Graphene, a two-dimensional (2D) honeycomb sheet composed of carbon atoms, has attracted intense interests worldwide because of its outstanding properties and promising prospects in both basic and applied science.
The great development of graphene is closely related to the unique electronic structure, that is, Dirac cones. The cone which represents linear energy dispersion at Fermi level gives graphene massless fermions, leading to various quantum Hall effects, ultra high carrier mobility, and many other novel phenomena and properties.
Dirac cone is special but might not unique to graphene. Recently, more and more 2D materials have been predicted to possess Dirac cones, such as silicene and germanene (graphene-like silicon and germanium, respectively), several graphynes (sp-sp2 carbon allotropes), and so on. But these 2D Dirac systems are so rare compared to the numerous 2D materials. A deep understanding of all known 2D Dirac systems and a strategy to seek for new ones are needed.
A new paper published in National Science Review presented the recent progress on theoretical studies of various 2D Dirac materials.
In this paper, the structural and electronic properties of graphene, silicene, germanene, graphynes, several boron and carbon allotropes, transition metal oxides, organic and organometallic crystals, square MoS2, and artificial lattices (electron gases and ultracold atoms) were summarized.
As the author stated, "most Dirac materials have spatial inversion symmetry", "Many of them are bipartite and composed of only one element", and "hexagonal honeycomb structure is common in atomic Dirac materials".
Since "the Dirac-cone structure gives graphene massless fermions, leading to half-integer/fractional/fractal quantum Hall effects, ultrahigh carrier mobility", other 2D Dirac systems were predicted to have similar properties, and some even possess new physics beyond graphene.
Based on the above discussions, the authors further investigated how Dirac points move and merge in these systems. They mentioned that strain can move the Dirac point to a new k (reciprocal) location. But "when two Dirac points with opposite Berry phases move in the k space under any perturbation and arrive at the same point, they merge and their Berry phases annihilate each other".
Moreover, the von Neumann-Wigner theorem was applied to explain the scarcity of 2D Dirac systems. Then rigorous requirements for a 2D system to achieve Dirac cones were deduced, which is related to the symmetry, parameters, Fermi level, and band overlap.
This paper noted that "Dirac cones are not only the linear energy dispersion around discrete points but also singularities in the spectrum of Hamiltonians and are topologically protected." The authors pointed out "Looking forward, we believe that more and more 2D Dirac materials will be discovered, and a thorough understanding on the existing conditions of Dirac cones is greatly helpful in seeking/designing new systems."
This research received funding from the National Natural Science Foundation of China (Grant No. 21373015).
See the article
Jinying Wang, Shibin Deng, Zhongfan Liu, and Zhirong Liu. "The Rare Two-Dimensional Materials with Dirac Cones".
National Science Review (March 2015) 2 (1): 22-39. (http://nsr.
The National Science Review is the first comprehensive scholarly journal released in English in China that is aimed at linking the country's rapidly advancing community of scientists with the global frontiers of science and technology. The journal also aims to shine a worldwide spotlight on scientific research advances across China.
Zhirong Liu | EurekAlert!
Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously
17.01.2017 | Sonderforschungsbereich 668
Manchester scientists tie the tightest knot ever achieved
13.01.2017 | University of Manchester
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).
Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...
Many pathogens use certain sugar compounds from their host to help conceal themselves against the immune system. Scientists at the University of Bonn have now, in cooperation with researchers at the University of York in the United Kingdom, analyzed the dynamics of a bacterial molecule that is involved in this process. They demonstrate that the protein grabs onto the sugar molecule with a Pac Man-like chewing motion and holds it until it can be used. Their results could help design therapeutics that could make the protein poorer at grabbing and holding and hence compromise the pathogen in the host. The study has now been published in “Biophysical Journal”.
The cells of the mouth, nose and intestinal mucosa produce large quantities of a chemical called sialic acid. Many bacteria possess a special transport system...
10.01.2017 | Event News
09.01.2017 | Event News
05.01.2017 | Event News
17.01.2017 | Earth Sciences
17.01.2017 | Materials Sciences
17.01.2017 | Architecture and Construction