Gate-controlled transitions in triple dots with interdot repulsion and magnetic field

Abstract

We study the quantum phase transition (QPT) and electronic transport in triple quantum dots for a wide range of the gate voltage ϵ. We focus on the effect of the interdot repulsion V and the magnetic field B. In the case of particle–hole (p–h) symmetry and B = 0, we find the local quadruplet–doublet transition of first order when V increases to a critical point Vc ≈ U, where U is the on-site repulsion. Beyond the p–h symmetry, the sequence of the QPTs depends on ϵ. For small ϵ, a first order doublet–singlet transition is observed. For middle ϵ, we find the quadruplet–triplet transition of first order at Vc1 and the triplet–singlet transition of the Kosterlitz–Thouless type at Vc2. For large ϵ, there are two kinds of first order QPT with phase sequence quadruplet–triplet–doublet. The magnetic field B compensates for the effect of V. For V > U, as B increases we find a first order or second order QPT from a low-spin state to a high-spin state. The restoring of the Kondo effect and a perfect spin filtering is realized in the appropriate regime of the magnetic field.