Scientists from the US Department of Energy’s Argonne National Laboratory have discovered an elusive collective vibration, a Higgs mode, within a semiconductor crystal.
The finding marks the first observation of this phenomenon in a semiconductor material. Researchers say the result improves understanding of how light interacts with matter at the atomic scale.
The study focused on a class of materials called metal halide perovskites. These materials have attracted significant attention because they can be engineered for different applications. Their properties make them promising candidates for advanced solar cells, sensors, light-emitting devices, and quantum technologies.
The research team used ultrafast laser pulses to excite a layered two-dimensional perovskite crystal. Instead of creating electrical excitations, the laser energy triggered coordinated atomic vibrations. These vibrations spread throughout the crystal, altering its internal structure.
Although solid materials appear motionless, their atoms constantly vibrate. Most of these vibrations occur randomly and independently. Under specific conditions, however, atoms can move in synchrony.
Scientists call these coordinated vibrations phonons. Phonons are often described as sound waves moving through a crystal lattice. Their behavior influences many material properties, including heat transfer, electrical conductivity, and structural stability.
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Researchers have long sought ways to control phonons with precision. The ability to manipulate atomic vibrations offers a pathway to changing material properties on demand. Such control is important for developing faster and more efficient technologies.
In the Argonne experiments, the laser pulses created a highly unusual collective vibration. The oscillation did more than move atoms back and forth. It changed the symmetry of the crystal itself.
Symmetry plays a central role in determining how materials behave. Different atomic arrangements can produce dramatically different properties. Even small symmetry changes can alter how a material conducts electricity or interacts with light.
The observed Higgs mode represented a repeating oscillation between different symmetry states. Researchers found that the crystal continuously shifted toward a higher-symmetry configuration and then returned. This cycle occurred repeatedly at extremely high speeds.
The discovery was reported in the journal Nature Materials. The study provides new information about the relationship between light, atomic motion, and material structure. It also highlights a previously unexplored mechanism for controlling quantum materials.
Observing the Higgs Mode
According to Argonne scientist Richard Schaller, the laser excitation caused atoms to oscillate in multiple ways at the same time. These coupled vibrations worked together to modify the crystal structure. As a result, the material moved toward a state with greater symmetry.
One of the most significant findings involved the material’s electronic properties. The changing atomic structure also changed the crystal’s bandgap. A bandgap determines which energies of light a semiconductor can absorb.
Bandgaps are essential for solar cells and electronic devices. Materials with different bandgaps absorb and process light differently. Controlling the bandgap can therefore change a material’s performance.
Researchers discovered that the light-induced Higgs mode pushed the crystal into a state with a much lower bandgap than its normal condition. This higher-symmetry phase cannot be reached simply by heating the material. The result demonstrates that light can access material states unavailable through conventional methods.
The Higgs mode has similarities to concepts found in several areas of physics. The most widely known example is the Higgs boson discovered in particle physics. Similar mathematical descriptions also appear in superconductors and other quantum systems.
Despite sharing a name, these phenomena occur in very different environments. What connects them is their relationship to symmetry. In each case, the Higgs mode describes oscillations associated with changes in order within a system.
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To explain the concept, researchers often use a simple analogy. Imagine a ball balanced on top of a perfectly round hill. The ball can roll down in any direction, but once it chooses a path, the original symmetry is lost.
This process is known as spontaneous symmetry breaking. It occurs throughout nature and physics. Many materials settle into lower-energy states by breaking an initially symmetric arrangement.
According to Argonne theorist Pierre Darancet, ideal perovskite structures are rarely found in nature. Real materials often lower their energy by forming structures with reduced symmetry. These lower-symmetry arrangements are generally more stable.
The Higgs mode observed in the experiment reflects this process. The coordinated vibrations repeatedly restore and break crystal symmetry. Scientists were able to observe this behavior through changes in the material’s electronic properties.
The team studied a specific perovskite known as butylammonium lead iodide. This layered semiconductor is relatively easy to manufacture. Its properties also align well with solar-energy applications.
During the experiment, researchers excited the material using light with energy below its bandgap. Because the energy was too low to generate charge carriers, the laser affected only atomic vibrations. This created an ideal environment for studying phonon behavior.
As the atoms moved, their positions changed the electronic structure of the crystal. The bandgap increased and decreased in a repeating pattern. Researchers observed these rapid oscillations using advanced spectroscopy techniques.
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The measurements were performed using impulsive stimulated Raman spectroscopy. The work took place at the Center for Nanoscale Materials, a Department of Energy user facility. This technique allowed scientists to track ultrafast changes occurring inside the crystal.
Researchers described the effect as a repeated color shift. The material effectively oscillated between states that absorbed different wavelengths of light. It became slightly redder and bluer as the crystal symmetry changed.
To better understand the observations, experimental data was compared with theoretical simulations. Argonne theorists mapped the electronic changes back to specific structural motions. This helped reveal the physical origin of the Higgs mode.
The simulations showed two distinct vibrational motions occurring simultaneously. In one motion, atomic groups rocked back and forth. In the other, they twisted in and out of a plane.
These two vibrations combined into a coherent motion across the crystal. Every laser pulse synchronized the oscillations. Together, they created the repeating symmetry changes characteristic of a Higgs mode.
Thousands of measurements confirmed the existence of the phenomenon. Researchers found two separate oscillation frequencies contributing to the overall behavior. Importantly, the frequencies remained locked together even when laser intensity increased.
This phase-locking indicates strong quantum mechanical interactions inside the material. The vibrations remained coordinated instead of drifting apart. Such coherence is a defining feature of the observed Higgs mode.
The discovery also highlights the growing importance of light-based control techniques. Scientists increasingly use ultrafast lasers to manipulate materials on extremely short timescales. These methods can reveal hidden behaviors that remain inaccessible under normal conditions.
The practical implications extend beyond fundamental science. Light-controlled changes in symmetry and bandgap could support new forms of electronic switching. Future devices may use these effects to process information faster and more efficiently.
Researchers also see potential benefits for photovoltaic technologies. Lower-bandgap states can enhance a material’s absorption of sunlight. Accessing and stabilizing these states may help improve solar-energy performance.
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Another area of interest involves quantum technologies. Many quantum systems depend on precise control of electronic and structural properties. Techniques demonstrated in this study offer a new tool for achieving that control.
The team now plans to explore ways to stabilize the higher-symmetry state for longer periods. Researchers also want to investigate additional light-induced phases in perovskite materials. Understanding these states could reveal entirely new functionalities.
The work received support from the US National Science Foundation and the Department of Energy’s Office of Basic Energy Sciences. Scientists from Argonne National Laboratory and Northwestern University contributed to the project. Their combined experimental and theoretical efforts helped uncover one of the most unusual vibrational phenomena yet observed in a semiconductor.
As researchers continue learning how to steer materials with light, discoveries such as this Higgs mode provide a deeper understanding of matter itself and bring advanced electronic, energy, and quantum technologies closer to practical reality.
