Elusive effect: New IR detectors

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  • Published: Jun 1, 2017
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Elusive effect: New IR detectors

A new spin

The existence of a previously unobserved phenomenon related to particles known as Weyl fermions has been observed in Weyl semi-metals by an international high-energy physics team. The discovery could lead to a new type of infrared detector. Image courtesy of researchers

The existence of a previously unobserved phenomenon related to particles known as Weyl fermions has been observed in Weyl semi-metals by an international high-energy physics team. The discovery could lead to a new type of infrared detector.

Physicist Qiong Ma of Massachusetts Institute of Technology, Cambridge, Massachusetts, USA and colleagues there and at the International Center for Quantum Materials at Peking University, Beijing, the Collaborative Innovation Center of Quantum Matter, also in Beijing, China, the National University of Singapore, and the Department of Chemistry, at Louisiana State University, Baton Rouge, Louisiana, USA, have carried out direct optical detection of Weyl fermion chirality in a topological semimetal.

By definition, all infrared frequencies are invisible to our eyes, but as we know are incredibly useful in night vision, thermal sensing, environmental monitoring, and a wide range of spectroscopic applications. A new phenomenon in an unconventional metal might now lead to a new type of highly sensitive detector for mid-infrared. The phenomenon pivots on the elusive Weyl fermion (a member of one of the two classes of fundamental sub-atomic particles, the fermions and bosons segregated by the property known as spin.

Camp fermion

The fermions themselves are divided into three camps - Dirac, Majorana, and Weyl fermions. Dirac fermions include the electrons in conventional metals such as copper and gold. While the other two types of fermion give rise to new phenomena that physicists are only now disentangling. These particles may well turn out to have a wide range of technological applications just as has the electron, once we grasp their subtleties.

For instance, the Weyl fermion, first postulated almost one hundred years ago by German physicist Hermann Weyl lies at the mathematical hearts of physics' Standard Model of subatomic physics but has never been observed experimentally. Theory predicts that Weyl fermions move at the speed of light but also have spin. There are thus two types depending on direction of spin and thus Weyl fermions have the property of handedness, or chirality. Although the particles themselves remain elusive, a phenomenon that mimics essential many of the anticipated properties of Weyl fermions has been observed in a class of unconventional metals known as Weyl semi-metals. If it were possible to determine the chirality of these particles in a Weyl semi-metal, then physics would be a step closer to observation.

Unconventional

Writing in the journal Nature Physics, the researchers explain how they have demonstrated an interesting effect in the material tantalum arsenide, or TaAs. It exhibits what they describe as an interesting optoelectronic property, the circular photogalvanic effect. In conventional materials, electrical conduction requires one to apply an external voltage across the two ends of a piece of copper, for example, in order to observe the effect. In TaAs, however, irradiation with mid-IR circularly polarized light generates an electric current with no external voltage. Moreover, the direction of this current is dictated by the chirality of the postulated Weyl fermions within. Change the polarization of the light from left to right and the flow of electric current flips direction. The current generated in this way is surprisingly large, about 10 to 100 times stronger than the response of other materials used in IR detectors.

"Despite being predicted a long time ago, Weyl fermions have never been observed as a fundamental particle in particle physics," team member Nuh Gedik of MIT explains. He points out that these new observations suggest that in unconventional metals, ordinary electrons can behave in a strange way so that their motion mimics the behaviour of Weyl fermions. Whether or not this makes the electrons in such material synonymous with Weyl fermions is a moot point, there are several other factors that must be taken into account in order that such an assertion be made, not least the posited velocity of Weyl fermions compared to the speed with which electrons might move in an unconventional metal.

Related Links

Nature Phys 2017, online: "Direct optical detection of Weyl fermion chirality in a topological semimetal."

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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