MIT Physicists Uncover Hidden Physics Behind Electronic Phase Transitions in Quantum Materials

The study of quantum materials has long been a topic of interest in the field of physics, with researchers seeking to understand the complex behavior of these materials at the atomic and subatomic level. Recently, a team of MIT physicists has made a significant breakthrough in this area, uncovering new details about how two distinct forms of electron organization can emerge within the same material.

The researchers, led by Nuh Gedik, the Donner Professor of Physics at MIT, investigated erbium tritelluride, a rare-earth material with unusual electronic behavior. Under ordinary conditions, electrons are distributed relatively evenly throughout erbium tritelluride. However, when the material is cooled to specific temperatures, the electrons begin organizing themselves into a wave-shaped arrangement known as a "charge density wave" (CDW) phase.

Cooling the material even further produces a second wave pattern running perpendicular to the first. Together, the two electronic phases form something resembling an atomic-scale checkerboard.

The researchers were able to separate the behavior of these two phases and observe how each one developed. The first phase appeared gradually across the material, somewhat like liquid water steadily becoming vapor. This matches the conventional picture of how many electronic phase transitions take place.

The second phase behaved very differently. Rather than appearing smoothly throughout the material, it began in isolated regions that expanded outward, resembling the way ice crystals begin forming in liquid water.

**An Atomic Checkerboard of Electrons**

The study's findings, published in *Nature Physics*, could improve scientists' understanding of materials that display superconductivity, magnetism, and other electronic phases. Learning how these phases arise and interact could eventually help researchers gain greater control over electronic properties and develop more powerful quantum devices.

The researchers used a combination of experimental techniques to study the behavior of the two CDW phases. They obtained small, atomically thin samples of erbium tritelluride created by collaborators at Stanford and cooled them to roughly -230 degrees Celsius, cold enough for both charge density waves to exist simultaneously in the checkerboard pattern.

They then disrupted or destroyed that electronic pattern and watched to see how the two waves returned. The experiment relied on two carefully timed laser pulses. The first laser pulse served as the "shake," breaking apart the electronic checkerboard. By changing the pulse intensity, the researchers could control how strongly they disturbed the charge density waves.

A second pulse containing high-energy photons then knocked electrons out of the material. The researchers applied this pulse at different intervals after the first one and measured the energy and momentum of the expelled electrons. Those measurements provided a series of snapshots showing how the electronic phases recovered over time.

**Two Very Different Ways to Rebuild**

The dominant charge density wave returned gradually and evenly, regardless of how strongly the researchers had initially disrupted the material. This smooth recovery is considered a textbook "second-order" phase transition. It resembles the gradual way a magnet loses its magnetism as its temperature rises.

The second charge density wave produced the bigger surprise. Instead of returning uniformly, the subdominant phase began forming in scattered pockets. Those regions then expanded through the material, much like ice crystals growing through liquid water. This behavior corresponds to a less common "first-order" transition and was not what the researchers expected.

The observations allowed the team to identify the long-debated mechanism responsible for the emergence of the subdominant CDW phase.

The findings may have implications well beyond erbium tritelluride. More complicated quantum materials can contain several electronic phases simultaneously, and scientists suspect that the way those phases interact may be responsible for some of their most unusual properties.

"In systems that are much more complex, like high-temperature superconductors, you see there are multiple phases -- magnetism, superconductivity, charge density waves, and they all exist together," Gedik says. "One of the theories is that, the way they interact with each other is key for their exotic properties. The lessons we learn here can be applied to much more complex materials."

| Material | Temperature for dominant CDW | Temperature for subdominant CDW | | --- | --- | --- | | Erbium tritelluride | -8 degrees Celsius | -113 degrees Celsius | | | | |

The research was supported by the U.S. Department of Energy, the U.S. National Science Foundation, and the Gordon and Betty Moore Foundation's EPiQS Initiative grant.

**Story Source:**

Materials provided by **Massachusetts Institute of Technology**. Original written by Jennifer Chu. *Note: Content may be edited for style and length.*