's disease (HD). Genetic evidence in humans and knock-in mouse models suggests that this gain-of-function involves an increase or deregulation of some aspect of huntingtin's normal function(s), which remains poorly understood. As huntingtin shows evolutionary conservation, a powerful approach to discovering its normal biochemical role(s) is to study the effects caused by its deficiency in a model organism with a short life-cycle that comprises both cellular and multicellular developmental stages. To facilitate studies aimed at detailed knowledge of huntingtin's normal function(s), we generated a null mutant of hd, the HD ortholog in Dictyostelium discoideum. Dictyostelium cells lacking endogenous huntingtin were viable but during development did not exhibit the typical polarized morphology of Dictyostelium cells, streamed poorly to form aggregates by accretion rather than chemotaxis, showed disorganized F-actin staining, exhibited extreme sensitivity to hypoosmotic stress, and failed to form EDTA-resistant cell–cell contacts. Surprisingly, chemotactic streaming could be rescued in the presence of the bivalent cations Ca2+ or Mg2+ but not pulses of cAMP. Although hd− cells completed development, it was delayed and proceeded asynchronously, producing small fruiting bodies with round, defective spores that germinated spontaneously within a glassy sorus. When developed as chimeras with wild-type cells, hd− cells failed to populate the pre-spore region of the slug. In Dictyostelium, huntingtin deficiency is compatible with survival of the organism but renders cells sensitive to low osmolarity, which produces pleiotropic cell autonomous defects that affect cAMP signaling and as a consequence development. Thus, Dictyostelium provides a novel haploid organism model for genetic, cell biological, and biochemical studies to delineate the functions of the HD protein
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