Transport through a double-quantum-dot system with noncollinearly polarized leads

Abstract

We investigate linear and nonlinear transport in a double quantum dot system weakly coupled to spin-polarized leads. In the linear regime, the conductance as well as the nonequilibrium spin accumulation are evaluated in analytic form. The conductance as a function of the gate voltage exhibits four peaks of different height, with mirror symmetry with respect to the charge neutrality point. As the polarization angle is varied, due to exchange effects, the position and shape of the peaks change in a characteristic way which preserves the electron-hole symmetry of the problem. In the nonlinear regime various spin-blockade effects are observed. Moreover, negative differential conductance features occur for noncollinear magnetizations of the leads. In the considered sequential tunneling limit, the tunneling magnetoresistance (TMR) is always positive with a characteristic gate voltage dependence for noncollinear magnetization. If a magnetic field is added to the system, the TMR can become negative.

Subgap features due to quasiparticle tunneling in quantum dots coupled to superconducting leads

“…Spin degeneracies can be neglected in the presence of unpolarized or collinearly polarized leads. 30,39 Orbital degeneracies are system dependent and they are not present in the single and double quantum dot systems discussed in this paper. A detailed discussion of their effects can be found, for example, in Refs.…”

“…29,30 In the second part, the case of independently coupled dots is discussed, where the detuning of the two gate voltages influences the level spacing of the energy spectrum. Thus, excited states can be observed only in detuning ranges where the difference between the energy level of the excited state and its ground state is less than 2| |.…”

“…In the case of site-independent on-site energies and on-site Coulomb interaction the Hamiltonian can be diagonalized analytically. 29,30 The superconducting leads are described by the mean-field formĤ MF B of the pairing Hamiltonian, where we additionally inserted a unity represented by a product of Cooper pair annihilation and creation operators,Ŝ ηŜ † η = 1, which will be specified later in Sec. II.…”

Sources of negative tunneling magnetoresistance in multilevel quantum dots with ferromagnetic contacts

“…Spin-dependent level shifts can originate to the lowest order in the tunneling Hamiltonian (sequential tunneling limit) if off-diagonal elements of the reduced density matrix participate in the dynamics. This is the case for noncollinearly polarized leads 14,16,17 but also for collinearly polarized leads for some double-dot setups. 18 Here we consider situations where coherences of the reduced density matrix do not play a crucial role in the dynamics.…”

Theory of spin blockade, charge ratchet effect, and thermoelectrical behavior in serially coupled quantum dot system

“…(A.3-14), we found that the one-particle retarded GF can be simply decomposed into a sum of contributions from eight spin-charge configurations of the SCQD system in the condition of t 12 /U ℓ ≪ 1 (after ignoring those small contributions proportional to t 12 /U .). It reads 15) where P m = p m 0 ; p m 's are probability weights defined in Eq. (3).…”