Cotunneling through two - level quantum dots weakly cou - pled to ferromagnetic leads

– The spin-polarized transport through two-level quantum dots weakly coupled to ferromagnetic leads is considered theoretically in the Coulomb blockade regime. It is assumed that the dot is doubly occupied, so that the current flows due to cotunneling through singlet and triplet states of the dot. It is shown that transport characteristics strongly depend on the ground state of quantum dot. If the ground state is a singlet, differential conductance (G) displays a broad minimum at low bias voltage, while tunnel magnetoresistance (TMR) is given by the Julliere value. If triplet is the ground state of the system, there is a maximum in differential conductance at zero bias when the leads are magnetized in antiparallel. The maximum is accompanied by a minimum in TMR. The different behavior of G and TMR may thus help to determine the ground state of the dot and the energy difference between the singlet and triplet states. Introduction. – Transport properties of quantum dots coupled to ferromagnetic leads have been a subject of thorough studies since a few years [1,2]. The considerations concerned both the strong coupling regime where the Kondo physics emerges [3], as well as the weak coupling regime. In the weak coupling regime most of theoretical works addressed the problem of spin-dependent sequential transport through quantum dots hosting single orbital level [4]. In the sequential tunneling regime, if the applied bias voltage exceeds a certain threshold voltage, electrons tunnel one by one through the system, otherwise the current is suppressed leading to the Coulomb blockade effect. Although in the Coulomb blockade regime the sequential transport is suppressed, the current can be still mediated by higher-order tunneling processes such as cotunneling which involves correlated tunneling of two electrons via virtual states of the dot [5]. Spin-polarized cotunneling transport has been addressed very recently. It has been shown that when the dot is singly occupied, a zero-bias anomaly appears in differential conductance when the magnetizations of the leads are aligned in antiparallel [6, 7]. These considerations were performed only for single-level quantum dots. In real systems, however, usually more than one energy level participate in transport, which can lead to further interesting effects [8]. The goal of this paper is to address the problem of spin-dependent transport through twolevel quantum dots coupled to ferromagnetic leads in the cotunneling regime. More specifically,