Tuesday, November 07, 2017

Observing Curved-Space Quantum Physics in Nano-Sized Metals

There’s a lot of room between the tiny world of the nanoscale and the grand scale over which we usually talk about Einstein’s general theory of relativity. Although the arenas seem vastly different, we may soon be able to observe the phenomena of general relativity in nano-sized metals.

In theoretical work published last week in the American Physical Society’s journal Physical Review X, Alex Westström and Teemu Ojanen from Aalto University in Finland introduce a new class of materials that could be used to study curved-space quantum physics in experiments that would fit on your kitchen table. In addition, they say, these materials could potentially form the basis for a powerful new type of electronic device.

Inhomogeneous Weyl semimetals (bottom) give rise to an effective curved space geometry that is experienced by the charge carriers that travel through them.
Image Credit: Alex Westström and Teemu Ojanen, Physical Review X.
To set the stage, let’s start by revisiting the standard model of particle physics, the theoretical framework that describes the fundamental forces and elementary particles in the universe. The model classifies quarks, electrons, protons, and many other particles as fermions. Fermions obey certain rules that the other class of particles in the standard model, the bosons, do not.

It turns out that when you study fundamental particles from a quantum mechanical perspective, things called Weyl fermions become important. Named after the German theoretical physicist who proposed them, Hermann Weyl, Weyl fermions have not yet been found in nature as elementary particles. However, Weyl fermions have been observed as quasiparticles—particle-like excitations that act like particles inside solid matter.

In 2015, for the first time, scientists experientially observed Weyl fermions carrying electric charges through a material. With the discovery of this so-called Weyl semimetal came a lot of excitement. As Ojanen explains, “The charge carriers in Weyl semimetals behave similarly to particles moving close to the speed of light, only with a velocity which is a hundred times smaller. Thus, Weyl semimetals display many phenomena related to high-energy particle physics,” he says. This opens the door to using Weyl semimetals to study general relativity and particle physics.

This new research develops the idea further. Westström and Ojanen are particularly interested in “simulating the fundamental phenomena of quantum physics in curved space,” says Ojanen, while also exploring new ideas for future electronics. To both ends, their work involves modifying semimetals or insulators to create what they call Weyl metamaterials. Metamaterials are materials designed to have specific properties not found in nature, often through the use of nanostructures.

In this work, the team mathematically shows that by modifying the magnetization and strain of a Weyl semimetal, you can create a metamaterial with a tunable effective geometry. In other words, you can create a structure through which Weyl fermions travel as if it they were experiencing the three-dimensional curved geometry of spacetime. Studying how the charged “particles” move through a Weyl metamaterial is therefore like studying a mini version of what happens in the cosmos—like bringing the physics of the very large and very high energy into the lab.

Taking inspiration from the field of optical metamaterials, which arranges structures that interact with light in specialized ways to form invisibility cloaks and superlenses, among other optical tools, the researchers also began exploring potential electronics applications of Weyl metamaterials. The basic idea is to create materials in which the path of the current is determined by the effective geometry. This could open up new possibilities for electronic systems, such as a three-dimensional Weyl electron lens that could focus charge carriers.

“This is just the first example of the new possibilities offered by Weyl metamaterials,” say the researchers in their paper. “An interesting venue for future work is to study what other exotic phenomena can be engineered in these systems. For example, a possibility to realize electronic cloaking devices from Weyl metamaterials is particular[ly] intriguing,” they write.

So, what’s the take home message? Physics isn’t a black and white field. The distinctions between applied research and fundamental research, high energy and low energy, classical and quantum, and nanoscale and cosmic are useful, but not set in stone. Bringing these topics together can result in surprising developments and exciting opportunities to simultaneously explore the fundamental nature of the universe and push technology beyond its existing limits.

Kendra Redmond

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