7,937 research outputs found
Covariant Helicity-Coupling Amplitudes: A New Formulation
We have worked out covariant amplitudes for any two-body decay of a resonance
with an arbitrary non-zero mass, which involves arbitrary integer spins in the
initial and the final states. One key new ingredient for this work is the
application of the total intrinsic spin operator which is given
directly in terms of the generators of the Poincar\'e group.
Using the results of this study, we show how to explore the Lorentz factors
which appear naturally, if the momentum-space wave functions are used to form
the covariant decay amplitudes. We have devised a method of constructing our
covariant decay amplitudes, such that they lead to the Zemach amplitudes when
the Lorentz factors are set one
Fracture toughness and crack-resistance curve behavior in metallic glass-matrix composites
Nonlinear-elastic fracture mechanics methods are used to assess the fracture toughness of bulk metallic glass (BMG) composites; results are compared with similar measurements for other monolithic and composite BMG alloys. Mechanistically, plastic shielding gives rise to characteristic resistance-curve behavior where the fracture resistance increases with crack extension. Specifically, confinement of damage by second-phase dendrites is shown to result in enhancement of the toughness by nearly an order of magnitude relative to unreinforced glass
Exploring the Oxygen Order in Hg-1223 and Hg-1201 by 199Hg MAS NMR
We demonstrate the use of a high-resolution solid-state fast (45 kHz) magic
angle spinning (MAS) NMR for mapping the oxygen distribution in Hg-based
cuprate superconductors. We identify observed three peaks in 199Hg spectrum as
belonging to the different chemical environments in the HgO? layer with no
oxygen neighbors, single oxygen neighbor, and two oxygen neighbors. We discuss
observed differences between Hg-1201 and Hg-1223 materials.Comment: 4 pages, 2 figures included. Submitted to NATO Advanced Research
Workshop Proceedings (Miami January 2004
Bremsstrahlung photon polarization for , and high energy collisions
The polarization of bremsstrahlung photon in the processes , and is calculated for peripheral
kinematics, in the high energy limit where the cross section does not decrease
with the incident energy. When the initial electron is
unpolarized(longitudinally polarized) the final photon can be linearly
(circularly) polarized. The Stokes parameters of the photon polarization are
calculated as a function of the kinematical variables of process: the energy of
recoil particle, the energy fraction of scattered electron, and the polar and
azimuthal angles of photon. Numerical results are given in form of tables, for
typical values of the relevant kinematic variables.Comment: 9 pages, 3 figure
Lichen Species as Bio-accumulator of some Halogens on Mount Cameroon Valcano, West Africa
Lichens diversities are informative indicators for assessing impacts of air pollution climate change environmental health volcanic activities habitat heterogeneity and continuity Lacking roots vascular tissues stomata and waxy cuticle they absorb and accumulate airborne nutrients pollutants from the atmosphere over their entire surface Halogens especially fluorides are released into the atmosphere in large amounts by volcanic eruptions and their pollutants levels in lichens can be determined quantitatively by chemical analysis of specie
Spin Dynamics in the LTT Phase of ~1/8 Doped Single Crystal La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4
We present La and Cu NMR relaxation measurements in single crystal
La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4. A strong peak in the La spin-lattice
relaxation rate observed in the spin ordered state is well-described by the BPP
mechanism[1] and arises from continuous slowing of electronic spin fluctuations
with decreasing temperature; these spin fluctuations exhibit XY-like anisotropy
in the ordered state. The spin pseudogap is enhanced by the static
charge-stripe order in the LTT phase.Comment: Four pages, three figure
An extreme [OIII] emitter at : a low metallicity Lyman continuum source
[Abridged] We investigate the physical properties of a Lyman continuum
emitter candidate at with photometric coverage from to MIPS
24m band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. Investigation of the
UV spectrum confirms a direct spectroscopic detection of the Lyman continuum
emission with . Non-zero Ly flux at the systemic redshift and
high Lyman- escape fraction suggest a low HI column density. The weak C
and Si low-ionization absorption lines are also consistent with a low covering
fraction along the line of sight. The
[OIII] equivalent width is one of the
largest reported for a galaxy at
(, rest-frame) and the NIR spectrum shows that this is mainly due to an
extremely strong [OIII] emission. The large observed [OIII]/[OII] ratio ()
and high ionization parameter are consistent with prediction from
photoionization models in case of a density-bounded nebula scenario.
Furthermore, the
is
comparable to recent measurements reported at , in the reionization
epoch. We also investigate the possibility of an AGN contribution to explain
the ionizing emission but most of the AGN identification diagnostics suggest
that stellar emission dominates instead. This source is currently the first
high- example of a Lyman continuum emitter exhibiting indirect and direct
evidences of a Lyman continuum leakage and having physical properties
consistent with theoretical expectation from Lyman continuum emission from a
density-bounded nebula.Comment: 14 pages, 11 figures, accepted for publication in A&A. Minor
modifications, Figure 2 updated, Figure 9 adde
Strong enhancement of spin fluctuations in the low-temperature-tetragonal phase of antiferromagnetically ordered La_{2-x-y}Eu_ySr_xCuO_4
Measurements of the static magnetization, susceptibility and ESR of Gd spin
probes have been performed to study the properties of antiferromagnetically
ordered La_{2-x-y}Eu_ySr_xCuO_4 (x less or equal 0.02) with the low temperature
tetragonal structure. According to the static magnetic measurements the CuO_2
planes are magnetically decoupled in this structural phase. The ESR study
reveals strong magnetic fluctuations at the ESR frequency which are not present
in the orthorhombic phase. It is argued that this drastic enhancement of the
spin fluctuations is due to a considerable weakening of the interlayer exchange
and a pronounced influence of hole motion on the antiferromagnetic properties
of lightly hole doped La_2CuO_4. No evidence for the stripe phase formation at
small hole doping is obtained in the present study.Comment: 10 pages, LaTeX, 3 EPS figures; to be published in Journal of
Physics: Condensed Matte
Structural Study of Binary Phosphate Glasses by X-ray and Neutron Diffraction
X-ray and neutron diffraction study on the structure of five binary metaphosphate glasses has been made by applying the pair function method coupled with the interference function refining technique. The distances and coordination numbers for the pairs of P-O, O-O and M-O (M=Li, Na, Zn, Mg, and Ca) were determined and a fundamental local ordering unit structure in these binary phosphate glasses has been confirmed to be a PO_4 tetrahedron and the particular features have also been recognized with respect to the numbers of oxygens around magnesium and zinc cations
Putative spin liquid in the triangle-based iridate BaIrTiO
We report on thermodynamic, magnetization, and muon spin relaxation
measurements of the strong spin-orbit coupled iridate BaIrTiO,
which constitutes a new frustration motif made up a mixture of edge- and
corner-sharing triangles. In spite of strong antiferromagnetic exchange
interaction of the order of 100~K, we find no hint for long-range magnetic
order down to 23 mK. The magnetic specific heat data unveil the -linear and
-squared dependences at low temperatures below 1~K. At the respective
temperatures, the zero-field muon spin relaxation features a persistent spin
dynamics, indicative of unconventional low-energy excitations. A comparison to
the isostructural compound BaRuTiO suggests that a concerted
interplay of compass-like magnetic interactions and frustrated geometry
promotes a dynamically fluctuating state in a triangle-based iridate.Comment: Physical Review B accepte
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