MIT physicists observe quantum electrons building competing phases in a material

MIT physicists have directly observed subtle red stripe patches forming alongside uniform blue stripes within a quantum material, revealing the coexistence of two distinct electron order phases. Using “pump” and “probe” laser beams, the researchers tracked emitted electrons in erbium tritelluride, a rare-earth material, to understand these phases.

Published in Nature Physics, the study draws a parallel to ice water existing as both liquid and solid to explain this complex duality at the quantum level. “People believe the cornerstone of replacing silicon lies in quantum materials that have multiple coexisting phases,” says co-author Alfred Zong, and this research provides a new method for studying them, potentially unlocking control over electronic behavior for high-performance quantum devices.

Erbium Tritelluride Reveals Coexisting Charge Density Waves

Erbium tritelluride exhibits a surprising duality at the quantum level; within this material, electrons spontaneously organize into two distinct wave-like patterns simultaneously. Researchers at MIT, led by Nuh Gedik, detailed how these charge density waves (CDWs) emerge and coexist, providing insight into the behavior of more complex quantum materials. The investigation centered on erbium tritelluride, a rare-earth material exhibiting CDW phases when cooled to extremely low temperatures.

Previous experiments have found that when erbium tritelluride is cooled to -8 degrees Celsius, the first of two charge density waves forms among the material’s electrons. This “dominant” wave stretches across the material in one direction.

Further cooling to -113 degrees Celsius introduces a “subdominant” wave, perpendicular to the first, creating an atomic checkerboard pattern of coexisting electron phases. Gedik explains, “One of the biggest questions in physics is why some materials host multiple phases while others do not, and when multiple phases do exist, how do they interact? Do they reinforce one another, compete, or coexist independently?” To unravel this interaction, the researchers employed a sophisticated “pump” and “probe” laser technique to disrupt and then observe the reformation of these phases.

This experimental approach revealed a striking difference in how each wave re-emerges after being disturbed. The dominant phase consistently reformed uniformly, aligning with the expected behavior of a textbook phase transition.

However, the subdominant phase exhibited unusual behavior; electrons reassembled the wave in isolated pockets that expanded outwards, resembling the crystallization of water into ice. “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 says. This behavior was unexpected and provides crucial insight into the material’s complex behavior.

As Alfred Zong, now an assistant professor at Stanford University, notes, the team’s work with CDWs, a relatively simple form of collective electron behavior, provides a foundational understanding that can be extended to more complex phenomena like high-temperature superconductivity. Yifan Su, the lead author, emphasizes this point, stating, “Just like superconductivity, charge density waves are a collective phenomena where electrons move together in certain ways.” The lessons learned from studying erbium tritelluride could ultimately lead to controlling electronic behavior and designing high-performance quantum devices.

Laser “Pump-Probe” Technique Visualizes Phase Transitions

The ability to directly visualize the emergence of competing electron phases within a quantum material represents a significant step forward in condensed matter physics, as demonstrated by recent work at MIT. Researchers employed a sophisticated “pump-probe” laser technique to observe how electrons re-organize themselves within erbium tritelluride, a material known to host multiple charge density wave (CDW) phases simultaneously. This experimental approach allowed the team to not only identify these phases, but to track their dynamic behavior following a disturbance, revealing unexpected differences in their reformation processes.

The team, led by Nuh Gedik, utilized laser pulses to disrupt the existing electron arrangement and then monitored the emitted electrons to map the recovery of each phase. The precision of this technique lies in its ability to capture the fleeting moments of phase transition.

By varying the intensity of the initial “pump” laser and carefully timing the “probe” pulse, the researchers could effectively create a stop-motion movie of the electron reassembly process. This allowed them to distinguish between gradual, uniform transitions and more localized, discrete events. Gedik adds that the lessons learned here can be applied to much more complex materials. This experimental approach, as Gedik states, “provides a neat way to study these multiple phases,” and offers a powerful tool for unraveling the mysteries of quantum materials.

Dominant Phase Transitions Uniformly, Subdominant Phase Nucleates

The research, published in Nature Physics, focused on how these electron arrangements emerge and reform following disruption, providing insights into phenomena like superconductivity and magnetism. Researchers utilized laser pulses to both destabilize and then monitor the reformation of charge density waves, patterns where electrons organize into wave-like structures, within the material. This approach allowed for visualization of two distinct phases: a dominant wave exhibiting uniform reformation and a subdominant wave displaying a unique nucleation process.

The team, led by Nuh Gedik, discovered that the primary charge density wave consistently re-established itself smoothly, aligning with expectations for a phase transition. This means the wave reformed uniformly across the material, much like a liquid transitioning to a vapor. “We see the destruction of these phases, and then if we wait long enough, they come back,” Gedik explains, highlighting the dynamic nature of the observed transitions.

The researchers were able to vary the intensity of the initial laser pulse, effectively controlling the degree of disruption to the electron arrangement and observing the subsequent recovery of each phase. This distinction in reformation, uniform for the dominant phase and nucleated for the subdominant, is significant because it suggests differing mechanisms driving their emergence.

Understanding CDWs as a Pathway to Quantum Material Control

The detailed observation of two distinct charge density wave (CDW) phases within erbium tritelluride provides a crucial step toward controlling electronic behavior in quantum materials. This approach allowed for visualization of a subtle red stripe pattern forming alongside a dominant, uniform blue stripe pattern, offering insight into the complex interplay of electron arrangements. The team’s experiments demonstrated that the primary CDW consistently reformed uniformly across the material, mirroring the transition of a liquid to a vapor.

However, the secondary wave exhibited a markedly different behavior; instead of gradual reformation, it coalesced first in isolated pockets before expanding, akin to the crystallization of liquid water into ice. “And depending on how you hit them, the two phases respond differently,” Gedik explains, highlighting the distinct responses to external stimuli. This unexpected nucleation process challenges conventional understanding of phase transitions and suggests a more complex mechanism at play.

Understanding these coexisting phases is paramount to designing future quantum devices. The ability to manipulate and control these electron arrangements could unlock advancements in superconductivity and magnetism, areas where current materials face significant limitations. They offer a playground for fundamental understanding,” makes them an ideal system for investigating the underlying principles governing more complex quantum phenomena.

This research builds on the observation that some materials host multiple phases, and seeks to understand how these phases interact, reinforcing or competing with one another. Gedik notes that this work serves as a “case study for us to understand much more complicated materials,” potentially paving the way for the creation of materials with tailored electronic properties.