Quan Yin

Origin of low thermal conductivity in nuclear fuels.

Using a novel many-body approach, we report lattice dynamical properties of UO2 and PuO2 and uncover various contributions to their thermal conductivities. Via calculated Grüneisen constants, we show that only longitudinal acoustic modes having large phonon group velocities are efficient heat carriers. Despite the fact that some optical modes also show their velocities which are extremely large, they do not participate in the heat transfer due to their unusual anharmonicity. Ways to improve thermal conductivity in these materials are discussed.

Electronic correlation and transport properties of nuclear fuel materials

The electronic structures and transport properties of a series of actinide monocarbides, mononitrides, and dioxides are studied systematically using a combination of density-functional theory and dynamical mean-field theory. The studied materials present different electronic correlation strength and degree of localization of 5f electrons, where a metal-insulator boundary naturally lies within. In the spectral function of Mott-insulating uranium oxide, a resonance peak is observed in both theory and experiment and may be understood as a generalized Zhang-Rice state. We also investigate the interplay between electron-electron and electron-phonon interactions, both of which are responsible for the transport in the metallic compounds. Our findings allow us to gain insight in the roles played by different scattering mechanisms, and suggest how to improve their thermal conductivities.

Anisotropy, itineracy, and magnetic frustration in high-Tc iron pnictides.

Using first-principles density functional theory calculations combined with insight from a tight-binding representation, dynamical mean field theory, and linear response theory, we have extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short range, and the nearest (J_{1a}) and next-nearest (J2) exchange constants follow the universal trend of J_{1a}/2J_{2} approximately 1, despite their itinerant origin and extreme sensitivity to the z position of As. These results bear on the discussion of itineracy versus magnetic frustration as the key factor in stabilizing the superconducting ground state. The calculated spin-wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast with cuprates.

Calculated momentum dependence of Zhang-Rice states in transition metal oxides.

Using a combination of local density functional theory and cluster exact diagonalization based dynamical mean field theory, we calculate many-body electronic structures of several Mott insulating oxides including undoped high T(c) materials. The dispersions of the lowest occupied electronic states are associated with the Zhang-Rice singlets in cuprates and with doublets, triplets, quadruplets, and quintets in more general cases. Our results agree with angle resolved photoemission experiments including the decrease of the spectral weight of the Zhang-Rice band as it approaches k=0.

Calculation of magnetic exchange interactions in Mott-Hubbard systems.

An efficient method of computing magnetic exchange interactions in systems with strong correlations is introduced. It is based on a magnetic force theorem that evaluates linear response due to rotations of magnetic moments and uses a generalized spectral density functional framework allowing us to explore several approximations ranging from local density functional to exact diagonalization based dynamical mean field theory. Applications to spin waves and magnetic transition temperatures of 3d metal mono-oxides as well as high-T(c) superconductors are in good agreement with experiment.

Multiple quantum phase transitions of plutonium compounds

We show by quantum Monte Carlo simulations of realistic Kondo lattice models derived from electronic-structure calculations that multiple quantum critical points can be realized in plutonium-based materials. We place representative systems, including PuCoGa

Spin and charge fluctuations in α -structure layered nitride superconductors

{}_{5}

Spin and charge fluctuations inα-structure layered nitride superconductors

, on a realistic Doniach phase diagram and identify the regions where the magnetically mediated superconductivity could occur. The solution of an inverse problem to restore the quasiparticle renormalization factor for

Spin and Charge Fluctuations in the

f

\alpha

electrons is shown to be sufficiently good to predict the trends among Sommerfeld coefficients and magnetism. A suggestion on the possible experimental verification for this scenario is given for PuAs.