787 research outputs found
On spherical twisted conjugacy classes
Let G be a simple algebraic group over an algebraically closed field of good
odd characteristic, and let theta be an automorphism of G arising from an
involution of its Dynkin diagram. We show that the spherical theta-twisted
conjugacy classes are precisely those intersecting only Bruhat cells
corresponding to twisted involutions in the Weyl group. We show how the
analogue of this statement fails in the triality case. We generalize to good
odd characteristic J-H. Lu's dimension formula for spherical twisted conjugacy
classes.Comment: proof of Lemma 6.4 polished. The journal version is available at
http://www.springerlink.com/content/k573l88256753640
Adaptive hard and tough mechanical response in single-crystal B1 VNx ceramics via control of anion vacancies
High hardness and toughness are generally considered mutually exclusive
properties for single-crystal ceramics. Combining experiments and ab initio
molecular dynamics (AIMD) atomistic simulations at room temperature, we
demonstrate that both the hardness and toughness of single-crystal
NaCl-structure VNx/MgO(001) thin films are simultaneously enhanced through the
incorporation of anion vacancies. Nanoindentation results show that VN0.8, here
considered as representative understoichiometric VNx system, is ~20% harder, as
well as more resistant to fracture than stoichiometric VN samples. AIMD
modeling of VN and VN0.8 supercells subjected to [001] and [110] elongation
reveal that the tensile strengths of the two materials are similar.
Nevertheless, while the stoichiometric VN phase systematically cleaves in a
brittle manner at tensile yield points, the understoichiometric compound
activates transformation-toughening mechanisms that dissipate accumulated
stresses. AIMD simulations also show that VN0.8 exhibits an initially greater
resistance to both {110} and {111} shear deformation than VN.
However, for progressively increasing shear strains, the VN0.8 mechanical
behavior gradually evolves from harder to more ductile than VN. The transition
is mediated by anion vacancies, which facilitate {110} and {111}
lattice slip by reducing activation shear stresses by as much as 35%.
Electronic-structure analyses show that the two-regime hard/tough mechanical
response of VN0.8 primarily stems from its intrinsic ability to transfer d
electrons between 2nd-neighbor and 4th-neighbor (i.e., across vacancy sites)
V-V metallic states. Our work offers a route for electronic-structure design of
hard materials in which a plastic mechanical response is triggered with
loading
Machine-learning potentials for nanoscale simulations of deformation and fracture: example of TiB ceramic
Machine-learning interatomic potentials (MLIPs) offer a powerful avenue for
simulations beyond length and timescales of ab initio methods. Their
development for investigation of mechanical properties and fracture, however,
is far from trivial since extended defects -- governing plasticity and crack
nucleation in most materials -- are too large to be included in the training
set. Using TiB as a model ceramic material, we propose a strategy for
fitting MLIPs suitable to simulate mechanical response of monocrystals until
fracture. Our MLIP accurately reproduces ab initio stresses and failure
mechanisms during room-temperature uniaxial tensile deformation of TiB at
the atomic scale ( atoms). More realistic tensile tests (low
strain rate, Poisson's contraction) at the nanoscale (--10
atoms) require MLIP up-fitting, i.e. learning from additional ab initio
configurations. Consequently, we elucidate trends in theoretical strength,
toughness, and crack initiation patterns under different loading directions. To
identify useful environments for further up-fitting, i.e., making the MLIP
applicable to a wider spectrum of simulations, we asses transferability to
other deformation conditions and phases not explicitly trained on
Cosmological Feedback from High-Redshift Dwarf Galaxies
We model how repeated supernova explosions in high-redshift dwarf starburst
galaxies drive superbubbles and winds out of the galaxies. We compute the
efficiencies of metal and mass ejection and energy transport from the galactic
potentials, including the effect of cosmological infall of external gas. The
starburst bubbles quickly blow out of small, high-redshift, galactic disks, but
must compete with the ram pressure of the infalling gas to escape into
intergalactic space. We show that the assumed efficiency of the star formation
rate dominates the bubble evolution and the metal, mass, and energy feedback
efficiencies. With star formation efficiency f*=0.01, the ram pressure of
infall can confine the bubbles around high-redshift dwarf galaxies with
circular velocities v_c>52 km/s. We can expect high metal and mass ejection
efficiencies, and moderate energy transport efficiencies in halos with
v_c~30-50 km/s and f*~0.01 as well as in halos with v_c~100 km/s and f*>>0.01.
