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Register a new userInteresting magnetic response of the nuclear fuel material UO2
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Magnetic response of uranium dioxide (UO2) has been investigated through temperature and magnetic field dependent dc magnetization measurements. UO2 is a paramagnet at room temperature. The magnetic susceptibility, however, deviates from Curie-Weiss (CW) like paramagnetic behavior below T = 280 K. Further down the temperature UO2 undergoes phase transition to an antiferromagnetic state below TN = 30.6 K. The zero field cooled (ZFC) and field cooled (FC) magnetizations exhibit some distinct thermomagnetic irreversibility below TN. The temperature dependence of the FC magnetization is more like a ferromagnet, whereas ZFC magnetization exhibits distinct structures not usually observed in the antiferromagnets. In low applied magnetic field this thermomagnetic irreversibility in magnetization exists in a subtle way even in the paramagnetic regime above TN up to a fairly high temperature, but vanishes in high applied magnetic fields. Deviation from CW law and irreversibility between ZFC and FC magnetization indicate that the paramagnetic state above TN is not a trivial one. Magnetic response below TN changes significantly with the increase in the applied magnetic field. Thermomagnetic irreversibility in magnetization initially increases with the increase in the strength of applied magnetic field, but then gets reduced in the high applied fields. A subtle signature of a magnetic field induced phase transition is also observed in the isothermal magnetic field vartaion of magnetization. All these experimetal results highlight the non-trivial nature of the antiferromagnetic state in UO2
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Phonon density of states (PDOS) measurements have been performed on polycrystalline UO2 at 295 and 1200 K using time-of-flight inelastic neutron scattering to investigate the impact of anharmonicity on the vibrational spectra and to benchmark ab initio PDOS simulations performed on this strongly correlated Mott-insulator. Time-of-flight PDOS measurements include anharmonic linewidth broadening inherently and the factor of ~ 7 enhancement of the oxygen spectrum relative to the uranium component by the neutron weighting increases sensitivity to the oxygen-dominated optical phonon modes. The first-principles simulations of quasi-harmonic PDOS spectra were neutron-weighted and anharmonicity was introduced in an approximate way by convolution with wavevector-weighted averages over our previously measured phonon linewidths for UO2 that are provided in numerical form. Comparisons between the PDOS measurements and the simulations show reasonable agreement overall, but they also reveal important areas of disagreement for both high and low temperatures. The discrepancies stem largely from an ~ 10 meV compression in the overall bandwidth (energy range) of the oxygen-dominated optical phonons in the simulations. A similar linewidth-convoluted comparison performed with the PDOS spectrum of Dolling et al. obtained by shell-model fitting to their historical phonon dispersion measurements shows excellent agreement with the time-of-flight PDOS measurements reported here. In contrast, we show by comparisons of spectra in linewidth-convoluted form that recent first-principles simulations for UO2 fail to account for the PDOS spectrum determined from the measurements of Dolling et al. These results demonstrate PDOS measurements to be stringent tests for ab initio simulations of phonon physics in UO2 and they indicate further the need for advances in theory to address lattice dynamics of UO2.
Using the Kadanoff-Baym-Keldysh formalism, we employ nonequilibrium dynamical mean-field theory to exactly solve for the nonlinear response of an electron-mediated charge-density-wave-ordered material. We examine both the dc current and the order parameter of the conduction electrons as the ordered system is driven by the electric field. Although the formalism we develop applies to all models, for concreteness, we examine the charge-density-wave phase of the Falicov-Kimball model, which displays a number of anomalous behaviors including the appearance of subgap density of states as the temperature increases. These subgap states should have a significant impact on transport properties, particularly the nonlinear response of the system to a large dc electric field.
An $S=1$ antiferromagnetic polymeric chain, [Ni(HF$_2$)(3-Clpy)$_4$]BF$_4$ (py = pyridine), has previously been identified to have intrachain, nearest-neighbor antiferromagnetic interaction strength $J/k_{mathrm{B}} = 4.86$ K and single-ion anisotropy (zero-field splitting) $D/k_{mathrm{B}} = 4.3$ K, so the ratio $D/J = 0.88$ places this system close to the $D/J approx 1$ gapless critical point between the topologically distinct Haldane and Large-$D$ phases. The magnetization was studied over a range of temperatures, 50 mK $leq T leq 1$ K, and magnetic fields, $B leq 10$ T, in an attempt to identify a critical field, $B_{mathrm{c}}$, associated with the closing of the Haldane gap, and the present work places an upper bound of $B_{mathrm{c}} leq (35 pm 10)$ mT. At higher fields, the observed magnetic response is qualitatively similar to the excess signal observed by other workers at 0.5 K and below 3 T. The high-field (up to 14.5 T), multi-frequency (nomially 200 GHz to 425 GHz) ESR spectra at 3 K reveal several broad features considered to be associated with the linear-chain sample.
In low-dimensional metallic systems, lattice distortion is usually coupled to a density-wave-like electronic instability due to Fermi surface nesting (FSN) and strong electron-phonon coupling. However, the ordering of other electronic degrees of freedom can also occur simultaneously with the lattice distortion thus challenges the aforementioned prevailing scenario. Recently, a hidden electronic reconstruction beyond FSN was revealed in a layered metallic compound BaTi2As2O below the structural transition temperature Ts ~ 200 K. The nature of this hidden electronic instability is under strong debate. Here, by measuring the local orbital polarization through 75As nuclear magnetic resonance experiment, we observe a p-d bond order between Ti and As atoms in BaTi2As2O single crystal. Below Ts, the bond order breaks both rotational and translational symmetry of the lattice. Meanwhile, the spin-lattice relaxation measurement indicates a substantial loss of density of states and an enhanced spin fluctuation in the bond-order state. Further first-principles calculations suggest that the mechanism of the bond order is due to the coupling of lattice and nematic instabilities. Our results strongly support a bond-order driven electronic reconstruction in BaTi2As2O and shed light on the mechanism of superconductivity in this family.
LiCu2O2 is the first multiferroic cuprate to be reported and its ferroelectricity is induced by complex magnetic ordering in ground state, which is still in controversy today. Herein, we have grown nearly untwinned LiCu2O2 single crystals of high quality and systematically investigated their dielectric and ferroelectric behaviours in external magnetic fields. The highly anisotropic response observed in different magnetic fields apparently contradicts the prevalent bc- or ab- plane cycloidal spin model. Our observations give strong evidence supporting a new helimagnetic picture in which the normal of the spin helix plane is along the diagonal of CuO4 squares which form the quasi-1D spin chains by edge-sharing. Further analysis suggests that the spin helix in the ground state is elliptical and in the intermediate state the present c-axis collinear SDW model is applicable with some appropriate modifications. In addition, our studies show that the dielectric and ferroelectric measurements could be used as probes for the characterization of the complex spin structures in multiferroic materials due to the close tie between their magnetic and electric orderings.
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