936 research outputs found
Strong Correlations in Electron Doped Phthalocyanine Conductors Near Half Filling
We propose that electron doped nontransition metal-phthalocyanines (MPc) like
ZnPc and MgPc, similar to those very recently reported, should constitute novel
strongly correlated metals. Due to orbital degeneracy, Jahn-Teller coupling and
Hund's rule exchange, and with a large on-site Coulomb repulsion, these
molecular conductors should display, particularly near half filling at two
electrons/molecule, very unconventional properties, including Mott insulators,
strongly correlated superconductivity, and other intriguing phases.Comment: 4 pages, 1 figure, submited to PR
Multiple origins of extra electron diffractions in fcc metals
Diffuse intensities in the electron diffraction patterns of concentrated
face-centered cubic solid solutions have been widely attributed to chemical
short-range order, although this connection has been recently questioned. This
article explores the many non-ordering origins of commonly reported features
using a combination of experimental electron microscopy and multislice
diffraction simulations, which suggest that diffuse intensities largely
represent thermal and static displacement scattering. A limited number of
observations may reflect additional contributions from planar defects, surface
terminations incommensurate with bulk periodicity, or weaker dynamical effectsComment: 8 pages, 3 figure
Extra electron reflections in concentrated alloys may originate from planar defects, not short-range order
In many concentrated alloys of current interest, the observation of diffuse
superlattice intensities by transmission electron microscopy has been
attributed to the presence of chemical short-range order. This interpretation
is questioned on the basis of crystallographic considerations and theoretical
predictions of ordering. The work of Xiao and Daykin [Ultramicroscopy 53
(1994)], which shows how planar defects can produce the exact set of observed
peaks, is highlighted as an alternative explanation that would impact the
conclusions of a number of recent studies.Comment: 5 pages, 3 figure
Overtaking CPU DBMSes with a GPU in whole-query analytic processing with parallelism-friendly execution plan optimization
Existing work on accelerating analytic DB query processing
with (discrete) GPUs fails to fully realize their potential for speedup through
parallelism: Published results do not achieve significant speedup over more
performant CPU-only DBMSes when processing complete queries.
This paper presents a successful
Plastic flow and failure resistance of metallic glass: Insight from \u3cem\u3ein situ\u3c/em\u3e compression of nanopillars
We report in situ nanocompression tests of Cu-Zr-Al metallic glass (MG) pillars in a transmission electron microscope. This technique is capable of spatially and temporally resolving the plastic flow in MGs. The observations reveal the intrinsic ability of fully glassy MGs to sustain large plastic strains, which would otherwise be preempted by catastrophic instability in macroscopic samples and conventional tests. The high ductility in volume-limited MGs and the sample size effects in suppressing the rapid failure common to MGs are analyzed by modeling the evolution of the collectivity of flow defects toward localization
Fabrication of GaNxAs1-x Quantum Structures by Focused Ion Beam Patterning
A novel approach to the fabrication of GaNxAs1-x quantum dots and wires via
ion beam patterning is presented. Photomodulated reflectance spectra confirm
that N can be released from the As sublattice of an MBE-grown GaNxAs1-x film by
amorphization through ion implantation followed by regrowth upon rapid thermal
annealing (RTA). Amorphization may be achieved with a focused ion beam (FIB),
which is used to implant Ga ions in patterned lines such that annealing
produces GaAs regions within a GaNxAs1-x film. The profiles of these amorphized
lines are dependent upon the dose implanted, and the film reaches a damage
threshold during RTA due to excess Ga. By altering the FIB implantation
pattern, quantum dots or wires may be fabricated.Comment: To appear in the proceedings of the 27th International Conference on
the Physics of Semiconductors (ICPS-27, Flagstaff, AZ, July 26-30, 2004
Exceptional fracture toughness of CrCoNi-based medium- and high-entropy alloys close to liquid helium temperatures
Medium- and high-entropy alloys based on the CrCoNi-system have been shown to
