The fundamental behavior of matter is often defined by its phase—the specific state in which its constituent particles organize themselves. In the macroscopic world, the coexistence of different phases is a familiar sight: a glass of ice water represents a delicate equilibrium where H2O exists simultaneously as both a solid and a liquid. However, in the realm of quantum materials, such coexistence is far more complex and elusive. A groundbreaking study conducted by physicists at the Massachusetts Institute of Technology (MIT) has provided a transformative look at how multiple electronic phases emerge and interact within a single material. Published today in the journal Nature Physics, the research offers a new lens through which scientists can view the "hidden" physics of quantum transitions, potentially paving the way for the next generation of high-performance quantum devices.
The study, led by Nuh Gedik, the Donner Professor of Physics at MIT, focuses on a rare-earth material known as erbium tritelluride (ErTe3). By utilizing ultra-fast laser pulses to "shake" the material’s electronic structure and observing its recovery, the team was able to tease apart two distinct "charge density wave" (CDW) phases. Their findings reveal that while these phases can coexist, they emerge through fundamentally different physical mechanisms—one forming gradually and uniformly, while the other forms in isolated "pockets" that expand like ice crystals in water.
The Architecture of Quantum Phases
In most conductive materials, electrons behave like a chaotic gas, moving randomly and scattering uniformly throughout the atomic lattice. However, when certain materials are cooled to cryogenic temperatures, their electrons begin to coordinate in highly structured ways. This collective behavior results in electronic phases such as superconductivity, where electrons pair up to flow without resistance, or magnetism, where electron spins align.
The focus of the MIT study is the charge density wave (CDW) phase. In a CDW, electrons spontaneously organize into a wave-like pattern of high and low density. The crests of these waves represent areas of high electron concentration, while the troughs represent areas of depletion. This "periodic modulation" of electron density is more than just a scientific curiosity; it is a fundamental state of matter that often competes or coexists with superconductivity.
"Just like superconductivity, charge density waves are a collective phenomenon where electrons move together in certain ways," explained lead author Yifan Su, who earned his PhD from MIT in 2024. "The power of CDWs is that they are a much simpler form of matter compared to superconductivity. They offer a playground for fundamental understanding."
Erbium Tritelluride: A Quantum Playground
The material of choice, erbium tritelluride, belongs to a family of rare-earth tritellurides that have long fascinated condensed matter physicists. These materials are characterized by their layered, quasi-two-dimensional structure, which allows researchers to study quantum effects in nearly flat planes.
In ErTe3, the electrons are known to undergo two distinct CDW transitions as the temperature drops. Previous research established a clear timeline for these transitions:
- The Dominant Phase: When cooled to approximately -8 degrees Celsius (265 Kelvin), the electrons first organize into a "dominant" charge density wave that stretches across the material in a single direction.
- The Subdominant Phase: As the temperature is further lowered to -113 degrees Celsius (160 Kelvin), a second, "subdominant" CDW emerges. This second wave forms perpendicular to the first, creating a complex, checkerboard-like pattern of electronic density.
The central mystery that Gedik’s team sought to solve was how these two phases manage to inhabit the same space. Do they cooperate, compete for the same electrons, or exist in total independence? Understanding this interaction is critical for engineers aiming to replace silicon with quantum materials that can handle multiple states of information simultaneously.
The "Shake and Listen" Methodology
To observe the birth of these phases, the MIT team employed a sophisticated technique known as ultrafast pump-probe spectroscopy. This method allows scientists to observe physical processes that occur on the scale of femtoseconds (one-quadrillionth of a second).
The researchers began by cooling atomically thin samples of ErTe3—synthesized by collaborators at Stanford University—to a staggering -230 degrees Celsius (about 43 Kelvin). At this temperature, the material is firmly in its "checkerboard" state, with both CDW phases fully formed.
The experiment then proceeded in two stages, which Professor Gedik describes as "shake and listen."
- The Shake: A "pump" laser pulse was fired at the sample. This intense burst of energy effectively dissolved the electronic checkerboard, knocking the electrons out of their ordered patterns and returning them to a disordered state. By varying the intensity of this pulse, the researchers could control how thoroughly they "melted" the CDW phases.
