Phason-Dominated Thermal Transport in Fresnoite.

Fast-propagating waves in the phase of incommensurate structures, called phasons, have long been argued to enhance thermal transport. Although supersonic phason velocities have been observed, the lifetimes, from which mean free paths can be determined, have not been resolved. Using inelastic neutron scattering and thermal conductivity measurements, we establish that phasons in piezoelectric fresnoite make a major contribution to thermal conductivity by propagating with higher group velocities and longer mean free paths than phonons. The phason contribution to thermal conductivity is maximum near room temperature, where it is the single largest contributing degree of freedom.

Observation of Magnon Polarization.

We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarization is a unique and unambiguous signature of magnets and is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. However, it has never been directly measured in any material until this work. The observation of both signs of magnon polarization in Y_{3}Fe_{5}O_{12} also gives direct proof of its ferrimagnetic nature. The experiments agree very well with atomistic simulations of the scattering cross section.

Anisotropic Exchange within Decoupled Tetrahedra in the Quantum Breathing Pyrochlore Ba_{3}Yb_{2}Zn_{5}O_{11}.

The low energy spin excitation spectrum of the breathing pyrochlore Ba_{3}Yb_{2}Zn_{5}O_{11} has been investigated with inelastic neutron scattering. Several nearly resolution limited modes with no observable dispersion are observed at 250 mK while, at elevated temperatures, transitions between excited levels become visible. To gain deeper insight, a theoretical model of isolated Yb^{3+} tetrahedra parametrized by four anisotropic exchange constants is constructed. The model reproduces the inelastic neutron scattering data, specific heat, and magnetic susceptibility with high fidelity. The fitted exchange parameters reveal a Heisenberg antiferromagnet with a very large Dzyaloshinskii-Moriya interaction. Using this model, we predict the appearance of an unusual octupolar paramagnet at low temperatures and speculate on the development of intertetrahedron correlations.

Implementation of a laser-neutron pump-probe capability for inelastic neutron scattering.

Knowledge about nonequilibrium dynamics in spin systems is of great importance to both fundamental science and technological applications. Inelastic neutron scattering (INS) is an indispensable tool to study spin excitations in complex magnetic materials. However, conventional INS spectrometers currently only perform steady-state measurements and probe averaged properties over many collision events between spin excitations in thermodynamic equilibrium, while the exact picture of re-equilibration of these excitations remains unknown. In this paper, we report on the design and implementation of a time-resolved laser-neutron pump-probe capability at hybrid spectrometer (beamline 14-B) at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. This capability allows us to excite out-of-equilibrium magnons with a nanosecond pulsed laser source and probe the resulting dynamics using INS. Here, we discussed technical aspects to implement such a capability in a neutron beamline, including choices of suitable neutron instrumentation and material systems, laser excitation scheme, experimental configurations, and relevant firmware and software development to allow for time-synchronized pump-probe measurements. We demonstrated that the laser-induced nonequilibrium structure factor is able to be resolved by INS in a quantum magnet. The method developed in this work will provide SNS with advanced capabilities for performing out-of-equilibrium measurements, opening up an entirely new research direction to study out-of-equilibrium phenomena using neutrons.

A flexible neutron spectrometer concept with a new ultra-high field steady-state vertical-bore magnet.

The proposed facility explores materials under ultra-high magnetic fields. By combining the power of high fields to tune materials and of neutron scattering to probe the resulting changes down to the atomic scale, this facility will enable transformative progress in the study of quantum materials and is named for the "TITAN" subset of Greek gods to reflect this transformation. TITAN will offer DC magnetic fields up to at least 20 T. Exploiting the record brightness and bandwidth of the Second Target Station at the Spallation Neutron Source, TITAN will probe atomic-scale responses through high efficiency neutron spectroscopy up to 80 meV energy transfer, high resolution diffraction, and small angle neutron scattering. Focusing neutron optics will maximize flux on accurately positioned samples, while radial collimation and optimized shielding and detection strategies will minimize backgrounds.

Multi-extreme conditions at the Second Target Station.

