By means of neutron scattering and magnetization measurements down to 90 mK, we determine the magnetic ground state of the spin-ice candidate Nd2Zr2O7. We show that, despite ferromagnetic ...interactions, Nd2Zr2O7 undergoes a transition around 285 mK towards an all-in-all-out antiferromagnetic state, with a strongly reduced ordered magnetic moment. We establish the (H,T) phase diagram in the three directions of the applied field and reveal a metamagnetic transition around 0.1 T, associated with an unexpected shape of the magnetization curves. We propose that this behavior results from the peculiar nature of the Nd3+ doublet, a dipolar-octupolar doublet, different from the standard Kramers doublet studied to date, thus revealing the importance of multipolar correlations in the properties of pyrochlore oxides.
We report on diffuse neutron scattering experiments providing evidence for the presence of random strains in the quantum spin-ice candidate Pr2Zr2O7 . Since Pr3+ is a non-Kramers ion, the strain ...deeply modifies the picture of Ising magnetic moments governing the low-temperature properties of this material. It is shown that the derived strain distribution accounts for the temperature dependence of the specific heat and of the spin-excitation spectra. Taking advantage of mean-field and spin-dynamics simulations, we argue that the randomness in Pr2Zr2O7 promotes a new state of matter, which is disordered yet characterized by short-range antiferroquadrupolar correlations, and from which emerge spin-ice-like excitations. Thus, this study gives an original research route in the field of quantum spin ice.
We present an experimental study of the quantum spin ice candidate pyrochlore compound Pr2Zr2O7 by means of magnetization measurements, specific heat, and neutron scattering up to 12 T and down to 60 ...mK. When the field is applied along the 111 and 11¯0 directions, k=0 field-induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for H∥11¯0, the ordered moment appears on the so-called α chains only. The spin excitation spectrum is essentially inelastic and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For H∥11¯0 (at least up to 2.5 T), we find that a well-defined mode forms from this broad response, whose energy increases with H, in the same way as the temperature of the specific-heat anomaly. We finally discuss these results in the light of mean field calculations and propose an interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those interactions. We then propose a range of acceptable parameters for Pr2Zr2O7 that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice-like excitations.
In this work, we revisit the crystal field acting on the non-Kramers Pr3+ ion (4f2) in the quantum spin-ice candidate Pr2Zr2O7 using both a standard calculation restricted to the ground spin-orbit ...multiplet and intermediate coupling states in the full basis of the f2 configuration. Analysis of the thermal variation of the polycrystal magnetic susceptibility and of the local susceptibilities χ⊥ and χ∥ determined by means of polarized neutron diffraction experiments reveals that the effective antiferromagnetic exchange is strongly depleted at low temperature with respect to its high-temperature value. We then discuss the influence of crystal field imperfections arising from residual strains, which are especially important for a non-Kramers ion. We find that they are an essential ingredient to account for the very low temperature M(H) magnetization curves, showing that the saturation is not achieved even at 8 T. Furthermore, as possible candidates to qualitatively understand the Curie-like behavior observed below 0.5 K, we discuss the influence of the magnetic hyperfine interaction.
Tb2Ti2O7 presents an ongoing conundrum in the study of rare-earth pyrochlores. Despite the expectation that it should be the prototypical unfrustrated noncollinear Ising antiferromagnet on the ...pyrochlore lattice, it presents a puzzling correlated state that persists to the lowest temperatures. Effects which can reintroduce frustration or fluctuations are therefore sought, and quadrupolar operators have been implicated. One consequence of strong quadrupolar effects is the possible coupling of magnetic and lattice degrees of freedom, and it has previously been shown that a hybrid magnetoelastic mode with both magnetic and phononic character is formed in Tb2Ti2O7 by the interaction of a crystal field excitation with a transverse-acoustic phonon. Here, using polarized and unpolarized inelastic neutron scattering, we present a detailed characterization of the magnetic and phononic branches of this magnetoelastic mode, particularly with respect to their composition, the anisotropy of any magnetic fluctuations, and also the temperature dependence of the different types of fluctuation that are involved. We also examine the dispersion relations of the exciton branches that develop from the crystal field excitation in the same temperature regime that the coupled mode appears, and find three quasidispersionless branches where four are expected, each with a distinctive structure factor indicating that they are nonetheless cooperative excitations. We interpret the overall structure of the spectrum as containing four branches, one hybridized with the phonons and gaining a strong dispersion, and three remaining dispersionless.
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Toute crise d’asthme aigu grave (CAAG) impose une prise en charge médicale préhospitalière spécialisée par un Smur.
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En dehors des critères cliniques, la gravité d’une CAAG repose avant tout sur ...l’existence d’un débit expiratoire de pointe (DEP, ou peak-flow) inférieur à 30 % de la valeur prédite par abaque ou de la valeur de référence du patient.
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Le traitement initial associe des nébulisations continues de bêta2-mimétiques et de bromure d’ipratropium, sous oxygène, et une injection de corticoïdes.
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Devant une crise réfractaire, sont recommandés en deuxième intention le sulfate de magnésium, les bêta2-mimétiques en perfusion au pousse-seringue électrique, ou de préférence l’adrénaline en cas de choc associé.
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En cas de recours à la ventilation mécanique, il faut utiliser de petits volumes courants, une fréquence basse et prolonger le temps expiratoire.
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Any life-threatening episode of asthma requires early pre-hospital specialized medical management by emergency medical crews.
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Gravity depends on both clinical criteria and a peak expiratory flow rate (PEFR) more than 30% below either the level predicted by the reference graph or the patient's reference value.
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Initial treatment combines continuous nebulizations containing a beta2-agonist and ipratropium bromide, with oxygen administration and intravenous corticosteroid bolus.
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Recommended as second-line treatment in the absence of adequate response are: intravenous magnesium sulphate and continuous-perfusion beta2-agonists (electric syringe), or, in the case of shock, epinephrine.
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If mechanical ventilation is required its settings should aim for low tidal volumes, low frequency, and increased expiratory time.
We present an experimental study of the quantum spin ice candidate pyrochlore coumpound \przr\ by means of magnetization measurements, specific heat and neutron scattering up to 12 T and down to 60 ...mK. When the field is applied along the \(111\) and \(1\bar{1}0\) directions, \({\bf k}=0\) field induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for \(H \parallel 1\bar{1}0\), the ordered moment appears on the so called \(\alpha\) chains only. The spin excitation spectrum is essentially {\it inelastic} and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For \(H \parallel 1\bar{1}0\) (at least up to 2.5 T), we find that a well defined mode forms from this broad response, whose energy increases with \(H\), in the same way as the temperature of the specific heat anomaly. We finally discuss these results in the light of mean field calculations and propose a new interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those new interactions. We then propose a range of acceptable parameters for \przr\, that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice like excitations.
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