14,433 research outputs found
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
Optical Study of the Free Carrier Response of LaTiO3/SrTiO3 Superlattices
We used infrared spectroscopic ellipsometry to investigate the electronic
properties of LaTiO3/SrTiO3 superlattices (SLs). Our results indicated that,
independent of the SL periodicity and individual layer-thickness, the SLs
exhibited a Drude metallic response with sheet carrier density per interface
~3x10^14 cm^-2. This is probably due to the leakage of d-electrons at
interfaces from the Mott insulator LaTiO3 to the band insulator SrTiO3. We
observed a carrier relaxation time ~ 35 fs and mobility ~ 35 cm^2V^-1s^-1 at 10
K, and an unusual temperature dependence of carrier density that was attributed
to the dielectric screening of quantum paraelectric SrTiO3.Comment: 4 pages, 4 figure
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
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
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.
Assembly and force measurement with SPM-like probes in holographic optical tweezers
We report a high fidelity tomographic reconstruction of the quantum state of photon pairs generated by parametric down-conversion with orbital angular momentum (OAM) entanglement. Our tomography method allows us to estimate an upper and lower bound for the entanglement between the down-converted photons. We investigate the two-dimensional state subspace defined by the OAM states ±â and superpositions thereof, with â=1, 2, ..., 30. We find that the reconstructed density matrix, even for OAMs up to around â=20, is close to that of a maximally entangled Bell state with a fidelity in the range between F=0.979 and F=0.814. This demonstrates that, although the single count-rate diminishes with increasing â, entanglement persists in a large dimensional state space
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
- âŠ