2D devices have hidden cavities that can modify electronic behavior

In the right combinations and conditions, two-dimensional materials can host intriguing and potentially valuable quantum phases, such as superconductivity and unique forms of magnetism. Why they occur and how they can be controlled are of considerable interest among physicists and engineers. Research published in Natural physics reveals a previously hidden feature that could explain how and why enigmatic quantum phases emerge.

Using a new terahertz (THz) spectroscopic technique, researchers have revealed that tiny stacks of 2D materials, found in research labs around the world, can naturally form so-called cavities. These cavities confine light and electrons to even smaller spaces, potentially changing their behavior drastically.

“We have discovered a hidden layer of control in quantum materials and opened the way for light-matter interactions to form in ways that can help us understand the exotic phases of matter and, ultimately, exploit them for future quantum technologies,” said James McIver, assistant professor of physics at Columbia and lead author of the paper.

The discovery began in Hamburg, when McIver was a group leader at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD), one of the institutions that make up the Max Planck-New York Center for Non-Equilibrium Quantum Phenomena. Researchers at the Center, which also includes Columbia, the Flatiron Institute and Cornell University, are interested in what happens when stable systems are thrown out of balance.

The McIver lab is turning to light. “2D materials, with their fascinating macroscopic properties, often behave like black boxes. By illuminating them, we can literally shine a light on the hidden behavior of their electrons, revealing details that would otherwise remain invisible,” said Gunda Kipp, a Ph.D. student at MPSD working with the McIver group and first author of the publication. The challenge is that the wavelengths of light needed to probe 2D materials are much larger than the materials themselves, which are typically smaller than a human hair.

To address this size mismatch, the team scaled things down significantly with a chip-sized spectroscope that confines THz light – the range in which enigmatic quantum phenomena are thought to occur – from 1 mm to just 3 micrometers. This allows the team to visualize the behavior of electrons in 2D systems. They began experiments with graphene to test how well the spectroscope could measure optical conductivity in a well-studied material.

They saw unexpected standing waves.

“Light can couple with electrons to form hybrid light-matter quasiparticles. These quasiparticles move in waves, and under certain conditions they can become confined, much like the standing wave on a guitar string that produces a distinct note,” explained Hope Bretscher, a postdoctoral researcher at MPSD and co-first author.

In the case of the guitar, the fixed ends of the string define the boundaries of the standing wave; holding your fingers on the strings shortens the wave at which a string can vibrate, thereby changing the note it produces. In optics, a similar effect can be achieved with two mirrors that trap light between them and create a confined standing wave inside what is called a cavity. When a material is placed between the mirrors, reflected light will interact with it, potentially changing its properties.

But mirrors can be optional.

“We found that the edges of the material already act like mirrors,” Kipp said. With their THz spectroscope, they observed that the excited electron streams reflect off the edges to form a type of light-matter hybrid quasiparticle called a plasmon polariton.

The McIver lab studied a device composed of several layers, each of which could act as a cavity separated by a few tens of nanometers. The plasmons that form in each layer can, in turn, interact, often strongly. “It’s like connecting two guitar strings: Once connected, the note changes,” Bretscher said. “In our case, this changes radically.”

The next question is what exactly determines the frequencies of vibrating quasiparticles and the force with which light and material interact. “With co-author and MPSD postdoctoral fellow Marios Michael, we developed an analytical theory that needed only a handful of geometric sample parameters to match the observations from our experiments,” Kipp said.

“With a single click, our theory can extract the properties of a material and will help us design and tailor future samples to achieve specific properties. For example, by tracking resonances as a function of carrier density, temperature or magnetic field, we can discover the mechanisms behind different quantum phases.”

While the published work captured plasmons, the new chip-scale THz spectroscope should be able to observe other types of quasiparticles oscillating in a wide variety of 2D materials. The team is already working on measuring new samples in Hamburg and New York.

“This whole project was sort of a serendipitous discovery. We didn’t expect to see these cavity effects, but we’re excited to use them to manipulate phenomena in quantum materials in the future,” Bretscher said. “And now that we have a technique to see them, we’re intrigued to know how they might affect other materials and phases.”