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The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H

Basalt fibre (BF)/acrylonitrile–butadiene rubber (NBR) composites with various BF contents and orientations were prepared, and the curing, physical and mechanical properties and wear performance under block-on-ring abrasion conditions were evaluated. The results showed that the curing time was not obviously affected by the BFs. The addition of BFs increased the hardness but had negative influence on the tensile/tear strength of the NBR composites, especially when the BFs were oriented perpendicular to the tensile/tear direction with a high content, which should be due to the weak connection between the BFs and the rubber matrix. The coefficient of friction (COF) of the composites decreased significantly when BFs were added, whereas the content (3–30 phr) and the orientation of the fibres did not mainly affect the COF. Both parallel- and perpendicular-oriented BFs were effective at improving the wear resistance of the composites, and the former showed a better effect. With the BF content increase, the wear rate initially decreased and then remained basically stable. The COF and wear rate of the composites decreased by 27% and 35% when 12 phr BFs were added in a parallel direction. The excessive BFs, especially the perpendicularly-oriented BFs, led to fracture of the worn surface due to the abrasive grain wear caused by the crushed fibres.

The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H 2 O molecular moieties. This is mirrored in a manifold of the H 2 O molecular potentials. Subnatural line width resonant inelastic X-ray scattering allowed us to quantify the manifold of molecular potential energy surfaces along the H 2 O symmetric normal mode and the local asymmetric O–H bond coordinate up to 1 and 1.5 Å, respectively. The comparison of the single H 2 O molecular potentials and spectroscopic signatures with the ambient conditions liquid phase H 2 O molecular potentials is done on various levels. In the gas phase, first principles, Morse potentials, and stepwise harmonic potential reconstruction have been employed and benchmarked. In the liquid phase the determination of the potential energy manifold along the local asymmetric O–H bond coordinate from resonant inelastic X-ray scattering via the bound state oxygen 1 s to 4 a 1 resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1 s to 2 b 2 resonance is based on stepwise harmonic reconstruction. We find in liquid water at ambient conditions H 2 O molecular potentials ranging from the weak interaction limit to strongly distorted potentials which are put into perspective to established parameters, i.e., intermolecular O–H, H–H, and O–O correlation lengths from neutron scattering.

The origin of the electronic nematicity in FeSe, which occurs below a tetragonal-to-orthorhombic structural transition temperature Ts ≈ 90 K, well above the superconducting transition temperature Tc = 9 K, is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at Q1 = (1; 0) and Q2 = (0; 1) of twin-free FeSe are expected to behave differently below Ts. Although anisotropic spin fluctuations below 10 meV between Q1 and Q2 have been unambiguously observed by inelastic neutron scattering around Tc(<< Ts) , it remains unclear whether such an anisotropy also persists at higher energies and associates with the nematic transition Ts. Here we use resonant inelastic x-ray scattering (RIXS) to probe the high-energy magnetic excitations of uniaxial-strain detwinned FeSe. A prominent anisotropy between the magnetic excitations along the H and K directions is found to persist to ~ 200 meV, which is even more pronounced than the anisotropy of spin waves in BaFe2As2. This anisotropy decreases gradually with increasing temperature and finally vanishes at a temperature around the nematic transition temperature Ts. Our results reveal an unprecedented strong spin-excitation anisotropy with a large energy scale well above the dxz/dyz orbital splitting, suggesting that the nematic phase transition is primarily spin-driven. Moreover, the measured high-energy spin excitations are dispersive and underdamped, which can be understood from a localmoment perspective. Our findings provide the much-needed understanding of the mechanism for the nematicity of FeSe and points to a unified description of the correlation physics across seemingly distinct classes of Fe-based superconductors.

