A new theory developed by physicists at the University of Heidelberg unites two long-competing ideas in quantum physics to provide a unified explanation of how unusual particles behave in crowded quantum environments. This work brings together two seemingly opposite depictions of a single impurity moving or remaining nearly motionless within a large ensemble of fermions, a system known as the Fermi Sea.
The framework, created by researchers at Heidelberg University’s Institute for Theoretical Physics, explains how quasiparticles arise and connect two previously separated quantum states. The researchers say this advance could have important implications for experiments exploring quantum materials.
New theory unites competing quantum models
Quantum object physics has long relied on a variety of models to explain how impurities such as exotic electrons and atoms interact with surrounding particles.
One well-established picture is based on quasiparticles. In this model, a single impurity moves through a sea of fermions containing electrons, protons, and neutrons, interacting with nearby particles. As it moves, it effectively carries neighboring particles with it, forming a complex called a Fermi polaron. Although this quasiparticle behaves like a single particle, it actually arises from the collective motion of the impurity and its surrounding particles.
According to Eugen Deiser, a doctoral candidate at the Institute for Theoretical Physics at Heidelberg University, this quasiparticle model has become a fundamental tool for understanding strongly interacting systems, such as ultracold atomic gases, solid materials, and nuclear matter.
Solving a decades-old quantum puzzle
A completely different situation presents itself when the impurity is so heavy that it is essentially immobile. In this situation, a phenomenon called Anderson’s orthogonality catastrophe takes over.
Rather than creating quasiparticles, heavy impurities dramatically change the quantum system, causing the wavefunctions of the surrounding fermions to lose their original shape. The resulting complex background prevents the coordinated motion required for the existence of quasiparticles.
For decades, physicists had no theory that could explain how these two very different descriptions fit together. