In a landmark achievement, an international team of researchers has successfully engineered the world’s first ideal Weyl semimetal, a quantum crystal that exhibits exotic electromagnetic properties. This innovative material, synthesized from a topological semiconductor, hosts a single pair of Weyl fermions without any irrelevant electronic states, paving the way for potential applications in terahertz devices, high-performance sensors, and low-power electronics.
The discovery, published in Nature, marks a major milestone in the decade-long pursuit of quantum materials, where researchers have been hindered by the presence of undesired electrons that obscure the unique properties of Weyl fermions. By revisiting a theoretically proposed strategy from 2011, the team has created a semimetal with a vanishing energy gap, enabling it to absorb low-frequency light and unlocking new possibilities for optoelectronics and quantum technology.
Introduction to Weyl Semimetals
Weyl semimetals are a class of quantum materials that have garnered significant attention in recent years due to their unique electronic properties. These materials are characterized by the presence of Weyl fermions, which are collective quantum excitations of electrons in crystals. Weyl fermions are predicted to exhibit exotic electromagnetic properties, making them an exciting area of research. However, despite intense study, most Weyl materials discovered to date have been found to be dominated by trivial electronic states, obscuring the Weyl fermions.
The synthesis of a material hosting a single pair of Weyl fermions, without any irrelevant electronic states, has been a long-standing goal in the field. Recently, an international team of researchers from the RIKEN Center for Emergent Matter Science (CEMS) and other institutions has achieved this milestone by engineering a Weyl semimetal from a topological semiconductor. This breakthrough was made possible through a collaboration between experimental and theoretical researchers, who worked together to design and synthesize a material with the desired properties.
The team’s approach involved revisiting a strategy that was first proposed in 2011 but had been largely forgotten by the community. By adjusting the chemical composition of the topological semiconductor bismuth telluride (Bi2Te3), the researchers were able to create a new material, (Cr,Bi)2Te3, which exhibited the desired Weyl semimetal properties. The discovery of this material has opened up new avenues for research into the properties and potential applications of Weyl semimetals.
Electronic Structure of Weyl Semimetals
Weyl semimetals are characterized by a unique electronic structure, in which the conduction and valence bands touch at a single point, known as a Weyl node. This touching point is protected by symmetry, making it robust against perturbations. The presence of Weyl nodes gives rise to exotic electromagnetic properties, such as a large anomalous Hall effect (AHE). The AHE is a phenomenon in which an electric current flowing through a material generates a magnetic field perpendicular to the current.
In the case of the newly synthesized material, (Cr,Bi)2Te3, the researchers observed a large AHE, which signaled the presence of Weyl fermions. The uniquely simple electronic structure of this material allowed the researchers to quantitatively explain their experiments using a precise theory. By tracing the large AHE back to emergent Weyl fermions, the team was able to confirm that (Cr,Bi)2Te3 is indeed a Weyl semimetal.
The electronic structure of Weyl semimetals is also characterized by the presence of Fermi arcs, which are surface states that connect the Weyl nodes. These Fermi arcs play a crucial role in determining the transport properties of Weyl semimetals and are a key feature that distinguishes them from other types of quantum materials.
Potential Applications of Weyl Semimetals
The discovery of a Weyl semimetal with a simple electronic structure has opened up new avenues for research into potential applications. One area of interest is in the development of terahertz (THz) devices. Semiconductors are typically unable to absorb photons with energy less than their energy gap, which rules out the THz frequency range. However, semimetals have a vanishing energy gap, allowing them to absorb low-frequency light down to THz frequencies.
The researchers are currently exploring the potential of (Cr,Bi)2Te3 for the generation and detection of THz light. This could lead to the development of new devices with applications in fields such as medicine, security, and materials science. Additionally, the team anticipates research into high-performance sensors, low-power electronics, and novel optoelectronics devices.
Research Environment and Future Directions
The discovery of (Cr,Bi)2Te3 was made possible by the unique combination of brilliant researchers, generous research funding, and a dynamic intellectual atmosphere at RIKEN. The institute’s Strong Correlation Quantum Transport Laboratory provided a collaborative environment that allowed experimental and theoretical researchers to work together to design and synthesize the material.
The availability of a Weyl semimetal with a simple electronic structure is expected to enable many exciting breakthroughs in the near future. Researchers are eager to explore the properties and potential applications of this new quantum phase of matter, and the discovery of (Cr,Bi)2Te3 has made this a particularly exciting time for research in the field.
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