  In contrast to the well recognized transverse-electric-field-induced
half-metallicity in zigzag graphene nanoribbons, here we demonstrate by
first-principles calculations that zigzag graphene nanoribbons sandwiched
between hexagonal boron nitride nanoribbons or sheets can be tuned into
half-metal simply by a bias voltage or a moderate compressive strain. The
half-metallicity is attributed to an enhanced coupling effect of spontaneous
polarization and asymmetrical exchange correlation along the ribbon width. The
findings should open a viable route for efficient spin-resolved band
engineering in graphene based devices that are compatible with the current
technology of semiconductor industry.
