4,996 research outputs found
Two-channel Kondo physics in tunnel-coupled double quantum dots
We investigate theoretically the possibility of observing two-channel Kondo
(2CK) physics in tunnel-coupled double quantum dots (TCDQDs), at both zero and
finite magnetic fields; taking the two-impurity Anderson model (2AIM) as the
basic TCDQD model, together with effective low-energy models arising from it by
Schrieffer-Wolff transformations to second and third order in the tunnel
couplings. The models are studied primarily using Wilson's numerical
renormalization group. At zero-field our basic conclusion is that while 2CK
physics arises in principle provided the system is sufficiently
strongly-correlated, the temperature window over which it could be observed is
much lower than is experimentally feasible. This finding disagrees with recent
work on the problem, and we explain why. At finite field, we show that the
quantum phase transition known to arise at zero-field in the two-impurity Kondo
model (2IKM), with an essentially 2CK quantum critical point, persists at
finite fields. This raises the prospect of access to 2CK physics by tuning a
magnetic field, although preliminary investigation suggests this to be even
less feasible than at zero field.Comment: 10 pages, 8 figures. Version as published in PR
Interplay between Kondo physics and spin-orbit coupling in carbon nanotube quantum dots
We investigate the influence of spin-orbit coupling on the Kondo effects in
carbon nanotube quantum dots, using the numerical renormalization group
technique. A sufficiently large spin-orbit coupling is shown to destroy the
SU(4) Kondo effects at zero magnetic field, leaving only two SU(2) Kondo
effects in the one- and three-electron Coulomb blockade valleys. On applying a
finite magnetic field, two additional, spin-orbit induced SU(2) Kondo effects
arise in the three- and two-electron valleys. Using physically realistic model
parameters, we calculate the differential conductance over a range of gate
voltages, temperatures and fields. The results agree well with measurements
from two different experimental devices in the literature, and explain a number
of observations that are not described within the standard framework of the
SU(4) Anderson impurity model.Comment: 15 pages, 11 figure
Chemical fracture and distribution of extreme values
When a corrosive solution reaches the limits of a solid sample, a chemical
fracture occurs. An analytical theory for the probability of this chemical
fracture is proposed and confirmed by extensive numerical experiments on a two
dimensional model. This theory follows from the general probability theory of
extreme events given by Gumbel. The analytic law differs from the Weibull law
commonly used to describe mechanical failures for brittle materials. However a
three parameters fit with the Weibull law gives good results, confirming the
empirical value of this kind of analysis.Comment: 7 pages, 5 figures, to appear in Europhysics Letter
Organizational response to disaster -the case of tsunami, December 2004
The Tsunami struck Sri Lanka on 26th December 2004, causing an enormous devastation of humanlives and property. State and non-state sectors being unprepared and poor coordination of internationaland local assistance left people internally displaced even after one year of the disaster. Using primaryand secondary data, the study assessed the responsiveness of the organizations to the Tsunami disasterin the Galle district and developed a model of action for effective disaster management.The study identified the response levels of the organizations at relief, recovery, reconstruction,rehabilitation and development stages. There was no pre-preparation for a major disaster in Galledistrict. The disaster reliefwas provided by unplanned emergent structures. The prevailing administrativestructures, political institutions, Center for National Operations, Non-governmental organizations,volunteers and community-based groups provided relief for two months. The government establishedthe emergency operation structures for national level coordinationAt the recovery stage community and the private sector organizations have been marginalized in theresponse system. Governmental and NGOs have focused on providing transitional shelters and dryrations. The reconstruction and rehabilitation stages have focused on housing, livelihoods, socialrehabilitation and infrastructure, which were in progress through September 2006. The Galle districtemergency operation center completed the Disaster management plan for the district in July 2005.The parliament of Sri Lanka approved the Sri Lanka Disaster Management Act, No 13 of2005, underwhich the National Disaster Management center has been established.The model identifies organizational structure to coordinate donor assistance and link to communityneeds, through national and local level coordinating institutions with the contribution of different sectorsand with proper monitoring. Getting the vulnerable community to actively participate in disastermanagement activities leading towards development will minimize the damage. Suggestions are madefor specific capacity building measures for the different levels of the institutional model