The parent compounds of iron-based superconductors are magnetically ordered bad metals, with superconductivity appearing near a putative magnetic quantum critical point. The presence of both Hubbard repulsion and Hund's coupling leads to rich physics in these multiorbital systems, and motivated descriptions of magnetism in terms of itinerant electrons or localized spins. The ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{As}$ series consists of magnetically ordered bad metal ($x=0$), superconducting ($x\ensuremath{\approx}0.02$) and magnetically ordered semiconducing/insulating ($x\ensuremath{\approx}0.5$) phases, providing a platform to investigate the connection between superconductivity, magnetism and electronic correlations. Here we use x-ray absorption spectroscopy and resonant inelastic x-ray scattering to study the valence state of Fe and spin dynamics in two ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{As}$ compounds ($x=0$ and 0.47). We find that magnetism in both compounds arises from ${\mathrm{Fe}}^{2+}$ atoms, and exhibits underdamped dispersive spin waves in their respective ordered states. The dispersion of spin excitations in ${\mathrm{NaFe}}_{0.53}{\mathrm{Cu}}_{0.47}\mathrm{As}$ is consistent with being quasi-one-dimensional. Compared to NaFeAs, the band top of spin waves in ${\mathrm{NaFe}}_{0.53}{\mathrm{Cu}}_{0.47}\mathrm{As}$ is slightly softened with significantly more spectral weight of the spin excitations. Our results indicate the spin dynamics in ${\mathrm{NaFe}}_{0.53}{\mathrm{Cu}}_{0.47}\mathrm{As}$ arise from localized magnetic moments and suggest the iron-based superconductors are proximate to a correlated insulating state with localized iron moments.

The origin of the electronic nematicity in FeSe is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at $\mathbf{Q}_1=(1, 0)$ and $\mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected to provide decisive criteria for clarifying this issue. Although a spin-fluctuation anisotropy below 10 meV between $\mathbf{Q}_1$ and $\mathbf{Q}_2$ has been observed by inelastic neutron scattering around $T_c\sim 9$ K ($<, Comment: 15 pages, 5 figures

It has been well established experimentally that the interplay of electronic correlations and spin-orbit interactions in Ir$^{4+}$ and Ir$^{5+}$ oxides results in insulating J$_{\rm eff}$=1/2 and J$_{\rm eff}$=0 ground states, respectively. However, in compounds where the structural dimerization of iridum ions is favourable, the direct Ir $d$--$d$ hybridisation can be significant and takes a key role. Here, we investigate the effects of direct Ir $d$--$d$ hybridisation in comparison with electronic correlations and spin-orbit coupling in Ba$_5$AlIr$_2$O$_{11}$, a compound with Ir dimers. Using a combination of $ab$ $initio$ many-body wave function quantum chemistry calculations and resonant inelastic X-ray scattering (RIXS) experiments, we elucidate the electronic structure of Ba$_5$AlIr$_2$O$_{11}$. We find excellent agreement between the calculated and the measured spin-orbit excitations. Contrary to the expectations, the analysis of the many-body wave function shows that the two Ir (Ir$^{4+}$ and Ir$^{5+}$) ions in the Ir$_2$O$_9$ dimer unit in this compound preserve their local J$_{\rm eff}$ character close to 1/2 and 0, respectively. The local point group symmetry at each of the Ir sites assumes an important role, significantly limiting the direct $d$--$d$ hybridisation. Our results emphasize that minute details in the local crystal field (CF) environment can lead to dramatic differences in electronic states in iridates and 5$d$ oxides in general., Comment: 5 pages with 3 figures

Quenching of vibrational excitations in resonant inelastic X-ray scattering (RIXS) spectra of liquid acetic acid is observed. At the oxygen core resonance associated with localized excitations at the O–H bond, the spectra lack the typical progression of vibrational excitations observed in RIXS spectra of comparable systems. We interpret this phenomenon as due to strong rehybridization of the unoccupied molecular orbitals as a result of hydrogen bonding, which however cannot be observed in x-ray absorption but only by means of RIXS. This allows us to address the molecular structure of the liquid, and to determine a lower limit for the average molecular chain length.

