315 research outputs found
Magnetism of PdNi alloys near the critical concentration for ferromagnetism
We report results of a muon spin rotation and relaxation (SR) study of
dilute PdNi alloys, with emphasis on Ni concentrations 0.0243
and 0.025. These are close to the critical value for the onset
of ferromagnetic long-range order (LRO), which is a candidate for a quantum
critical point. The 2.43 and 2.5 at.% Ni alloys exhibit similar SR
properties. Both samples are fully magnetic, with average muon local fields
2.0 and 3.8 mT and Curie temperatures
1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively, at . The
temperature dependence of suggests ordering of
Ni spin clusters rather than isolated spins. Just above a two-phase
region is found with separate volume fractions of quasistatic short-range order
(SRO) and paramagnetism. The SRO fraction decreases to zero with increasing
temperature a few kelvin above . This mixture of SRO and paramagnetism is
consistent with the notion of an inhomogeneous alloy with Ni clustering. The
measured values of extrapolate to = 0.0236 0.0027.
The dynamic muon spin relaxation in the vicinity of differs for the two
samples: a relaxation-rate maximum at is observed for = 0.0243,
reminiscent of critical slowing down, whereas for 0.025 no dynamic
relaxation is observed within the SR time window. The data suggest a
mean-field-like transition in this alloy.Comment: 15 pages, 15 figures, to be published in Phys. Rev.
The development and general morphology of the telencephalon of actinopterygian fishes: synopsis, documentation and commentary
The Actinopterygii or ray-finned fishes comprise, in addition to the large superorder of teleosts, four other superorders, namely the cladistians, the chondrosteans, the ginglymodes, and the halecomorphs, each with a limited number of species. The telencephalon of actinopterygian fishes differs from that in all other vertebrates in that it consists of a pair of solid lobes. Lateral ventricles surrounded by nervous tissue are entirely lacking. At the end of the nineteenth century, the theory was advanced that the unusual configuration of the forebrain in actinopterygians results from an outward bending or eversion of its lateral walls. This theory was accepted by some authors, rejected or neglected by others, and modified by some other authors. The present paper is based on the data derived from the literature, complemented by new observations on a large collection of histological material comprising specimens of all five actinopterygian superorders. The paper consists of three parts. In the first, a survey of the development of the telencephalon in actinopterygian fishes is presented. The data collected show clearly that an outward bending or eversion of the pallial parts of the solid hemispheres is the principal morphogenetic event in all five actinopterygian superorders. In all of these superorders, except for the cladistians, eversion is coupled with a marked thickening of the pallial walls. In the second part, some aspects of the general morphology of the telencephalon in mature actinopterygians are highlighted. It is pointed out that (1) the degree of eversion varies considerably among the various actinopterygian groups; (2) eversion leads to the transformation of the telencephalic roof plate into a wide membrane or tela choroidea, which is bilaterally attached to the lateral or ventrolateral aspect of the solid hemispheres; (3) the lines of attachment or taeniae of the tela choroidea form the most important landmarks in the telencephalon of actinopterygians, indicating the sites where the greatly enlarged ventricular surface of the hemispheres ends and its reduced meningeal surface begins; (4) the meningeal surface of the telencephalon shows in most actinopterygians bilaterally a longitudinally oriented sulcus externus, the depth of which is generally positively correlated with the degree of eversion; (5) a distinct lateral olfactory tract, occupying a constant topological position close to the taenia, is present in all actinopterygians studied; and (6) this tract is not homologous to the tract of the same name in the evaginated and inverted forebrains of other groups of vertebrates. In the third and final section, the concept that the structural organization of the pallium in actinopterygians can be fully explained by a simple eversion of its walls, and the various theories, according to which the eversion is complicated by extensive shifts of its constituent cell groups, are discussed and evaluated. It is concluded that there are no reasons to doubt that the pallium of actinopterygian fishes is the product of a simple and complete eversion
Spin dynamics in a structurally ordered non-Fermi liquid compound: YbRh_2Si_2
Muon spin relaxation (muSR) experiments have been carried out at low
