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An efficient and fast microwave-assisted extraction (MAE) technique was developed for extracting gallic acid as an indicative biomarker for the quality control of Acacia arabica bark. The MAE technique was optimized and compared with other conventional extraction techniques. The optimal conditions of MAE were 20% methanol as solvent, solid/liquid ratio 1:40 (g/mL), irradiation power 20% and two extraction cycles, 5 min each. The proposed extraction technique produced a maximum yield of 10.59 (mg/g) gallic acid in 10 min, which was 1.03 and 1.15 times more efficient than 6 h of heat reflux and 24 h of maceration extraction, respectively. This high yield, along with saving of time, energy, and solvent would position MAE as a valuable and cost-effective technology suitable for today's highly competitive industries, with growing demand for increased productivity, improved efficiency, and reduced cycle time. Moreover, a new high-performance liquid chromatography method was developed and validated for the determination of gallic acid in Acacia arabica bark extract. The method was found to be rapid, sensitive, accurate, precise, and robust. The method showed good linearity over concentration range 1-100 (µg/mL) with LOD 16.08 (ng/mL) and LOQ 48.73 (ng/mL). The average recovery obtained using standard addition technique was 100.36% with a low value of RSD% (1.19%) indicating the accuracy of the proposed method for determination of gallic acid in Acacia arabica bark extract.

CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuations in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported in [Sarte et al. Phys. Rev. B 98 024415 (2018)], combined with the single-ion spin-orbit coupling of similar magnitude [Cowley et al. Phys. Rev. B 88, 205117 (2013)] results in significant mixing between $j_{eff}$ spin-orbit levels in the low temperature magnetically ordered phase. The high degree of entanglement, combined with the structural domains originating from the Jahn-Teller structural distortion at $\sim$ 300 K, make the magnetic excitation spectrum highly structured in both energy and momentum. We extend previous theoretical work on PrTl$_{3}$ [Buyers et al. Phys. Rev. B 11, 266 (1975)] to construct a mean-field and multi-level spin exciton model employing the aforementioned spin exchange and spin-orbit coupling parameters for coupled Co$^{2+}$ ions on a rocksalt lattice. This parameterization, based on a tetragonally distorted type-II antiferromagnetic unit cell, captures both the sharp low energy excitations at the magnetic zone center, and the energy broadened peaks at the zone boundary. However, the model fails to describe the momentum dependence of the excitations at high energy transfers, where the neutron response decays faster with momentum than the Co$^{2+}$ form factor. We discuss such a failure in terms of a possible breakdown of localized spin-orbit excitons at high energy transfers., (main text - 21 pages, 12 figures; supplementary information - 15 pages, 3 figures, to be published in Phys. Rev. B)

We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides ${\mathrm{La}}_{2}{\mathrm{O}}_{2}{\mathrm{Fe}}_{2}\mathrm{O}M{}_{2}(M=\text{S},\text{Se})$. Elastic neutron scattering reveals that $M=\text{S}$ and Se have similar nuclear structures at room and low temperatures. Our neutron diffraction data reveals that both materials obtain antiferromagnetic ordering at a N\'eel temperature ${T}_{N}90.1\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}0.16$ K and $107.2\ifmmode\pm\else\textpm\fi{}0.06$ K for $M=\text{Se}$ and S, respectively. The magnetic arrangements for both $M=\text{S}$, Se compounds are obtained through Rietveld refinement. We find the order parameter exponent $\ensuremath{\beta}$ to be 0.$129\ifmmode\pm\else\textpm\fi{}0.006$ for $M=\text{Se}$ and $0.133\ifmmode\pm\else\textpm\fi{}0.007$ for $M=\text{S}$. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen replacement, the nature of the magnetic interactions is similar in these materials.

