Cotunneling through two-level quantum dots weakly coupled to ferromagnetic leads

Abstract

The spin-polarized transport through two-level quantum dots weaklycoupled to ferromagnetic leads is considered theoretically in theCoulomb blockade regime. It is assumed that the dot is doublyoccupied, so that the current flows due to cotunneling throughsinglet and triplet states of the dot. It is shown that transportcharacteristics strongly depend on the ground state of the quantumdot. If the ground state is a singlet, differential conductance(G) displays a broad minimum at low bias voltage, while tunnelmagnetoresistance (TMR) is given by the Julliere value. If the ground state of the system is a triplet, there is a maximum indifferential conductance at zero bias when the leads form the antiparallel configuration. The maximum is accompanied by aminimum in TMR. The different behavior of G and TMR may thushelp to determine the ground state of the dot and the energydifference between the singlet and triplet states.