Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO_4 . 1/2 D_2O

Magnetic excitations have been investigated in the spin dimer material VODPO_4 \cdot 1/2 D_2O using inelastic neutron scattering. A dispersionless magnetic mode was observed at an energy of 7.81(4) meV. The wavevector dependence of the scattering intensityfrom this mode is consistent with the excitation of isolated V^{4+} spin dimers with a V-V separation of 4.43(7) \AA. This result is unexpected since the V-V pair previously thought to constitute themagnetic dimer has a separation of 3.09 \AA. We identify an alternative V-V pair as the likely magnetic dimer, which involves superexchange pathways through a covalently bonded PO_4 group. This surprising result casts doubt on the interpretation of (VO)_2P_2O_7 as a spin ladder.Comment: 4 pages, 4 postscript figures - identical to previous paper but figure 2 and 3 hopefully more compatible .p

Crystal structures of superconducting sodium intercalates of hafnium nitride chloride

Abstract Sodium intercalation compounds of HfNCl have been prepared at room temperature in naphtyl sodium solutions in tetrahydrofuran and their crystal structure has been investigated by Rietveld refinement using X-ray powder diffraction data and high-resolution electron microscopy. The structure of two intercalates with space group R3m and lattice parameters a = 3.58131(6) Å , c = 57.752(6) Å , and a = 3.58791(8) Å , c = 29.6785(17) Å is reported, corresponding to the stages 2 and 1, respectively, of Na x HfNCl. For the stage 2 phase an ordered model is presented, showing two crystallographically independent [HfNCl] units with an alternation of the Hf-Hf interlayer distance along the c-axis, according with the occupation by sodium atoms of one out of two van der Waals gaps. Both stages 1 and 2 phases are superconducting with critical temperatures between 20 and 24 K, they coexist in different samples with proportions depending on the synthesis conditions, and show a variation in c spacing that can be correlated with the sodium stoichiometry. High-resolution electron microscopy images of the host and intercalated samples show bending of the HfNCl bilayers as well as stacking faults in some regions, which coexist in the same crystal with ordered domains.

Coexistence of double alternating antiferromagnetic chains in (VO)_2P_2O_7 : NMR study

Nuclear magnetic resonance (NMR) of 31P and 51V nuclei has been measured in a spin-1/2 alternating-chain compound (VO)_2P_2O_7. By analyzing the temperature variation of the 31P NMR spectra, we have found that (VO)_2P_2O_7 has two independent spin components with different spin-gap energies. The spin gaps are determined from the temperature dependence of the shifts at 31P and 51V sites to be 35 K and 68 K, which are in excellent agreement with those observed in the recent inelastic neutron scattering experiments [A.W. Garrett et al., Phys. Rev. Lett. 79, 745 (1997)]. This suggests that (VO)_2P_2O_7 is composed of two magnetic subsystems showing distinct magnetic excitations, which are associated with the two crystallographically-inequivalent V chains running along the b axis. The difference of the spin-gap energies between the chains is attributed to the small differences in the V-V distances, which may result in the different exchange alternation in each magnetic chain. The exchange interactions in each alternating chain are estimated and are discussed based on the empirical relation between the exchange interaction and the interatomic distance.Comment: 10 pages, 11 embedded eps figures, REVTeX, Submitted to Phys. Rev.

Ab initio investigation of VOSeO3, a spin gap system with coupled spin dimers

Motivated by an early experimental study of VOSeO3, which suggested that it is a quasi-2D system of weakly coupled spin dimers with a small spin gap, we have investigated the electronic structure of this material via density-functional calculations. These ab initio results indicate that the system is better thought of as an alternating spin-1/2 chain with moderate interchain interactions, an analog of (VO)2P2O7. The potential interest of this system for studies in high magnetic field given the presumably small value of the spin gap is emphasized.Comment: 4 pages, 5 figure

Spin dynamics and transport in gapped one-dimensional Heisenberg antiferromagnets at nonzero temperatures

We present the theory of nonzero temperature ($T$) spin dynamics and transport in one-dimensional Heisenberg antiferromagnets with an energy gap $\Delta$. For $T << \Delta$, we develop a semiclassical picture of thermally excited particles. Multiple inelastic collisions between the particles are crucial, and are described by a two-particle S-matrix which has a super-universal form at low momenta. This is established by computations on the O(3) $\sigma$-model, and strong and weak coupling expansions (the latter using a Majorana fermion representation) for the two-leg S=1/2 Heisenberg antiferromagnetic ladder. As an aside, we note that the strong-coupling calculation reveals a S=1, two particle bound state which leads to the presence of a second peak in the T=0 inelastic neutron scattering (INS) cross-section for a range of values of momentum transfer. We obtain exact, or numerically exact, universal expressions for the thermal broadening of the quasi-particle peak in the INS cross-section, for the magnetization transport, and for the field dependence of the NMR relaxation rate $1/T_1$ of the effective semiclassical model: these are expected to be asymptotically exact for the quantum antiferromagnets. The results for $1/T_1$ are compared with the experimental findings of Takigawa et al and the agreement is quite good. In the regime $\Delta < T < (a typical microscopic exchange)$ we argue that a complementary description in terms of semiclassical waves applies, and give some exact results for the thermodynamics and dynamics.Comment: REVTEX, 53 pages and 23 postscript figures; added additional reference and associated clarificatio