We have carried out inelastic neutron scattering experiments to study magnetic excitations in the ordered double perovskite ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$. We have found a well-defined magnon mode with a bandwidth of $\ensuremath{\sim}50$ meV below the ferrimagnetic ordering temperature (${T}_{c}\ensuremath{\sim}520$ K), similar to the previously studied ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$. The spin excitation is gapless for most temperatures within the magnetically ordered phase. However, a spin gap of $\ensuremath{\sim}10$ meV opens up below $\ensuremath{\sim}150$ K, which is well below the magnetic ordering temperature but coincides with a previously reported metal-insulator transition and onset of structural distortion. The observed temperature dependence of the spin gap provides strong evidence for ordering of Re orbitals at $\ensuremath{\sim}150$ K, in accordance with an earlier proposal put forward by K. Oikawa et al. based on neutron diffraction [J. Phys. Soc. Jpn. 72, 1411 (2003)] as well as recent theoretical work by Lee and Marianetti [Phys. Rev. B 97, 045102 (2018)]. The presence of separate orbital and magnetic ordering in ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ suggests weak coupling between spin and orbital degrees of freedom and hints at a subdominant role played by spin-orbit coupling in describing its magnetism. In addition, we observed only one well-defined magnon band near the magnetic zone boundary, which is incompatible with simple ferrimagnetic spin waves arising from Fe and Re local moments but suggests a strong damping of the Re magnon mode.

Neutron spectroscopy was applied to study the magnetic interactions of orbitally degenerate Co2+ on a host MgO rocksalt lattice where no long-range spin or orbital order exists. The paramagnetic nature of the substituted monoxide Co0.03Mg0.97O allows for the disentanglement of spin exchange and spin-orbit interactions. By considering the prevalent excitations from Co2+ spin pairs, we extract seven exchange constants out to the fourth coordination shell. An antiferromagnetic next-nearest-neighbor 180◦ exchange interaction is dominant; however, dual ferromagnetic and antiferromagnetic interactions are observed for pairings with other pathways. These interactions can be understood in terms of a combination of orbital degeneracy in the t2g channel and the Goodenough-Kanamori-Anderson rules. Our work suggest that such a hierarchy of exchange interactions exists in transition-metal-based oxides with a t2g orbital degeneracy.

Inelastic neutron scattering has been used to study the magneto-elastic excitations in the multiferroic manganite hexagonal YMnO$_3$. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the $(a,b)$-plane. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the $c$-axis is observed to cause a linear field-induced splitting of one of the spin wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes and a magneto-elastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magneto-elastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites., Comment: 12 pages, 10 figures

We carried out detailed studies of the magnetic structure, magnetoelastic coupling, and thermal properties of ${\mathrm{EuCrO}}_{3}$ nanopowders from room temperature to liquid helium temperature. Our neutron powder diffraction and x-ray powder diffraction measurements provide precise atomic positions of all atoms in the cell, especially for the light oxygen atoms. The low-temperature neutron powder diffraction data revealed extra Bragg peaks of magnetic origin, which can be attributed to a ${G}_{x}$ antiferromagnetic structure with an ordered moment of $\ensuremath{\sim}2.4{\ensuremath{\mu}}_{\mathrm{B}}$ consistent with the $3{d}^{3}$ electronic configuration of the ${\mathrm{Cr}}^{3+}$ cations. Apart from previously reported antiferromagnetic and ferromagnetic transitions in ${\mathrm{EuCrO}}_{3}$ at low temperatures, we also observed an anomaly at about 100 K. This anomaly was observed in the temperature dependence of the sample's, lattice parameters, thermal expansion, Raman spectroscopy, permittivity, and conductance measurements. This anomaly is attributed to the magnetoelastic distortion in the ${\mathrm{EuCrO}}_{3}$ crystal.

Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN ∼290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hund’s rule model, the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom, whose bandwidth is controlled by interionic superexchange. The different ordering domains give rise to several magnetic peaks at each wavevector transfer.

We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeO$\emph{Ch}$) series La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest among the Fe-based materials with the potential \ to show unconventional high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra, as well as resitivity data, reveal that the parent FeO$\emph{Ch}$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that unravel a Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that FeO$\emph{Ch}$ may constitute a new, ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.

The electronic phase diagram of the weak spin-orbit Mott insulator (Sr(1-x)Lax)3Ir2O7 is determined via an exhaustive experimental study. Upon doping electrons via La substitution, an immediate collapse in resistivity occurs along with a narrow regime of nanoscale phase separation comprised of antiferromagnetic, insulating regions and paramagnetic, metallic puddles persisting until x~0.04. Continued electron doping results in an abrupt, first-order phase boundary where the Neel state is suppressed and a homogenous, correlated, metallic state appears with an enhanced spin susceptibility and local moments. As the metallic state is stabilized, a weak structural distortion develops and suggests a competing instability with the parent spin-orbit Mott state., 5 pages, 4 figures

In this study, we use nuclear magnetic resonance (NMR), high-resolution x-ray and neutron scattering to study structural and magnetic phase transitions in phosphorus-doped BaFe2(As1-xPx)2. Thus, previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x = 0.3. However, we show that the tetragonal-to-orthorhombic structural (Ts) and paramagnetic to antiferromagnetic (AF, TN) transitions in BaFe2(As1-xPx)2 are always coupled and approach to TN ≈ Ts ≥ Tc (≈ 29 K) for x = 0.29 before vanishing abruptly for x ≥ 0.3. These results suggest that AF order in BaFe2(As1-xPx)2 disappears in a weakly first order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.

Author(s): Dhital, C; Hogan, T; Yamani, Z; Birgeneau, RJ; Tian, W; Matsuda, M; Sefat, AS; Wang, Z; Wilson, SD

Neutron diffraction measurements are presented measuring the responses of both magnetic and structural order parameters of parent and lightly Co-doped Ba(${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$)${}_{2}{\mathrm{As}}_{2}$ under the application of uniaxial pressure. We find that the uniaxial pressure induces a thermal shift in the onset of antiferromagnetic order that grows as a percentage of ${T}_{N}$ as Co doping is increased and the superconducting phase is approached. Additionally, as uniaxial pressure is increased within parent and lightly doped Ba(${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}$)${}_{2}{\mathrm{As}}_{2}$ on the first-order side of the tricritical point, we observe a decoupling between the onsets of the orthorhombic structural distortion and antiferromagnetism. Our findings place needed constraints on models exploring the nematic susceptibility of the bilayer pnictides in the tetragonal, paramagnetic regime.

Neutron diffraction is used to examine the polarization of weak static antiferromagnetism in high quality single crystalline URu2Si2. As previously documented, elastic Bragg-like diffraction develops for temperature T

Neutron scattering is used to investigate the single-ion spin and orbital excitations below the Mott-Hubbard gap in CoO. Three excitations are reported at $0.870\ifmmode\pm\else\textpm\fi{}0.009$ eV, $1.84\ifmmode\pm\else\textpm\fi{}0.03$, and $2.30\ifmmode\pm\else\textpm\fi{}0.15$ eV. These were parametrized within a weak crystal field scheme with an intraorbital exchange of $J(dd)=1.3\ifmmode\pm\else\textpm\fi{}0.2$ eV and a crystal field splitting $10Dq=0.94\ifmmode\pm\else\textpm\fi{}0.10$ eV. A reduced spin-orbit coupling of $\ensuremath{\lambda}=\ensuremath{-}0.016\ifmmode\pm\else\textpm\fi{}0.003$ eV is derived from dilute samples of Mg${}_{0.97}$Co${}_{0.03}$O, measured to remove effects due to spin exchange and structural distortion parameters which break the cubic phase degeneracy of the orbital excitations complicating the inelastic spectrum. The peak at 1.84 eV, while reported using resonant x-ray and optical techniques, was absent or weak for nonresonant x-ray experiments and overlaps with the expected position of a ${}^{4}{A}_{2}$ level. This transition is absent in the dipolar approximation but expected to have a finite quadrupolar matrix element that can be observed with neutron scattering techniques at larger momentum transfers. Our results agree with a crystal field analysis (in terms of Racah parameters and Tanabe-Sugano diagrams) and with previous calculations performed using local-density band theory for Mott insulating transition metal oxides. The results also demonstrate the use of neutron scattering for measuring dipole forbidden transitions in transition metal oxide systems.

Interest in many strongly spin-orbit coupled 5d-transition metal oxide insulators stems from mapping their electronic structures to a J=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems' potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J=1/2 Mott insulator, carriers remain localized within a nanoscale phase separated ground state. A percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond the critical region. Our results demonstrate a surprising parallel between doped 5d- and 3d-electron Mott systems and suggest either through the near degeneracy of nearby electronic phases or direct carrier localization that U is essential to the carrier response of this doped spin-orbit Mott insulator., 25 pages, 4 figures in main text, 4 figures in supplemental text

Interest in many strongly spin-orbit-coupled 5d-transition metal oxide insulators stems from mapping their electronic structures to a J(eff)=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems' potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J(eff)=1/2 Mott insulator, carriers remain localized within a nanoscale phase-separated ground state. A percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond the critical region. Our results demonstrate a surprising parallel between doped 5d- and 3d-electron Mott systems and suggest either through the near-degeneracy of nearby electronic phases or direct carrier localization that U is essential to the carrier response of this doped spin-orbit Mott insulator.

