June 2009

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Non-Abelian Josephson Effect Discovered

Recently, Non-Abelian Josephson effect was discovered by Prof. Liu Wumings group, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, CAS. Beginning with original paper on Josephson's Effecty in 1962, the Josephson effect has become a paradigm of the phase coherence manifestation in a macroscopic quantum system. With the rapid experimental progress in cold atom physics, the Josephson junction has been realized for the trapped Bose-Einstein condensates of Rb-87 and Na-23. However, most of extensive studies about this effect focus on the Abelian case so far, in terms of a junction of two systems with spontaneously broken Abelian symmetry.

The non-Abelian Josephson Effect emerges in a junction of two weakly coupled systems with spontaneously broken non-Abelian symmetries, which often involves multi-component order parameters. The non-Abelian nature of the symmetry will induce more than one kind of tunneling modes in the Josephson Effect. These different tunneling modes can be characterized by the excitation of so called pseudo Goldstone bosons which have small but finite masses. The emergence of pseudo Goldstone bosons is a consequence of the symmetry breaking term due to the coupling between the two condensates. Theoretically speaking, non-Abelian Josephson Effect is ubiquitous in nature, covering many topics from particle physics to condensed matter physics. However, there are no specific experimental constructions so far. How to design an experimental protocol to observe this novel effect in future experiments? They have briefly introduced the system about a spinor atomic BEC in a double-well optical traps, and focus on the pseudo Goldstone modes due to the non-Abelian symmetry breaking, which is at the heart of Josephson Effect. They reveal a novel Josephson effect in spin-2 Bose system which involves non-Abelian symmetry and propose an experimental protocol to realize the so called non-Abelian Josephson effect in this system. They find that the frequencies of pseudo Goldstone modes do not only relate to the coupling parameter but also to the interacting strengthes, which is a nonlinear effect due to the spin dependent interaction. Their results are of particular significance for exploring the new features of the non-Abelian Josephson effect which are very distinct from the Abelian case (From R. Qi, X. L. Yu, Z. B. Li, W. M. Liu, Non-Abelian Josephson effect between two F=2 spinor condensates in double optical traps, Phys. Rev. Lett. 102, 185301 (2009)).

New Materials for UV Detectors

For potential integration of ZnO-based optoelectronic devices with the well-developed Si technologies, the epitaxial growth of ZnO films on Si substrate is highly desirable. However, the easy oxidation of silicon surface, the formation of silicides even at room temperature (RT), and the big lattice mismatch between ZnO and Si severely influence the quality of ZnO films. The preparation of high-quality ZnO epitaxial film on Si remains a formidable challenge though many attempts have been performed to develop an appropriate buffer technique. Recently, a group led by Xiaolong Du from Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matters Physics, achieved a reproducible growth process for single crysatlline ZnO films by using a three-step low temperature (LT) interface engineering method, involving LT deposition of Mg, oxidation of Mg film, and further homoepitaxy of MgO by molecular beam epitaxy (MBE). The formation of a well-defined double heterostructure of MgO(111)/Mg(0001)/Si(111) at ?10 C prevents the Si surface from oxidation and serves as an excellent template for single-domain epitaxy of ZnO which shows high crystal and optical quality. This technique has also been suggested very useful to control various reactive metal/Si interfaces to obtain highquality oxide templates on Si substrate.
Furthermore, by using the above-mentioned growth technique, Dr. Guo, Dr. Zhang, et al, from Dus group, designed and fabricated a visible-blind ultraviolet (UV) photodetector based on a double heterojunction of n-ZnO/insulator-MgO p-Si grown by molecular beam epitaxy (MBE). The photoresponse spectrum indicates a visible-blind UV detectivity of the devices with a sharp cutoff of responsivity at the wavelength of 378 nm, which corresponds to the near band edge absorption of ZnO. Moreover, an obvious suppression of photoresponse to visible light is observed in this device. And the high UV/visible rejection ratio at ?2 V indicates a high signal-to-noise ratio. It is revealed that middle i-MgO layer takes the dual role, i.e., a buffer layer for the epitaxial growth of the p-insulator-n (PIN) double heterojunction and a barrier layer for the realization of visible-blind UV detectivity of the PIN photodetector with a high UV/visible rejection ratio. In comparison with the Si UV photodiode, no visible light filter is necessary for this novel device which is of practical applications in many areas (Appl. Phys. Lett. 94 (2009) 113508).


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