Nature of Spin Polarization in a Quantum Point Contact
Katsumoto Group
Creation of spin current in quantum structures of non-magnetic materials is a key technique in semiconductor spin-electronics (spintronics). The use of spin-orbit interaction (SOI) and quantum point contact (QPC) structure is a promising candidate. So called Rashba-type SOI splits electronic spin states according to the direction of momentum. Electronic states in a QPC structure are a set of one-dimensional bands with discrete edges due to quantization transverse to the one-dimensional motion. As a result, ordinary conductance of a QPC is quantized with the unit of 2e2/h, which is called conductance quantum. There are a number of experimental reports on the anomalous quantization of conductance at a half conductance quantum, i.e. e2/h, which phenomenon, called 0.5 conductance plateau has been attributed to the result of perfect spin polarization [1] though direct evidence was not obtained. Such a high spin polarization was also predicted on the 1.0 plateau (i.e. 2e2/h) [2] but due to the lack of detection method, there have been no experimental confirmation.
We have developed a new technique for the detection of spin-polarization with time-domain spin blockade. For that the two-electron tunneling process from the target device to a quantum dot (QD) is used. When the process is to a single orbital level, the probability is suppressed while it is not for the tunneling into two orbital levels. Hence the degree of suppression has a direct relationship to the spin-polarization in the target with the spin-relaxation time in the QD as a parameter.
As the base system, two-dimensional electron gas (2DEG) in an (In, Ga)As quantum well was adopted. The sample consists of a QPC with a side-coupled QD with another QPC for the remote charge detection (Fig. 1). The conductance of the QPC as a function of the gate voltage shown in Fig. 2 clearly exhibits 0.5 anomalous plateau structure in addition to the ordinary quantized plateau at 2e2/h. Spin-polarization measured with the QD detector is shown as a function of the QPC bias voltage VQPC in the insets of Fig. 2, both for 0.5 and 1.0 plateaus. Here the definition of spin-polarization P is P = (Nu–Nd) / (Nu+Nd), where Nu and Nd correspond to the concentration of up-spin electrons and down-spin electrons respectively. At zero-bias voltage, P is high on 0.5 plateau while it decreases with increasing VQPC. The differential conductance of the QPC, on the contrary, increases with VQPC indicating that the mechanism of the spin-polarization is a kind of spin-filtering, which cuts the flow of spin-down electrons. On the other hand at 1.0 plateau, P gradually increases with VQPC, indicating that the mechanism here is spin-rotation, which turns up the down spin of electrons. There is a small dip structure in P near the zero-bias at 0.5 plateau, which reflects the formation of the Kondo cloud. As just described we have demonstrated the powerfulness of the present method and revealed the mechanisms of spin-polarization in a QPC with Rashba-type SOI.
References
- [1] P. Debray et al., Nature Nanotechnology 4, 759 (2009).
- [2] M. Eto, T. Hayashi, and Y. Kurotani, J. Phys. Soc. Jpn. 74, 1934 (2005).
- [3] M. Kohda et al., Nature Communications 3, 1082 (2012).