228 research outputs found
Crystal Field, Magnetic Anisotropy and Excitations in Rare-Earth Hexaborides
We clarify the role of crystalline electric field (CEF) induced magnetic
anisotropy in the ground state and spin-wave spectrum of cubic rare-earth
materials with dominating isotropic magnetic exchange interactions. In
particular we study the hexaboride NdB_6 which is shown to exhibit strong
spin-quadrupolar coupling. The CEF scheme is analyzed and a non-collinear
magnetization response is found. The spin orientation in the
antiferromagnetically ordered ground-state is identified. Moreover, the spin
excitations are evaluated and in agreement with inelastic neutron scattering a
suppression of one of the two magnetic modes in the strong-coupling regime is
predicted.Comment: 4 pages, 1 eps-figur
Non-Collinear Magnetism due to Orbital Degeneracy and Multipolar Interactions
The origin of non-collinear magnetism under quadrupolar ordering is
investigated with CeB6 taken as a target system. The mode-mixing effect among
15 multipoles is analyzed based on the Ginzburg-Landau free energy. Then the
lower magnetic transition temperature and the order parameters are derived
within the mean-field approximation. In the presence of pseudo-dipole-type
interactions for the next-nearest neighbors, the observed pattern of
non-collinear ordering is indeed stabilized for certain set of interaction
parameters. The stability of the phase III' in the magnetic field is also
explained, which points to the importance of the next-nearest-neighbor
octupole-octupole interaction. Concerning the phase IV in CexLa1-xB6 with x ~
0.75, a possibility of pure octupole ordering is discussed based on slight
modifications of the strength of interactions.Comment: 12 pages, 7 figures, 3 tables, to appear in J. Phys. Soc. Jpn. 70 (6)
(2001
Interpocket polarization model for magnetic structures in rare-earth hexaborides
The origin of peculiar magnetic structures in cubic rare-earth (R)
hexaborides RB_6 is traced back to their characteristic band structure. The
three sphere-like Fermi surfaces induce interpocket polarization of the
conduction band as a part of a RKKY-type interaction. It is shown for the
free-electron-like model that the interpocket polarization gives rise to a
broad maximum in the intersite interaction I(q) around q=(1/4,1/4,1/2) in the
Brillouin zone. This maximum is consistent with the superstructure observed in
R=Ce, Gd and Dy. The wave-number dependence of I(q) is independently extracted
from analysis of the spin-wave spectrum measured for NdB_6. It is found that
I(q) obtained from fitting the data has a similarly to that derived by the
interpocket polarization model, except that the absolute maximum now occurs at
(0,0,1/2) in consistency with the A-type structure. The overall shape of I(q)
gives a hint toward understanding an incommensurate structure in PrB_6 as well.Comment: 5 pages, 3 figures, submitted to J.Phys.Soc.Jp
Fermi Surface Properties of Low Concentration CeLaB: dHvA
The de Haas-van Alphen effect is used to study angular dependent extremal
areas of the Fermi Surfaces (FS) and effective masses of CeLaB alloys for between 0 and 0.05. The FS of these alloys was previously
observed to be spin polarized at low Ce concentration ( = 0.05). This work
gives the details of the initial development of the topology and spin
polarization of the FS from that of unpolarized metallic LaB to that of
spin polarized heavy Fermion CeB .Comment: 7 pages, 9 figures, submitted to PR
Multipolar Interactions in the Anderson Lattice with Orbital Degeneracy
Microscopic investigation is performed for intersite multipolar interactions
in the orbitally degenerate Anderson lattice, with CeB taken as an
exemplary target. In addition to the intermediate state,
Hund's-rule ground states are included as intermediate states for the
interactions. The conduction-band states are taken as plane waves and the
hybridization as spherically symmetric. The spatial dependences of multipolar
interactions are given by the relative weight of partial wave components along
the pair of sites. It is clarified how the the anisotropy arises in the
interactions depending on the orbital degeneracy and the spatial configuration.
The stability of the antiferro-quadrupole order in the phase II of
CeB is consistent with our model. Moreover, the pseudo-dipole interactions
follow a tendency required by the phenomenological model for the phase III.Comment: 30 pages, 4 figure
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Accommodation and vergence response gains to different near cues characterize specific esotropias
Aim. To describe preliminary findings of how the profile of the use of blur, disparity and proximal cues varies between non-strabismic groups and those with different types of esotropia.
