Basic Science

Big Step Forward in Research of Topological Semi-metal State

Through years of efforts, Prof. Fang Zhong and Prof. Dai Xi’s Group obtained a breakthrough in finding the topological semi-metal state for the first time from the theoretical prediction to experimental observation, in collaboration with the experimental group. In 2012, doctoral student Wang Zhijun, with guidance of Associate Prof. Wong Hongming, Profs. Fang Zhong and Dai Xi from the State Key Laboratory of Condensed Physics in collaboration with Prof. Chen Xinqiu at the Institute of Metal Research predicted that the three-dimensional Dirac cone like semi-metal state may exist in the chemical compound Na3Bi, which is protected by lattice symmetry through theoretical calculation. The three-dimensional Dirac is the singular point of quantum state space, from which quite a few novel quantum state of matters can be created using different adjustment measures, and thus it is an ideal material for the adjustment of the quanta. Such work published on the Pysical Review (Bulleting Phys. Rev. B 85, 195320 (2012)), which attracted great emphasis by the experimental physicists, and many research groups started the verification work. Through a year’s hard work, the Chinese professors took lead in achieving success in this regard in collaboration with Prof. Yulin Chen from the Oxford University, Prof. Zhixun Shen from the Stanford University and other researchers from the Lawrence Berkeley National Laboratory and Stanford Linear Accelerator Center. They observed the predicted three-dimensional Dirac cone like electronic energy band, which is different with the two dimensional Dirac existing in graphene. The results were published by Science (Science 343, 864 (2014)). Their work was also reported by Nature entitled “Wonders of flat physics now seen in 3D”, while Physics World briefed their progress as Physicists discover 3D versions of graphene. In 2013, The Chinese professors found that the traditional semi-conductor materials proved also to be Three-dimensional Dirac cone like semi-metal, moreover its mobility at room temperature is as high as 15,000cm2/V/s, which can be compared with silicon, which thus has more direct application value and prospect. The relevant result published on the Physical Review Bulleting (Phys. Rev. B 88, 125427 (2013)). Their achievements have been already verified by themselves and many other groups.

Topological Phase in GaAs/Ge/GaAs Ultra-thin Quantum Wells

Chang Kai’s group in the State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, CAS, recently proposed to realize topological phase in conventional semicoductors GaAs/Ge/GaAs quantum wells utilizing interface polarization caused by the charge transfer between Ga and As atoms. Using first-principles calculation and the multi-band k?p theory, they demonstrate theoretically that the interface polarization in GaAs along (111) direction can induce topological insulating phase in GaAs/Ge/GaAs quantum wells. This work makes an important step toward potential application of topological insulator-based electronic devices. In their previous works, they found topological insulating phase can be induced in GaN/InN system (Phys. Rev. Lett. 109, 186803(2012)), without heavy atoms and narrow gap. But the strain between GaN and InN is very strong and makes the MBE growth of GaN/InN system a very challenging task. In GaAs/Ge/GaAs quantum wells, the lattices are perfectly matched. More importantly, GaAs and Ge are both conventional semiconductors with well-developed growth and fabrication techniques. Topological phase in Ge could make TI-based electronic device more easily combined with integrated circuit. This work reveal that the band structure and band gap can be engineered in large energy scale using polarized interface (1eV), shed a new light on exploring new topological phase in conventional semiconductors. This work was published in Physical Review Letters 111, 156402 (2013).

Another Wonder: Giant Star Cluster Revealed

W49A is located about 36,000 light-years from Earth, on the opposite side of the Milky Way. It represents a nearby example of the sort of vigorous star formation seen in so-called "starburst" galaxies, where stars form 100 times faster than in our galaxy. The purple mountain observatory (PMO) 13.7m telescope has peered through the dusty fog to provide the first clear view of this stellar nursery. The study revealed an active site of star formation being fed by streamers of in falling gas, with combined observations with PMO 13.7m, Submillimeter Array (SMA) and IRAM 30m telescopes. "We were amazed by all the features we saw in these images," says lead author Roberto Galván-Madrid, who conducted this research at the Harvard-Smithsonian Center for Astrophysics (CfA) and the European Southern Observatory (ESO). The heart of W49A holds a giant yet surprisingly compact star cluster. About 100,000 stars already exist within a space only 10 light-years on a side. In contrast, fewer than 10 stars lie within 10 light-years of our Sun. In a few million years, the giant star cluster in W49A will be almost as crowded as a globular cluster.

The PMO 13.7m also revealed an intricate network of filaments feeding gas into the center, much like tributaries feed water into mighty rivers on Earth. The gaseous filaments in W49A form three big streamers, which funnel star-building material inward at speeds of about 2 km/sec. The data from PMO, SMA, and IRAMrevealed the molecular gas within W49A in exquisite detail. It showed that central 30 light-years of W49A are several hundred times denser than the average molecular cloud in the Milky Way. This also explains why star formation is so active in this region. In total, the nebula contains about 1 million suns' worth of gas, mostly molecular hydrogen. Their research was published in the December 2013 issue of Astrophysical Journal.