作為一個粒子的本徵量子數，自旋在物理的許多領域中已經有了相當廣泛的研究。其中在電子學的應用即是被視為取代傳統電子元件的候選之ㄧ:自旋場效電晶體。在以自旋為訊號的系統中，自旋極化電流的產生是一個非常重要的課題。藉由鐵磁性材料與外加磁場的作用下，自旋極化電流的產生、操控以及偵測已經有了廣泛的應用。但是，鐵磁性材料的應用卻與半導體科技不能和諧共存。
自旋軌道耦合效應或許能成為另一個有力的方法用以操縱以及分析電子自旋，並且不需要鐵磁性材料以及外加磁場的應用。近期的研究指出，藉由施加直流偏壓在由量子點接觸形成的一維量子線之源極與汲極能破壞動量的簡併態，進而產生自旋極化電流。量子點接觸因此而能當作自旋極化電流的產生器與偵測器。但是產生自旋極化電流的機制仍然尚未明瞭。
在我們的研究中，存在於砷化鎵銦/砷化鋁銦的異質結構介面所形成二維電子氣中的電子，由於自旋軌道耦合效應的存在以及磁聚焦技術的應用，帶有不同自旋的電子在空間中被分離，我們能夠實際偵測到通過量子線的電子所帶的自旋資訊。磁聚焦的量測結果提供了透過施加直流偏壓在一維量子線的兩端所引發的電子自旋極化的直接證據。

Spin, as an intrinsic quantum number of the particle, is extensively investigated in many fields in physics. One of the applications in electronics is thought to be a candidate for substituting the conventional electronic devices: the spin field-effect transistor. The generation of the spin-polarized current is therefore important in such system. The ferromagnet together with external magnetic field has been widely used for the creation, manipulation and detection of the spin-polarized current. However, it is incompatible with the semiconductor technology.
Spin-orbit interaction may be another powerful tool for controlling and analyzing the spin without an external magnetic field. Recently, it was shown that applying the dc source-drain voltage on a semiconductor-based one- dimensional (1D) conductor would induce the spin polarization by breaking the momentum degeneracy. The quantum point contact is therefore expected to be a promising spin polarizer and analyzer. However, the mechanism for its spin polarization is still an open problem.
In our research, we apply the magnetic focusing technique to assess the spin content of the biased 1D conductor in an InGaAs/InAlAs heterostructure because the opposite spins are spatially separated in cyclotron motion due to the Rashba spin-orbit interaction. Our focusing spectrum provides the direct evidence that source-drain bias triggers a spin symmetry breaking and gives rise to spin polarization.

[1] S. A. Wolf, D. D. Awchalom, R. A. Buhrman, J. M. Daughton, S. von Moln’ar, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Science 294, 1488 (2001).
[2] S. Datta and B. Das, Applied Physics Letters, 56, 665 (1990).
[3] H. C. Koo, J. H. Kwon, J. Eom, J. Chang, S. H. Han, and M. Johnson, Science 325, 1515(2009).
[4] P. Debray, S. M. Rahman, J. Wan, R. S. Newrock, M. Cahay, A. T. Ngo, S.E.Ulloa,S. T. Herbert, M. Muhammad, and M. Johnson, Nature Nanotechnology, 4, 759 (2009).
[5] T. -M. Chen, A. C. Graham, M. Pepper, I. Farrer, and D. A. Ritchie, Applied Physics Letters, 93, 032102 (2008).
[6] T. Koga, J. Nitta, T. Akazaki, and T. Enoki, Phys. Rev. Lett. 89, 046801 (2002).
[7] K. Berggren and M. Pepper, Physics World 15, 37 (2002).
[8] H. van Houten and C. Beenakker, Physics Today 49, 22 (1996).
[9] T.-M. Chen, Electron-electron interactions in GaAs quantum wires. University of Cambridge, 2009
[10] B. Das, D. C. Miller, S. Datta, R. Reifenberger, W. P. Hong, P. K. Bhattacharya, J. Singh, and M. Jaffe, Phys. Rev. B 39, 1411 (1989).
[11] S. D. Ganichev, V. V. Bel’kov, L. E. Golub, E. L. Ivchenko, P. Schneider, D. Weiss, and W. Prettl, Phys. Rev. Lett. 92, 256601 (2004).
[12] R. M. Potok, J. A. Folk, C. M. Marcus, and V. Umansky, Phys. Rev. Lett. 89, 266602 (2002).
[13] L. P. Rokhinson, L. N. Pfeiffer, and K.W.West, Phys. Rev. Lett. 96, 156602 (2006).
[14] T.-M. Chen, M. Pepper, I. Farrer, G. A. C. Jones, and D. A. Ritchie, Phys. Rev. Lett. 109, 177202 (2012).
[15] S. Ihnatsenka, and I. V. Zozoulenko, Phys. Rev. B 79, 235313 (2009).
[16] P. J. Simmonds, F. Sfigakis, H. E. Beere, D. A. Ritchie, M. Pepper, D. Anderson, and G. A. C. Jones, Applied Physics Letters 92, 152108 (2008).
[17] M. Akabori, S. Hidaka, H. Iwase, S. Yamada, and U. Ekenberg, Journal of Applied Physics, 112, 113711 (2012).
[18] L. P. Rokhinson, V. Larkina, West Lafayette, Y. B. Lyanda-Geller, L. N. Pfeiffer and K.W. West, Phys. Rev. Lett. 93, 146601 (2004).
[19] T.-M. Chen, M. Pepper, I. Farrer, D. A. Ritchie, and G. A. C. Jones, Applied Physics Letters 103, 093503 (2013).
[20] Shun-Tsung Lo, Chin-Hung Chen, Ju-Chun Fan, L.W. Smith, G.L. Creeth, Che-Wei Chang, M. Pepper, J.P. Griffiths, I. Farrer, H.E. Beere, G.A.C. Jones, D.A. Ritchie & Tse-Ming Chen, Nature Communications 8, 15997 (2017).
[21] H. van Houten, C. W. J. Beenakker, J. G. Williamson, M. E.I. Broekaart, and P. H. M. van Loosdrecht, Phys. Rev. B 39, 8556–8575 (1989).
[22] A. C. Graham, M. Pepper, M. Y. Simmons, and D. A. Ritchie, Physica E (Amsterdam) 34, 588 (2006).
[23] K. J. Thomas, J. T. Nicholls, M. Y. Simmons, M. Pepper, D. R. Mace, and D. A. Ritchie, Philos. Mag. B 77, 1213 (1998).
[24] Pojen Chuang, Sheng-Chin Ho, L. W. Smith, F. Sfigakis, M. Pepper, Chin-Hung Chen, Ju-Chun Fan, J. P. Griffiths, I. Farrer, H. E. Beere, G. A. C. Jones, D. A. Ritchie and Tse-Ming Chen, Nature Nanotechnology 10, 35–39 (2015).
[25] Andreas Lassl, Peter Schlagheck, and Klaus Richter, Phys. Rev. B 75, 045346 (2007).
[26] A. Reynoso, Gonzalo Usaj, and C. A. Balseiro Phys. Rev. B 75, 085321 (2007).