Vienna University of Technology in Austria has been the first developed the diode made by tungsten diselenide (WSe2). According to the experiment, this material can be used for ultra-thin flexible solar cells. It was expected in the market that it will take the place of graphene to become the next new-material concept, scientific experts and related enterprises indicates that the application of prospective studies are currently unclear.

 

Graphene is considered as one of the most promising electronic material, but it’s not suitable for build solar cells, which is why the Vienna University of Technology research team began to look for other similar materials like graphene, they want to find the one material that can be arranged by ultra-thin layers and has better electronic properties. Therefore, the researchers found tungsten diselenide, and its main structure is that the upper and lower layer of selenium connected to an intermediate layer of tungsten atoms, as graphene can absorb light, about 95% of the light can pass through it, but one-tenth of the remaining 5% of the light will be absorbed by the material, and converted into electricity.

Recently, Chinese Academy of Sciences Hefei Physical Institute of Solid State Physics, and the collaborative research team of Nanjing University and a strong magnetic field center, made new progress on the study of the WTe2 material physical properties under high pressure high magnetic field extreme conditions. The team used high-pressure diamond anvil cell technology, by studying the electrical transport and magnetic susceptibility, for the first time that observed the superconductivity of high-pressure-induced and magnetic resistance calculated by the theory of evolution and the superconductivity with pressure has been systematically studied, related to the results of "superconductivity and evolution of the electronic structure of the type" and titled by "tungsten telluride pressure-driven", published in "Nature - communication" on July 23, the solid researcher YangZhaorong and professor at Nanjing University Song Fengqi, Wan Xiangang are co-corresponding authors of this article.

 

High pressure is a clean, pure and powerful tool in researching new material properties, especially in the exploration of new superconducting materials. WTe2 is a kind of layered semi-metallic material, when the temperature is 0.53K, under strong magnetic field at 60T, the magnetoresistance can be up to 13,000,000%, and it still did not reach the saturation. This strange phenomenon has aroused wide attention of researchers at home and abroad. Recent theoretical studies suggest that it may be a kind of Weyl semi-metallic, magnetic resistance is considered to be related to the perfect match of the hole and electron carrier concentration of the Fermi surface nearby in the system, of which this perfect equilibrium is extremely sensitive for external perturbation, such as doping and stress.

Stress can make lattice shrink, increase energy band overlap, destroy this balance between carriers, which may induce a new electronic phase transition. Based on this consideration, cooperation team at Nanjing University of Artificial Microstructure Collaborative Innovation Center under the framework of the research carried out quickly both experimental and theoretical high pressure.

 

It was found that: at a pressure of 2.7GPa, when the temperature dropped to 3.1K or less, along with the gradual disappearance of the giant magnetoresistance effect, there was a sharp decline of the resistance. With further increase of the pressure, the emergence of zero resistance appear gradually, indicating the resistance steep drop corresponds to a stress-induced superconducting transition. At about 17GPa, the superconducting transition temperature TC reaches the maximum of 7K, and the accompanying pressures continue to increase, TC gradually decreased, showing a phase diagrama of superconducting pressure as a "dome" type. The study of different external magnetic field at high pressure resistance and magnetic susceptibility has been the further evidence of the existence of superconductivity. On the other hand, Theoretical calculations of high-pressure related show that TC beginning to increase with increasing pressure is the results of increasing density of states near the Fermi surface, and decreases of TC under higher pressure is attributed to the lattice structure instability.

Monolayer films of tungsten disulfide, just three atoms thick, have unique electronic valleys which can be manipulated with laser light. This finding, by MIT physics graduate student Edbert Jarvis Sie, Associate Professor Nuh Gedik, and colleagues, was significant enough to warrant placement on the cover of Nature Materials earlier this year.

 

The cover illustrates a tornado-like whorl of light, lifting an electronic band in the material to a higher energy state, which widens the band gap in the material. This widening is known as the optical Stark effect. The researchers, under senior author Nuh Gedik, the Lawrence C. (1944) and Sarah W. Biedenharn Career Development Associate Professor of Physics at MIT, found that applying circularly polarized laser light lifted the energy in one valley while leaving the energy in the other valley unaffected. "There are two valleys. If we switch the laser polarization, the effect switches to the other valley," Sie says. Gedik spoke about his group's research on topological insulators at the Materials Day Symposium, on Oct. 14, in Kresge Auditorium.

 

"In materials, electrons travelling in different directions experience different scattering potential with the atoms. This establishes an energy landscape as a function of electron's momentum, which can form a local minimum that we call a valley," Sie explains. "The phenomenon of the valleys only occurs in these extremely thin, monolayer forms of the tungsten sulfur compound, not in its bulk form. These valleys normally have the same energy. But as we apply this circularly polarized light, we can lift the energy of one valley relative to the other."