政治大學應用物理研究所 郭光宇
(2011/08/24)Visit to Nanjing University: I left 桃園機場 for Nanjing via 立榮 direct cross-strait flight on June 29. It would take only about 2.0 hours and is very convenient. There were no direct flights until two years ago, and I usually went to any cities in the Mainland of China via Hong Kong which would need a whole day. After arriving in the Nanjing Airport, I was sent by a shuttle bus to a Hotel near Nanjing University. I was then greeted by two young Professors: Xiangang Wan and Jian Zhou from the Physics Department. On the next day (6/30), I had intensive discussions with both Prof. Wan and Prof. Zhou, respectively, on the developments of the ab initio many-body dynamic mean-field theory for strongly correlated oxides, and on optical property calculations using the WIEN2k and VASP packages. During the lunch break, I took the opportunity to say “hello” to Prof. Ding-Yu Xing and other “old friends” in the Physics Department. On the Friday, July 1, I gave a talk in the Physics Department titled “Physics and Applications of Spin Hall Effect” (see also the enclosed abstract below). This has been a hot topic in recent years and thus a lot of people attended my seminar. Also before and after my seminar, I had extensive discussions with Profs. Jimming Dong and Jian Zhou on possible collaborations. In particular, we agreed to join our forces to work together on ab initio studies on the anomalous Hall effect and also magneto-optical properties of new materials.
ISMMM2011 in Shanghai: I left Nanjing for Shanghai on Sunday, July 3. I took new Chinese High Speed Trains from Nanjing to Shanghai. It was rather quick. I took a taxi to a hotel near Shanghai Institute of Ceramics, Chinese Academy of Sciences, where the Symposium was held. It was very stimulating symposium. Experts in the field from many countries in world come to participate this Workshop, including USA, Austria, Japan as well as many places in the Mainland China. The topics covered include fundamental theories, computational methods and algorithms as well as applications to many presently interesting subjects (see the program attached below). My talk was scheduled on the first day morning (the second talk in fact) (July 4) of the Symposium. My talk was on another currently hot topic of nano-materials titled “Ab Initio Computational Studies of the Optical Properties and Excitonic Effects in SiC Nanotubes” (see the abstract enclosed below) and was well received by the audience. The program of the Symposium was rather relaxed in order to allow time for many intensive discussions. It was a timely and helpful Symposium. I enjoyed it every minute. In particular, it was pleasing to talk many overseas old friends and collaborators, e.g., Profs.
Ding-Sheng Wang, Xingao Gong, Wenqing Zhang (China), Wai-Yim Ching (Missouri), David Singh (Oak Ridge), Peihong Zhang (New York). It was equally useful to meet and make new friends, e.g., Profs. Martijn Marsman (Vienna), Anton van der Ven (Michigan), Boawen Li (Singapore), and Chuang-Tian Chen (Chinese Academy of Sciences).
The Symposium finished in the later afternoon of July 5. This was then followed by the 2 day Tutorial Workshop on Ab Initio Simulations Methods, focusing on Vianna Ab-Initio Simulation Package (VASP). I attended the morning sessions of the Workshop on July 6. However, I went to Fudan University to see several old friends, e.g., Profs. Xiaofeng Jin and Lei Zhou. My discussion on anomalous Hall effect in metals (Fe, Ni) with Prof. Jin was especially helpful. After the discussion, I was able to finish our manuscript in the intrinsic anomalous Hall effect in Ni and this manuscript was submitted to the Physical Review B soon afterwards (see arXiv:1107.3242). I left Shanghai for Suzhou on July 7. I returned to Taiwan on July 17 via 上海虹橋機場 by Eva Airline. I arrived at 松 山機場in the late evening of July 17.
My research certainly benefits quite a lot from this trip to Nanjing, Shanghai and Suzhou. I therefore want to thank the National Science Council for the financial support for this trip.
2011/7/1 在
南京大學物理系學術
演講論文摘要:Physics and Applications of Spin Hall Effect
Guang-Yu Guo (郭光宇)1,2
1Graduate Institute of Applied Physics, National Chengchi University, Taipei 116, Taiwan
2Department of Physics, National Taiwan University, Taipei 106, Taiwan
Spin Hall effect refers to the generation of transverse spin current in a solid by an electric field or current. Spin current generation is an important issue in the emerging spintronics technology. Therefore, spin Hall effect has recently become an issue of intensive interests both theoretically and experimentally since the theoretical proposals of the intrinsic spin Hall effect [1-2]. In this talk, I will first give an introduction to spin Hall effect and its potential
applications. I will then describe ab initio calculation approaches to the various issues in the field of spin Hall effect, in particular, Berry phase theory and ab initio relativistic band structure method for studying intrinsic spin Hall effect [3]. This will be followed by a review on our recent ab initio relativistic band theoretical studies on the intrinsic spin Hall effect in platinum, aluminum [4], palladium, gold [5] and Mo. In particular, our ab initio calculations revealed that the resonant contribution from the spin-orbit splitting of the doubly degenerated d bands at high-symmetry points near the Fermi level gives rise to a large intrinsic spin Hall conductivity in Pt and Pd. These predictions are confirmed by latest experimental measurements [6].
More recently, our ab initio electronic structure calculations for various transition metal impurities in gold indicated possible orbital-dependent Kondo effect in Fe impurity in Au [7]. Thus, the gigantic spin Hall effect observed recently in FePt/Au system [8] was attributed to extrinsic resonant skew scattering mechanism due to multi-orbital Kondo effect [7,9]. Our estimated spin Hall angle is about 0.1, in agreement with the measured value [8].
