Abstract
Quantum mechanics states that hopping integral between local orbitals makes the energy band dispersive. However, in some special cases, there are bands with no dispersion due to quantum interference. These bands are called as flat band. Many models having flat band have been proposed, and many interesting physical properties are predicted. However, no real compound having flat band has been found yet despite the 25 years of vigorous researches. We have found that some pyrochlore oxides have quasi-flat band just below the Fermi level by first principles calculation. Moreover, their valence bands are well described by a tight-binding model of pyrochlore lattice with isotropic nearest neighbor hopping integral. This model belongs to a class of Mielke model, whose ground state is known to be ferromagnetic with appropriate carrier doping and on-site repulsive Coulomb interaction. We have also performed a spin-polarized band calculation for the hole-doped system from first principles and found that the ground state is ferromagnetic for some doping region. Interestingly, these compounds do not include magnetic element, such as transition metal and rare-earth elements.
Highlights
Electronic and magnetic properties of materials are mostly determined by their energy dispersion
We have used a full-potential augmented planewave (FLAPW) scheme and the exchange-correlation potential was constructed within the general gradient approximation [25]
The obtained band dispersion well agrees with the previous studies, while the existence of the quasi-flat band was not referred [19, 28]
Summary
Electronic and magnetic properties of materials are mostly determined by their energy dispersion. Electronic conductivity is high when the valence/ conduction band has large energy dispersion. Magnetic properties arise when the band dispersion is small. The band dispersion is determined by the character of atomic wave function. Most magnetic compound contains magnetic elements, such as transition metal elements and rare-earth elements. If we can synthesize a magnetic material containing no magnetic element, its impact is immeasurable
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