We present neutron-scattering measurements on single crystals of lightly doped La${}_{2\ensuremath{-}x}$Ba${}_{x}$CuO${}_{4}$, with $0\ensuremath{\le}x\ensuremath{\le}0.035$. These reveal the evolution of the magnetism in this prototypical doped Mott insulator from a three-dimensional (3D) commensurate (C) antiferromagnetic ground state, which orders at a relatively high ${T}_{N}$, to a two-dimensional (2D) incommensurate (IC) ground state with finite-ranged static correlations, which appear below a relatively low effective ${T}_{N}$. At low temperatures, the 2D IC magnetism coexists with the 3D C magnetism for doping concentrations as low as $\ensuremath{\sim}$0.0125. We find no signal of a 3D C magnetic ground state by $x\ensuremath{\sim}0.025$, consistent with the upper limit of $x\ensuremath{\sim}0.02$ observed in the sister family of doped Mott insulators, La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$. The 2D IC ground states observed for $0.0125\ensuremath{\le}x\ensuremath{\le}0.035$ are diagonal, and are rotated by 45 degrees within the orthorhombic basal plane compared with those previously reported for samples with superconducting ground states: La${}_{2\ensuremath{-}x}$Ba${}_{x}$CuO${}_{4}$, with $0.05\ensuremath{\le}x\ensuremath{\le}0.095$. We construct a phase diagram based solely on magnetic order-parameter measurements, which displays much of the complexity of standard high-temperature superconductivity phase diagrams discussed in the literature. Analysis of high-energy resolution inelastic neutron scattering at moderately low temperatures shows a progressive depletion of the very low-energy dynamic magnetic susceptibility as $x$ increases from 0.0125 to 0.035. This low-energy, dynamic susceptibility falls off with increasing temperature on a scale much higher than the effective 2D IC ${T}_{N}$ appropriate to these materials. Appreciable dynamic 2D IC magnetic fluctuations inhabit much of the ``pseudogap'' regime of the phase diagram.

We study the structural and magnetic orders in electron-doped BaFe2-xNixAs2 by high-resolution synchrotron x-ray and neutron scatterings. Upon Ni doping x, the nearly simultaneous tetragonal-to-orthorhombic structural (T-s) and antiferromagnetic (T-N) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in T-s > T-N with increasing x, as was previously observed. However, the temperature separation between T-s and T-N decreases with increasing x for x >= 0.065, tending toward a quantum bicritical point near optimal superconductivity at x approximate to 0.1. The zero-temperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0: 088 less than or similar to x less than or similar to 0.104, resulting in a finite value of T-N approximate to T-c + 10 K above the superconducting dome around x approximate to 0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.

Neutron diffraction measurements exploring the magnetic and structural phase behaviors of the candidate ${J}_{\mathrm{eff}}=1/2$ Mott insulating iridate Sr${}_{2}$IrO${}_{4}$ are presented. Comparisons are drawn between the correlated magnetism in this single-layer system and its bilayer analog Sr${}_{3}$Ir${}_{2}$O${}_{7}$, where both materials exhibit magnetic domains originating from crystallographic twinning and comparable moment sizes. Weakly temperature-dependent superlattice peaks violating the reported tetragonal space group of Sr${}_{2}$IrO${}_{4}$ are observed, supporting the notion of a lower structural symmetry arising from a high-temperature lattice distortion, and we use this to argue that moments orient along a unique in-plane axis demonstrating an orthorhombic symmetry in the resulting spin structure. Our results demonstrate that the correlated spin order and structural phase behaviors in both single-layer and bilayer Sr${}_{n+1}$Ir${}_{n}$O${}_{3n+1}$ systems are remarkably similar and suggest comparable correlation strengths in each system.

We report inelastic neutron scattering measurements in the hidden order state of URu${}_{2\ensuremath{-}x}$Re${}_{x}$Si${}_{2}$ with $x=0.10$. We observe that towards the ferromagnetic quantum critical point induced by the negative chemical pressure of Re doping, the gapped incommensurate fluctuations are robust and comparable in intensity to the parent material. As the Re doping moves the system toward the quantum critical point, the commensurate spin fluctuations related to hidden order weaken, display a shortened lifetime, and slow down. Halfway to the quantum critical point, the hidden order phase survives, albeit weakened, in contrast to its destruction by hydrostatic pressure and by positive chemical pressure from Rh doping.

