MIT Physicists Reveal Mechanisms Behind Coexisting Electron Phases in Quantum Materials

Researchers from MIT have made significant strides in understanding how distinct forms of electron organization can coexist within the same quantum material. Their findings, published in Nature Physics, have the potential to enhance the scientific community’s grasp of materials displaying superconductivity, magnetism, and other electronic phases.

Led by Nuh Gedik, the Donner Professor of Physics at MIT, the team focused on erbium tritelluride, a rare-earth material known for its peculiar electronic behavior. Under typical conditions, electrons in erbium tritelluride are relatively evenly distributed, but as the material is cooled, they reorganize into a wave-shaped arrangement termed a “charge density wave” (CDW) phase. Further cooling introduces a second CDW pattern perpendicular to the first, creating an atomic-scale checkerboard structure.

“The mechanism responsible for the emergence of this second phase has long been debated, and our approach provides a powerful new way to uncover the hidden physics behind phase transitions in quantum materials,” Gedik said.

Co-author Alfred Zong PhD ’20 stated, “People believe the cornerstone of replacing silicon lies in quantum materials that have multiple coexisting phases. Our experiment provides a very neat way to study these multiple phases.”

Understanding Charge Density Waves

A charge density wave occurs when electric charges, such as electrons, spontaneously arrange into a repeating wave pattern, with higher concentrations at the crests and lower concentrations at the troughs. This organized state can appear only at extremely low temperatures in some materials.

Despite years of study on charge density waves, recent research has highlighted their occurrence in materials that also exhibit more complex collective electron behavior, including superconductivity. “The power of CDWs is that they are a much simpler form of matter compared to superconductivity. They offer a playground for fundamental understanding,” explained lead author Yifan Su.

To examine how two CDW phases coexist in erbium tritelluride, the researchers obtained atomically thin samples and cooled them to approximately -230 degrees Celsius, a temperature that supports the checkerboard pattern of both waves.

Experimental Insights Using Laser Pulses

In the experiment, the researchers disrupted the electronic pattern using two carefully timed laser pulses. The first pulse served as the “shake,” breaking apart the checkerboard structure. By varying the pulse intensity, they manipulated the disturbance of the charge density waves. The second pulse, containing high-energy photons, knocked electrons out of the material, allowing them to measure how the two waves returned over time.

They observed that the dominant charge density wave returned gradually and evenly, reflecting a typical “second-order” phase transition. In contrast, the subdominant phase reformed in isolated pockets, expanding through the material like ice crystals in water, indicative of a “first-order” transition.

These observations helped identify a mechanism responsible for the emergence of the subdominant CDW phase, shedding light on the ongoing debate among physicists regarding the nature of these phase transitions.

The implications of this research extend beyond erbium tritelluride; it may provide insights into the exotic properties of more complex quantum materials that host multiple phases simultaneously. Gedik noted, “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.”

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