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe$_2$As$_2$, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature $T_s$. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature $T_N$ ($, Comment: 11 pages, 4 figures, Supplementary information is available upon request

Significance Water is the matrix of life and behaves anomalously in many of its properties. Since Wilhelm Conrad Röntgen, two distinct separate phases have been argued to coexist in ambient water, competing with the alternative view of the single-phase liquid, footing on X-ray scattering experiment and theory. We conducted a quantitative and high-resolution X-ray spectroscopic multimethod investigation and analysis (X-ray absorption, X-ray emission, and resonant inelastic X-ray scattering). We find that all known X-ray spectroscopic observables can be fully and consistently described with continuous-distribution models of near-tetrahedral liquid water at ambient conditions with 1.74 ± 2.1% donated and accepted H-bonds per molecule., The phase diagram of water harbors controversial views on underlying structural properties of its constituting molecular moieties, its fluctuating hydrogen-bonding network, as well as pair-correlation functions. In this work, long energy-range detection of the X-ray absorption allows us to unambiguously calibrate the spectra for water gas, liquid, and ice by the experimental atomic ionization cross-section. In liquid water, we extract the mean value of 1.74 ± 2.1% donated and accepted hydrogen bonds per molecule, pointing to a continuous-distribution model. In addition, resonant inelastic X-ray scattering with unprecedented energy resolution also supports continuous distribution of molecular neighborhoods within liquid water, as do X-ray emission spectra once the femtosecond scattering duration and proton dynamics in resonant X-ray–matter interaction are taken into account. Thus, X-ray spectra of liquid water in ambient conditions can be understood without a two-structure model, whereas the occurrence of nanoscale-length correlations within the continuous distribution remains open.

The iron-chalcogenide superconductor FeTe1–x Se x displays a variety of exotic features distinct from iron pnictides. Although much effort has been devoted to understanding the interplay between magnetism and superconductivity near x = 0.5, the existence of a spin glass phase with short-range magnetic order in the doping range (x ∼ 0.1–0.3) has rarely been studied. Here, we use DC/AC magnetization and (quasi) elastic neutron scattering to confirm the spin-glass nature of the short-range magnetic order in a Fe1.07Te0.8Se0.2 sample. The AC-frequency dependent spin-freezing temperature T f generates a frequency sensitivity ΔT f(ω)/[T f(ω)Δlog10 ω] ≈ 0.028 and the description of the critical slowing down with τ = τ 0(T f/T SG – 1)−z v gives T SG ≈ 22 K and zv ≈ 10, comparable to that of a classical spin-glass system. We have also extended the frequency-dependent T f to the smaller time scale using energy-resolution-dependent neutron diffraction measurements, in which the T N of the short-range magnetic order increases systematically with increasing energy resolution. By removing the excess iron through annealing in oxygen, the spin-freezing behavior disappears, and bulk superconductivity is realized. Thus, the excess Fe is the driving force for the formation of the spin-glass phase detrimental to bulk superconductivity.

Triangular lattice of rare-earth ions with interacting effective spin-$1/2$ local moments is an ideal platform to explore the physics of quantum spin liquids (QSLs) in the presence of strong spin-orbit coupling, crystal electric fields, and geometrical frustration. The Yb delafossites, NaYbCh$_2$ (Ch=O, S, Se) with Yb ions forming a perfect triangular lattice, have been suggested to be candidates for QSLs. Previous thermodynamics, nuclear magnetic resonance, and muon spin rotation measurements on NaYbCh$_2$ have supported the suggestion of the QSL ground states. The key signature of a QSL, the spin excitation continuum, arising from the spin quantum number fractionalization, has not been observed. Here we perform both elastic and inelastic neutron scattering measurements as well as detailed thermodynamic measurements on high-quality single-crystalline NaYbSe$_2$ samples to confirm the absence of long-range magnetic order down to 40 mK, and further reveal a clear signature of magnetic excitation continuum extending from 0.1 to 2.5 meV. The comparison between the structure of the magnetic excitation spectra and the theoretical expectation from the spinon continuum suggests that the ground state of NaYbSe$_2$ is a QSL with a spinon Fermi surface., Comment: 18 pages, 6 figures

