閘極局域量子元件的自旋極化操控與偵測

Abstract

我們將針對一系列以閘極局域的量子線與量子點做系統性的自旋極化研究，自我存在的自旋束縛態或自旋軌道耦合都有可能產生自旋極化電流，即使在沒有鐵磁性接點和外加磁場下；我們將利用磁場聚焦技術來測量樣品的自旋極化值，希望藉由此研究工作，學會以電性測量有效地製造、操控、與偵測自旋極化電流。 過去數年，我們已建立一套電子束微影技術，並成功地在砷化鎵異質結構上高mobility的二維電子氣製作各式奈米尺度的表面閘極，除此之外，我們也可以加製作上電極來控制二維電子氣與一維、零維結構的電子密度，上電極與表面閘極中間夾著約100nm的絕緣電子阻劑(PMMA)，在微結構中的局域電子自旋極化來自多體之間的交互作用，當電子密度減少，多體之間的交互作用就越強，同時我們也知道它也會受微結構的幾何形狀影響，像量子線的長度就會增加其內部電子的背向散射，因此量子線的自旋極化值會因電子密度與幾何長度而有所不同。 一系列以閘極局域的量子線與量子點將會投入於研究因電子密度與幾何長度不同而導致的自旋極化，我們將系統性地歸類其物理屬性，藉此也一併釐清0.7結構與一維量子線的基態，之後，我們會接著探討含量子點與量子線耦合的複式系統，對大一些的量子點，我們將對因自旋向上與向下電子束不等產生的自旋堆積做電性的控制與測量，在非區域性自旋閥線路中，藉由調變量子線使之與量子點的四端接觸產生選擇性自旋傳輸，達成自旋之注入與偵測。最後，我們將研究複合式奈米結構的傳輸機制，量子點的電荷傳輸在適量的Zeeman分裂下將可成為相位同調的自旋極化，組合量子點與量子尖端接點可形成自旋極化電流源與偵測器，我們將研究自旋軌道作用在自旋鬆弛與非同調性的扮演角色。

In all, we propose to investigate spin polarization systematically in gate-confined quantum wires and quantum dots. Either intrinsic spin bound state or spin-orbit coupling may generates spin polarization current in nanostructures without ferromagnetic contacts and applied magnetic fields. A magnetic focusing technique will be used to measure spin polarization. Through this work, we will learn how to effectively create, manipulate, and detect spin polarized currents by electrical means. We have been set up e-beam lithography that allows us to make different arrangements of surface gates atop GaAs-based 2DEG. Moreover, on top of the split gates, being isolated by a ~100nm thick dielectric layer of cross-linked Polymethylmethacrylate (PMMA), a top metallic gate (top) can be fabricated to control the carrier concentration. Spin polarization of carriers in nanostructures can result from strongly enhanced many-body interactions, which arise when the carriers are confined in a quantum wire or a quantum dot. Many body interactions are predominantly influenced by carrier density in quantum wires. Later we also found that electron backscattering depends on the wire geometry. Hence, QWs of different carrier density and different geometry will have different values of spin polarization. A series of QWs and QDs are employed to study the carrier density and wire geometry dependent spin polarization. The degree of spin polarization among samples will be cataloged. Through this, we would figure out the origin of 0.7 structures and ground state of 1D interacting wires. Later, a complex system consist of a QD and a QW will be studied. We will make electric control and measurement of spin accumulation-an imbalance of spin up and spin down electrons –in large QD. Spin injection an detection can be achieved with a QW tuned to have spin selective transport with four contacts per dot for realizing a non-local spin valve circuit. It has also been known that a quantum-dot based charge transport in the presence of sizable Zeeman splitting would function as a phase-coherent spin current. Combination of a quantum dot and a quantum point contact can act as a spin-polarized current generator and detector. The role of spin-orbit interaction in spin relaxation and decoherence will be explored