Three concepts for the application of multi-extreme conditions under in situ neutron scattering are described here. The first concept is a neutron diamond anvil cell made from a non-magnetic alloy. It is shrunk in size to fit existing magnets and future magnet designs and is designed for best pressure stability upon cooling. This will allow for maximum pressures above 10 GPa to be applied simultaneously with (steady-state) high magnetic field and (ultra-)low temperature. Additionally, an implementation of miniature coils for neutron diamond cells is presented for pulsed-field applications. The second concept presents a set-up for laser-heating a neutron diamond cell using a defocused CO2 laser. Cell, anvil, and gasket stability will be achieved through stroboscopic measurements and maximum temperatures of 1500 K are anticipated at pressures to the megabar. The third concept presents a hybrid levitator to enable measurements of solids and liquids at temperatures in excess of 4000 K. This will be accomplished by a combination of bulk induction and surface laser heating and hyperbaric conditions to reduce evaporation rates. The potential for deployment of these multi-extreme environments within this first instrument suite of the Second Target Station is described with a special focus on VERDI, PIONEER, CENTAUR, and CHESS. Furthermore, considerations for deployment on future instruments, such as the one proposed as TITAN, are discussed. Overall, the development of these multi-extremes at the Second Target Station, but also beyond, will be highly advantageous for future experimentation and will give access to parameter space previously not possible for neutron scattering.

Lymphocyte migration into three-dimensional collagen matrices: a quantitative study.

Lymphocytes have been plated onto the surface of three-dimensional gels of native collagen fibers, and their distribution throughout the three-dimensional collagen matrix has been determined in a quantitative fashion at various times thereafter. Information regarding the total number of applied cells may be obtained by this means. Lymphocyte penetration into the collagen gel does not appear to involve the expression of collagenolytic activity, nor does it require the presence of serum. Analysis of the kinetics of lymphocyte penetration into the gel matrix indicates that lymphocytes are migrating in a "random-walk" fashion. Our objective has been to establish a model system for studying the cell-matrix and cell-cell interactions which influence the pattern of lymphocyte recirculation in vivo and the results presented here are discussed in this context.

Spin-liquid-like state in pure and Mn-doped TbInO3 with a nearly triangular lattice.

Inelastic neutron scattering studies in single crystals of TbInO3 and TbIn0.95Mn0.05O3 with nearly triangular antiferromagnetic lattice are reported. At low energies, a broad and apparently gapless continuum of magnetic excitations, located at the triangular lattice (TL) Brillouin zone boundary, is observed. The data are well described by the uncorrelated nearest-neighbor valence bonds model. At higher energies, a broad excitation branch dispersing from the TL zone boundary is observed. No signs of static magnetic order are found down to the temperatures two orders of magnitude smaller than the effective interaction energy. The fluctuating magnetic moment exceeds two-thirds of the Tb3+ free-ion value and is confined to the TL plane. These observations are consistent with a TL-based spin liquid state in TbInO3.

Continuum of quantum fluctuations in a three-dimensional S = 1 Heisenberg magnet.

Conventional crystalline magnets are characterized by symmetry breaking and normal modes of excitation called magnons, with quantized angular momentum ℏ. Neutron scattering correspondingly features extra magnetic Bragg diffraction at low temperatures and dispersive inelastic scattering associated with single magnon creation and annihilation. Exceptions are anticipated in so-called quantum spin liquids, as exemplified by the one-dimensional spin-1/2 chain, which has no magnetic order and where magnons accordingly fractionalize into spinons with angular momentum ℏ/2. This is spectacularly revealed by a continuum of inelastic neutron scattering associated with two-spinon processes. Here, we report evidence for these key features of a quantum spin liquid in the three-dimensional antiferromagnet NaCaNi2F7. We show that despite the complication of random Na1+-Ca2+ charge disorder, NaCaNi2F7 is an almost ideal realization of the spin-1 antiferromagnetic Heisenberg model on a pyrochlore lattice. Magnetic Bragg diffraction is absent and 90% of the neutron spectral weight forms a continuum of magnetic scattering with low-energy pinch points, indicating NaCaNi2F7 is in a Coulomb-like phase. Our results demonstrate that disorder can act to freeze only the lowest-energy magnetic degrees of freedom; at higher energies, a magnetic excitation continuum characteristic of fractionalized excitations persists.

Interpreting the multifocal visual evoked potential: the effects of refractive errors, cataracts, and fixation errors.

AIM: To understand how refractive errors, cataracts, and fixation errors affect multifocal visual evoked potential (mfVEP) responses. METHODS: Monocular mfVEP responses were obtained using a pattern reversal dartboard display. For the control condition, visual acuity was corrected to > or =20/20 and foveal fixation was maintained. The right eye was tested under the following conditions: simulated refractive error, simulated cataract, steady eccentric fixation, and unsteady fixation. RESULTS: No subject demonstrated significant abnormalities under control conditions. For the simulated refractive error condition, significant centrally located abnormalities were seen for all subjects. For the simulated cataract condition, significant abnormalities were found for three subjects. The steady eccentric fixation condition yielded abnormalities in both eyes for all subjects while the unsteady fixation condition yielded significant central abnormalities in the tested eye. With eccentric and unsteady fixation conditions, all subjects had at least one sector with a waveform polarity reversal. CONCLUSIONS: While the mfVEP is a useful tool for identifying local optic nerve damage or ruling out non-organic aetiology of visual field defects, factors such as uncorrected refractive errors, cataract, eccentric fixation, and unsteady fixation can produce apparent field defects on the mfVEP. With care, these problems can be correctly identified.

Multifocal visual evoked potentials to cone specific stimuli in patients with retinitis pigmentosa.

Our aim was to determine whether patients with retinitis pigmentosa show differences in L- and M-cone multifocal visual evoked potential (mfVEP) responses that are eccentricity dependent, as has been shown for control subjects. Second, we compared the losses for mfVEPs to losses on achromatic visual field and multifocal electroretinogram (mfERG) measures in the patients. Monocular mfVEPs were recorded to a pattern reversing display that modulated only the L- or M-cones. Also, standard automated achromatic visual fields and mfERGs were obtained. For the control subjects, the ratio of L-cone to M-cone mfVEP amplitudes increased as a function of retinal eccentricity. For the patients, the ratio did not vary with eccentricity. For all measures, responses were least affected for the first ring (central 2.4 degrees ) and most affected for the third ring (11.6 degrees - 44.4 degrees ). For the first ring, mfERG amplitudes were more impaired than were the mfVEPs or the visual field thresholds. For most of the patients, there was local response correspondence among our measures of visual function.

Designing and evaluating Brain Powered Games for cognitive training and rehabilitation in at-risk African children.

BACKGROUND: Valid, reliable, accessible, and cost-effective computer-training approaches can be important components in scaling up educational support across resource-poor settings, such as sub-Saharan Africa. The goal of the current study was to develop a computer-based training platform, the Michigan State University Games for Entertainment and Learning laboratory's Brain Powered Games (BPG) package that would be suitable for use with at-risk children within a rural Ugandan context and then complete an initial field trial of that package. METHODS: After game development was completed with the use of local stimuli and sounds to match the context of the games as closely as possible to the rural Ugandan setting, an initial field study was completed with 33 children (mean age = 8.55 ± 2.29 years, range 6-12 years of age) with HIV in rural Uganda. The Test of Variables of Attention (TOVA), CogState computer battery, and the Non-Verbal Index from the Kaufman Assessment Battery for Children, 2nd edition (KABC-II) were chosen as the outcome measures for pre- and post-intervention testing. The children received approximately 45 min of BPG training several days per week for 2 months (24 sessions). RESULTS: Although some improvements in test scores were evident prior to BPG training, following training, children demonstrated clinically significant changes (significant repeated-measures outcomes with moderate to large effect sizes) on specific TOVA and CogState measures reflecting processing speed, attention, visual-motor coordination, maze learning, and problem solving. CONCLUSIONS: Results provide preliminary support for the acceptability, feasibility, and neurocognitive benefit of BPG and its utility as a model platform for computerized cognitive training in cross-cultural low-resource settings.

Dynamic accommodation and myopia.

PURPOSE: Accommodative effort during nearwork is thought to be a causative factor in the development of myopia. It has been proposed that an anomaly in autonomic control may be a precursor to the development of myopia. In the present study the closed-loop accommodation response after variations in fixation period was investigated in emmetropes, early-onset myopes and late-onset myopes to determine characteristics of reflex accommodation for each refractive group. METHODS: Closed-loop accommodation responses were measured in a group of emmetropes (n = 7), early-onset myopes (n = 7), and late-onset myopes (n = 7) by use of a dynamic tracking infrared optometer. A variation in fixation period (10 seconds, 60 seconds, and 180 seconds) before an accommodative step was used to stimulate the accommodation control mechanism differentially. RESULTS: Group results of accommodative response times showed that late-onset myopes were significantly affected by the duration of fixation before the change in stimulus vergence. Accommodative response times after 3 minutes of sustained near vision were significantly longer than those observed for other groups for the near-to-far condition. Reaction time appears to be independent of refractive grouping, prior fixation period, and direction of step change. CONCLUSIONS: Late-onset myopes showed significantly extended accommodation response times after a sustained near vision task that was demonstrable under well-controlled experimental conditions. The extended response times observed in the present study were consistent with previous reports of refractive shifts in late-onset myopes and early-onset myopes and provide a corollary between reflex and adaptive components of the accommodation response. Potential mechanisms are discussed in an attempt to explain the resultant hysteresis under closed-loop viewing conditions.

Human dynamic closed-loop accommodation augmented by sympathetic inhibition.

PURPOSE: A ciliary alpha-adrenoceptor accommodative effect has been proposed, caused by a small population of alpha1-inhibitory receptors in excised human ciliary muscle. This study was intended to investigate the effect on the closed-loop dynamic accommodative process of modulating alpha1-adrenoceptor activity by topical instillation of the alpha1-adrenergic agonist, phenylephrine hydrochloride. METHODS: A group of 10 visually normal subjects viewed a photopic (30 candela/m2) high-contrast Maltese cross, which was modulated sinusoidally (0.05-0.6Hz) and stepwise over a 2-D range (2-4 D). Monocular temporal accommodation responses were measured using a continuously recording dynamic tracking infrared optometer under two trial conditions: after instillation of saline control solution and 50 minutes subsequent to the instillation of 0.27 microl 0.4% benoxinate hydrochloride and 0.27 microl 2.5% phenylephrine hydrochloride. Pupil size and accommodative amplitude were measured at 90-second intervals for 50 minutes after drug instillation. All accommodative measurements were recorded through a fixed 4-mm pupil. RESULTS: A significant reduction in accommodative amplitude (11%; P < 0.05) was recorded, whereas pupil size showed a significant increase (33%; P < 0.05). No significant change in step-response dynamics was observed. However, phenylephrine hydrochloride caused a significant increase in accommodative gain in the low and midtemporal frequency ranges compared with the effect of a saline control treatment. No significant variation in phase lag was observed. CONCLUSIONS: For the first time in humans, this study shows that augmentation of the alpha1-inhibitory sympathetic contribution results in increased accommodative gain at low and midtemporal frequencies, which is consistent with findings in animal studies.

Assessment of retinal-neural function before neodymium:YAG laser capsulotomy.

PURPOSE: The aim of this study was to evaluate a clinical test of hyperacuity in the assessment of retinal-neural function in patients with posterior capsular opacification. METHODS: Neodymium (Nd):YAG laser capsulotomy was performed on 39 subjects (mean age, 76.72 years +/- 10.41 years). Measurements of refractive error, logMAR acuity, and displacement threshold hyperacuity (DTH) were made before and 3 weeks after Nd:YAG therapy. The DTH task involved measurement of the smallest detectable displacement of an object relative to two stationary references. In addition, an independent fundus examination was performed before and after therapy to determine the presence of retinal disease. By ophthalmoscopic examination, a blind protocol was adopted for the classification of subjects as normal or as having retinal disease. RESULTS: Preoperative measures of logMAR visual acuity were of no value in distinguishing between patients with retinal disease and normals (P > 0.1) and were a poor indicator of postoperative logMAR acuity (r2 = 0.2). Preoperative DTH could be used to distinguish patients with retinal disease from normals (P < 0.005) and were found to be correlated with measures of postoperative logMAR acuity (r2 = 0.4). Preoperative DTH correlated well with postoperative DTH (r2 = 0.7), which is consistent with its resistance to optical image degradation. CONCLUSION: The results of this study indicate that DTH is of value in the presurgical assessment of visual function in patients with media opacification if adequate fundus examination is not possible.

Factors affecting light-adapted pupil size in normal human subjects.

PURPOSE: To investigate the effect of age, gender, refractive error, and iris color on light-adapted pupil size in humans. METHODS: Pupil diameters of 91 subjects (age range, 17 to 83 years) with normal, healthy eyes were measured using an objective infrared-based continuous recording technique. Five photopic ocular illuminance levels were used (2.15 to 1050 lumens m-2), and the accommodative status of each subject was precisely controlled at a constant level. RESULTS: Pupil size decreased linearly as a function of age at all illuminance levels. Even at the highest illuminance level, there was still a significant effect of age upon pupil size. The rate of change of pupil diameter with age decreased from 0.043 mm per year at the lowest illuminance level to 0.015 mm per year at the highest. In addition, the variability between pupil sizes of subjects of the same age decreased by a factor of approximately two as luminance was increased over the range investigated. Pupil size was found to be independent of gender, refractive error, or iris color (P > 0.1). CONCLUSIONS: Of the factors investigated, only chronologic age had a significant effect on the size of the pupil. The phenomenon of senile miosis is present over a wide range of ocular illuminance levels.

Objective concurrent measures of open-loop accommodation and vergence under photopic conditions.

PURPOSE: To investigate the relationship between photopic open-loop accommodation and vergence and the effect of mental effort on these positions. METHODS: Twenty subjects (11 men and 9 women) viewed monocularly a photopic (25 cdm-2), high-contrast (90%) Maltese cross-target for 3 minutes, through a 0.5-mm pinhole drilled into an infrared filter. Accommodation was measured objectively at 1-second intervals using a Canon Autoref R-1 infrared optometer, and vergence was recorded continuously and objectively using a differential infrared eye tracking system. RESULTS: Under passive viewing conditions there was a significant correlation between photopic open-loop accommodation and vergence (R = 0.671, P = 0.0012); for the majority of subjects the imposition of mental effort shifted the passive levels of both open-loop accommodation and vergence, but these shifts were unsystematic and uncorrelated (R = 0.259, P = 0.270). The active open-loop positions of accommodation and vergence were also found to be correlated (R = 0.692, P = 0.0007). CONCLUSIONS: The influence of proximal stimuli can explain the correlations identified between photopic open-loop positions of accommodation and vergence. The uncorrelated responses of the accommodation and vergence systems to mental effort are likely to reflect interactions between various spatiotopic stimuli including mental effort and perceived proximity.

Functional visual loss in amblyopia and the effect of occlusion therapy.

PURPOSE: The aim of this study was to define the nature of functional visual loss in amblyopia and to identify those subjects whose amblyopia is chiefly due to one or more of the following deficits: abnormal contour interaction, abnormal eye movements, abnormal contrast perception, or positional uncertainty. METHODS: Fifty amblyopic children with a mean age of 5.6+/-1.3 years were referred from diverse sources. In addition to routine orthoptic and optometric evaluation the principal visual deficits in the amblyopic eye of each subject were identified using the following measures of visual acuity: high contrast linear, single optotype, repeat letter and low contrast linear, plus Vernier and displacement thresholds. These measures were repeated as the children underwent a prescribed occlusion therapy regime, after parental consent. RESULTS: All amblyopic subjects demonstrated a functional loss in each of the tests used, and occlusion therapy appeared to improve all aspects of the amblyopia. High contrast visual acuity was not always the primary deficit in visual function, and when amblyopic subjects were divided according to their primary visual loss, this visual function was found to show the greatest improvement with treatment. CONCLUSIONS: These results suggest that to successfully identify the primary visual deficit and monitor the success of occlusion therapy it is necessary to assess other aspects of visual function in amblyopia.

Accommodative microfluctuations and pupil diameter.

The nominally steady-state accommodation response exhibits temporal variations which can be characterized by two dominant regions of activity; a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.0 < or = HFC < or = 2.1 Hz). There is no consensus as to the relative contribution made by each of the frequency components of the microfluctuations to the control of steady-state accommodation. We investigate the effect of variations in artificial pupil diameter (0.5, 1, 2, 3, 4 and 5 mm pupils) on the microfluctuations of accommodation, while three young emmetropic subjects view, monocularly, a photopic high contrast Maltese cross target placed at a dioptric distance equal to their open-loop accommodation level. Average power spectra were calculated for five accommodation signals, each of 10 sec duration, collected for each viewing condition at a sampling rate of 102.4 Hz using a continuously recording infrared objective optometer. For artificial pupil diameters < or = 2 mm the power of the LFC was found to increase as a function of reducing pupil diameter, while for artificial pupil diameters > 2 mm the LFC was found to be relatively constant. No systematic change in the HFC with varying artificial pupil diameter was observed. Changes in the root-mean-square (r.m.s.) value of the fluctuations with varying pupil diameter were significant (one-way ANOVA, F = 8.507, P = 0.0001, d.f. = 89) and showed a similar form to the changes in the LFC.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of pupil response to blur-driven accommodation.

The drive to the pupil constriction associated with near fixation has generally been attributed to accommodation with convergence and fusional convergence having secondary roles. However, our previous investigations have shown that significant changes in accommodation can take place without concomitant pupil response. To investigate further, the present study recorded pupil and accommodation responses to a blur-only accommodative stimulus using a target moved sinusoidally at a range of temporal frequencies. Care was taken to minimise target size change and apparent lateral or vertical target displacement. Results show that pupil response could be very much reduced or absent irrespective of stimulus temporal frequency and despite maintained accommodation response. The results suggest that blur-driven accommodation alone is not sufficient to drive pupil near response and that the presence of cues such as size change and lateral or vertical displacement of an approaching object may be necessary to elicit a response.