  Generating a nonequilibrium spin polarization with a driving force has been
first realized by the electric current in a system with broken inversion
symmetry and extended to that induced by the thermal current and that appearing
in an inversion-symmetric system with locally-broken inversion symmetry. This
paper theoretically explores the spin polarization generated by the thermal
current and the locally-broken inversion symmetry in a symmetric
double-quantum-well structure (DQWS). This thermally-induced spin polarization
(TISP) appears in the antiparallel configuration with the TISP of two wells in
opposite directions. The calculation using the Boltzmann equation in the
relaxation-time approximation under the condition of zero charge current shows
that the local TISP exhibits the maximum at a finite Rashba spin-orbit
interaction when the electron density is fixed. This is because the local TISP
in the DQWS is enhanced at the chemical potential near the bottom of the
first-excited subband. This enhancement also occurs in a single quantum well
with globally-broken inversion symmetry. Another finding is that the maximum of
the local TISP appears at a nonzero interwell coupling. The spin current by the
diffusion of the local TISP into an adjacent electrode is also calculated.
