584,135 research outputs found
Magentically-Induced Lattice Distortions and Ferroelectricity in Magnetoelectric GdMnO3
In this work we investigate the magnetic field dependence of Ag octahedra
rotation (tilt) and B2g symmetric stretching modes frequency at different
temperatures. Our field-dependent Raman investigation at 10K is interpreted by
an ionic displacive nature of the magnetically induced ferroelectric phase
transition. The frequency change of the Ag tilt is in agreement with the
stabilization of the Mn-Gd spin arrangement, yielding the necessary conditions
for the onset of ferroelectricity on the basis of the inverse
Dzyaloshinskii-Moriya interaction. The role of the Jahn-Teller cooperative
interaction is also evidenced by the change of the B2g mode frequency at the
ferroelectric phase transition. This frequency change allows estimating the
shift of the oxygen position at the ferroelectric phase transition and the
corresponding spontaneous polarization of 480 {\mu}C/m2, which agrees with
earlier reported values in single crystals. Our study also confirms the
existence of a large magnetic hysteresis at the lowest temperatures, which is a
manifestation of magnetrostiction.Comment: 5 pages, 3 figure
Estimating the sensitivity of wide-parameter-space searches for gravitational-wave pulsars
This paper presents an in-depth study of how to estimate the sensitivity of
searches for gravitational-wave pulsars -- rapidly-rotating neutron stars which
emit quasi-sinusoidal gravitational waves. It is particularly concerned with
searches over a wide range of possible source parameters, such as searches over
the entire sky and broad frequency bands. Traditional approaches to estimating
the sensitivity of such searches use either computationally-expensive Monte
Carlo simulations, or analytic methods which sacrifice accuracy by making an
unphysical assumption about the population of sources being searched for. This
paper develops a new, analytic method of estimating search sensitivity which
does not rely upon this unphysical assumption. Unlike previous analytic
methods, the new method accurately predicts the sensitivity obtained using
Monte Carlo simulations, while avoiding their computational expense. The change
in estimated sensitivity due to properties of the search template bank, and the
geographic configuration of the gravitational wave detector network, are also
investigated.Comment: 16 figures, 2 tables, REVTeX 4.1; minor typos corrected from v2,
updated reference
Least squares volatility change point estimation for partially observed diffusion processes
A one dimensional diffusion process , with drift
and diffusion coefficient
known up to , is supposed to switch volatility regime at some point
. On the basis of discrete time observations from , the
problem is the one of estimating the instant of change in the volatility
structure as well as the two values of , say and
, before and after the change point. It is assumed that the sampling
occurs at regularly spaced times intervals of length with
. To work out our statistical problem we use a least squares
approach. Consistency, rates of convergence and distributional results of the
estimators are presented under an high frequency scheme. We also study the case
of a diffusion process with unknown drift and unknown volatility but constant
Prediction of spiralling in BTA deep-hole drilling: estimating the system's eigenfrequencies
One serious problem in deep-hole drilling is the formation of a dynamic disturbance called spiralling which causes holes with several lobes. Since such lobes are a severe impairment of the bore hole quality the formation of spiralling has to be prevented. Gessesse et al. [2] explain spiralling by the coincidence of bending modes and multiples of the rotation frequency. They derive this from an elaborate finite elements model of the process. In online measurements we detected slowly changing frequency patterns similar to those calculated by Gessesse et al. We therefore propose a method to estimate the parameters determining the change of frequencies over time from spectrogram data. This significantly simplifies the explanation of spiralling for practical applications compared to finite elements models which have to be correctly modified for each machine and tool assembly. It turns out that this simpler model achieves to explain the observed frequency patterns quite well. We use this for estimating the variation of the frequencies as good as possible. This opens up the opportunity to prevent spiralling by e.g. changing the rotary frequency. --
Inference of historical population-size changes with allele-frequency data
With up to millions of nearly neutral polymorphisms now being routinely sampled in population-genomic surveys, it is possible to estimate the site-frequency spectrum of such sites with high precision. Each frequency class reflects a mixture of potentially unique demographic histories, which can be revealed using theory for the probability distributions of the starting and ending points of branch segments over all possible coalescence trees. Such distributions are completely independent of past population history, which only influences the segment lengths, providing the basis for estimating average population sizes separating tree-wide coalescence events. The history of population-size change experienced by a sample of polymorphisms can then be dissected in a model-flexible fashion, and extension of this theory allows estimation of the mean and full distribution of long-term effective population sizes and ages of alleles of specific frequencies. Here, we outline the basic theory underlying the conceptual approach, develop and test an efficient statistical procedure for parameter estimation, and apply this to multiple population-genomic datasets for the microcrustacean Daphnia pulex
Measuring Impacts of New Highways Capacity – A Discussion of Potential Survey Methods
The paper reviews survey methods that might be used to detect the various impacts of new highway capacity (changes in flow and network travel times; behavioural responses such as rerouting, change in departure times, change of mode, redistribution and change in trip frequency; and changes in land use). The review was conducted in the context of a study for TRRL which sought to establish the feasibility of measuring responses to new highway capacity.
The paper considers, in turn, surveys of traffic flow, public transport usage and network travel times, methods of estimating origin-destination matrices and a variety of questionnaire and interview techniques which might be used to collect individual travel data (roadside interviews; stopline surveys; household interviews; trip-end interviews; self completion questionnaires; retrospective, prospective and stated preference questions; panel surveys and indepth interviews). There is also a brief discussion of methods to determine bight movements and land use effects.
The paper should not be regarded as a source of detailed information about the various types of survey but rather as a review of their comparative strengths and weaknesses in the given context
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