Quantum transport through circularly coupled triple quantum dots

We report a theoretical investigation into the Kondo transport properties of circularly coupled triple quantum dots (QDs), composed of one Coulomb-type QD and two Kondo-type QDs. These two Kondo-type QDs are coupled together to form an Aharonov-Bohm ( AB) ring by two channels: the direct coupling t(0) and the indirect one t via a Coulomb-type QD-M; these respectively serve as the continuous and discrete channels for observing the Fano effect. It is particularly interesting that this QD system may be seen as a powerful platform for studying the coexistence and interplay of Kondo, Fano and AB effects. First we study the device conductance in the absence of a magnetic field, and striking competition between t(0) and t is obtained. It is shown that the Kondo-induced conductance peak pattern without direct coupling is completely changed into a new pattern characteristic of one deep Fano-induced valley when t(0) becomes sufficiently strong. Furthermore, we show that the position of the bottom of the Fano valley is determined only by the specific values of the direct and indirect couplings. Then by applying a magnetic field to this QD ring we explore the AB oscillations. It is shown that the phase-locking effect still exists even in the Kondo regime, and the conductance is an even function of the reduced magnetic flux. The AB oscillation pattern becomes very complex for the QD-M level vertical bar epsilon(M)vertical bar approximate to 0 and strong t, t(0) couplings. In addition, the Fano-type valley can be strongly varied by the magnetic field in the AB ring.