Conductance plateau in quantum spin transport through an inter- acting quantum dot

Quantum spin transport is studied in an interacting quantum dot. It is found that a conductance ”plateau” emerges in the non-linear charge conductance by a spin bias in the Kondo regime. The conductance plateau, as a complementary to the Kondo peak, originates from the strong electron correlation and exchange processes in the quantum dot, and can be regarded as one of the characteristics in quantum spin transport. Introduction. – The quantum charge transport has been extensively investigated in a wide class of correlated electron systems, which involves the interaction between a localized spin and free conduction electrons, such as in metals containing magnetic impurities [1]. Among them, the Kondo effect was of fundamental importance and was proposed to emerge in the device consisting of a singleelectron quantum dot (QD) coupled to electrodes (i.e. leads) [2,3], and its conductance can be strongly enhanced in the Kondo regime due to the frequent occurrence of spin exchange processes. Rapid progresses in nano-technology have made it available to control the number of electrons in QD and the parameters (e.g. the energy level, coupling strength between QD and leads, and so on) of the QD [4]. The Kondo effect in the QD was observed experimentally [5,6], which was in excellent agreement with theoretical predictions [2,3]. Most early works focus on the Kondo physics driven by the charge current, which was generated in two non-magnetic leads and under the bias voltage V (hereafter the bias voltage is named as charge bias). Recently the Kondo effect of the spin-polarized transport in QD with the ferromagnetic leads was investigated [7, 8]. It reveals novel properties of the Kondo effect in the spin aspect. Since electron has both charge and spin, it would be imcomplete for any physical picture of the Kondo effect without full consideration of quantum spin transport. On the other hand, the spin current, in which electrons with spin up and down move in opposite directions, has attracted a lot of theoretical and experimental attentions. By using the magnetic tunneling injection, and electrical or optical injection technique, a pure spin current without accompanying the charge current can be generated and detected experimentally [9, 10]. These experimental progresses make it practical to investigate the properties of quantum spin transport in correlated systems. In the present paper, we introduce a spin bias instead of a charge bias to study the quantum transport in Kondo regime in an interacting QD system. The spin bias means the spin-dependent chemical potential in the two leads, and may produce a spin current flowing through the QD (see Fig.2) [11]. Consider a common-used device, which consists of a QD coupled to two non-magnetic leads. It is found that a conductance plateau emerges in the nonlinear charge conductance while a double peak appears in the non-linear spin conductance. The charge conductance plateau and spin conductance double peak only exist in the Kondo regime, and its width is equal to twice of the spin splitting of the QD’s energy levels. It is believed that these phenomena can be regarded as one of the intrinsic characteristics in quantum spin transport. General formalism. –