The fundamental behavior of matter is often defined by its phase—the specific state, such as solid, liquid, or gas, in which molecules arrange themselves. While a glass of ice water serves as a common example of coexisting phases, where solid and liquid H2O reside in the same vessel, the world of quantum materials offers a far more enigmatic version of this duality. In these advanced materials, electrons can organize into distinct "phases" of behavior that dictate properties like conductivity, magnetism, and superconductivity. A groundbreaking study led by physicists at the Massachusetts Institute of Technology (MIT) has now pulled back the curtain on how these exotic electronic phases emerge and interact within a single material, potentially paving the way for the next generation of high-performance quantum devices.
The research, published today in the journal Nature Physics, focuses on the rare-earth material erbium tritelluride (ErTe3). By utilizing "ultrafast" laser techniques, the team observed how two different "charge density wave" (CDW) phases coexist and reform after being disturbed. Their findings challenge traditional textbook assumptions about electronic transitions, revealing that different phases within the same material can emerge through entirely different physical mechanisms. One phase forms gradually and uniformly, while the other emerges in isolated "pockets" that expand like ice crystals in water.
The Quest to Move Beyond Silicon
For decades, the semiconductor industry has relied on silicon as the backbone of modern electronics. However, as transistors approach the atomic scale, silicon faces physical limitations, particularly regarding heat dissipation and energy efficiency. The scientific community increasingly views quantum materials—substances where electron-electron interactions lead to macroscopic quantum effects—as the key to overcoming these hurdles.
"People believe the cornerstone of replacing silicon lies in quantum materials that have multiple coexisting phases," explains Alfred Zong, an assistant professor at Stanford University and co-lead author of the study, who began this work as a PhD student at MIT. "Our experiment provides a very neat way to study these multiple phases."
The ability to control these phases is the holy grail of condensed matter physics. If engineers can master the "switches" that turn superconductivity or magnetism on and off, they can design devices that operate with near-zero friction or process information at speeds currently unattainable by classical silicon-based architectures.
Understanding Charge Density Waves
To understand the MIT team’s discovery, one must first understand the concept of a Charge Density Wave (CDW). In most conductive materials, electrons are distributed more or less uniformly throughout the atomic lattice. However, in certain materials cooled to extreme temperatures, electrons undergo a collective transition. They spontaneously organize into a wave-like pattern, with peaks of high electron density and troughs of low density.
This CDW phase is a collective phenomenon, much like superconductivity. Lead author Yifan Su, a recent MIT PhD graduate, notes that CDWs serve as an ideal "playground" for fundamental physics. "The power of CDWs is that they are a much simpler form of matter compared to superconductivity. They offer a playground for fundamental understanding," Su explains.
In erbium tritelluride, this phenomenon is doubled. The material hosts two distinct CDW phases that exist simultaneously, creating what researchers describe as an "atomic checkerboard."
The Experiment: A Chronology of Discovery
The study of erbium tritelluride is not new, but the MIT team’s approach to "teasing apart" its phases represents a significant leap forward. Previously, physicists knew that ErTe3 underwent two distinct transitions as it was cooled:
- The Dominant Phase: At approximately -8 degrees Celsius (265 Kelvin), the first CDW forms, with electron waves stretching across the material in a specific horizontal direction.
- The Subdominant Phase: As the material is cooled further to -113 degrees Celsius (160 Kelvin), a second CDW emerges, oriented perpendicularly to the first.
Until now, the exact mechanism of how the second, subdominant phase emerged remained a subject of intense debate. To resolve this, the MIT team, led by Nuh Gedik, the Donner Professor of Physics, employed a "shake and listen" methodology using ultrafast spectroscopy.
The team obtained atomically thin samples of ErTe3 synthesized by collaborators at Stanford University. In Gedik’s lab, they cooled these samples to a staggering -230 degrees Celsius, ensuring both CDW phases were fully established in their checkerboard pattern. They then used a high-intensity laser pulse—the "shake"—to momentarily shatter the electronic order.
