Magnetic properties of the Ising-like rare earth pyrosilicate: D-Er2Si2O7.

Ising-like spin-1/2 magnetic materials are of interest for their ready connection to theory, particularly in the context of quantum critical behavior. In this work we report detailed studies of the magnetic properties of a member of the rare earth pyrosilicate family, D-Er2Si2O7, which is known to display a highly anisotropic Ising-likeg-tensor and effective spin-1/2 magnetic moments. We used powder neutron diffraction, powder inelastic neutron spectroscopy (INS), and single crystal AC susceptibility to characterize its magnetic properties. Neutron diffraction enabled us to determine the magnetic structure below the known transition temperature (TN= 1.9 K) in zero field, confirming that the magnetic state is a four-sublattice antiferromagnetic structure with two non-collinear Ising axes, as was previously hypothesized. Our powder INS data revealed a gapped excitation at zero field, consistent with anisotropic (possibly Ising) exchange. An applied field of 1 T produces a mode softening, which is consistent with a field-induced second order phase transition. To assess the relevance of D-Er2Si2O7to the transverse field Ising model, we performed AC susceptibility measurements on a single crystal with the magnetic field oriented in the direction transverse to the Ising axes. This revealed a transition at 2.65 T at 0.1 K, a field significantly higher than the mode-softening field observed by powder INS, showing that the field-induced phase transitions are highly field-direction dependent as expected. These measurements suggest that D-Er2Si2O7may be a candidate for further exploration related to the transverse field Ising model.

Néel ordering in the distorted honeycomb pyrosilicate: C-Er2Si2O7.

The rare-earth pyrosilicate family of compounds (RE2Si2O7) hosts a variety of polymorphs, some with honeycomb-like geometries of the rare-earth sublattices, and the magnetism has yet to be deeply explored in many of the cases. Here we report on the ground state properties of C-Er2Si2O7. C-Er2Si2O7crystallizes in the C2/m space group and the Er3+atoms form a distorted honeycomb lattice in thea-bplane. We have utilized specific heat, DC susceptibility, and neutron diffraction measurements to characterize C-Er2Si2O7. Our specific heat and DC susceptibility measurements show signatures of antiferromagnetic ordering at 2.3 K. Neutron powder diffraction confirms this transition temperature and the relative intensities of the magnetic Bragg peaks are consistent with a collinear Néel state in the magnetic space group C2'/m, with ordered moment of 6.61µBcanted 13° away from thec-axis toward thea-axis. These results are discussed in relation to the isostructural quantum dimer magnet compound Yb2Si2O7.

Understanding Reentrance in Frustrated Magnets: The Case of the Er_{2}Sn_{2}O_{7} Pyrochlore.

Reentrance, the return of a system from an ordered phase to a previously encountered less-ordered one as a controlled parameter is continuously varied, is a recurring theme found in disparate physical systems, yet its microscopic cause is often not investigated thoroughly. Here, through detailed characterization and theoretical modeling, we uncover the microscopic mechanism behind reentrance in the strongly frustrated pyrochlore antiferromagnet Er_{2}Sn_{2}O_{7}. We use single crystal heat capacity measurements to expose that Er_{2}Sn_{2}O_{7} exhibits multiple instances of reentrance in its magnetic field B vs temperature T phase diagram for magnetic fields along three cubic high symmetry directions. Through classical Monte Carlo simulations, mean field theory, and classical linear spin-wave expansions, we argue that the origins of the multiple occurrences of reentrance observed in Er_{2}Sn_{2}O_{7} are linked to soft modes. These soft modes arise from phase competition and enhance thermal fluctuations that entropically stabilize a specific ordered phase, resulting in an increased transition temperature for certain field values and thus the reentrant behavior. Our work represents a detailed examination into the mechanisms responsible for reentrance in a frustrated magnet and may serve as a template for the interpretation of reentrant phenomena in other physical systems.

Evaluation of CpG-ODN-adjuvanted polyanhydride-based intranasal influenza nanovaccine in pigs.

Influenza results in significant economic loss in the swine industry each year. A broadly protective swine influenza vaccine would have the dual benefit of protecting pigs from influenza A viruses (IAVs) and limiting their possible zoonotic transmission to humans. In this study, we developed polyanhydride nanoparticles-based swine influenza vaccine (KAg + CpG-nanovaccine) co-encapsulating inacticated/killed soluble antigen (KAg) and Toll-like receptor (TLR)-9 agonist (CpG-ODN). The immunogenicity and protective efficacy of KAg + CpG-nanovaccine was compared with KAg vaccine containing five-times greater quantity of antigens following heterologous virus challenge. Prime-boost intranasally delivered KAg + CpG-nanovaccine induced significantly higher levels of cross-reactive antigen-specific IgA antibody responses in the nasal cavity, greater lymphoproliferative response in peripheral blood mononuclear cells (PBMCs), and higher IFN-γ secretion during antigen-induced recall responses of PBMCs and tracheobronchial lymph nodes cells compared to those immunized with KAg alone. Importantly, KAg + CpG-nanovaccine provided better protective efficacy through a significant reduction in influenza-induced fever, 16-fold reduction of nasal virus shedding and 80-fold reduction in lung virus titers compared to those immunized with soluble KAg. Our results indicated that CpG-ODN-adjuvanted polyanhydride nanovaccine can induce higher mucosal antibody and cellular immune responses in pigs; and provide better protection as compared with intranasally delivered soluble KAg.

Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb_{2}Si_{2}O_{7}.

The quantum dimer magnet (QDM) is the canonical example of quantum magnetism. The QDM state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced triplon Bose-Einstein condensate (BEC) phase. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb_{2}Si_{2}O_{7}. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet ground state at zero field with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of H_{c1}∼0.4 and H_{c2}∼1.4 T. Using inelastic neutron scattering in an applied magnetic field we observe a Goldstone mode (gapless to within δE=0.037 meV) that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well-known cases of this phase, the high-field (µ_{0}H≥1.2 T) part of the phase diagram in Yb_{2}Si_{2}O_{7} is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.

Quantum Spin Ice Dynamics in the Dipole-Octupole Pyrochlore Magnet Ce_{2}Zr_{2}O_{7}.

Neutron scattering measurements on the pyrochlore magnet Ce_{2}Zr_{2}O_{7} reveal an unusual crystal field splitting of its lowest J=5/2 multiplet, such that its ground-state doublet is composed of m_{J}=±3/2, giving these doublets a dipole-octupole (DO) character with local Ising anisotropy. Its magnetic susceptibility shows weak antiferromagnetic correlations with θ_{CW}=-0.4(2) K, leading to a naive expectation of an all-in, all-out ordered state at low temperatures. Instead, our low-energy inelastic neutron scattering measurements show a dynamic quantum spin ice state, with suppressed scattering near |Q|=0, and no long-range order at low temperatures. This is consistent with recent theory predicting symmetry-enriched U(1) quantum spin liquids for such DO doublets decorating the pyrochlore lattice. Finally, we show that disorder, especially oxidation of powder samples, is important in Ce_{2}Zr_{2}O_{7} and could play an important role in the low-temperature behavior of this material.

Behavior of the breathing pyrochlore lattice Ba3Yb2Zn5O11 in applied magnetic field.

The breathing pyrochlore lattice material Ba3Yb2Zn5O11 exists in the nearly decoupled limit, in contrast to most other well-studied breathing pyrochlore compounds. As a result, it constitutes a useful platform to benchmark theoretical calculations of exchange interactions in insulating Yb3+ magnets. Here we study Ba3Yb2Zn5O11 at low temperatures in applied magnetic fields as a further probe of the physics of this model system. Experimentally, we consider the behavior of polycrystalline samples of Ba3Yb2Zn5O11 with a combination of inelastic neutron scattering and heat capacity measurements down to 75 mK and up to fields of 10 T. Consistent with previous work, inelastic neutron scattering finds a level crossing near 3 T, but no significant dispersion of the spin excitations is detected up to the highest applied fields. Refinement of the theoretical model previously determined at zero field can reproduce much of the inelastic neutron scattering spectra and specific heat data. A notable exception is a low temperature peak in the specific heat at ∼0.1 K. This may indicate the scale of interactions between tetrahedra or may reflect undetected disorder in Ba3Yb2Zn5O11.

Ground state selection under pressure in the quantum pyrochlore magnet Yb2Ti2O7.

A quantum spin liquid is a state of matter characterized by quantum entanglement and the absence of any broken symmetry. In condensed matter, the frustrated rare-earth pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7, so-called spin ices, exhibit a classical spin liquid state with fractionalized thermal excitations (magnetic monopoles). Evidence for a quantum spin ice, in which the magnetic monopoles become long range entangled and an emergent quantum electrodynamics arises, seems within reach. The magnetic properties of the quantum spin ice candidate Yb2Ti2O7 have eluded a global understanding and even the presence or absence of static magnetic order at low temperatures is controversial. Here we show that sensitivity to pressure is the missing key to the low temperature behaviour of Yb2Ti2O7. By combining neutron diffraction and muon spin relaxation on a stoichiometric sample under pressure, we evidence a magnetic transition from a disordered, non-magnetic, ground state to a splayed ferromagnetic ground state.

Disordered Route to the Coulomb Quantum Spin Liquid: Random Transverse Fields on Spin Ice in Pr_{2}Zr_{2}O_{7}.

Inelastic neutron scattering reveals a broad continuum of excitations in Pr_{2}Zr_{2}O_{7}, the temperature and magnetic field dependence of which indicate a continuous distribution of quenched transverse fields (Δ) acting on the non-Kramers Pr^{3+} crystal field ground state doublets. Spin-ice correlations are apparent within 0.2 meV of the Zeeman energy. A random phase approximation provides an excellent account of the data with a transverse field distribution ρ(Δ)∝(Δ^{2}+Γ^{2})^{-1}, where Γ=0.27(1) meV. Established during high temperature synthesis due to an underlying structural instability, it appears disorder in Pr_{2}Zr_{2}O_{7} actually induces a quantum spin liquid.

Iridium containing honeycomb Delafossites by topotactic cation exchange.

We report the structure and magnetic properties of two new iridium-based honeycomb Delafossite compounds, Cu3NaIr2O6 and Cu3LiIr2O6, formed by a topotactic cation exchange reaction. The starting materials Na2IrO3 and Li2IrO3, which are based on layers of IrO6 octahedra in a honeycomb lattice separated by layers of alkali ions, are transformed to the title compounds by a topotactic exchange reaction through heating with CuCl below 450 °C; higher temperature reactions cause decomposition. The new compounds display dramatically different magnetic behavior from their parent compounds - Cu3NaIr2O6 has a ferromagnetic like magnetic transition at 10 K, while Cu3LiIr2O6 retains the antiferromagnetic transition temperature of its parent compound but displays significantly stronger dominance of antiferromagnetic coupling between spins. These results reveal that a surprising difference in the magnetic interactions between the magnetic Ir ions has been induced by a change in the non-magnetic interlayer species. A combination of neutron and X-ray powder diffraction is used for the structure refinement of Cu3NaIr2O6 and both compounds are compared to their parent materials.

Structure and magnetic properties of the spin-1/2-based honeycomb NaNi2BiO(6-δ) and its hydrate NaNi2BiO(6-δ)·1.7H2O.

We present the structure and magnetic properties of the honeycomb anhydrate NaNi2BiO6-δ and its monolayer hydrate NaNi2BiO6-δ·1.7H2O, synthesized by deintercalation of the layered α-NaFeO2-type honeycomb compound Na3Ni2BiO6. The anhydrate adopts ABAB-type oxygen packing and a one-layer hexagonal unit cell, whereas the hydrate adopts an oxygen packing sequence based on a three-layer rhombohedral subcell. The metal-oxide layer separations are 5.7 Å in the anhydrate and 7.1 Å in the hydrate, making the hydrate a quasi 2-D honeycomb system. The compounds were characterized through single crystal diffraction, powder X-ray diffraction, thermogravimetric analysis, and elemental analysis. Temperature-dependent magnetic susceptibility measurements show both to have negative Weiss temperatures (-18.5 and -14.6 K, respectively) and similar magnetic moments (2.21 and 2.26 µB/Ni, respectively), though the field-dependent magnetization and heat capacity data suggest subtle differences in their magnetic behavior. The magnetic moments per Ni are relatively high, which we suggest is due to the presence of a mixture of Ni(2+) and Ni(3+) caused by oxygen vacancies.

Order by disorder spin wave gap in the XY pyrochlore magnet Er2Ti2O7.

The recent determination of a robust spin Hamiltonian for the antiferromagnetic XY pyrochlore Er2Ti2O7 reveals a most convincing case of the "Order-by-Quantum-Disorder" mechanism for ground state selection. This mechanism relies on quantum fluctuations to remove an accidental symmetry of the magnetic ground state, and selects a particular ordered spin structure below TN=1.2 K. The removal of the continuous degeneracy results in an energy gap in the spectrum of spin wave excitations, long wavelength pseudo-Goldstone modes. We have measured the Order-by-Quantum-Disorder spin wave gap at a zone center in Er2Ti2O7, using low incident energy neutrons and the time-of-flight inelastic scattering method. We report a gap of Δ=0.053±0.006 meV, which is consistent with upper bounds placed on it from heat capacity measurements and roughly consistent with the theoretical estimate of ∼0.02 meV, further validating the spin Hamiltonian that led to that prediction. The gap is observed to vary with the square of the order parameter, and goes to zero for T∼TN.

What are you looking at? Moving toward an attentional timeline in insomnia: a novel semantic eye tracking study.

STUDY OBJECTIVES: To date, cognitive probe paradigms have been used in different guises to obtain reaction time measurements suggestive of an attention bias towards sleep in insomnia. This study adopts a methodology which is novel to sleep research to obtain a continual record of where the eyes-and therefore attention-are being allocated with regard to sleep and neutral stimuli. DESIGN: A head mounted eye tracker (Eyelink II,SR Research, Ontario, Canada) was used to monitor eye movements in respect to two words presented on a computer screen, with one word being a sleep positive, sleep negative, or neutral word above or below a second distracter pseudoword. Probability and reaction times were the outcome measures. PARTICIPANTS: Sleep group classification was determined by screening interview and PSQI (> 8 = insomnia, < 3 = good sleeper) score. MEASUREMENTS AND RESULTS: Those individuals with insomnia took longer to fixate on the target word and remained fixated for less time than the good sleep controls. Word saliency had an effect with longer first fixations on positive and negative sleep words in both sleep groups, with largest effect sizes seen with the insomnia group. CONCLUSIONS: This overall delay in those with insomnia with regard to vigilance and maintaining attention on the target words moves away from previous attention bias work showing a bias towards sleep, particularly negative, stimuli but is suggestive of a neurocognitive deficit in line with recent research.

Thermal conductivity of Ho2Ti2O7 along the [111] direction.

Thermal transport measurements have been made on the spin-ice material Ho(2)Ti(2)O(7) in an applied magnetic field with both the heat current and the field parallel to the [111] direction for temperatures from 50 mK to 1.2 K. A large magnetic field >6 T is applied to suppress the magnetic contribution to the thermal conductivity in order to extract the lattice conductivity. The low field thermal conductivity thus reveals a magnetic field dependent contribution to the conductivity which both transfers heat and scatters phonons. We interpret these magnetic excitations as monopolelike excitations and describe their behavior via existing Debye-Hückel theory.

Neutron Laue diffraction study on the magnetic phase diagram of multiferroic MnWO4 under pulsed high magnetic fields.

We have combined time-of-flight neutron Laue diffraction and pulsed high magnetic fields at the Spallation Neutron Source to study the phase diagram of the multiferroic material MnWO(4). The control of the field-pulse timing enabled an exploration of magnetic Bragg scattering through the time dependence of both the neutron wavelength and the pulsed magnetic field. This allowed us to observe several magnetic Bragg peaks in different field-induced phases of MnWO(4) with a single instrument configuration. These phases were not previously amenable to neutron diffraction studies due to the large fields involved.

The impact of ractopamine hydrochloride on growth and metabolism, with special consideration of its role on nitrogen balance and water utilization in pork production.

Two experiments were conducted to determine if ractopamine hydrochloride (RAC) could improve nutrient utilization and decrease water utilization, thus reducing the environmental footprint of hog operations. The tissue accretion experiment used comparative slaughter involving 120 barrows (95 ± 3 kg of BW), including 12 assigned to an initial slaughter group; the remaining pigs were slaughtered at 108 or 120 kg. Growth performance and nutrient retention were determined. The 15-d metabolism experiment consisted of 54 pigs (95 ± 3 kg of BW). Growth performance, feed and water intake, and urine and fecal output were measured. The metabolism experiment used 9 dietary treatments arranged as a 3 × 3 factorial: 3 quantities of RAC (0, 5, and 10 mg/kg) and 3 standardized ileal digestible-Lys:DE ratios (1.73, 2.14, and 2.63 g/Mcal of DE). The tissue accretion study was designed as a 3 × 3 × 2 factorial arrangement of treatments using the same 9 dietary treatments to include slaughter BW (108 and 120 kg of BW) as an additional factor. In the tissue accretion experiment, RAC had no effect on ADG, ADFI, or G:F (P>0.10). With increased Lys, G:F improved (P=0.029), but not ADG or ADFI (P>0.10). Protein deposition rates increased numerically (P=0.11); water deposition rates increased (P=0.050), whereas lipid deposition tended to decrease with RAC inclusion (P=0.055). With greater RAC and Lys, the pigs had improved ADG (P=0.002) and G:F (P<0.001) in the metabolism experiment. Daily water intake (P=0.017.) and water output (P=0.033) decreased with RAC inclusion. Lysine inclusion did not alter the water balance (P>0.10). Urinary N excretion (P<0.001), total N excretion (P=0.003), and the urine N:fecal N ratio (P<0.001) decreased with the addition of RAC; fecal N (P=0.008) increased with RAC inclusion. Retention of N improved with addition of RAC to the diet (P=0.003). With greater dietary Lys, fecal N was reduced (P<0.001). The pigs fed the 2.14 g of Lys/Mcal tended to have the least urinary N (P = 0.069) and total N excretion (P=0.086) and to have the greatest N retention (P=0.086) and urinary N:fecal N ratio (P=0.009). A RAC × Lys interaction was observed for N digestibility (P=0.001), excretion (P=0.001), and retention (P=0.002) and for fecal (P=0.001) and urinary N (P=0.036). By improving N and water utilization in finishing pigs, RAC-containing diets supplemented with sufficient Lys can reduce N excretion into the environment from swine facilities.

Neurocognitive sequelae in African American and Caucasian children with multiple sclerosis.

OBJECTIVE: To examine domain-specific neurocognitive differences between African American (AA) and Caucasian (CA) patients with pediatric-onset multiple sclerosis (POMS). METHODS: An extensive battery of neuropsychological tests was given to each subject, including tests in all major domains of cognitive function. Point-biserial correlations between ethnicity and test performance were computed. Significant correlations were followed up with hierarchical multiple regression analysis, accounting for clinical and demographic variables before examining ethnic differences. RESULTS: Forty-two patients with POMS including 20 AA and 22 CA subjects were assessed. The cohorts did not differ in age, gender, socioeconomic status, disease duration, disability score, immunoglobulin G index, or number of relapses in the first 2 years of disease. Retaining some of these variables as covariates in the hierarchical regression analysis, the AA cohort performed worse on measures of language (p < 0.001) and complex attention (p < 0.01) than their CA peers. CONCLUSION: AA patients with POMS may be at higher risk for adverse cognitive impact in the areas of language and complex attention. Longitudinal characterization of cognitive pathology is critical for the development of effective intervention strategies to prolong cognitive functioning in POMS cohorts.

Magnetoelastics of a spin liquid: X-ray diffraction studies of Tb2Ti2O7 in pulsed magnetic fields.

We report high resolution single crystal x-ray diffraction measurements of the frustrated pyrochlore magnet Tb2Ti2O7, collected using a novel low temperature pulsed magnet system. This instrument allows characterization of structural degrees of freedom to temperatures as low as 4.4 K, and in applied magnetic fields as large as 30 T. We show that Tb2Ti2O7 manifests intriguing structural effects under the application of magnetic fields, including strongly anisotropic giant magnetostriction, a restoration of perfect pyrochlore symmetry in low magnetic fields, and ultimately a structural phase transition in high magnetic fields. It is suggested that the magnetoelastic coupling thus revealed plays a significant role in the spin liquid physics of Tb2Ti2O7 at low temperatures.

Two-dimensional kagome correlations and field induced order in the ferromagnetic XY pyrochlore Yb2Ti2O7.

Neutron scattering measurements show the ferromagnetic XY pyrochlore Yb2Ti2O7 to display strong quasi-two-dimensional (2D) spin correlations at low temperature, which give way to long range order (LRO) under the application of modest magnetic fields. Rods of scattering along 111 directions due to these 2D spin correlations imply a magnetic decomposition of the cubic pyrochlore system into decoupled kagome planes. A magnetic field of approximately 0.5 T applied along the [110] direction induces a transition to a 3D LRO state characterized by long-lived, dispersive spin waves. Our measurements map out a complex low temperature-field phase diagram for this exotic pyrochlore magnet.