Possibility of Flat-Band Ferromagnetism in Hole-Doped Pyrochlore Oxides Sn_{2}Nb_{2}O_{7} and Sn_{2}Ta_{2}O_{7}.

Quantum mechanics tells us that the hopping integral between local orbitals makes the energy band dispersive. In a lattice with geometric frustration, however, dispersionless flat bands may appear due to quantum interference. Several models possessing flat bands have been proposed theoretically, and many attracting magnetic and electronic properties are predicted. However, despite many attempts to realize these models experimentally, compounds that are appropriately described by this model have not been found so far. Here we show that pyrochlore oxides Sn_{2}Nb_{2}O_{7} and Sn_{2}Ta_{2}O_{7} are such examples, by performing first-principles band calculation and several tight-binding analyses. Moreover, spin-polarized band calculation shows that the hole-doped systems Sn_{2}Nb_{2}O_{6}N and Sn_{2}Ta_{2}O_{6}N have complete spin polarization, and their magnetic moments are mostly carried by Sn-s and N-p orbitals, which are usually nonmagnetic. These compounds are not only candidates for ferromagnets without a magnetic element, but also will provide an experimental platform for a flat-band model which shows a wide range of physical properties.