9,608 research outputs found
Thermal activation energy of 3D vortex matter in NaFe1-xCoxAs (x=0.01, 0.03 and 0.07) single crystals
We report on the thermally activated flux flow dependency on the doping
dependent mixed state in NaFe1-xCoxAs (x=0.01, 0.03, and 0.07) crystals using
the magnetoresistivity in the case of B//c-axis and B//ab-plane. It was found
clearly that irrespective of the doping ratio, magnetoresistivity showed a
distinct tail just above the Tc, offset associated with the thermally activated
flux flow (TAFF) in our crystals. Furthermore, in TAFF region the temperature
dependence of the activation energy follows the relation U(T, B)=U_0 (B)
(1-T/T_c )^q with q=1.5 in all studied crystals. The magnetic field dependence
of the activation energy follows a power law of U_0 (B)~B^(-{\alpha}) where the
exponent {\alpha} is changed from a low value to a high value at a crossover
field of B=~2T, indicating the transition from collective to plastic pinning in
the crystals. Finally, it is suggested that the 3D vortex phase is the dominant
phase in the low-temperature region as compared to the TAFF region in our
series samples
Spectroscopy of a narrow-line laser cooling transition in atomic dysprosium
The laser cooling and trapping of ultracold neutral dysprosium has been
recently demonstrated using the broad, open 421-nm cycling transition.
Narrow-line magneto-optical trapping of Dy on longer wavelength transitions
would enable the preparation of ultracold Dy samples suitable for loading
optical dipole traps and subsequent evaporative cooling. We have identified the
closed 741-nm cycling transition as a candidate for the narrow-line cooling of
Dy. We present experimental data on the isotope shifts, the hyperfine constants
A and B, and the decay rate of the 741-nm transition. In addition, we report a
measurement of the 421-nm transition's linewidth, which agrees with previous
measurements. We summarize the laser cooling characteristics of these
transitions as well as other narrow cycling transitions that may prove useful
for cooling Dy.Comment: 6+ pages, 5 figures, 5 table
Dielectric constants of Ir, Ru, Pt, and IrO2: Contributions from bound charges
We investigated the dielectric functions () of Ir, Ru, Pt,
and IrO, which are commonly used as electrodes in ferroelectric thin film
applications. In particular, we investigated the contributions from bound
charges (), since these are important scientifically as
well as technologically: the (0) of a metal electrode is one of
the major factors determining the depolarization field inside a ferroelectric
capacitor. To obtain (0), we measured reflectivity spectra of
sputtered Pt, Ir, Ru, and IrO2 films in a wide photon energy range between 3.7
meV and 20 eV. We used a Kramers-Kronig transformation to obtain real and
imaginary dielectric functions, and then used Drude-Lorentz oscillator fittings
to extract (0) values. Ir, Ru, Pt, and IrO produced
experimental (0) values of 4810, 8210, 5810, and
295, respectively, which are in good agreement with values obtained using
first-principles calculations. These values are much higher than those for
noble metals such as Cu, Ag, and Au because transition metals and IrO have
such strong d-d transitions below 2.0 eV. High (0) values will
reduce the depolarization field in ferroelectric capacitors, making these
materials good candidates for use as electrodes in ferroelectric applications.Comment: 26 pages, 6 figures, 2 table
Multiresolution spatiotemporal mechanical model of the heart as a prior to constrain the solution for 4D models of the heart.
In several nuclear cardiac imaging applications (SPECT and PET), images are formed by reconstructing tomographic data using an iterative reconstruction algorithm with corrections for physical factors involved in the imaging detection process and with corrections for cardiac and respiratory motion. The physical factors are modeled as coefficients in the matrix of a system of linear equations and include attenuation, scatter, and spatially varying geometric response. The solution to the tomographic problem involves solving the inverse of this system matrix. This requires the design of an iterative reconstruction algorithm with a statistical model that best fits the data acquisition. The most appropriate model is based on a Poisson distribution. Using Bayes Theorem, an iterative reconstruction algorithm is designed to determine the maximum a posteriori estimate of the reconstructed image with constraints that maximizes the Bayesian likelihood function for the Poisson statistical model. The a priori distribution is formulated as the joint entropy (JE) to measure the similarity between the gated cardiac PET image and the cardiac MRI cine image modeled as a FE mechanical model. The developed algorithm shows the potential of using a FE mechanical model of the heart derived from a cardiac MRI cine scan to constrain solutions of gated cardiac PET images
Non-Gaussian errors of baryonic acoustic oscillations
We revisit the uncertainty in baryon acoustic oscillation (BAO) forecasts and
data analyses. In particular, we study how much the uncertainties on both the
measured mean dilation scale and the associated error bar are affected by the
non-Gaussianity of the non-linear density field. We examine two possible
impacts of non-Gaussian analysis: (1) we derive the distance estimators from
Gaussian theory, but use 1000 N-Body simulations to measure the actual errors,
and compare this to the Gaussian prediction, and (2) we compute new optimal
estimators, which requires the inverse of the non-Gaussian covariance matrix of
the matter power spectrum. Obtaining an accurate and precise inversion is
challenging, and we opted for a noise reduction technique applied on the