Through a neutron scattering, charge transport, and magnetization study, the correlated ground state in the bilayer iridium oxide Sr${}_{3}$Ir${}_{2}$O${}_{7}$ is explored. Our combined results resolve scattering consistent with a high temperature magnetic phase that persists above 600 K, reorients at the previously defined ${T}_{\text{AF}}=280$ K, and coexists with an electronic ground state whose phase behavior suggests the formation of a fluctuating charge or orbital phase that freezes below ${T}^{*}\ensuremath{\approx}70$ K. Our study provides a window into the emergence of multiple electronic order parameters near the boundary of the metal to insulator phase transition of the $5d$ ${J}_{\text{eff}}=1/2$ Mott phase.

Elastic and inelastic neutron scattering results show that SrCu${}_{2}$(BO${}_{3}$)${}_{2}$ enters its low-temperature singlet ground state below 10 K without an obvious accompanying structural phase transition, despite suggestions emanating from earlier heat capacity measurements. However, evidence for significant spin-phonon coupling is found in the energy widths, and the corresponding lifetimes, of transverse acoustic phonons propagating in the ($H$00) direction of the Shastry-Sutherland, tetragonal basal plane. Transverse acoustic phonons with energies comparable to and higher than the onset of the two-triplet continuum show substantially increased lifetimes on entering the singlet ground state below $\ensuremath{\sim}$10 K. This is qualitatively consistent with the removal of a decay channel for the phonons due to the gapping of the spin excitation spectrum in SrCu${}_{2}$(BO${}_{3}$)${}_{2}$ at low temperatures.

Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_y$O$_4$ ($x=0.25$, $y\approx0.01$) are measured via inelastic neutron scattering techniques. As Zn$^{2+}$ is substituted onto the Cu$^{2+}$-sites, an anomalous enhancement of the local spin susceptibility $\chi^{\prime\prime}(\omega)$ appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector \textbf{Q}$=(\pi, \pi, 0)$ that coexists with the known incommensurate SDW excitations at \textbf{Q}$_{HK}=(\pi\pm\delta,\pi), (\pi,\pi\pm\delta)$. Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu$^{2+}$-spins, and the simultaneous suppression of T$_c$ as AF fluctuations are slowed in proximity to Zn-impurities suggests the continued importance of high energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.

While the high transition temperatures suggest that the conventional BCS phononmediated mechanism may not provide the main pairing mechanism in the recently discovered RFeAsO1−xFx (1111-type) superconductors, there is, as yet, no consensus, despite extensive experimental and theoretical study. We report here the results of an inelastic neutron scattering investigation of an overdoped polycrystalline sample of LaFeAsO1−xFx with x = 0.15 (Tc = 26 K). Four excitation peaks were observed at 13.6±1.5, 24.2±0.8, 32.2±0.5, and 41.4±1.0 meV. They were identified as phonon modes based on their wavevector and temperature dependence. The peak positions agree well with first-principles calculations of phonon density of states as well as experimental data on both the insulating parent and optimally doped LaFeAsO1−xFx compounds. No evidence for the presence of a resonance mode was found. We found that the phonon density of states of the x = 0.15 sample remains unchanged below Tc and is similar to samples with other fluorine concentrations. This suggests that a standard electron-phonon pairing mechanism cannot explain the high transition temperatures observed in these materials., 5th European Conference on Neutron Scattering (ECNS 2011), July 17-22, 2011, Prague, Czech Republic

We report neutron scattering experiments probing the influence of uniaxial strain on both the magnetic and structural order parameters in the parent iron pnictide compound, BaFe$_2$As$_2$. Our data show that modest strain fields along the in-plane orthorhombic b-axis can affect significant changes in phase behavior simultaneous to the removal of structural twinning effects. As a result, we demonstrate in BaFe$_2$As$_2$ samples detwinned via uniaxial strain that the in-plane C$_4$ symmetry is broken by \textit{both} the structural lattice distortion \textit{and} long-range spin ordering at temperatures far above the nominal (strain-free), phase transition temperatures. Surprising changes in the magnetic order parameter of this system under relatively small strain fields also suggest the inherent presence of magnetic domains fluctuating above the strain-free ordering temperature in this material., 4 pages, 3 figures