Design. Case control study
Methodology. A remote haploscopic photorefractor measured simultaneous convergence and accommodation to a range of targets containing all combinations of binocular disparity, blur and proximal (looming) cues. 13 constant esotropes, 16 fully accommodative esotropes, and 8 convergence excess esotropes were compared with age and refractive error matched controls, and 27 young adult emmetropic controls. All wore full refractive correction if not emmetropic. Response AC/A and CA/C ratios were also assessed.
Results. Cue use differed between the groups. Even esotropes with constant suppression and no binocular vision (BV) responded to disparity in cues. The constant esotropes with weak BV showed trends for more stable responses and better vergence and accommodation than those without any BV. The accommodative esotropes made less use of disparity cues to drive accommodation (p=0.04) and more use of blur to drive vergence (p=0.008) than controls. All esotropic groups failed to show the strong bias for better responses to disparity cues found in the controls, with convergence excess esotropes favoring blur cues. AC/A and CA/C ratios existed in an inverse relationship in the different groups. Accommodative lag of >1.0D at 33cm was common (46%) in the pooled esotropia groups compared with 11% in typical children (p=0.05).
Conclusion. Esotropic children use near cues differently from matched non-esotropic children in ways characteristic to their deviations. Relatively higher weighting for blur cues was found in accommodative esotropia compared to matched controls
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A demonstration of 'broken' visual space
It has long been assumed that there is a distorted mapping between real and ‘perceived’ space, based on demonstrations of systematic errors in judgements of slant, curvature, direction and separation. Here, we have applied a direct test to the notion of a coherent visual space. In an immersive virtual environment, participants judged the relative distance of two squares displayed in separate intervals. On some trials, the virtual scene expanded by a factor of four between intervals although, in line with recent results, participants did not report any noticeable change in the scene. We found that there was no consistent depth ordering of objects that can explain the distance matches participants made in this environment (e.g. A > B > D yet also A < C < D) and hence no single one-to-one mapping between participants’ perceived space and any real 3D environment. Instead, factors that affect pairwise comparisons of distances dictate participants’ performance. These data contradict, more directly than previous experiments, the idea that the visual system builds and uses a coherent 3D internal representation of a scene
Relative contributions to vergence eye movements of two binocular cues for motion-in-depth
When we track an object moving in depth, our eyes rotate in opposite directions. This type of "disjunctive" eye movement is called horizontal vergence. The sensory control signals for vergence arise from multiple visual cues, two of which, changing binocular disparity (CD) and inter-ocular velocity differences (IOVD), are specifically binocular. While it is well known that the CD cue triggers horizontal vergence eye movements, the role of the IOVD cue has only recently been explored. To better understand the relative contribution of CD and IOVD cues in driving horizontal vergence, we recorded vergence eye movements from ten observers in response to four types of stimuli that isolated or combined the two cues to motion-in-depth, using stimulus conditions and CD/IOVD stimuli typical of behavioural motion-in-depth experiments. An analysis of the slopes of the vergence traces and the consistency of the directions of vergence and stimulus movements showed that under our conditions IOVD cues provided very little input to vergence mechanisms. The eye movements that did occur coinciding with the presentation of IOVD stimuli were likely not a response to stimulus motion, but a phoria initiated by the absence of a disparity signal
Quadrupolar and magnetic ordering in CeB6
The quadrupolar ordering in CeB_6 is explained in terms of the electrostatic
interaction of quadrupolar moments arranged into a simple cubic lattice. The
representation of magnetic and quadrupolar moments by means of quasispins of
two kinds is employed. A linear increase of the quadrupolar transition
temperature T_Q(H) with applied magnetic field and its further re-entrance are
described using a generalized spherical model which is well adjusted to a
particular problem of the quadrupolar ordering in CeB_6. The theory naturally
explains the growing specific heat jump at T_Q(H) with increasing magnetic
field. The role of the quadrupolar ordering in the formation of the magnetic
ordering, as well as the possible critical experiments and applications to
other rare-earth compounds, are discussed.Comment: 40 pages, 9 Postscript figures, to appear in Phys.Rev.
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