First-principles study on the electronic structure of bismuth transition-metal oxides
The electronic structure, magnetic and electric properties, and lattice stability of multiferroic BiMnO 3 as a typical system in perovskite Bi transition-metal oxides (BiMO 3 ) are studied from first principles. It is demonstrated theoretically for the first time that the orbital ordering within the Mn eg orbitals is actually realized in BiMnO 3 , being consistent with crystallographic data, and plays a crucial role in the appearance of ferromagnetism. Total-energy calculation shows the ferromagnetic state is indeed stabilized. Electrical polarization of BiMnO 3 is also estimated based on the Berry phase theory. Lattice instability to off-centred displacement, which is driven by strong covalent bonding between Bi 6p and O 2p states, is found to be rather common in the BiMO 3 series.
A novel microstrip bandpass filter with four transmission zeros is proposed. Four transmission zeros close to the passband are realized to improve the selectivity for the passband. A sixth- order passband is realized with two shorted stubs and a dual-mode ring resonator. The transmission zeros near the passband can be adjusted conveniently by only changing the characteristic impedances of the coupled lines. For demonstration, a planar bandpass filter (3-dB bandwidth 21.9%) was designed and fabricated.