11,191 research outputs found
Optical Conductivity of the Trellis-Lattice t-J Model: Charge Fluctuations in NaV_2O_5
Optical conductivity of the trellis lattice t-J model at quarter filling is
calculated by an exact-diagonalization technique on small clusters, whereby the
valence state of V ions of NaV_2O_5 is considered. We show that the
experimental features at \sim 1 eV, including peak positions, presence of
shoulders, and anisotropic spectral weight, can be reproduced in reasonable
range of parameter values, only by assuming that the system is in the charge
disproportionated ground state. Possible reconciliation with experimental data
suggesting the presence of uniform ladders at T>T_c is discussed.Comment: 4 pages, 4 gif figures. Minor revisions have been made. Hardcopies of
figures (or the entire manuscript) can be obtained by e-mail request to
[email protected]
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
First-Principles Study of Electronic Structure in -(BEDT-TTF)I at Ambient Pressure and with Uniaxial Strain
Within the framework of the density functional theory, we calculate the
electronic structure of -(BEDT-TTF)I at 8K and room temperature
at ambient pressure and with uniaxial strain along the - and -axes. We
confirm the existence of anisotropic Dirac cone dispersion near the chemical
potential. We also extract the orthogonal tight-binding parameters to analyze
physical properties. An investigation of the electronic structure near the
chemical potential clarifies that effects of uniaxial strain along the a-axis
is different from that along the b-axis. The carrier densities show
dependence at low temperatures, which may explain the experimental findings not
only qualitatively but also quantitatively.Comment: 10 pages, 7 figure
One-Dimensional Confinement and Enhanced Jahn-Teller Instability in LaVO
Ordering and quantum fluctuations of orbital degrees of freedom are studied
theoretically for LaVO in spin-C-type antiferromagnetic state. The
effective Hamiltonian for the orbital pseudospin shows strong one-dimensional
anisotropy due to the negative interference among various exchange processes.
This significantly enhances the instability toward lattice distortions for the
realistic estimate of the Jahn-Teller coupling by first-principle LDA+
calculations, instead of favoring the orbital singlet formation. This explains
well the experimental results on the anisotropic optical spectra as well as the
proximity of the two transition temperatures for spin and orbital orderings.Comment: 4 pages including 4 figure
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
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
Asynchronous Graph Pattern Matching on Multiprocessor Systems
Pattern matching on large graphs is the foundation for a variety of
application domains. Strict latency requirements and continuously increasing
graph sizes demand the usage of highly parallel in-memory graph processing
engines that need to consider non-uniform memory access (NUMA) and concurrency
issues to scale up on modern multiprocessor systems. To tackle these aspects,
graph partitioning becomes increasingly important. Hence, we present a
technique to process graph pattern matching on NUMA systems in this paper. As a
scalable pattern matching processing infrastructure, we leverage a
data-oriented architecture that preserves data locality and minimizes
concurrency-related bottlenecks on NUMA systems. We show in detail, how graph
pattern matching can be asynchronously processed on a multiprocessor system.Comment: 14 Pages, Extended version for ADBIS 201
Charge Ordering in Organic ET Compounds
The charge ordering phenomena in quasi two-dimensional 1/4-filled organic
compounds (ET)_2X (ET=BEDT-TTF) are investigated theoretically for the
and -type structures, based on the Hartree approximation for the
extended Hubbard models with both on-site and intersite Coulomb interactions.
It is found that charge ordered states of stripe-type are stabilized for the
relevant values of Coulomb energies, while the spatial pattern of the stripes
sensitively depends on the anisotropy of the models. By comparing the results
of calculations with the experimental facts, where the effects of quantum
fluctuation is incorporated by mapping the stripe-type charge ordered states to
the S=1/2 Heisenberg Hamiltonians, the actual charge patterns in the insulating
phases of -(ET)_2MM'(SCN)_4 and -(ET)_2I_3 are deduced.
Furthermore, to obtain a unified view among the , and
-(ET)_2X families, the stability of the charge ordered state in
competition with the dimeric antiferromagnetic state viewed as the Mott
insulating state, which is typically realized in -type compounds, and
with the paramagnetic metallic state, is also pursued by extracting essential
parameters.Comment: 35 pages, 27 figures, submitted to J. Phys. Soc. Jp
Zigzag Charge Ordering in alpha'-NaV2O5
23Na NMR spectrum measurements in alpha'-NaV2O5 with a single- crystalline
sample are reported. In the charge-ordered phase, the number of inequivalent Na
sites observed is more than that expected from the low-temperature structures
of space group Fmm2 reported so far. This disagreement indicates that the real
structure including both atomic displacement and charge disproportionation is
of lower symmetry. It is suggested that zigzag ordering is the most probable.
The temperature variation of the NMR spectra near the transition temperature is
incompatible with that of second-order transitions. It is thus concluded that
the charge ordering transition is first-order.Comment: 4 pages, 5 eps figures, submitted to J. Phys. Soc. Jp
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AKARI observation of early-type galaxies in Abell 2218
We describe the AKARI InfraRed Camera (IRC) imaging observation of early-type galaxies (ETGs) in A2218 at z ≅ 0.175. With the imaging capability at 11 and 15 μm, we investigate mid-infrared (MIR) properties of ETGs in the cluster environment. Among our flux-limited sample of 22 optical red sequence ETGs, we find that more than 50% have MIR-excess emission, and the most likely cause of the MIR excess is the circumstellar dust emission from asymptotic giant branch (AGB) stars. The MIR-excess galaxies reveal a wide spread in N3-S11 (3 and 11 μm) colors, indicative of a significant spread (2–11 Gyr) in the mean ages of stellar populations. They are also preferentially located in the outer region, suggesting the environment dependence of MIR-excess ETGs over an area out to a half virial radius
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