Two research teams have sensed a subatomic particle with bizarre properties that could revolutionize electronics.
The Weyl particle was first predicted more than 85 years ago but had thus far eluded physicists. Because it has no mass and does not scatter like an electron, it promises to make electronics faster and more efficient. Think of it as a greased-up ghost electron that zips through materials without stopping or being knocked off course.
"At this stage, we know that their physics is very different from what we are used to," explains Sefaattin Tongay, assistant professor in Arizona State University's School for Engineering of Matter, Transport and Energy, via email. "Since they are massless particles, unlike electrons, we can foresee ultrafast electronic and even photonic applications."
Electrons, conversely, have mass and can scatter inside of metals, semiconductors and insulators anywhere from zero to many times, due to electrostatic forces (the tendency for like charges to repulse and opposite charges to attract) and, occasionally, magnetism. This contributes to power loss, a major limitation facing the field of electronics that Weyl particles could help to overcome.
"Power dissipation is the main obstacle in modern electronics, and the unique non-dissipative nature of currents carried by Weyl fermions may well offer the solution," says Dmitri Kharzeev, a senior scientist at Brookhaven National Laboratory who has studied the electrical and magnetic properties of Weyl semimetals, also by email.
"But much still has to be done before this opportunity is realized."
The Weyl particle is a fermion (actually, it's more complicated than that, but see below), the class of particles that make up matter. Leptons (including electrons) and quarks are examples of fermions. Together with force-transmitting bosons, such as photons, they make up the Standard Model that has dominated the "physics of the very small" for decades.
Paul Dirac helped to lay out the principles governing traditional, or Dirac, fermions in 1928. Not long after, scientists theorized that two more types of fermions might exist, based on other solutions of the English physicist's equation.
One of those scientists, Hermann Weyl, a contemporary and schoolmate of Einstein's, predicted the possibility of a massless fermion in 1929. The idea has intrigued physicists ever since.
First, it helps to realize that Weyl particles aren't fundamental particles like electrons. They're more like a set of characteristics that pop up under the right conditions — typically oscillations or vibrations — inside a material. Like a bubble in a glass of beer, a quasiparticle doesn't exist as a concrete object outside of the material in which it arises.
"The observed Weyl fermion is an emergent 'quasiparticle' rather than a fundamental object," adds Kharzeev. "As such, it does not exist outside of the crystal — much like sound does not exist in the vacuum."
As complex as quasiparticles sound, their behavior is actually much simpler than that of fundamental particles, because their properties allow them to shrug off the same forces that knock their counterparts around.
Scientists are able to find them because they can predict what they ought to look like, that is, what kinds of characteristics they should produce when a photon or electron moves through certain special substances.
According to Ling Lu, a research scientist who worked on the MIT study, one of the major challenges in tickling the Weyl into being involved choosing, and making, the right material for the experiments.
The MIT team, working with Zhejiang University in China, confirmed Weyl particles using a double-gyroid photonic crystal. This holey, plastic material has special properties that control and limit how certain wavelengths of photons — in this case, microwaves — move through it.
By moving and rotating the crystal while also varying the wavelength of the microwaves beamed at it, researchers could detect which microwave bands were "forbidden" or "allowed," producing a frequency-wavelength graph that showed a Weyl point — the predicted fingerprint of a Weyl particle.
The Princeton team used this method, which we've simplified:
1. Create special synthetic crystals out of a semimetal called tantalum arsenide.
2. Make sure that the crystals can hold Weyl fermions.
3. Examine the crystals using a scanning tunneling spectromicroscope to make sure they are up to spec.
4. Expose the crystals to high-energy photon beams from an accelerator at Lawrence Berkeley National Laboratory in California.
5. Check to make sure that what happens to the beam — its shape, size and direction — is consistent with the presence of Weyl fermions.
Tongay believes that this might be only "the tip of the iceberg." Comparing the discovery to Dirac fermions discovered in graphene, a one-atom thick network of graphite with exciting physical and electronic properties of its own, he points out that Weyl particles exist "in three-dimensions, which is more industry compatible."
"Based on their fundamental properties, we will have more insight into what they are truly capable of in the upcoming months," he says.