  The ferroelectric polarization of triangular-lattice antiferromagnets induced
by helical spin-spiral order is not explained by any existing model of
magnetic-order-driven ferroelectricity. We resolve this problem by developing a
general theory for the ferroelectric polarization induced by spin-spiral order
and then by evaluating the coefficients needed to specify the general theory on
the basis of density functional calculations. Our theory correctly describes
the ferroelectricity of triangular-lattice antiferromagnets driven by helical
spin-spiral order, and incorporates known models of magnetic-order-driven
ferroelectricity as special cases.