temperatures in the non-Fermi-liquid heavy-fermion compound YbRh_2Si_2. The
longitudinal-field muSR relaxation function is exponential, indicative that the
dynamic spin fluctuations are homogeneous. The relaxation rate 1/T_1 varies
with applied field as H^{-y}, y = 1.0 \pm 0.1, which implies a scaling law of
the form \chi''(\omega) \propto \omega^{-y} f(\omega/T), \lim_{x\to0} f(x) = x
for the dynamic spin susceptibility.Comment: 5 pages, 2 figures. To be published in proceedings of musr2002
(Physica B
Challenging empowerment: AIDS-affected southern African children and the need for a multi-level relational approach
Critics of empowerment have highlighted the concept's mutability, focus on individual transformation, one-dimensionality and challenges of operationalisation. Relating these critiques to children's empowerment raises new challenges. Drawing on scholarship on children's subjecthood and exercise of power, alongside empirical research with children affected by AIDS, I argue that empowerment envisaged as individual self-transformation and increased capacity to act independently offers little basis for progressive change. Rather it is essential to adopt a relational approach that recognises the need to transform power relationships at multiple levels. This analysis has implications for our wider understanding of empowerment in the 21st century. © The Author(s) 2013.This research was funded by DFID
Spin glass behavior in URh_2Ge_2
URh_2Ge_2 occupies an extraordinary position among the heavy-electron
122-compounds, by exhibiting a previously unidentified form of magnetic
correlations at low temperatures, instead of the usual antiferromagnetism. Here
we present new results of ac and dc susceptibilities, specific heat and neutron
diffraction on single-crystalline as-grown URh_2Ge_2. These data clearly
indicate that crystallographic disorder on a local scale produces spin glass
behavior in the sample. We therefore conclude that URh_2Ge_2 is a 3D
Ising-like, random-bond, heavy-fermion spin glass.Comment: 10 pages, RevTeX, with 4 postscript figures, accepted by Physical
Review Letters Nov 15, 199
Disorder, inhomogeneity and spin dynamics in f-electron non-Fermi liquid systems
Muon spin rotation and relaxation (SR) experiments have yielded evidence
that structural disorder is an important factor in many f-electron-based
non-Fermi-liquid (NFL) systems. Disorder-driven mechanisms for NFL behaviour
are suggested by the observed broad and strongly temperature-dependent SR
(and NMR) linewidths in several NFL compounds and alloys. Local disorder-driven
theories (Kondo disorder, Griffiths-McCoy singularity) are, however, not
capable of describing the time-field scaling seen in muon spin relaxation
experiments, which suggest cooperative and critical spin fluctuations rather
than a distribution of local fluctuation rates. A strong empirical correlation
is established between electronic disorder and slow spin fluctuations in NFL
materialsComment: 24 pages, 15 figures, submitted to J. Phys.: Condens. Matte
29-Si NMR and Hidden Order in URu2Si2
We present new 29-Si NMR spectra in URu2Si2 for varying temperature T, and
external field H. On lowering T, the systematics of the low-field lineshape and
width reveal an extra component (lambda) to the linewidth below T_N ~ 17 K not
observed previously. We find that lambda is magnetic-field independent and
dominates the low-field lineshape for all orientations of H with respect to the
tetragonal c axis. The behavior of lambda indicates a direct relationship
between the 29-Si spin and the transition at T_N, but it is inconsistent with a
coupling of the nuclei to static antiferromagnetic order/disorder of the U-spin
magnetization. This leads us to conjecture that lambda is due to a coupling of
29-Si to the system's hidden-order parameter. A possible coupling mechanism
involving charge degrees of freedom and indirect nuclear spin/spin interactions
is proposed. We also propose further experiments to test for the existence of
this coupling mechanism.Comment: 4 pages, 4 figures, submitted to PR
Continuous-Time Quantum Monte Carlo Approach to Singlet-Triplet Kondo Systems
Dynamical properties are studied numerically for a variant of the Kondo model
with singlet and triplet crystalline electric field (CEF) levels where Kondo
and CEF singlets compete for the ground state. Using the continuous-time
quantum Monte Carlo method, we derive the -matrix of conduction electrons
and dynamical susceptibilities of local electrons without encountering the
negative sign problem. When the CEF splitting is comparable to the Kondo
temperature, the dynamical response has only a quasi-elastic peak.
Nevertheless, the local single-particle spectrum shows an energy gap in strong
contrast with the ordinary Kondo model.Comment: 9 pages, 10 figure
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