covariance matrices. By measuring the bootstrap error on the inverted matrix,
this work quantifies for the first time the significance of the non-Gaussian
error corrections on the BAO dilation scale. We find that the variance (error
squared) on distance measurements can deviate by up to 12% between both
estimators, an effect that requires a large number of simulations to be
resolved. We next apply a reconstruction algorithm to recover some of the BAO
signal that had been smeared by non-linear evolution, and we rerun the
analysis. We find that after reconstruction, the rms error on the distance
measurement improves by a factor of ~1.7 at low redshift (consistent with
previous results), and the variance ({\sigma}^2) shows a change of up to 18%
between optimal and sub-optimal cases (note, however, that these discrepancies
may depend in detail on the procedure used to isolate the BAO signal). We
finally discuss the impact of this work on current data analyses.Comment: 13 pages, 11 figures, MNRAS accepte
A performance comparison of the contiguous allocation strategies in 3D mesh connected multicomputers
The performance of contiguous allocation strategies can be significantly affected by the distribution of job execution times. In this paper, the performance of the existing contiguous allocation strategies for 3D mesh multicomputers is re-visited in the context of heavy-tailed distributions (e.g., a Bounded Pareto distribution). The strategies are evaluated and compared using simulation experiments for both First-Come-First-Served (FCFS) and Shortest-Service-Demand (SSD) scheduling strategies under a variety of system loads and system sizes. The results show that the performance of the allocation strategies degrades considerably when job execution times follow a heavy-tailed distribution. Moreover, SSD copes much better than FCFS scheduling strategy in the presence of heavy-tailed job execution times. The results also show that the strategies that depend on a list of allocated sub-meshes for both allocation and deallocation have lower allocation overhead and deliver good system performance in terms of average turnaround time and mean system utilization
Tunable magnetic interaction at the atomic scale in oxide heterostructures
We report on a systematic study of a number of structurally identical but
chemically distinct transition metal oxides in order to determine how the
material-specific properties such as the composition and the strain affect the
properties at the interface of heterostructures. Our study considers a series
of structures containing two layers of ferromagnetic SrRuO3, with
antiferromagnetic insulating manganites sandwiched in between. The results
demonstrate how to control the strength and relative orientation of interfacial
ferromagnetism in correlated electron materials by means of valence state
variation and substrate-induced strain, respectively
Charge ordering in quarter-filled ladder systems coupled to the lattice
We investigate charge ordering in the presence of electron-phonon coupling
for quarter-filled ladder systems by using Exact Diagonalization. As an example
we consider NaV2O5 using model parameters obtained from first-principles
band-structure calculations. The relevant Holstein coupling to the lattice
considerably reduces the critical value of the nearest-neighbor Coulomb
repulsion at which formation of the zig-zag charge-ordered state occurs, which
is then accompanied by a static lattice distortion. Energy and length of a
kink-like excitation on the background of the distorted lattice are calculated.
Spin and charge spectra on ladders with and without static distortion are
obtained, and the charge gap and the effective spin-spin exchange parameter J
are extracted. J agrees well with experimental results. Analysis of the
dynamical Holstein model, restricted to a small number of phonons, shows that
low frequency lattice vibrations increase the charge order, accompanied by
dynamically produced zig-zag lattice distortions.Comment: 11 pages, 17 figures, revised version as to appear in Phys. Rev.
Charge Fluctuations in Geometrically Frustrated Charge Ordering System
Effects of geometrical frustration in low-dimensional charge ordering systems
are theoretically studied, mainly focusing on dynamical properties. We treat
extended Hubbard models at quarter-filling, where the frustration arises from
competing charge ordered patterns favored by different intersite Coulomb
interactions, which are effective models for various charge transfer-type
molecular conductors and transition metal oxides. Two different lattice
structures are considered: (a) one-dimensional chain with intersite Coulomb
interaction of nearest neighbor V_1 and that of next-nearest neighbor V_2, and
(b) two-dimensional square lattice with V_1 along the squares and V_2 along one
of the diagonals. From previous studies, charge ordered insulating states are
known to be unstable in the frustrated region, i.e., V_1 \simeq 2V_2 for case
(a) and V_1 \simeq V_2 for case (b), resulting in a robust metallic phase even
when the interaction strenghs are strong. By applying the Lanczos exact
diagonalization to finite-size clusters, we have found that fluctuations of
different charge order patterns exist in the frustration-induced metallic
phase, showing up as characteristic low energy modes in dynamical correlation
functions. Comparison of such features between the two models are discussed,
whose difference will be ascribed to the dimensionality effect. We also point
out incommensurate correlation in the charge sector due to the frustration,
found in one-dimensional clusters.Comment: 8 pages, 9 figure
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