Atomic mean-square displacements in f.c.c. metals

Abstract

In two recent letters, one by Zoli in 1991, and the other by Schober in 1992, the evaluation of the quasiharmonic and anharmonic contributions to the atomic mean-square displacement (MSD) for f.c.c. metals has been discussed. In Zoli's work, the difference in the two contributions is found to be 91%. Schober, on the other hand, has not evaluated the explicit anharmonic contribution to MSD. The huge difference in Zoli's work is shown here to be due to an inaccurate evaluation of the explicit anharmonic contribution to MSD. A proper self-contained method as presented here, which employs the same model in the quasiharmonic and anharmonic calculations of MSD or Debye-Waller factor, indeed shows that the two contributions differ from each other by 10–15% depending on the temperature. Larger differences exist at higher temperatures. Some numerical results are given for a model of the f.c.c. lattice, namely a nearest-neighbour central force model employing a Lennard-Jones potential (applicable to rare-gas solids) and the Morse potential as a model for Cu.     

Scaling law for the low-frequency a.c. conductivity of fluid-saturated porous media

Abstract

An empirical scaling law for the real part of the frequency-dependent electrical conductivitσ′(ω) of brine-saturated porous materials in relation to its d.c. limit has been identified. This scaling law relates the conductivity as a function of frequency, porosity and salinity to a function of only one variable that is the ratio of the frequency to the d.c. conductivity in appropriate units. This result is very similar to what has been reported in hopping semiconducting systems.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

Properties of 5f6d electron complexes in the light actinides

Recently temperature-sensitive 5f6d electron complexes were postulated to explain certain properties of the light actinide metals Pa, U, Np and Pu, but the sizes of these complexes remained undefined. Here it is shown that these complexes are localized in the short (covalent) bonds of α-U, govern the anomalous properties of α-Pa and explain the stabilization of certain binary alloy phases between light actinide metals. The systematic variation in certain properties in the light actinide series can be correlated with the presence of 5f6d complexes. In the first three metals (Th, Pa and U), typical 5f effects are more or less suppressed. In contrast these effects exist with clearly different strengths in Np and Pu. Experiments are proposed by which the properties of 5f6d complexes might be studied further.     

Modelling of irradiation-induced hardening in metals using dislocation dynamics

A new simulation technique (three-dimensional dislocation dynamics) enabling the capture of a hardening effect in metals due to irradiation is reported. When bombarded with high-energy particles, metals accrue internal damage. In irradiated Pd, for example, damage takes the form of interstitial loops. Such loops are nano-sized and typically have a high number density. The stress field of a loop is given from dislocation theory. It is shown here the hardening is due to the elastic interaction of gliding dislocations with a high number of spatially dispersed interstitial loops. Results are found to correlate well with experiments.     

Qualifications for Membership in the A.G.P.A.

A model of the development of group therapists is presented. It is proposed that student group therapists proceed through a sequence of stages, each of which is marked by characteristic therapist behavior. During the initial Group shock phase, the student reacts to the novelty of group therapy stimuli with individual therapy or content-oriented approaches. Reappraisal is a period of some therapist inhibition, frustration, or self-doubt. One step behind is a phase characterized by a gradual recognition of the process level of analysis, despite difficulty supplying the interventions autonomously. Using the here-and-now is a period of active intervention about interpersonal phenomena among group members. The techniques for highlighting this material are refined in the Polishing skills phase. The usefulness of models in training is discussed.     

Internal stress effect on grain-boundary diffusion in submicrocrystalline metals

Grain-boundary diffusion in the regime affected by internal stresses of triple-line disclinations is analysed. The concentration profiles in a stressed solid averaged over a distribution of hydrostatic stress gradients are calculated. It is shown that the stresses from triple-line disclinations in as-prepared submicrocrystalline metals can significantly increase the effective grain-boundary diffusion coefficient, which is obtained by fitting the concentration profiles to the solution of a diffusion equation for an unstressed polycrystal.     

Estimation of density of charged defects in amorphous chalcogenides from a.c. conductivity: Random-walk approach for bipolarons based on correlated barrier hopping

Abstract

Although the pair approximation (PA) based on correlated bamer hopping (CBH) is now fairly well established to explain the a.c. conductivity of amorphous chalcogenides, the density of charged states deduced from PA is large compared with those estimated from other studies, namely light-induced electron spin resonance and drift mobility. We have used the continuous-time random-walk approximation based on CBH to estimate the density of charged defects and have applied it to experimental data for amorphous As₂Se₃, and a density of charged defects consistent with other measurements has been estimated.     

Thanks to 2010 Reviewers

Abstract

The electronic conductivity, diffusion coefficient and the self-energy in the vicinity of the energy gap of amorphous Si at T=0K have been computed as a function of energy for a fully bonded model of Si. We have used the equation-of-motion method in κ-space to show, for the first time, the probable position and existence of mobility edges. The conductivity rises rapidly away from the mobility edges to take values typical of those found in liquid metals. The behaviour of the self-energy indicates that a resonance phenomenon is responsible for the formation of a gap.     