This explains why the gigantic spin Hall effect in Au with Fe impurities was observed in recent experiments, while it is not visible in the anomalous Hall effect. Indeed, recent quantum Monte Carlo simulations for the realistic three-orbital Anderson impurity model demonstrated two Kondo temperatures in Fe in Au: one very low Kondo temperature for Fe deg-states (a few Ks) and the other high Kondo temperature for Fe t2g-states (above room temperature) [10].
Furthermore, latest experiments on Au films with well controlled Fe impurity concentrations confirmed that the spin Hall angle is about 0.07 and importantly, is independent of Fe impurity concentration [11], thereby showing the extrinsic nature of the spin Hall effect observed in FePt/Au systems.
The speaker thanks Qian Niu, Naoto Nagaosa, Bo Gu, Sadamichi Maekawa, Tsung-Wei Chen, Shuichi Murakami for stimulating discussions and collaborations. He also thanks the National Science Council of Taiwan for financial supports.
[1] S. Murakami, N. Nagaosa, and S.-C. Zhang, Science 301, 1348 (2003).
[2] J. Sinova et al., Phys. Rev. Lett. 92, 126603 (2004).
[3] G. Y. Guo, Y. Yao and Q. Niu, Phys. Rev. Lett. 94, 226601 (2005).
[4] G. Y. Guo, S. Murakami, T. W. Chen, and N. Nagaosa, Phys. Rev. Lett. 100, 096401 (2008).
[5] G. Y. Guo, J. Appl. Phys. 105, 07C701 (2009).
[6] O. Mosendz, et al., Phys. Rev. B 82, 214403 (2010).
[7] G. Y. Guo, S. Maekawa, and N. Nagaosa, Phys. Rev. Lett. 102, 036401 (2009).
[8] T. Seki et al., Nature Mater. 7, 125 (2008).
[9] P. Coleman, Physics 2, 6 (2009).
[10] B. Gu, J.-Y. Gan, N. Bulut, T. Ziman, G. Y. Guo, N. Nagaosa and S. Maekawa, Phys. Rev. Lett. 105, 086401 (2010).
[11] I. Sugai, S. Mitani, and K. Takanashi, IEEE Trans. Magn. 46, 2559 (2010).
2011 CMRS International Symposium on Multiscale Materials Modelling 研討會議程(July 4-7, 2011, Shanghai):
2011 CMRS International Symposium on Multiscale Materials Modelling 研討會演講論文摘要 (July 4-7, 2011, Shanghai):
Ab Initio Computational Studies of the Optical Properties and Excitonic Effects in SiC Nanotubes
Guang-Yu Guo1,2,*
1Graduate Institute of Applied Physics, National Chengchi University, Taipei 11605, Taiwan
2Department of Physics, National Taiwan University, Taipei 10617, Taiwan
Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, carbon and other nanotubes have attracted
considerable interest worldwide because of their unusual properties and also great potentials for technological applications. Apart from CNTs, inorganic tubukar materials, such as BN, AlN, GaN and SiC, have also been predicted and synthesized [1,2]. Though CNTs continue to attract great interest, other inorganic nanotubes may offer different opportunities that CNTs cannot provide. Therefore, we have carried out systematic computational studies of the optical and nonlinear optical properties of SiC nanotubes within the density functional theory (DFT) with the local density approximation (LDA) [3,4]. In the first part of this talk, we will present the results of these DFT-LDA calculations.
However, many-body interactions are known to play an important role in low-dimensional systems such as nanotubes [5], due to reduced charge screening and hence enhanced electron-electron correlation. Therefore, in the second part of this talk, we will present recent ab initio quasiparticle and excitonic calculations of the electronic and optical properties of both SiC sheet and nanotubes [6]. We find the signicant band-gap correction of up to 1eV to the LDA of semiconducting SiC-NTs and SiC sheet, mainly due to the many-body screening effect which is included in our GW approximation [7] for the valence electron self-energy. Furthermore, the excitons with a large binding energy a strong anisotropy are present in our GW+BSE (Bethe-Salpeter equation) calculations [8] of the low-dimensional SiC systems. The characters of these strongly bonded excitons in SiC-NTs is also analyzed in terms of the obtained excitonic wavefunctions.
* Electronic address: [email protected]
[1] X. Blase, A. Rubio, S. G. Louie, and M. L. Cohen, Europhys. Lett. 28, 335 (1994)
[2] N. G. Chopra, R. J. Luyken, K. Cherrey, V. H. Crespi, M. L. Cohen, S. G. Louie, and A. Zettl, Science 28, 335 (1994)
[3] I. J. Wu and G. Y. Guo, Physical Review B 76, 035343 (2007) [4] I. J. Wu and G. Y. Guo, Physical Review B 78, 035447 (2008)
[5] C.-H. Park, C. D. Spataru and S. G. Louie, Phys. Rev. Lett. 96, 126105 (2006) [6] H. C. Hsueh, G. Y. Guo and S. G. Louie, Phys. Rev. B (submitted, 2011).
[7] M. S. Hybertsen and S. G. Louie, Phys. Rev. B 34, 5390 (1986) [8] M. Rohlfing and S. G. Louie, Phys. Rev. B 62, 4927 (2000)