CuMoO${}_{4}$ is a triclinic quantum magnet based on $S=1/2$ moments at the Cu${}^{2+}$ site. It has recently attracted interest due to the remarkable changes in its chromic and volumetric properties at high temperatures and in its magnetic properties at low temperatures. This material exhibits a first-order structural phase transition at ${T}_{\mathrm{C}}$ $\ensuremath{\sim}$ 190 K as well as a magnetic phase transition at ${T}_{\mathrm{N}}$ $\ensuremath{\sim}$ 1.75 K. We report low-temperature heat capacity measurements as well as extensive elastic and inelastic neutron scattering measurements on powder samples taken above and below ${T}_{\mathrm{N}}$. We observe neutron diffraction consistent with a simple (1/2, 0, 0) antiferromagnetic structure indicating a doubling of the $a$-axis periodicity below ${T}_{\mathrm{N}}$. In addition, inelastic neutron scattering above a spin gap of $\ensuremath{\sim}$2.3 meV is consistent with triplet excitations out of paired $S=1/2$ moments which form singlet dimers. Low-lying spin wave excitations are also observed and these originate from ordered $S=1/2$ moments below ${T}_{\mathrm{N}}$. Taken together these measurements show the ground state of CuMoO${}_{4}$ to display both nonmagnetic singlets and ferromagnetically coupled spins coexisting within an antiferromagnetic structure below ${T}_{\mathrm{N}}$ $\ensuremath{\sim}$ 1.75 K.

Commonalities among the order parameters of the ubiquitous antiferromagnetism present in the parent compounds of the iron arsenide high temperature superconductors are explored. Additionally, comparison is made between the well established two-dimensional Heisenberg-Ising magnet, K$_2$NiF$_4$ and iron arsenide systems residing at a critical point whose structural and magnetic phase transitions coincide. In particular, analysis is presented regarding two distinct classes of phase transition behavior reflected in the development of antiferromagnetic and structural order in the three main classes of iron arsenide superconductors. Two distinct universality classes are mirrored in their magnetic phase transitions which empirically are determined by the proximity of the coupled structural and magnetic phase transitions in these materials., Comment: 6 pages, 4 figures

Magnetic correlations near the magnetostructural phase transition in the bilayer iron-pnictide parent compound, ${\text{BaFe}}_{2}{\text{As}}_{2}$, are measured. In close proximity to the antiferromagnetic phase transition in ${\text{BaFe}}_{2}{\text{As}}_{2}$, a crossover to three-dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about ${T}_{N}$. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of ${\text{BaFe}}_{2}{\text{As}}_{2}$ and their evolution near the anticipated crossover to three-dimensional critical behavior and long-range order are discussed.

Inelastic neutron-scattering measurements were performed on single-crystal ${\text{Co}}_{3}{\text{V}}_{2}{\text{O}}_{8}$ wherein magnetic cobalt ions reside on distinct spine and cross-tie sites within kagome staircase planes. This system displays a rich magnetic phase diagram which culminates in a ferromagnetic ground state below ${T}_{C}\ensuremath{\sim}6\text{ }\text{K}$. We have studied the low-lying magnetic excitations in this phase within the kagome plane. Despite the complexity of the system at higher temperatures, linear spin-wave theory describes most of the quantitative detail of the inelastic neutron measurements. Our results show two spin-wave branches, the higher energy of which displays finite spin-wave lifetimes well below ${T}_{C}$, and that, surprisingly, the magnetic exchange coupling between Co moments on the spine sites is negligible.

