1,877 research outputs found
Learning to detect an oddball target with observations from an exponential family
The problem of detecting an odd arm from a set of K arms of a multi-armed
bandit, with fixed confidence, is studied in a sequential decision-making
scenario. Each arm's signal follows a distribution from a vector exponential
family. All arms have the same parameters except the odd arm. The actual
parameters of the odd and non-odd arms are unknown to the decision maker.
Further, the decision maker incurs a cost for switching from one arm to
another. This is a sequential decision making problem where the decision maker
gets only a limited view of the true state of nature at each stage, but can
control his view by choosing the arm to observe at each stage. Of interest are
policies that satisfy a given constraint on the probability of false detection.
An information-theoretic lower bound on the total cost (expected time for a
reliable decision plus total switching cost) is first identified, and a
variation on a sequential policy based on the generalised likelihood ratio
statistic is then studied. Thanks to the vector exponential family assumption,
the signal processing in this policy at each stage turns out to be very simple,
in that the associated conjugate prior enables easy updates of the posterior
distribution of the model parameters. The policy, with a suitable threshold, is
shown to satisfy the given constraint on the probability of false detection.
Further, the proposed policy is asymptotically optimal in terms of the total
cost among all policies that satisfy the constraint on the probability of false
detection
Ferromagnetic Behavior of High Purity ZnO nanoparticles
ZnO nanoparticles with Wurtzite structure were prepared by chemical methods
at low temperature in aqueous solution. Nanoparticles are in the range from
about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room
temperature and above. Magnetization vs temperature, M(T) and isothermal
measurements M(H) were determined. The coercive field clearly shows
ferromagnetism above room temperature. The chemical synthesis, structural
defects in the bulk related to oxygen vacancies are the main factors for the
observed magnetic behavior.
PACS numbers: 61.46.Hk Nanocrystals, 75.50.Pp Magnetic semiconductors,
81.05.Dz II-VI semiconductor
Can a gravitational wave and a magnetic monopole coexist?
We investigate the behavior of small perturbations around the Kaluza-Klein
monopole in the five dimensional space-time. We find that the even parity
gravitational wave does not propagate in the five dimensional space-time with
Kaluza-Klein monopole provided that the gravitational wave is constant in the
fifth direction. We conclude that a gravitational wave and a U(1) magnetic
monopole do not coexist in five dimensional Kaluza-Klein spacetime.Comment: 10 pages, LaTeX. To appear in Modern Physics Letters
The dollar squeeze of the financial crisis
By Covered Interest rate Parity (CIP), the FX swap implied currrency interest rates should coincide with actual interest rates. When a difference occurs, the residual is referred to as the cross currency basis. We link the Euro- Dollar currency basis (e.g. in 2008) to shadow prices of dollar funding constraints and interpret the basis as the relative physical possession value of the scarcer currency, or the “convenience yield” associated with that currency. This is similar to specialness in repo markets, expressing the physical possession value of a security. We examine how the coordinated central banks intervention can reduce the currency basis.FX swaps, Repo, Euro-Dollar currency basis, The 2008 dollar squeeze, Possession
Interplay of 4f-3d Magnetism and Ferroelectricity in DyFeO3
DyFeO3 exhibits a weak ferromagnetism (TNFe ~ 645 K) that disappears below a
spin-reorientation (Morin) transition at TSRFe ~ 50 K. It is also known that
applied magnetic field induces ferroelectricity at the magnetic ordering
temperature of Dy-ions (TNDy ~ 4.5 K). Here, we show that the ferroelectricity
exists in the weak ferromagnetic state (TSRFe < T < TN,C) without applying
magnetic field, indicating the crucial role of weak ferromagnetism in inducing
ferroelectricity. 57Fe M\"ossbauer studies show that hyperfine field (Bhf)
deviates from mean field-like behaviour that is observed in the weak
ferromagnetic state and decreases below the onset of spin-reorientation
transition (80 K), implying that the Bhf above TSR had additional contribution
from Dy-ions due to induced magnetization by the weak ferromagnetic moment of
Fe-sublattice and below TSR, this contribution decreases due to collinear
ordering of Fe-sublattice. These results clearly demonstrate the presence of
magnetic interactions between Dy(4f) and Fe(3d) and their correlation with
ferroelectricity in the weak ferromagnetic state of DyFeO3.Comment: 5 pages, 6 figures, published in EP
Field-induced Polar Order at the N\'eel Temperature of Chromium in Rare-earth Orthochromites: Interplay of Rare-earth and Cr Magnetism
We report field-induced switchable polarization (P = 0.2 ~ 0.8 microC/cm2)
below the N\'eel temperature of chromium (TN Cr) in weakly ferromagnetic
rareearth orthochromites, RCrO3 (R=rareearth) but only when the rareearth ion
is magnetic. Intriguingly, the polarization in ErCrO3 (TC ~ 133 K) disappears
at a spin reorientation (Morin) transition (TSR ~ 22 K) below which the weak
ferromagnetism associated with the Cr sublattice also disappears, demonstrating
the crucial role of weak ferromagnetism in inducing the polar order. Further,
the polarization (P) is strongly influenced by applied magnetic field,
indicating a strong magneto electric effect. We suggest that the polar order
occurs in RCrO3, due to the combined effect of poling field that breaks the
symmetry and the exchange field on R ion from Cr sublattice stabilizes the
polar state. We propose that a similar mechanism could work in the
isostructural rareearth orthoferrites, RFeO3 as well.Comment: 31 pages (Manuscript(6 figures)+supplemental information(8 figures)
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