We use inelastic neutron scattering to study spin excitation anisotropy in mechanically detwinned Ba(Fe1-xCox)2As2 with x=0.048 and 0.054. Both samples exhibit a tetragonal-to-orthorhombic structural transition at Ts, a collinear static antiferromagnetic order at wave vector Q1=QAF=(1,0) below the Neel temperature TN, and superconductivity below Tc (Ts>TN>Tc). In the high-temperature paramagnetic tetragonal phase (T≫Ts), spin excitations centered at Q1 and Q2=(0,1) are gapless and have fourfold (C4) rotational symmetry. On cooling to below TN but above Tc, spin excitations become highly anisotropic, developing a gap at Q2 but still are gapless at Q1. Upon entering into the superconducting state, a neutron spin resonance appears at Q1 with no magnetic scattering at Q2. By comparing these results with those from angle-resolved photoemission spectroscopy experiments, we conclude that the anisotropic shift of the dyz and dxz bands in detwinned Ba(Fe1-xCox)2As2 below Ts is associated with the spin excitation anisotropy, and the superconductivity-induced resonance arises from the electron-hole Fermi surface nesting of quasiparticles with the dyz orbital characters.

In this paper, we revisit the generic construction of ring signatures from hash-then-one-way type (Type-H) signatures proposed by Abe et al. (AOS) in 2004 and made the following contributions. First, we give a proof for the generic construction, in a strengthened security model. Previously, this was only done for concrete instantiations, in a weaker model. Second, we extend AOS's framework to generically construct one-time linkable ring signatures from Type-H signatures and one-time signatures. Lastly, we instantiate the generic construction with an NTRU-based Type-H signature: Falcon~and obtain a post-quantum linkable ring signature scheme. Our analysis shows that the resulting linkable signature is more efficient than any existing lattice based solutions for small to moderate number of users.

Spin nematics break spin-rotational symmetry while maintaining time-reversal symmetry, analogous to liquid crystal nematics that break spatial rotational symmetry while maintaining translational symmetry. Although several candidate spin nematics have been proposed, the identification and characterization of such a state remain challenging because the spin-nematic order parameter does not couple directly to experimental probes. KFe_{0.8}Ag_{1.2}Te_{2} (K_{5}Fe_{4}Ag_{6}Te_{10}, KFAT) is a local-moment magnet consisting of well-separated 2×2 Fe clusters, and in its ground state the clusters order magnetically, breaking both spin-rotational and time-reversal symmetries. Using uniform magnetic susceptibility and neutron scattering measurements, we find a small strain induces sizable spin anisotropy in the paramagnetic state of KFAT, manifestly breaking spin-rotational symmetry while retaining time-reversal symmetry, resulting in a strain-induced spin-nematic state in which the 2×2 clusters act as the spin analog of molecules in a liquid crystal nematic. The strain-induced spin anisotropy in KFAT allows us to probe its nematic susceptibility, revealing a divergentlike increase upon cooling, indicating the ordered ground state is driven by a spin-orbital entangled nematic order parameter.

Understanding and controlling the electronic structure of thin layers of quantum materials is a crucial first step towards designing heterostructures where new phases and phenomena, including the metal-insulator transition (MIT), emerge. Here, we demonstrate control of the MIT via tuning electronic bandwidth and local site environment through selection of the number of atomic layers deposited. We take CaVO3, a correlated metal in its bulk form that has only a single electron in its V4+ 3d manifold, as a representative example. We find that thick films and ultrathin films (≤6 unit cells, u.c.) are metallic and insulating, respectively, while a 10 u.c. CaVO3 film exhibits a clear thermal MIT. Our combined X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) study reveals that the thickness-induced MIT is triggered by electronic bandwidth reduction and local moment formation from V3+ ions, that are both a consequence of the thickness confinement. The thermal MIT in our 10 u.c. CaVO3 film exhibits similar changes in the RIXS response to that of the thickness-induced MIT in terms of reduction of bandwidth and V 3d–O 2p hybridization., npj Quantum Materials, 4 (1), ISSN:2397-4648

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for the quantum nuclear wave packet simulations, showing the important role it plays in RIXS.

