The SU(4)-SU(2) crossover and spin filter properties of a double quantum dot nanosystem

The $SU(4)-SU(2)$ crossover, driven by an external magnetic field $h$, is analyzed in a capacitively-coupled double-quantum-dot device connected to independent leads. As one continuously charges the dots from empty to quarter-filled, by varying the gate potential $V_g$, the crossover starts when the magnitude of the spin polarization of the double quantum dot, as measured by $\langle n_{\uparrow}\rangle -\langle n_{\downarrow}\rangle$, becomes finite. Although the external magnetic field breaks the $SU(4)$ symmetry of the Hamiltonian, the ground state preserves it in a region of $V_g$, where $\langle n_{\uparrow}\rangle -\langle n_{\downarrow}\rangle =0$. Once the spin polarization becomes finite, it initially increases slowly until a sudden change occurs, in which $\langle n_{\downarrow}\rangle$ (polarization direction opposite to the magnetic field) reaches a maximum and then decreases to negligible values abruptly, at which point an orbital $SU(2)$ ground state is fully established. This crossover from one Kondo state, with emergent $SU(4)$ symmetry, where spin and orbital degrees of freedom all play a role, to another, with $SU(2)$ symmetry, where only orbital degrees of freedom participate, is triggered by a competition between $g\mu_Bh$, the energy gain by the Zeeman-split polarized state and the Kondo temperature $T_K^{SU(4)}$, the gain provided by the $SU(4)$ unpolarized Kondo-singlet state.