Light Pulses Uncover Higgs Mode in Perovskite Crystal

Scientists have discovered an elusive Higgs mode within a perovskite crystal. This mode reshapes the crystal's symmetry. Researchers at the U.S. Department of Energy's (DOE) Argonne National Laboratory used ultrafast laser pulses in their experiments. They observed complex vibrations across the crystal structure.

This collective atomic movement modulated the crystal's symmetry. It demonstrates how light can directly control symmetry changes in quantum materials. The atoms oscillated in multiple ways when excited. This collective motion changed the material's structure. It drove the material toward a state with higher crystal symmetry.

The study, published in *Nature Materials*, provides insight into light-phonon interactions. These interactions impact structural, electronic, and optical properties in perovskite crystals. The light-induced Higgs mode steered the material toward a crystal phase. This phase cannot be achieved through heating alone. This suggests light can excite materials in ways distinct from thermal excitations. It may offer a path to new material phases and properties.

This marks the first demonstration of a Higgs mode in a semiconductor. A Higgs mode is an oscillation in a system's degree of symmetry. It emerges when a system undergoes a phase transition caused by spontaneous symmetry-breaking. The Higgs mode in this study represents oscillations of atoms across the crystal sample. These oscillations are a manifestation of broken crystal symmetries.

The scientists studied butylammonium lead iodide, a two-dimensional (2D) metal halide perovskite crystal. These crystals are promising for future photovoltaics. When excited below its bandgap, the light pulse excited only vibrations. Small groups of atoms began to oscillate about their ideal, symmetric orientations. The material's bandgap changed as the angles between the oscillating atoms changed.

Researchers detected the changing bandgap using impulsive stimulated Raman spectroscopy. This technique was performed at the Center for Nanoscale Materials, a DOE Office of Science user facility. The bandgap increased and decreased periodically and rapidly. The color of the sample oscillated as it rocked through different crystal symmetries.