display outstanding strength, tensile ductility and fracture toughness
(damage-tolerance properties), especially at cryogenic temperatures. Here we
examine the JIc and (back-calculated) KJIc fracture toughness values of the
face-centered cubic, equiatomic CrCoNi and CrMnFeCoNi alloys at 20 K. At flow
stress values of ~1.5 GPa, crack-initiation KJIc toughnesses were found to be
exceptionally high, respectively 235 and 415 MPa(square-root)m for CrMnFeCoNi
and CrCoNi, with the latter displaying a crack-growth toughness Kss exceeding
540 MPa(square-root)m after 2.25 mm of stable cracking, which to our knowledge
is the highest such value ever reported. Characterization of the crack-tip
regions in CrCoNi by scanning electron and transmission electron microscopy
reveal deformation structures at 20 K that are quite distinct from those at
higher temperatures and involve heterogeneous nucleation, but restricted
growth, of stacking faults and fine nano-twins, together with transformation to
the hexagonal closed-packed phase. The coherent interfaces of these features
can promote both the arrest and transmission of dislocations to generate
respectively strength and ductility which strongly contributes to sustained
strain hardening. Indeed, we believe that these nominally single-phase,
concentrated solid-solution alloys develop their fracture resistance through a
progressive synergy of deformation mechanisms, including dislocation glide,
stacking-fault formation, nano-twinning and eventually in situ phase
transformation, all of which serve to extend continuous strain hardening which
simultaneously elevates strength and ductility (by delaying plastic
instability), leading to truly exceptional resistance to fracture.Comment: 31 pages, 10 figures, including Supplementary Informatio
Transition to Long Range Magnetic Order in the Highly Frustrated Insulating Pyrochlore Antiferromagnet Gd_2Ti_2O_7
Experimental evidence from measurements of the a.c. and d.c. susceptibility,
and heat capacity data show that the pyrochlore structure oxide, Gd_2Ti_2O_7,
exhibits short range order that starts developing at 30K, as well as long range
magnetic order at K. The Curie-Weiss temperature, =
-9.6K, is largely due to exchange interactions. Deviations from the Curie-Weiss
law occur below 10K while magnetic heat capacity contributions are found
at temperatures above 20K. A sharp maximum in the heat capacity at K
signals a transition to a long range ordered state, with the magnetic specific
accounting for only 50% of the magnetic entropy. The heat capacity above
the phase transition can be modeled by assuming that a distribution of random
fields acts on the ground state for Gd. There is no
frequency dependence to the a.c. susceptibility in either the short range or
long range ordered regimes, hence suggesting the absence of any spin-glassy
behavior. Mean field theoretical calculations show that no long range ordered
ground state exists for the conditions of nearest-neighbor antiferromagnetic
exchange and long range dipolar couplings. At the mean-field level, long range
order at various commensurate or incommensurate wave vectors is found only upon
inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar
coupling. The properties of Gd$_2Ti_2O_7 are compared with other geometrically
frustrated antiferromagnets such as the Gd_3Ga_5O_{12} gadolinium gallium
garnet, RE_2Ti_2O_7 pyrochlores where RE = Tb, Ho and Tm, and Heisenberg-type
pyrochlore such as Y_2Mo_2O_7, Tb_2Mo_2O_7, and spinels such as ZnFe_2O_4Comment: Letter, 6 POSTSCRIPT figures included. (NOTE: Figure 5 is not
included --) To appear in Physical Review B. Contact:
[email protected]
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Metal-induced assembly of a semiconductor-island lattice: Getruncated pyramids on Au-patterned Si
We report the two-dimensional alignment of semiconductor islands using rudimentary metal patterning to control nucleation and growth. In the Ge on Si system, a square array of sub-micron Au dots on the Si (001) surface induces the assembly of deposited Ge adatoms into an extensive island lattice. Remarkably, these highly ordered Ge islands form between the patterned Au dots and are characterized by a unique truncated pyramidal shape. A model based on patterned diffusion barriers explains the observed ordering and establishes general criteria for the broader applicability of such a directed assembly process to quantum dot ordering
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