Physicists Observe Coherent Exciton-Phonon Interaction in Perovskites

An international research team has directly observed the synchronized quantum dynamics of excitons and phonons in perovskite nanocrystals. This finding advances the understanding of quantum behavior in semiconductor materials. The researchers published their results in *Nature Communications*.

An exciton forms when light excites an electron in a semiconductor. The electron leaves behind a positively charged hole. The electron and hole bind together, moving as a single quantum object. A phonon is a quantum of crystal lattice vibration. In perovskites, these two distinct quantum objects are strongly linked, evolving together as a coupled quantum system.

Perovskite nanocrystals are tiny crystals, only a few nanometers in size. These nanocrystals act as nanoscale boxes, trapping both excitons and phonons. This confinement enhances the interaction between them. An exciton within the nanocrystal strongly couples with the vibrations of the surrounding crystal lattice.

A short laser pulse creates an exciton. This exciton slightly distorts the surrounding crystal lattice, generating phonons. The electronic excitation and the lattice vibration then form a joint quantum state called an exciton-polaron. In most solids, lattice interaction quickly destroys fragile quantum states. However, lead-halide perovskite nanocrystals show an exception. At a low temperature of two Kelvin, crystal vibrations remain well-defined. This allows the quantum state to evolve coherently for about ten picoseconds.

Researchers used very short laser pulses, lasting about one hundred femtoseconds, to track this evolution. They observed pronounced quantum beats. These beats occur when a system exists in a coherent superposition of different quantum states simultaneously. The oscillations revealed how excitons and crystal vibrations exchange energy and evolve together on ultrafast timescales. The strong amplitude and long coherence of these quantum beats are particularly notable.

The research team, including theorists from TU Dortmund University and Jackson State University, demonstrated that the effect can be tuned by changing the nanocrystal size. Smaller nanocrystals show stronger coupling between excitons and lattice vibrations. Larger nanocrystals preserve oscillations for longer periods. This provides a practical method to engineer and control the quantum dynamics of this system. Perovskite nanocrystals may serve as a platform for future quantum devices. This could lead to new methods for semiconductor quantum information processing, quantum light sources, and the generation of single phonons.