Blinded by the light: on the relationship between CO first overtone emission and mass accretion rate in massive young stellar objects

To date, there is no explanation as to why disc-tracing CO first overtone (or ‘bandhead’) emission is not a ubiquitous feature in low- to medium-resolution spectra of massive young stellar objects (MYSOs), but instead is only detected towards approximately 25 per cent of their spectra. In this paper, we investigate the hypothesis that only certain mass accretion rates result in detectable bandhead emission in the near-infrared spectra of MYSOs. Using an analytic disc model combined with an LTE model of the CO emission, we find that high accretion rates (≳10⁻⁴ M⊙ yr⁻¹) result in large dust sublimation radii, a larger contribution to the K-band continuum from hot dust at the dust sublimation radius, and therefore correspondingly lower CO emission with respect to the continuum. On the other hand, low accretion rates (≲10⁻⁶ M⊙ yr⁻¹) result in smaller dust sublimation radii, a correspondingly smaller emitting area of CO, and thus also lower CO emission with respect to the continuum. In general, moderate accretion rates produce the most prominent, and therefore detectable, CO first overtone emission. We compare our findings to a recent near-infrared spectroscopic survey of MYSOs, finding results consistent with our hypothesis. We conclude that the detection rate of CO bandhead emission in the spectra of MYSOs could be the result of MYSOs exhibiting a range of mass accretion rates, perhaps due to the variable accretion suggested by recent multi-epoch observations of these objects

A combination of three surface modifiers for the optimal generation and application of natural hybrid nanopigments in a biodegradable resin

Our purpose was to improve the thermal, mechanical and optimal properties of an epoxy bioresin using optimum hybrid natural pigments previously synthesised in our lab. Next, we searched for the best combinations of factors in the synthesis of natural hybrid nanopigments and then incorporated them into the bioresin. We combined three structural modifiers in the nanopigment synthesis, surfactant, coupling agent (silane) and a mordant salt (alum), selected to replicate mordant textile dyeing with natural dyes. We used Taguchi s design L8 to seek final performance optimisation. We selected three natural dyes, chlorophyll, beta-carotene and beetroot extract, and used two laminar nanoclay types, montmorillonite and hydrotalcite. The thermal, mechanical and colorimetric characterisation of the composite obtained by mixing natural hybrid nanopigments (bionanocomposite) was made. The natural dye interactions with both nanoclays improved the thermal stabilities, colour performance and UV VIS light exposure stability of natural dyes and bioresins. The best bionanocomposite materials were found in an acidic pH [3, 4] environment and by modifying nanoclays with mordant and surfactant during the nanopigment synthesis processWe thank the Spanish Ministry of Economy and Competitiveness for funding Projects DPI2011-30090-C02-02 and DPI2015-68514-R.Micó Vicent, B.; Jordán Núñez, J.; Martinez Verdu, FM.; Balart Gimeno, RA. (2017). A combination of three surface modifiers for the optimal generation and application of natural hybrid nanopigments in a biodegradable resin. Journal of Materials Science. 52(2):889-898. https://doi.org/10.1007/s10853-016-0384-8S889898522Majdzadeh-Ardakani K, Nazari B (2010) Improving the mechanical properties of thermoplastic starch/poly(vinyl alcohol)/clay nanocomposites. 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Etude magnétique et par résonance Mössbauer de l'orthophosphate Na 3Fe2(PO4)3α et d'une phase vitreuse dérivée

A study of Na3Fe2(PO4)3α by magnetic susceptibility measurements and Môssbauer spectroscopy has shown antiferromagnetic ordering at 45.7 K with some weak ferromagnetism. A vitreous phase has been formed by quenching the molten material. The presence of both divalent and trivalent iron was detected. It has been established that this reduction of iron was necessary to the vitrification process. The magnetic structure of the glass has been discussed.L'étude magnétique et par résonance Mössbauer de Na3Fe 2(PO4)3α a permis de mettre en évidence un ordre antiferromagnétique à 45,7 K accompagné d'un faible ferromagnétisme. Une trempe après fusion conduit à une phase vitreuse. Son étude indique la présence simultanée de fer divalent et trivalent. Il a été établi que cette réduction du fer était indispensable pour que se produise le processus de vitrification. La structure magnétique du verre est discutée