A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density ($J_c$) and the vortex-pinning behaviour in these compounds. The observed $J_c$ is higher than the threshold limit (10$^5$ A/cm$^2$) considered for the technological potential and even greater than 1 MA/cm$^2$ for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen $et$ $al$, which suggest the dominant role of $\delta l$-type pinning., Comment: 26 pages, 13 figures, Accepted for publication in ACS Applied Electronic Materials 2019

We use inelastic neutron scattering to show that long-range spin waves arising from the static bicollinear antiferromagnetic (AF) order in FeTe, which have twofold rotational symmetry in a fully detwinned crystal, rapidly dissolve above $E\approx 26$ meV into ridges of scattering with fourfold rotational symmetry and a nearly isotropic magnetic fluctuation spectrum. With increasing temperature above $T_N\approx 68$ K, the twofold spin waves change into broad regions of scattering with fourfold symmetry. Since the scattering patterns from plaquette magnetic order generated within a bilinear biquadratic Hamiltonian have fourfold rotational symmetry consistent with the high-energy, spin-isotropic spin waves of FeTe, we conclude that the bicollinear AF state in FeTe is quasidegenerate with plaquette magnetic order, providing evidence for the strongly frustrated nature of the local moments in iron chalcogenide family of iron-based superconductors., Comment: 12 pages, 11 figures

We present Raptor, the first practical lattice-based (linkable) ring signature scheme with implementation. Raptor is as fast as classical solutions; while the size of the signature is roughly 1.3 KB per user. Prior to our work, all existing lattice-based solutions are analogues of their discrete-log or pairing-based counterparts. We develop a generic construction of (linkable) ring signatures based on the well-known generic construction from Rivest et al., which is not fully compatible with lattices. Our generic construction is provably secure in random oracle model. We also give instantiations from both standard lattice, as a proof of concept, and NTRU lattice, as an efficient instantiation. We show that the latter construction, called Raptor, is almost as efficient as the classical RST ring signatures and thus may be of practical interest.

Elucidating the nature of spin excitations is important to understanding the mechanism of superconductivity in the iron pnictides. Motivated by recent inelastic neutron scattering measurements in the nearly 100% detwinned BaFe$_{2}$As$_{2}$, we study the spin dynamics of an $S=1$ frustrated bilinear-biquadratic Heisenberg model in the antiferromagnetic phase with wavevector $(\pi,0)$. The biquadratic interactions are treated in a dynamical way using a flavor-wave theory in an $SU(3)$ representation. Besides the magnon (dipolar) excitations, the biquadratic interactions give rise to quadrupolar excitations at high energies. We find that the quadrupolar wave significantly influences, in an energy dependent way, the anisotropy between the spin excitation spectra along the $(\pi,0)$ and $(0,\pi)$ directions in the wave vector space. Our theoretical results capture the essential behavior of the spin dynamics measured in the antiferromagnetic phase of the detwinned BaFe$_2$As$_2$. More generally, our results underscore the importance of electron correlation effects for the microscopic physics of the iron pnictides., Comment: 8 pages, 4 figures, published version

Superconductivity in FeSe emerges from a nematic phase that breaks four-fold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations, or hidden magnetic quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe detwinned by mounting on a BaFe2As2 substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors QAF = (1, 0) at low energies E = 6-11 meV in the normal state. This two-fold (C2) anisotropy is reduced at lower energies 3-5 meV, indicating a gapped four-fold (C4) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.7 meV) at QAF with incommensurate scattering around 5-6 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations., Comment: accepted for Nature Materials

In this paper, we propose a secure blockchain-based electronic voting (e-voting) system with distributed authorities. Specifically, we employ threshold blind signature to distribute trust for registration; and threshold ElGamal decryption to distribute trust in ballots tallying. Combining these techniques with decentralized blockchain technology, our system can achieve verifiability, eligibility, fairness and anonymity with reduced trust. We also analyze the efficiency of our system by implementing the proposed e-voting system on a typical laptop for performance evaluation. Our system is efficient enough to be employed in practice based on the experiment results.

