Spin thermoelectric effects in Kondo quantum dots coupled to ferromagnetic leads

Thermoelectric effects in transport through a quantum dot coupled to external ferromagnetic leads are investigated theoretically. The basic thermoelectric transport characteristics, such as thermopower, electronic contribution to the heat conductance, and the corresponding figure of merit, are calculated in the linear response regime by means of the density-matrix numerical renormalization group method. The case of a nonzero spin splitting of the electrochemical potential in the electrodes is also considered and the associated spin thermoelectric effects are analyzed. It is shown that the spin-dependent thermoelectric phenomena in the local moment regime depend generally on the exchange field induced by ferromagnetic contacts. In addition, the temperature dependence of the Seebeck coefficient is rather nontrivial, and depends on the spin polarization and spin relaxation in the leads. In the presence of ferromagnetic leads, the thermopower as a function of temperature may change sign more times than the thermopower for nonmagnetic leads. These changes can be thus used to determine the relevant Kondo behavior and Kondo energy scale in the system. Moreover, the effects of external magnetic field and different spin polarization of ferromagnetic leads are also analyzed.