- The Listen: A second "probe" pulse of high-energy photons was then delivered at precisely timed intervals after the first. This pulse kicked electrons out of the material, a process known as photoemission. By measuring the energy and momentum of these ejected electrons, the team could reconstruct snapshots of the electronic structure as it attempted to reorganize.
"We see the destroying of these phases, and then if we wait long enough, they come back," Gedik explained. "And depending on how you hit them, the two phases respond differently."
Two Phases, Two Different Births
The data revealed a striking disparity in how the two CDW phases recovered from the laser-induced chaos.
The first, dominant phase reemerged in a manner consistent with a "second-order" phase transition. This is the classic, textbook mechanism where the order parameter grows gradually and uniformly across the entire material. It is analogous to the way a piece of iron slowly gains magnetism as it cools below its Curie temperature. No matter how hard the researchers "kicked" the material with the laser, this dominant phase always returned in a smooth, predictable fashion.
However, the second, subdominant phase behaved in an entirely unexpected way. It did not emerge uniformly. Instead, it formed via a "first-order" transition, characterized by the nucleation of small pockets or "seeds" of the CDW phase. These pockets then expanded until they filled the material. This is remarkably similar to the way liquid water freezes into ice, where crystals form at specific points and grow outward.
This discovery of a first-order transition for the subdominant CDW phase is a major breakthrough. It suggests that the presence of the first CDW phase fundamentally changes the landscape for the second, forcing it to "tunnel" through an energy barrier to establish itself. This "hidden physics" explains why the two phases can coexist without one completely obliterating the other.
Implications for the Future of Electronics
The ability to tease apart and control these coexisting phases has profound implications for the future of technology. As the limits of silicon-based semiconductors become increasingly apparent, the industry is looking toward quantum materials to provide faster, smaller, and more energy-efficient devices.
"People believe the cornerstone of replacing silicon lies in quantum materials that have multiple coexisting phases," says co-author Alfred Zong, a former MIT graduate student and current assistant professor at Stanford University. "Our experiment provides a very neat way to study these multiple phases."
If engineers can master the "switches" that govern these phase transitions, they could create transistors that operate on quantum principles. For instance, a device could use light or electricity to toggle between a CDW state and a conductive state, representing the 1s and 0s of binary code at speeds far exceeding current technology.
Furthermore, the study provides a roadmap for understanding high-temperature superconductors. In those materials, superconductivity often coexists with magnetism and charge density waves. The interactions between these phases are believed to be the key to their "exotic" properties, such as the ability to conduct electricity without loss at relatively high temperatures. By applying the "shake and listen" technique to these more complex systems, scientists may finally unlock the secret to room-temperature superconductivity.
A Collaborative Scientific Effort
The success of the study was the result of a multi-institutional collaboration. Along with Gedik and Su, the MIT team included former postdocs Bai-Qing Lv, Doron Azoury, and Masataka Mogi, as well as Dongsung Choi. The high-quality erbium tritelluride samples were synthesized by researchers at Stanford University, whose expertise in material growth was essential for the precision of the experiments.
The research was supported by several major scientific bodies, including the U.S. Department of Energy (DOE), the U.S. National Science Foundation (NSF), and the Gordon and Betty Moore Foundation’s EPiQS Initiative.
Conclusion: A New Era of Quantum Observation
The MIT study marks a significant milestone in the field of condensed matter physics. By proving that coexisting electronic phases can emerge through radically different mechanisms within the same crystal lattice, the team has challenged long-held assumptions about the uniformity of phase transitions.
"One of the biggest questions in physics is why some materials host multiple phases while others do not," Gedik says. "And when multiple phases do exist, how do they interact? Do they reinforce one another, compete, or coexist independently? This is like a case study for us to understand much more complicated materials."
As the scientific community continues to probe the boundaries of quantum matter, the "shake and listen" approach will likely become a standard tool for uncovering the subtle, hidden dynamics that define the materials of the future. The transformation of erbium tritelluride from a simple rare-earth material into a detailed map of quantum phase behavior is just the beginning of a new era in material science.