Understanding magnetic interactions in the parent compounds of high-temperature superconductors forms the basis for determining their role for the mechanism of superconductivity. For parent compounds of iron pnictide superconductors such as $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{Ba}$, Ca, Sr), although spin excitations have been mapped out throughout the entire Brillouin zone, the respective measurements were carried out on twinned samples and did not allow for a conclusive determination of the spin dynamics. Here we use inelastic neutron scattering to completely map out spin excitations of $\ensuremath{\sim}100%$ detwinned ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$. By comparing observed spectra with theoretical calculations, we conclude that the spin excitations can be well described by an itinerant model when taking into account moderate electronic correlation effects.

Understanding magnetic interactions in the parent compounds of high-temperature superconductors forms the basis for determining their role for the mechanism of superconductivity. For parent compounds of iron pnictide superconductors such as AFe_{2}As_{2} (A=Ba, Ca, Sr), although spin excitations have been mapped out throughout the entire Brillouin zone, the respective measurements were carried out on twinned samples and did not allow for a conclusive determination of the spin dynamics. Here we use inelastic neutron scattering to completely map out spin excitations of ∼100% detwinned BaFe_{2}As_{2}. By comparing observed spectra with theoretical calculations, we conclude that the spin excitations can be well described by an itinerant model when taking into account moderate electronic correlation effects.

We investigated the doping dependence of magnetic excitations in the lightly doped cuprate La2−xSrxCuO4 via combined studies of resonant inelastic x-ray scattering (RIXS) at the Cu L3 edge and theoretical calculations. With increasing doping, the magnon dispersion is found to be essentially unchanged, but the spectral width broadens and the spectral weight varies differently at different momenta. Near the Brillouin zone center, we directly observe bimagnon excitations that possess the same energy scale and doping dependence as previously observed by Raman spectroscopy. They disperse weakly in energy-momentum space, and they are consistent with a bimagnon dispersion that is renormalized by the magnon-magnon interaction at the zone center.

We use transport and inelastic neutron scattering measurements to investigate single crystals of iron pnictide BaFe$_{2-x}$Ni$_{x}$As$_{2}$ ($x=0,0.03$), which exhibit a tetragonal-to-orthorhombic structural transition at $T_s$ and stripe antiferromagnetic order at $T_N$ ($T_s\geq T_N$). Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave vector $S_1(1,0)$ and its 90$^\circ$ rotated wave vector $S_2(0,1)$ under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain single domain orthorhombic antiferromagnetic phase of BaFe$_{2-x}$Ni$_{x}$As$_{2}$ induces resistivity and spin excitation anisotropy at temperatures above zero pressure $T_s$. In uniaxial pressure-free detwinned sample, spin excitation anisotropy between $S_1(1,0)$ and $S_2(0,1)$ first appear in the paramagnetic orthorhombic phase below $T_s$. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above $T_s$ in iron pnictides.

“Ambient water properties have been shown to require heterogeneity” (1) is the imperative followed by Pettersson et al. (2) to relate X-ray spectroscopic findings to a heterogeneous or 2-phase model of ambient water. In ref. 3 we question this hypothesis based on quantitative X-ray spectroscopic evidence. We come to conclude that X-ray spectroscopies support no observations related to heterogeneous, distinct structural motives in ambient water. The critique of sum rules by Pettersson et al. (2) is unjustified: Through normalization to the asymptotic behavior we avoid sum rule normalization of X-ray absorption spectroscopy (XAS). In liquid water, extended X-ray absorption fine structure oscillations are less … [↵][1]1To whom correspondence may be addressed. Email: alexander.foehlisch{at}helmholtz-berlin.de. [1]: #xref-corresp-1-1

The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2\(p\) orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li\(^+\)–O(2\(p\))–Li\(^+\) interactions). Na\(_{2/3}\)[Mg\(_{0.28}\)Mn\(_{0.72}\)]O\(_2\) exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg\(^{2+}\) resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na\(_{2/3}\)[Mg\(_{0.28}\)Mn\(_{0.72}\)]O\(_2\) does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg\(^{2+}\) remains in Na\(_{2/3}\)[Mg\(_{0.28}\)Mn\(_{0.72}\)]O\(_2\), which stabilizes oxygen.