Limiting relations for atomic transport in lattice-gas models of solids

Abstract

The computer simulation of lattice-gas models of solids by Monte Carlo methods is significant for the theory of atomic migration and transport in alloys, solid solutions of hydrogen in metals, fast-ion conductors, etc. This paper presents certain exact limiting relations which provide useful checks on such simulations on complex models. These relations specify the way macroscopic transport coefficients and associated quantities (‘correlation factors’) depend upon isotopic ratios and vacancy fraction.     

On the detection of time-reversal symmetry breaking by photoemission with circularly polarized light in the high- T c superconductor Bi2212

We argue that that in their recent experiment in which they claim to have found evidence for a time-reversal symmetry broken state, Kaminski et al. () (Nature, 416, p. 610) overlooked small temperature-dependent changes in the superstructure of Bi2212. These subtle changes may manifest themselves by changing the final state configurations of the photoemission process and thus invalidate their ultimate conclusions.     

Common barriers to healthcare for transgender people in the U.S. Southeast

Abstract

Background: Transgender and non-binary people are more likely to face barriers to healthcare than their cisgender counterparts. The majority of work in this area centers on the experiences of transgender people in northern cities and urban enclaves, yet over 500,000 transgender people live in the U.S. Southeast.

Aims: The purpose of this study is to explore barriers to healthcare among transgender people in the U.S. Southeast.

Methods: The research team conducted four 120-minute focus groups (eligibility criteria: 18?years or older, self-identify as transgender, live in the U.S. Southeast). Participants completed a demographic questionnaire prior to the start of the focus group. Each focus group explored access to and experiences of receiving basic healthcare as a transgender person in the U.S. Southeast. Established qualitative methods were used to conduct the focus groups and data analysis.

Results: Participants (n?=?48) ranged in age from 19 to 65, with the majority identifying as trans women (43.8%) and non-binary (33.3%). The sample was racially diverse: White (50%), Black (37.5%), and Latinx or Multiracial (12.5%). Multiple barriers to care were identified: (1) fear and mistrust of providers; (2) inconsistency in access to healthcare; (3) disrespect from providers; and, (4) mistreatment due to intersecting experiences of gender, race, class, and location.

Discussion: Transgender Southerners face barriers to care at the structural, cultural, and interpersonal levels. The study results have implications for researchers, as well as providers, practices, and health care systems throughout the region.     

Toxic Environment of war: Maternal prenatal heavy metal load predicts infant emotional development

BackgroundPeople in war zones are exposed to heavy metal contamination deriving from new-generation weapons, in addition to exposure to psychologically traumatizing war events. Pregnant women and their children-to-be are particularly vulnerable to both biological and psychological war effects.ObjectiveThe aim of the current study was to analyse the impact of maternal prenatal heavy metal contamination on infant emotional development and to examine the potential moderating role of maternal symptoms of post-traumatic stress disorder (PTSD) in the association between heavy metal load and infant emotional development.MethodsThe participants were 502 Palestinian mothers, pregnant in their first trimester during the 2014 War on Gaza. The mothers were recruited at their delivery (T1) and followed at the infants’ age of 6–7 months (T2; N = 392). The load of five weapon-related heavy metals (chromium, mercury, vanadium, strontium, and uranium) was analysed by Inductively Coupled Plasma Mass Spectrometry (ICP/MS) from mothers’ hair samples at childbirth (T1). Assessment of maternal PTSD symptoms was based on the Harvard Trauma Questionnaire (HTQ) and infant emotional development on the Infant Behavior Questionnaire (IBQ), both reported by mothers (T2).ResultsTwo of the analysed metals, chromium and uranium, adversely predicted children’s early emotional development, indicated by decreased positive affectivity, increased negative emotionality, and problems in early orientation and regulation. Mother’s PTSD did not moderate the impact of heavy metal contamination on children’s emotional development.ConclusionsAdverse impact of war is not limited to those who experience it directly, but is passed on to future generations through multiple mechanisms. International organizations are obliged to protect parents and infants from the modern weaponry in wars.     

Threading dislocation with b=c+2a in 4H-SiC as determined by LACBED

The Burgers vectors of the so-called threading screw dislocations (a total of 28 dislocations) in 4H-SiC were determined by large-angle convergent-beam electron diffraction. A new type of TSD, that is, b = c + 2a dislocation, was identified. Thus, all of the four types of TSD predicted by Onda et al. [Phil. Mag. Lett. 93 (2013) p.591] were identified.     