Data production models for the CDF experiment
The data production for the CDF experiment is conducted on a large Linux PC
farm designed to meet the needs of data collection at a maximum rate of 40
MByte/sec. We present two data production models that exploits advances in
computing and communication technology. The first production farm is a
centralized system that has achieved a stable data processing rate of
approximately 2 TByte per day. The recently upgraded farm is migrated to the
SAM (Sequential Access to data via Metadata) data handling system. The software
and hardware of the CDF production farms has been successful in providing large
computing and data throughput capacity to the experiment.Comment: 8 pages, 9 figures; presented at HPC Asia2005, Beijing, China, Nov 30
- Dec 3, 200
Mainstreaming Grassroots Adaptation and Building Climate Resilient Agriculture in Sri Lanka
Climate change in recent decades has become a
major concern of the global community. The Inter-
Governmental Panel on Climate Change (IPCC)
established in 1988 by the United Nations has
highlighted alarming trends in changes in global
temperatures, shifts in rainfall patterns, rising sea levels
and the impact of these changes on the livelihoods of
people, especially the poor. The changes will affect
sub-Saharan Africa the greatest and also Asia with its
very large population, most of whom are dependent on
agriculture
Data processing model for the CDF experiment
The data processing model for the CDF experiment is described. Data
processing reconstructs events from parallel data streams taken with different
combinations of physics event triggers and further splits the events into
datasets of specialized physics datasets. The design of the processing control
system faces strict requirements on bookkeeping records, which trace the status
of data files and event contents during processing and storage. The computing
architecture was updated to meet the mass data flow of the Run II data
collection, recently upgraded to a maximum rate of 40 MByte/sec. The data
processing facility consists of a large cluster of Linux computers with data
movement managed by the CDF data handling system to a multi-petaByte Enstore
tape library. The latest processing cycle has achieved a stable speed of 35
MByte/sec (3 TByte/day). It can be readily scaled by increasing CPU and
data-handling capacity as required.Comment: 12 pages, 10 figures, submitted to IEEE-TN
Helping Communities Adapt: Climate Change Perceptions and Policy in Asia
Recent studies on climate change and the 4th
assessment report of the Intergovernmental Panel
on Climate Change (IPCC)1 provide the latest
understanding on the status, impacts, and measures
required to address the effects of climate change.
The global climate change outlook highlighted serious
concerns about the future at a macro perspective,
ie, continental, national, and regional levels. Further,
the state of knowledge available at the regional
and sub-regional levels is still inadequate (INCAA
2010)
Metabolic changes in schizophrenia and human brain evolution.
BACKGROUND: Despite decades of research, the molecular changes responsible for the evolution of human cognitive abilities remain unknown. Comparative evolutionary studies provide detailed information about DNA sequence and mRNA expression differences between humans and other primates but, in the absence of other information, it has proved very difficult to identify molecular pathways relevant to human cognition. RESULTS: Here, we compare changes in gene expression and metabolite concentrations in the human brain and compare them to the changes seen in a disorder known to affect human cognitive abilities, schizophrenia. We find that both genes and metabolites relating to energy metabolism and energy-expensive brain functions are altered in schizophrenia and, at the same time, appear to have changed rapidly during recent human evolution, probably as a result of positive selection. CONCLUSION: Our findings, along with several previous studies, suggest that the evolution of human cognitive abilities was accompanied by adaptive changes in brain metabolism, potentially pushing the human brain to the limit of its metabolic capabilities.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are
Search for Neutral Higgs Bosons in Events with Multiple Bottom Quarks at the Tevatron
The combination of searches performed by the CDF and D0 collaborations at the
Fermilab Tevatron Collider for neutral Higgs bosons produced in association
with b quarks is reported. The data, corresponding to 2.6 fb-1 of integrated
luminosity at CDF and 5.2 fb-1 at D0, have been collected in final states
containing three or more b jets. Upper limits are set on the cross section
multiplied by the branching ratio varying between 44 pb and 0.7 pb in the Higgs
boson mass range 90 to 300 GeV, assuming production of a narrow scalar boson.
Significant enhancements to the production of Higgs bosons can be found in
theories beyond the standard model, for example in supersymmetry. The results
are interpreted as upper limits in the parameter space of the minimal
supersymmetric standard model in a benchmark scenario favoring this decay mode.Comment: 10 pages, 2 figure
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