Wei Ku

Exchange Coupling in Eu Monochalcogenides from First Principles

Using a density functional method with explicit account for strong Coulomb repulsion within the 4 f shell, we calculate effective exchange parameters and the corresponding ordering temperatures of the (ferro)magnetic insulating Eu monochalcogenides (EuX; X = O, S, Se, Te) at ambient and elevated pressure conditions. Our results provide quantitative account of the many-fold increase of the Curie temperatures with applied pressure and reproduce well the decrease of ferromagnetic coupling across the EuO–EuTe series. The first J 1 and second J 2 neighbor effective exchange are found to follow different pressure dependencies. Finally, our calculations show explicitly that the mixing of Eu 4 f orbitals with the ligand states is necessary for the ferromagnetic ordering to take place at any pressure.

Insulating ferromagnetism in La4Ba2Cu2O10: an Ab initio wannier function analysis.

Microscopic mechanisms of the puzzling insulating ferromagnetism of half-filled La4Ba2Cu2O10 are elucidated with energy-resolved Wannier states. The dominant magnetic coupling, revealed through evaluated parameters (t, U, and J), turns out to be the intersite direct exchange, a currently ignored mechanism that overwhelms the antiferromagnetic superexchange. By contrast, the isostructural Nd4Ba2Cu2O10 develops the observed antiferromagnetic order via its characteristics of a 1D chain. Surprisingly, the in-plane order of both cases is not controlled by coupling between nearest neighbors. An intriguing pressure-induced ferromagnetic to antiferromagnetic transition is predicted.

Ab Initio Investigation of Collective Charge Excitations in MgB2

A sharp collective charge excitation is predicted in MgB2 at approximately 2.5 eV for q perpendicular to the boron layers, based on an all-electron analysis of the dynamical density response within time-dependent density functional theory. This novel excitation, consisting of coherent charge fluctuation between Mg and B sheets, induces an abrupt plasma edge in the experimentally observable reflectivity. The existence of this mode reflects the unique electronic structure of MgB2 that is also responsible for strong electron-phonon coupling. By contrast, the acoustic plasmon, recently suggested to explain the high T(c), is not realized when realistic transition strengths are incorporated.

Microscopic analysis of insulating magnetism of La4Ba2Cu2O10 and Nd4Ba2Cu2O10

Abstract The rare-earth ions La and Nd evidently play a crucial role in the spin ordering of the Cu(2+) moments in (RE)4Ba2Cu2O10 (“RE422”), because La422 is a ferromagnetic (FM) insulator (a rare occurrence anyway) while Nd422 is antiferromagnetic (AF). Here we introduce explicit Wannier function (the “spin orbital”) for the problem that illustrates the unusually strong effect of covalency and extends to (and beyond) the rare-earth ions. Material constants t, U, and (interatomic) J are calculated (not fitted) directly from the Wannier functions and show that the FM interatomic direct exchange can overwhelm the usual AF superexchange. Computationally, La422 is compressed to the volume of Nd422 to demonstrate that volume difference is not the determining factor; rather it is local “chemistry” on the rare-earth ion. This viewpoint is couched within a more general approach to the magnetic insulator problem.

Insulating Ferromagnetism in La

Microscopic mechanisms of the puzzling insulating ferromagnetism of half-filled La4Ba2Cu2O10 are elucidated with energy-resolved Wannier states. The dominant magnetic coupling, revealed through evaluated parameters (t, U, and J), turns out to be the intersite direct exchange, a currently ignored mechanism that overwhelms the antiferromagnetic superexchange. By contrast, the isostructural Nd4Ba2Cu2O10 develops the observed antiferromagnetic order via its characteristics of a 1D chain. Surprisingly, the in-plane order of both cases is not controlled by coupling between nearest neighbors. An intriguing pressure-induced ferromagnetic to antiferromagnetic transition is predicted.

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Microscopic mechanisms of the puzzling insulating ferromagnetism of half-filled La4Ba2Cu2O10 are elucidated with energy-resolved Wannier states. The dominant magnetic coupling, revealed through evaluated parameters (t, U, and J), turns out to be the intersite direct exchange, a currently ignored mechanism that overwhelms the antiferromagnetic superexchange. By contrast, the isostructural Nd4Ba2Cu2O10 develops the observed antiferromagnetic order via its characteristics of a 1D chain. Surprisingly, the in-plane order of both cases is not controlled by coupling between nearest neighbors. An intriguing pressure-induced ferromagnetic to antiferromagnetic transition is predicted.