66 research outputs found
Pricing ambiguity in catastrophe risk insurance
Ambiguity about the probability of loss is a salient feature of catastrophe risk insurance. Evidence shows that insurers charge higher premiums under ambiguity, but that they rely on simple heuristics to do so, rather than being able to turn to pricing tools that formally link ambiguity with the insurerâs underlying economic objective. In this paper, we apply an α-maxmin model of insurance pricing to two catastrophe model data sets relating to hurricane risk. The pricing model considers an insurer who maximises expected profit, but is sensitive to how ambiguity affects its risk of ruin. We estimate ambiguity loads and show how these depend on the insurerâs attitude to ambiguity, α. We also compare these results with those derived from applying model blending techniques that have recently gained popularity in the actuarial profession, and show that model blending can imply relatively low aversion to ambiguity, possibly ambiguity seeking
Memristive Tunnelbarrieren und ihre Integration in neuromorphe Systeme
Niehörster S. Memristive Tunnelbarrieren und ihre Integration in neuromorphe Systeme. Bielefeld: UniversitÀt Bielefeld; 2016
Shadowing the rotating annulus. Part I: Measuring candidate trajectory shadowing times
An intuitively necessary requirement of models used to provide forecasts of a
system's future is the existence of shadowing trajectories that are consistent
with past observations of the system: given a system-model pair, do model
trajectories exist that stay reasonably close to a sequence of observations of
the system? Techniques for finding such trajectories are well-understood in
low-dimensional systems, but there is significant interest in their application
to high-dimensional weather and climate models. We build on work by Smith et
al. [2010, Phys. Lett. A, 374, 2618-2623] and develop a method for measuring
the time that individual "candidate" trajectories of high-dimensional models
shadow observations, using a model of the thermally-driven rotating annulus in
the perfect model scenario. Models of the annulus are intermediate in
complexity between low-dimensional systems and global atmospheric models. We
demonstrate our method by measuring shadowing times against
artificially-generated observations for candidate trajectories beginning a
fixed distance from truth in one of the annulus' chaotic flow regimes. The
distribution of candidate shadowing times we calculated using our method
corresponds closely to (1) the range of times over which the trajectories
visually diverge from the observations and (2) the divergence time using a
simple metric based on the distance between model trajectory and observations.
An empirical relationship between the expected candidate shadowing times and
the initial distance from truth confirms that the method behaves reasonably as
parameters are varied.Comment: This paper was submitted to Physica D in 2010, but, after review, was
not accepted. We no longer have the time or resources to work on this topic,
but would like this record of our work to be available for others to read,
cite, and follow up. 19 pages, 9 figure
Shadowing the rotating annulus. Part II: Gradient descent in the perfect model scenario
Shadowing trajectories are model trajectories consistent with a sequence of
observations of a system, given a distribution of observational noise. The
existence of such trajectories is a desirable property of any forecast model.
Gradient descent of indeterminism is a well-established technique for finding
shadowing trajectories in low-dimensional analytical systems. Here we apply it
to the thermally-driven rotating annulus, a laboratory experiment intermediate
in model complexity and physical idealisation between analytical systems and
global, comprehensive atmospheric models. We work in the perfect model scenario
using the MORALS model to generate a sequence of noisy observations in a
chaotic flow regime. We demonstrate that the gradient descent technique
recovers a pseudo-orbit of model states significantly closer to a model
trajectory than the initial sequence. Gradient-free descent is used, where the
adjoint model is set to I in the absence of a full adjoint model. The
indeterminism of the pseudo-orbit falls by two orders of magnitude during the
descent, but we find that the distance between the pseudo-orbit and the
initial, true, model trajectory reaches a minimum and then diverges from truth.
We attribute this to the use of the -adjoint, which is well suited to
noise reduction but not to finely-tuned convergence towards a model trajectory.
We find that gives optimal results, and that candidate model
trajectories begun from this pseudo-orbit shadow the observations for up to 80
s, about the length of the longest timescale of the system, and similar to
expected shadowing times based on the distance between the pseudo-orbit and the
truth. There is great potential for using this method with real laboratory
data.Comment: This paper was originally prepared for submission in 2011; but, after
Part I was not accepted, it was not submitted. It has not been peer-reviewed.
We no longer have the time or resources to work on this topic, but would like
this record of our work to be available for others to read, cite, and follow
up. 22 pages, 11 figure
Sign change in the tunnel magnetoresistance of Fe3O4/MgO/Co-Fe-B magnetic tunnel junctions depending on the annealing temperature and the interface treatment
Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions
(MTJs) since it shows a high spin polarization at the Fermi level as well as a
high Curie temperature of 585{\deg}C. In this study, Fe3O4/MgO/Co-Fe-B MTJs
were manufactured. A sign change in the TMR is observed after annealing the
MTJs at temperatures between 200{\deg}C and 280{\deg}C. Our findings suggest an
Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the
change of the TMR. Additionally, different treatments of the magnetite
interface (argon bombardment, annealing at 200{\deg}C in oxygen atmosphere)
during the preparation of the MTJs have been studied regarding their effect on
the performance of the MTJs. A maximum TMR of up to -12% could be observed
using both argon bombardment and annealing in oxygen atmosphere, despite
exposing the magnetite surface to atmospheric conditions before the deposition
of the MgO barrier.Comment: 5 pages, 5 figures, 2 table
Tunnel junction based memristors as artificial synapses
Thomas A, Niehörster S, Fabretti S, et al. Tunnel junction based memristors as artificial synapses. Frontiers in Neuroscience. 2015;9: 241.We prepared magnesia, tantalum oxide and barium titanate based junction structures and investigated their memristive properties. The low amplitudes of the resistance change in these types of junctions are the major obstacle for their use. Here, we increased the amplitude of the resistance change from 10% up to 100%. Utilizing the memristive properties, we looked into the use of the junction structures as artificial synapses. We observed analogs of longterm potentiation, long-term depression and spike-time dependent plasticity in these simple two terminal devices. Finally, we suggest a possible pathway of these devices towards their integration in neuromorphic systems for storing analog synaptic weights and supporting the implementation of biologically plausible learning mechanisms
Sign change in the tunnel magnetoresistance of Fe3O4/MgO/Co-Fe-B magnetic tunnel junctions depending on the annealing temperature and the interface treatment
Marnitz L, Rott K, Niehörster S, et al. Sign change in the tunnel magnetoresistance of Fe3O4/MgO/Co-Fe-B magnetic tunnel junctions depending on the annealing temperature and the interface treatment. AIP Advances. 2015;5(4): 047103.Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe3O4/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the preparation of the MTJs have been studied regarding their effect on the performance of the MTJs. A maximum TMR of up to -12% could be observed using both argon bombardment and annealing in oxygen atmosphere, despite exposing the magnetite surface to atmospheric conditions before the deposition of the MgO barrier
Supermultiplexed optical imaging and barcoding with engineered polyynes
Optical multiplexing has a large impact in photonics, the life sciences and biomedicine. However, current technology is limited by a 'multiplexing ceiling' from existing optical materials. Here we engineered a class of polyyne-based materials for optical supermultiplexing. We achieved 20 distinct Raman frequencies, as 'Carbon rainbow', through rational engineering of conjugation length, bond-selective isotope doping and end-capping substitution of polyynes. With further probe functionalization, we demonstrated ten-color organelle imaging in individual living cells with high specificity, sensitivity and photostability. Moreover, we realized optical data storage and identification by combinatorial barcoding, yielding to our knowledge the largest number of distinct spectral barcodes to date. Therefore, these polyynes hold great promise in live-cell imaging and sorting as well as in high-throughput diagnostics and screening
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