Such haloes collapse successively from 1-2 sigma peaks in LambdaCDM Gaussian
density perturbations as time progresses. These dwarf galaxies can probably
enrich low and high-density regions of intergalactic space with metals to
10^-3-10^-2 Zsun as they collapse at z~8 and z<5 respectively. They also may be
able to provide adequate turbulent energy to prevent the collapse of other
nearby halos, as well as to significantly broaden Lyman-alpha absorption lines
to v_rms~20-40 km/s. We compute the timescales for the next starbursts if gas
freely falls back after a starburst, and find that, for star formation
efficiencies as low as f*<0.01, the next starburst should occur in less than
half the Hubble time at the collapse redshift. This suggests that episodic star
formation may be ubiquitous in dwarf galaxies.Comment: Accepted for ApJ v613, 60 pages, 15 figure
Luminescence related to high density of Mg-induced stacking faults in homoepitaxially grown GaN
Solubility limit and precipitate formation in Al-doped 4H-SiC epitaxial material
Heavily Al-doped 4H–SiC structures have been prepared by vapor phase epitaxy. Subsequent anneals have been carried out in an Ar atmosphere in a rf-heated furnace between 1500 °C and 2000 °C for 0.5 to 3 h. Secondary ion mass spectrometry has been utilized to obtain Al concentration versus depth as well as lateral distributions (ion images). Transmission electron microscopy(TEM) has been employed to study the crystallinity and determine phase composition after heat treatment. A solubility limit of ∼2×10²⁰ Al/cm³ (1900 °C) is extracted. Three-dimensional ion images show that the Al distribution does not remain homogeneous in layers heat treated at 1700 °C or above when the Al concentration exceeds 2×10²⁰ cm⁻³. Al-containing precipitates are identified by energy-filtered TEM.Financial support was partly received
from the Swedish Foundation for Strategic Research (SSF)
SiCEP program
Ab Initio Molecular Dynamics Simulations of Nitrogen/VN(001) Surface Reactions: Vacancy-Catalyzed N 2
Raman scattering in C_{60} and C_{48}N_{12} aza-fullerene: First-principles study
We carry out large scale {\sl ab initio} calculations of Raman scattering
activities and Raman-active frequencies (RAFs) in
aza-fullerene. The results are compared with those of .
Twenty-nine non-degenerate polarized and 29 doubly-degenerate unpolarized RAFs
are predicted for . The RAF of the strongest Raman
signal in the low- and high-frequency regions and the lowest and highest RAFs
for are almost the same as those of .
The study of reveals the importance of electron correlations and
the choice of basis sets in the {\sl ab initio} calculations. Our best
calculated results for with the B3LYP hybrid density functional
theory are in excellent agreement with experiment and demonstrate the desirable
efficiency and accuracy of this theory for obtaining quantitative information
on the vibrational properties of these molecules.Comment: submitted to Phys.Rev.
Do Child Soldiers Influence UN Peacekeeping?
The use of child soldiers in conflicts has received increasing academic attention in recent years. This article examines post-conflict periods to see whether the use of child soldiers mobilizes United Nations peacekeeping operations (UN PKO) in the aftermath of a conflict. Taking into consideration how child soldiers affect conflict and how important their reintegration is to sustainable peace and post-conflict development, we analyse whether the presence of child soldiers in a civil war increases the likelihood of the presence of a PKO. We argue that the UN deems a conflict with child soldiers as a difficult case for conflict resolution, necessitating a response from the international community. This is in line with our empirical results confirming that the use of child soldiers significantly increases the likelihood of peacekeeping
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