By using neutron scattering techniques, we have studied the incommensurate spin ordering as well as the low energy spin dynamics in single crystal underdoped ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ with $x\ensuremath{\sim}0.095$ and 0.08, which are high temperature superconductors with ${T}_{C}\ensuremath{\sim}27$ and 29 K, respectively. Static two dimensional incommensurate magnetic order appears below ${T}_{N}=39.5\ifmmode\pm\else\textpm\fi{}0.3\text{ }\text{K}$ in ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ $(x=0.095)$ and a similar temperature for $x=0.08$ within the low temperature tetragonal phase. The spin order is unaffected by either the onset of superconductivity or the application of magnetic fields of up to 7 T applied along the $c$ axis in the $x=0.095$ sample. Such a magnetic field independent behavior is in marked contrast to the field induced enhancement of the staggered magnetization observed in the related ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$ system, which indicates that this phenomenon is not a universal property of cuprate superconductors. Surprisingly, we find that the incommensurability $\ensuremath{\delta}$ is only weakly dependent on doping relative to ${\text{La}}_{2\ensuremath{-}x}{\text{Sr}}_{x}{\text{CuO}}_{4}$. Dispersive excitations in ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ $(x=0.095)$ at the same incommensurate wave vector persist up to at least 60 K. The dynamical spin susceptibility of the low energy spin excitations saturates below ${T}_{C}$ in a similar manner to that seen in the superconducting state of ${\text{La}}_{2}{\text{CuO}}_{4+y}$.

By means of neutron scattering we have determined new branches of magnetic excitations in orbitally active CoO (TN=290 K) up to 15 THz and for temperatures from 6 K to 450 K. Data were taken in the (111) direction in six single-crystal zones. From the dependence on temperature and Q we have identified several branches of magnetic excitation. We describe a model for the coupled orbital and spin states of Co2+ subject to a crystal field and tetragonal distortion., Comment: To be published in Physica B (Proceedings of SCES07 conference in Houston)

Nuclear magnetic resonance (NMR) measurements of $\mathrm{CuO}$ chains of detwinned ortho-II ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ (YBCO6.5) single crystals reveal unusual and remarkable properties. The chain Cu resonance broadens significantly, but gradually, on cooling from room temperature. The line shape and its temperature dependence are substantially different from that of a conventional spin/charge density wave (S/CDW) phase transition. Instead, the line broadening is attributed to small amplitude static spin and charge density oscillations with spatially varying amplitudes connected with the ends of the finite length chains. The influence of this $\mathrm{CuO}$ chain phenomenon is also clearly manifested in the plane Cu NMR.

We have mapped the neutron scattering spin spectrum at low energies in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.353}$ $({T}_{c}=18\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, where the doping $\ensuremath{\sim}0.06$ is near the critical value $({p}_{c}=0.055)$ for superconductivity. No coexistence with long-range-ordered antiferromagnetism is found. The spins fluctuate on two energy scales: one a damped spin response with a $\ensuremath{\simeq}2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ relaxation rate and the other a central mode with a relaxation rate that slows to less than $0.08\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below ${T}_{c}$. The spectrum mirrors that of a soft mode driving a central mode. Extremely short correlation lengths, $42\ifmmode\pm\else\textpm\fi{}5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ in-plane and $8\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ along the $c$ direction, and isotropic spin orientations for the central mode indicate that the correlations are subcritical with respect to any second-order transition to N\'eel order. The dynamics follows a model where damped spin fluctuations are coupled to the slow fluctuations of regions with correlations shortened by the hole doping.

Copper NMR has been used as a local probe of the oxygen ordering in Ortho--II YBa2Cu3O6.5 crystals grown in BaZrO3 crucibles. Line assignments have been made to each of the expected crystallographically inequivalent sites. The presence of distinct and narrow lines for these sites as well as the lack of a line known to be associated with oxygen defects indicates that these crystals are highly stoichiometric. Our estimate of the lower limit on the chain length is consistent with that derived from X-ray diffraction measurements. In addition, we have found no evidence for static magnetic moments, in contrast to some previous results., Comment: 11 pages, 7 figures, submitted to Physica C

Neutron scattering from high-quality YBCO6.334 single crystals with a T$_c$ of 8.4 K shows that there is no coexistence with long-range antiferromagnetic order at this very low, near-critical doping of $\sim$0.055, in contrast to claims based on local probe techniques. We find that the neutron resonance seen in optimally doped YBCO7 and underdoped YBCO6.5, has undergone large softening and damping. It appears that the overdamped resonance, with a relaxation rate of 2 meV, is coupled to a zero-energy central mode that grows with cooling and eventually saturates with no change at or below T$_c$. Although a similar qualitative behaviour is found for YBCO6.35, our study shows that the central mode is stronger in YBCO6.334 than YBCO6.35. The system remains subcritical with short-ranged three dimensional correlations., Comment: In press Physica C 2007 (proceedings of M2S Conference, Dresden 2006)