At the atomic scale, nothing is ever truly still. Materials that appear perfectly rigid and motionless to the naked eye are in fact swarms of vibrating atoms. This motion is generally random and uncoordinated, but with the right input, the atoms in certain materials will start to move together, vibrating in sync.
These collective vibrations are a form of sound called phonons, and when tuned just right, they can influence a material’s structure and behavior in dramatic and useful ways. Researchers are working to understand and control this effect to optimize material properties and even access hidden phases of matter.
Scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory are using light to drive phonon activity in a class of materials called metal halide perovskites, whose customizable structures and photosensitivity hold promise for use in next-generation solar cells, advanced sensors and quantum information technologies.
In recent experiments, Argonne scientists exposed a layered, 2D perovskite crystal to ultrafast laser pulses. When studying the material’s response, they discovered the emergence of a complex and elusive type of vibration across its structure - a Higgs mode. This collective atomic movement modulated the symmetry of the crystal’s structure itself, demonstrating the ability of light to directly drive and control symmetry changes in quantum materials.
“When we excite this material, the atoms that make up its structure start to oscillate in more ways than one,” said Richard Schaller, an Argonne scientist and author on the study. “Because of the ways those atomic vibrations are coupled with each other, the collective motion actually changes the material’s structure, driving it toward a state with higher crystal symmetry.”
Published in Nature Materials, the study provides insight into the interactions between light and phonons and how they impact structural, electronic and optical properties in perovskite crystals.
The scientists also found that the light-induced Higgs mode steers the material toward a crystal phase that cannot be achieved by merely heating it up. This means that light can excite materials in ways that are distinct from thermal excitations, potentially offering a path to entirely new and otherwise inaccessible material phases and properties.
The Higgs Mode - A Wiggle in Symmetry
The Argonne study marks the first demonstration of a Higgs mode in a semiconductor - a material whose electric conductivity can be switched on and off, or otherwise controlled.
Through the years, researchers have uncovered mathematical analogs of the Higgs mode within a range of systems spanning seemingly disparate scientific domains - perhaps most famously the Higgs boson in particle physics. Another Higgs mode serves as the mechanism behind superconductivity, a state in which a material can conduct electricity with virtually no resistance.
In general, a Higgs mode is an oscillation in the degree to which a system displays some sort of symmetry, or order. It emerges when a system experiences a phase transition caused by something called spontaneous symmetry-breaking.
Imagine letting go of a ball positioned at the very peak of a perfectly round hill surrounded by a valley. The peak is a precarious place to be. Any little disturbance will cause the ball to roll down the hill and settle into a more stable spot.
There are many possible paths the ball could take from the hilltop into the valley. When it selects a particular path, the ball-hill system’s symmetry is spontaneously broken. In other words, the system ends up in a state that no longer reflects the symmetry of the equations used to describe its underlying nature.
“You can simulate an ideal perovskite structure, but you won’t find most perovskites in that configuration in nature,” said Argonne scientist Pierre Darancet, a theorist on the study. “They tend to lower their energy by creating secondary structures that decrease their crystal symmetries.”
In the ball-hill system, a Higgs mode represents oscillations the ball might make in the valley after rolling down the hill. In the Argonne study, the Higgs mode is the manifestation of broken crystal symmetries, and it takes the form of distinctive, synchronized oscillations of atoms across the crystal sample.
Good Vibrations and Photoexcitations
The scientists studied butylammonium lead iodide, a 2D metal halide perovskite crystal. These single-layer semiconducting crystals are relatively easy to fabricate, and their highly tunable properties - particularly their bandgaps - conveniently align with solar energy, making them promising for use in future photovoltaics.
The bandgap of a semiconductor dictates which energies of light it absorbs and converts into electricity. Light with energy higher than the bandgap is absorbed by the material, and light with energy lower than the bandgap passes through, scattering off the material’s structure.
“In these experiments, when we excite the sample below its bandgap, there’s not enough energy to create electric excitations. Instead, at these very low energies, the light pulse can excite only vibrations,” said Schaller.
When exposed to ultrafast laser pulses during the experiments, small groups of atoms repeated across the sample’s crystal structure began to oscillate about their ideal, symmetric orientations. Due to the interactions between their electrons, as the angles between the oscillating atoms changed, so did the material’s bandgap.
The researchers detected the changing bandgap using a technique called impulsive stimulated Raman spectroscopy at the Center for Nanoscale Materials, a DOE Office of Science user facility at Argonne.
“We found that the bandgap increased and decreased periodically and rapidly,” said Schaller, who has a joint appointment with Northwestern University. “Essentially, the color of the sample oscillated as it rocked through different crystal symmetries - turning redder and then bluer, over and over.”
After the experimentalists detected the material’s oscillating bandgap, they brought the results to Argonne theorists, who used simulations to map the changes in bandgap to structural changes in the material.
When exposed to the light, the oscillating groups of atoms across the crystal’s structure start to move in two distinct ways: sometimes, they wave back and forth along a certain plane; other times, they twist in and out of that plane. These two vibrational modes available to the crystal are like the multiple paths available to the ball at the hill’s peak.
With every pulse, the laser light prompts the atomic structures across the sample to move in a particular way that combines the two vibrational modes. This collective motion temporarily restores crystal symmetry, or kicks the system’s metaphorical ball back up the hill. Almost immediately, the atomic structures across the sample select one way of oscillating over another, spontaneously breaking symmetry. In this rapid cycle, the sample’s crystal symmetry is restored and broken again and again.
Together, thousands of spectroscopic measurements reveal the overall effect of these light-induced, coupled vibrations - a Higgs mode.
“Two frequencies were involved in the bandgap oscillations, and that’s where this material is special. Instead of just one simple vibration, the material displayed a coherent superposition of harmonics, resonating similarly to a violin when you bow its strings,” Darancet said.
When exciting the material below its bandgap, even when the scientists increased the intensity of the laser pulses, the frequencies of the two oscillations contributing to the Higgs mode stayed the same and remained phase-locked - or coherent - with each other. “There are quantum mechanical origins to the forces that result in this coherent oscillation,” Darancet added.
Learning to Shine
Different crystal symmetries can yield significantly different material properties. Take carbon atoms, for example. Arranged in one way, they can form diamond, a hard and transparent electric insulator. Arranged with different symmetries, the same carbon atoms can form graphite, a soft and opaque electric conductor.
“In this study, the oscillations steer the material toward a state with higher symmetry - and with a much lower bandgap - than its ground state,” said Sraddha Agrawal, a postdoctoral researcher at Argonne and theorist on the study. “Our next steps are to try and actually achieve that higher symmetry state, and to explore other light-induced phases in perovskite materials.”
When excited above bandgap, charge carriers released in the sample from photoexcitations counteracted the Higgs effect, driving the system away from the light-induced, higher-symmetry phase.
“If we can use light to control structural and electronic changes in materials on ultrafast timescales - for example, switching a material between conducting and insulating states every picosecond - they might find use as optical switches in modern microelectronics and quantum technologies,” said Argonne postdoctoral researcher and experimentalist, Ayushi Shukla. A picosecond is equal to one trillionth of a second. “Also, stabilizing novel, high-symmetry phases with low bandgaps could open exciting opportunities for photovoltaic applications.”
This research was supported by the U.S. National Science Foundation and DOE’s Office of Basic Energy Sciences. Other authors include Shoshanna Peifer from Northwestern University and Mercouri Kanatzidis, who has a joint appointment with Argonne and Northwestern University.