Advances in research on localization of electrons in topological insulators such as physics

Illustration: The upper left panel is a schematic diagram of the device for regulating and measuring the transport properties of the topological insulator film. The upper right diagram shows that in the ultra thin film, there is an energy gap near the Dirac point due to the coupling of the upper and lower surfaces. The following figure shows a set of resistance-temperature relationships: When the disorder increases gradually, the temperature dependence changes from a logarithmic to an exponential type, which indicates that the electron transport has changed from the diffusion type in the weak local area to the strong local condition. The next transition transition type. Quoted in J. Liao et al., PRL 114, 216601 (2015).

Anderson localization refers to the behavior that waves cannot propagate in disordered media. Since PW Anderson proposed this concept in 1958, it has profoundly affected the human phenomenon of conduction, or more broadly, various waves (such as electron waves, microwaves, light waves, sound waves, etc.) in the condensed state system and even cold Understanding of the Spreading Behavior in the Atomic System. The scale theory developed by Abrahams et al. in 1979 further deepened people's understanding of localization and metal-insulator transitions in electronic systems. Important concepts such as weak localization and anti-weak locality were successively proposed and experimentally verified. For more than three decades, disorder and electronic localization have been an important topic in condensed matter physics research.

The three-dimensional topological insulators that have been discovered in recent years provide a valuable new system for studying the localization of electrons. The three-dimensional topological insulator is internally insulated, but the surface state is a unique two-dimensional electronic system with features such as linear dispersion and spin-momentum lock.

Different from traditional topologically mediocre two-dimensional electronic systems, the theory predicts that surface state electrons of three-dimensional topological insulators will not be localized, and appear as anti-weak localized behaviors on electron transport. Interestingly, when the thickness of the three-dimensional topological insulator film is small (typically a few nanometers), the upper and lower surfaces open an energy gap near the Dirac point due to the wave function hybrid, and the theory predicts that this will result from three-dimensional topological insulators to two The transformation of dimensional topological insulators or topological mediocre insulators may generate quantum spin Hall effect, anti-weak local-weak local transition, and metal-insulator transition in electron transport. But so far, the above predictions lacked clear experimental support.

Recently, Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter Physics (CIC), Li Yongqing Research Group, Key Laboratory of Nanophysics and Devices, Department of Physics, Tsinghua University/National Key Laboratory of Low Dimension Physics Xue Qikun-He Wei Research Group and Chinese Academy of Sciences The Wu Kehui Research Group of the State Key Laboratory of Surface Physics of the Institute of Physics collaborated with them to accumulate many years of experiments on topological insulator film growth, device fabrication, gate voltage regulation, and transport measurement, successfully observed in ultra-thin topological insulator films. The electron transport transitions from the diffusion type in the case of anti-weak locality to the transition type in the case of strong locality (that is, Anderson localization).

They also found that as the resistivity (degree of disorder) of the sample increases, the negative magnetic conductance (ie, positive magnetoresistance) caused by the anti-weak local area is gradually suppressed, and in the case of strong localization, it is converted to a positive magnetic conductance (negative). Magnetoresistive). These results reveal that the disorder plays a crucial role in the transport properties of topological insulators, and can provide a valuable reference for the further study of topological quantum phase transitions in ultrathin films.

The doctoral students who undertake this research work include Liao Jian, Ou Yunbo, and Feng Yan of the Tsinghua University at the Physics Institute of the Chinese Academy of Sciences. This work was supported by the National Natural Science Foundation of China, the National Major Basic Research (“973”) Program of the Ministry of Science and Technology and the Class B Special Project of the Chinese Academy of Sciences. This work was published in the recent Physical Review Letters 114, 216601 (2015).

Coal  and coke
Coal is a naturally-found fossil fuel that is formed by the anaerobic decay of plant life taking place over millions of years. The energy released when the fuel is burned comes from the initial solar energy that remains stored in the hydrocarbons contained in coal. Coke is what remains when the impurities in coal, such as coal-tar and coal-gas, are removed by high temperatures in an oxygen-free furnace. The absence of oxygen during the process prevents the coal from burning. Unlike coal, coke can be burned with little or no smoke.