Fabrication and characterization of hot- pressed tantalum carbide

Microstructure and chemistry of hot pressed powder compacts of tantalum carbid

The flavour projection of staggered fermions and the quarter-root trick

It is shown that the flavour projection of staggered fermions can be written as a projection between the fields on four separate, but parallel, lattices, where the fields on each are modified forms of the standard staggered fermion field. Because the staggered Dirac operator acts equally on each lattice, it respects this flavour projection. We show that the system can be gauged in the usual fashion and that this does not interfere with flavour projection. We also consider the path integral, showing that, prior to flavour projection, it evaluates to the same form on each lattice and that this form is equal to that used in the quarter-root trick. The flavour projection leaves a path integral for a single flavour of field on each lattice.Comment: 8 pages, including title pag

Microscopic Model versus Systematic Low-Energy Effective Field Theory for a Doped Quantum Ferromagnet

We consider a microscopic model for a doped quantum ferromagnet as a test case for the systematic low-energy effective field theory for magnons and holes, which is constructed in complete analogy to the case of quantum antiferromagnets. In contrast to antiferromagnets, for which the effective field theory approach can be tested only numerically, in the ferromagnetic case both the microscopic and the effective theory can be solved analytically. In this way the low-energy parameters of the effective theory are determined exactly by matching to the underlying microscopic model. The low-energy behavior at half-filling as well as in the single- and two-hole sectors is described exactly by the systematic low-energy effective field theory. In particular, for weakly bound two-hole states the effective field theory even works beyond perturbation theory. This lends strong support to the quantitative success of the systematic low-energy effective field theory method not only in the ferromagnetic but also in the physically most interesting antiferromagnetic case.Comment: 34 pages, 1 figur

R-Curve Response Of Silicon Carbide Whisker-Reinforced Alumina: Microstructural Influence

Rising fracture resistance with crack extension (R-curve response) can lead to improvements in the mechanical reliability of ceramics. To understand how microstructures influence the R-curve behavior, direct observations of crack interactions with microstructural features were conducted on SiC whisker-reinforced alumina. The contribution of the dominant toughening mechanisms to the R-curve behavior of these composites is discussed using experimental and theoretical studies

Homogeneous versus Spiral Phases of Hole-doped Antiferromagnets: A Systematic Effective Field Theory Investigation

Using the low-energy effective field theory for magnons and holes -- the condensed matter analog of baryon chiral perturbation theory for pions and nucleons in QCD -- we study different phases of doped antiferromagnets. We systematically investigate configurations of the staggered magnetization that provide a constant background field for doped holes. The most general configuration of this type is either constant itself or it represents a spiral in the staggered magnetization. Depending on the values of the low-energy parameters, a homogeneous phase, a spiral phase, or an inhomogeneous phase is energetically favored. The reduction of the staggered magnetization upon doping is also investigated.Comment: 35 pages, 5 figure

Direct photon production with effective field theory

The production of hard photons in hadronic collisions is studied using Soft-Collinear Effective Theory (SCET). This is the first application of SCET to a physical, observable cross section involving energetic partons in more than two directions. A factorization formula is derived which involves a non-trivial interplay of the angular dependence in the hard and soft functions, both quark and gluon jet functions, and multiple partonic channels. The relevant hard, jet and soft functions are computed to one loop and their anomalous dimensions are determined to three loops. The final resummed inclusive direct photon distribution is valid to next-to-next-to-leading logarithmic order (NNLL), one order beyond previous work. The result is improved by including non-logarithmic terms and photon isolation cuts through matching, and compared to Tevatron data and to fixed order results at the Tevatron and the LHC. The resummed cross section has a significantly smaller theoretical uncertainty than the next-to-leading fixed-order result, particularly at high transverse momentum.Comment: 42 pages, 9 figures; v2: references added, minor changes; v3: typos; v4: typos, corrections in (16), (47), (72

