Phonon Density of States and Anharmonicity of UO2

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.Comment: Text slightly modified, results unchange

Phonon Lifetime Investigation of Anharmonicity and Thermal Conductivity of UO₂ by Neutron Scattering and Theory

Inelastic neutron scattering measurements of individual phonon lifetimes and dispersion at 295 and 1200 K have been used to probe anharmonicity and thermal conductivity in UO2. They show that longitudinal optic phonon modes carry the largest amount of heat, in contrast to past simulations and that the total conductivity demonstrates a quantitative correspondence between microscopic and macroscopic phonon physics. We have further performed first-principles simulations for UO2 showing semiquantitative agreement with phonon lifetimes at 295 K, but larger anharmonicity than measured at 1200 K

Emergent Spin Topology in Cuprate Superconductors Seen with Neutrons

Peer reviewed: YesNRC publication: Ye

Neutron study of magnetic ordering of planar spins in a random anisotropy antiferromagnet: Evidence for quantum tunnelling?

Large As/V substitutions in DyVO4 result in planar Dy spins ordering antiferromagnetically in the presence of strong random anisotropy disorder. Neutron measurements on DyAs0.35V0.65O4 show the development of sharp but not resolution-limited magnetic diffraction peaks below 1.6?K. Magnetic ordering appears to equilibrate on a time scale of hours as the temperature is reduced in small steps, but, remarkably, the time scale is nearly independent of temperature.NRC publication: Ye

Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu2Si2

Many correlated electron materials, such as high-temperature superconductors1, geometrically frustrated oxides2 and low-dimensional magnets3, 4, are still objects of fruitful study because of the unique properties that arise owing to poorly understood many-body effects. Heavy-fermion metals5\u2014materials that have high effective electron masses due to those effects\u2014represent a class of materials with exotic properties, ranging from unusual magnetism, unconventional superconductivity and 'hidden' order parameters6. The heavy-fermion superconductor URu2Si2 has held the attention of physicists for the past two decades owing to the presence of a 'hidden-order' phase below 17.5 K. Neutron scattering measurements indicate that the ordered moment is 0.03muB, much too small to account for the large heat-capacity anomaly at 17.5 K. We present recent neutron scattering experiments that unveil a new piece of this puzzle\u2014the spin-excitation spectrum above 17.5 K exhibits well-correlated, itinerant-like spin excitations up to at least 10 meV, emanating from incommensurate wavevectors. The large entropy change associated with the presence of an energy gap in the excitations explains the reduction in the electronic specific heat through the transition.NRC publication: Ye

Central mode and spin confinement near the boundary of the superconducting phase in YBa[sub 2]Cu[sub 3]O[sub 6.353] (T[sub c] = 18 K)

We have mapped the neutron scattering spin spectrum at low energies in YBa2Cu3O6.353 (Tc=18 K), where the doping ~0.06 is near the critical value (pc=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 ~=2 meV relaxation rate and the other a central mode with a relaxation rate that slows to less than 0.08 meV below Tc. The spectrum mirrors that of a soft mode driving a central mode. Extremely short correlation lengths, 42\ufffd5 \ufffd in-plane and 8\ufffd2 \ufffd 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\ufffdel 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.NRC publication: Ye