Following this disruption, they delivered a second laser pulse consisting of high-energy photons. This second pulse kicked electrons out of the material, allowing the researchers to measure their energy and momentum. By varying the timing between the two pulses, the team could take "snapshots" of the material as the electronic phases recovered and re-organized.
A Tale of Two Transitions
The results revealed a startling disparity in how the two phases returned to equilibrium. The first, dominant phase behaved exactly as expected according to "second-order" phase transition theory. Regardless of how hard the material was "kicked" by the laser, the dominant CDW re-emerged gradually and uniformly across the entire sample. This is analogous to a gas cooling down and turning into a liquid in a smooth, continuous process.
However, the second, subdominant phase followed a "first-order" transition—a much rarer occurrence in this context. Instead of forming everywhere at once, the electrons organized into small, isolated pockets or "nuclei." These pockets then expanded and merged until the phase covered the material. This is strikingly similar to how liquid water crystallizes into ice, where freezing begins at specific points and spreads outward.
"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," says Gedik.
Technical Analysis of the "Checkerboard" Interaction
The coexistence of these two phases is not merely a matter of proximity; it is a matter of competition and cooperation. One of the central questions in quantum physics is whether multiple phases in a material reinforce one another, compete for the same electrons, or exist independently.
In the case of erbium tritelluride, the MIT study suggests a complex interplay. The dominant phase, by forming first and occupying a large portion of the electronic "real estate," forces the subdominant phase to find a different path toward emergence. The fact that the second phase must form via nucleation (pockets) suggests that the dominant phase creates a landscape where the second phase cannot easily take hold everywhere at once.
This "case study" provides vital data for theorists working on high-temperature superconductors. In those materials, magnetism and superconductivity often coexist or compete. Understanding why one phase "wins" or how they find a way to share the material is essential for increasing the temperatures at which these materials can operate.
Broader Implications and Future Research
The implications of this research extend far beyond the laboratory. By demonstrating a reliable way to observe and distinguish between different types of electronic phase transitions, the MIT team has provided engineers with a new set of tools for material design.
Quantum Computing and Sensors
Quantum devices rely on the precise manipulation of electronic states. Knowing that a phase emerges via nucleation rather than uniform transition allows designers to account for potential "defects" or boundaries between pockets, which could affect the performance of a quantum sensor or a qubit in a quantum computer.
High-Temperature Superconductivity
One of the greatest challenges in modern physics is developing materials that can conduct electricity without resistance at room temperature. Many candidate materials exhibit CDWs alongside superconductivity. The MIT study’s ability to untangle these phases helps scientists determine if CDWs are a "competitor" that suppresses superconductivity or a "precursor" that helps it form.
New Experimental Paradigms
The "shake and listen" technique used by Gedik’s team is likely to become a standard protocol for studying other complex materials. By moving away from static observations and toward "time-resolved" snapshots, physicists can see the "how" and "why" of material behavior, not just the final state.
Conclusion
The study conducted by MIT, Stanford, and their collaborators represents a landmark in the study of quantum matter. By identifying that erbium tritelluride’s subdominant electronic phase emerges through a first-order nucleation process—an unexpected find in a system previously thought to be well-understood—the researchers have opened a new chapter in condensed matter physics.
As the industry looks toward a post-silicon future, the ability to engineer materials with "designer phases" will be paramount. The "atomic checkerboard" of erbium tritelluride, once a mystery of quantum coexistence, now serves as a roadmap for the development of the high-performance devices of tomorrow.
The research was supported by the U.S. Department of Energy, the National Science Foundation, and the Gordon and Betty Moore Foundation’s EPiQS Initiative. Additional co-authors from MIT include Dongsung Choi, Doron Azoury, Masataka Mogi, and Bai-Qing Lv, alongside international collaborators who provided the essential material synthesis and theoretical framework.