SCET sum rules for B->P and B->V transition form factors

We investigate sum rules for heavy-to-light transition form factors at large recoil derived from correlation functions with interpolating currents for light pseudoscalar or vector fields in soft-collinear effective theory (SCET). We consider both, factorizable and non-factorizable contributions at leading power in the Lambda/m_b expansion and to first order in the strong coupling constant alpha_s, neglecting contributions from 3-particle distribution amplitudes in the B-meson. We pay particular attention to various sources of parametric and systematic uncertainties. We also discuss certain form factor ratios where part of the hadronic uncertainties related to the B-meson distribution amplitude and to logarithmically enhanced alpha_s corrections cancel.Comment: 27 pages, 19 figures, minor corrections, matches journal versio

Elevated temperature static fatigue of a Nicalon fiber-reinforced SiC composite

Static fatigue tests of a Nicalon fiber-reinforced SiC matrix composite were conducted in four-point bending over a temperature range of 425 to 1,150 C in air at selected stress levels. The composite consisted of a Nicalon cloth with a 0.3 {mu}m graphite interfacial coating and a Forced Chemical Vapor Infiltration (FCVI) SiC matrix composite; samples were tested with or without a final protective SiC seal coat. The results indicated that the fatigue life of the Nicalon-SiC composite decreased with an increase in either applied stresses or test temperatures. However, the composite exhibited a fatigue limit of {approximately} 100 MPa at temperatures < 950 C which decreased to {approximately} 70 MPa at 1,150 C. Both electron microscopy and thermogravimetric studies suggested that the lifetime of the composites was dictated by the oxidation of graphite interfacial layer at temperatures {le} 700 C and by oxidation of graphite coating accompanied by formation of silicate interfacial layer via oxidation of the Nicalon fiber (and the SiC matrix) at temperatures {ge} 950 C. Use of a SiC seal coat effectively retarded the oxidation reactions and increased the lifetime by at least one order of magnitude at 425 C. On the other hand, the SiC seal coat made little (if any) difference in fatigue life at 950 C

Longitudinal Differences Observed in the Ionospheric F-Region During the Major Geomagnetic Storm of March 31, 2001

A new ionospheric sounding station using a Canadian Advanced Digital Ionosonde (CADI) was established for routine measurements by the “Universidade do Vale do Paraiba (UNIVAP)” at S˜ao Jos´e dos Campos (23.2_ S, 45.9_ W), Brazil, in August 2000. A major geomagnetic storm with gradual commencement at about 01:00 UT was observed on 31 March 2001. In this paper, we present and discuss salient features from the ionospheric sounding measurements carried out at S. J. Campos on the three consecutive UT days 30 March (quiet), 31 March (disturbed) and 1 April (recovery) 2001. During most of the storm period, the foF2 values showed negative phase, whereas during the two storm-time peaks, large F-region height variations were observed. In order to study the longitudinal differences observed in the F-region during the storm, the simultaneous ionospheric sounding measurements carried out at S. J. Campos, El Arenosillo (37.1_ N, 6.7_W), Spain, Okinawa (26.3_ N, 127.8_ E), Japan and Wakkanai (45.5_ N, 141.7_ E), Japan, during the period 30 March–1 April 2001, have been analyzed. A comparison of the observed ionospheric parameters (h0F and foF2) in the two longitudinal zones (1. Japanese and 2. Brazilian-Spanish) shows both similarities and differences associated with the geomagnetic disturbances. Some latitudinal differences are also observed in the two longitudinal zones. In addition, global ionospheric TEC maps from the worldwide network of GPS receivers are presented, showing widespread TEC changes during both the main and recovery phases of the storm. The ionospheric sounding measurements are compared with the ASPEN-TIMEGCM model runs appropriate for the storm conditions. The model results produce better agreement during the quiet period. During the disturbed period, some of the observed F-region height variations are well reproduced by the model results. The model foF2 and TEC results differ considerably during the recovery period and indicate much stronger negative phase at all the stations, particularly at the low-latitude ones