Extended energy-loss fine structure analysis of 3d transition metals using L ionization edges

The extended energy-loss fine structure (EXELFS) in electron-energy-loss spectroscopy has been compared experimentally with the extended X-ray absorption fine structure (EXAFS), for determining local structure around 3d transition metals. Since the EXELFS spectrum is acquired in an analytical transmission electron microscope, the probing beam can be focused to offer a lateral spatial resolution in the nanometre range, which is several orders of magnitude better than that of X-rays. Also, the microscope allows the area of interest to be imaged and analysed by other methods. However, difficulties in the analysis of EXELFS of 3d transition metals arise from the overlap of the edges in the accessible L series. EXELFS of L ionization edges of crystalline Ni and Cu were examined as test specimens. The overlapped L₁,L₂ and L₃ edges were separated and compared with the K-edge EXAFS of the same samples. The first- and second-nearest-neighbour distances from EXELFS are in agreement with those measured from EXAFS and X-ray diffraction. As a structural probe, however, the accuracy of quantitative analysis of EXAFS is still superior since the L-edge EXELFS decays more rapidly than K-edge EXAFS.     

A Negationless Interpretation of Intuitionistic Theories. I

The present work contains an axiomatic treatment of some parts of the restricted version of intuitionistic mathematics advocated by G. F. C. Griss, also known as negationless intuitionistic mathematics. Formal systems NPC, NA, and FIM ^N for negationless predicate logic, arithmetic, and analysis are proposed. Our Theorem 4 in Section 2 asserts the translatability of Heyting's arithmetic HAinto NA. The result can in fact be extended to a large class of intuitionistic theories based on HAand their negationless counterparts. For instance, in Section 3 this is shown for Kleene's system of intuitionistic analysis FIMand our FIM ^N . This revised version was published online in June 2006 with corrections to the Cover Date.     

Evidence of enhanced self-organization in the work-hardening stage V of fcc metals

Stages IV and V of the stress-strain curves of fcc metals, τ = τ (γ), are shownto be clearly discernible in the measurements of Taylor and Quinney onpolycrystalline copper strongly deformed at room temperature. The differentialstored-energy fraction determined by these workers may serve as an indicator ofthe degree of self-organization during the deformation process and hence of thedislocation mechanisms involved. In the data there is no indication of a change inmechanism at the stress at which the slope of the work-hardening curve becomesstrain independent, conventionally associated with the onset of stage IV. Only athigher stresses is there evidence of enhanced self-organization and hence of amechanism that diOEers significantly from stage-III hardening. It is argued thatthe enhanced self-organization is related to the onset of stage V, characterized byvanishing work-hardening. In this context, a basic difficulty in obtainingquantitative information on stage V from the widely used torsion tests ispointed out.     

The orientation and temperature dependences of dislocation velocity in Ni 3 Al single crystals

The average velocities of screw dislocations in Ni 3 Al single crystals have been directly measured as a function of resolved shear stress (RSS) and orientation in the temperature domain of the flow stress anomaly using the etch-pit technique. The velocity was found to be extremely sensitive to the RSS in all cases. In contrast with ordinary metals, the screw dislocation velocities in Ni 3 Al show anomalous behaviour; under a constant RSS, the velocities decrease dramatically with increasing temperature. Furthermore, the velocities and the tension-compression asymmetry of the velocities depend on the orientation of applied stress.     

Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations

High-speed heavy plastic deformation of thin foils of fcc metals, including aluminium, is found to produce a high density of small vacancy clusters, in the form of stacking-fault tetrahedra. The dependences of the density of the clusters on the deformation temperature and deformation rate indicate the production of vacancy clusters from deformation-induced dispersed vacancies. Neither dislocations nor any indication of the reaction of dislocations are present in the regions containing a high density of vacancy clusters. A possible model is proposed that describes, at extremely high strain rates where dislocation generation is difficult, how a high concentration of point defects is produced by a large number of parallel shifts of atomic planes without dislocations.     

Neurodevelopmental Disorders Across the Lifespan: A Neuroconstructivist Approach.

Objectives. To investigate neuropsychological and neurobehavioral outcome in children with arterial ischemic stroke (AIS).

Background. Childhood stroke can have consequences on motor, cognitive, and behavioral development. We present a cross-sectional study of neuropsychological and neurobehavioral outcome at least one year poststroke in a uniquely homogeneous sample of children who had experienced AIS.

Method. Forty-nine children with AIS aged 6 to 18 years were recruited from a specialist clinic. Neuropsychological measures of intelligence, reading comprehension, attention, and executive function were administered. A triangulation of data collection included questionnaires completed by the children, their parents, and teachers, rating behavior, executive functions, and emotions.

Key Findings. Focal neuropsychological vulnerabilities in attention (response inhibition and dual attention) and executive function were found, beyond general intellectual functioning, irrespective of hemispheric side of stroke. Difficulties with emotional and behavioral regulation were also found. Consistent with an “early plasticity” hypothesis, earlier age of stroke was associated with better performance on measures of executive function.

Conclusions. A significant proportion of children poststroke are at long-term risk of difficulties with emotional regulation, executive function, and attention. Data also suggest that executive functions are represented in widespread networks in the developing brain and are vulnerable to unilateral injury.