  An atomistic effective Hamiltonian technique is used to investigate the
finite-temperature energy storage properties of a ferroelectric nanocomposite
consisting of an array of BaTiO$_{3}$ nanowires embedded in a SrTiO$_{3}$
matrix, for electric field applied along the long axis of the nanowires. We
find that the energy density \textit{versus} temperature curve adopts a
nonlinear, mostly temperature-independent response when the system exhibits
phases possessing an out-of-plane polarization and vortices while the energy
density more linearly increases with temperature when the nanocomposite either
only possesses vortices (and thus no spontaneous polarization) or is in a
paraelectric and paratoroidic phase for its equilibrium state. Ultrahigh energy
density up to $\simeq$140 J/cm$^{3}$ and an ideal 100% efficiency are also
predicted in this nanocomposite. A phenomenological model, involving a coupling
between polarization and toroidal moment, is further proposed to interpret
these energy density results.
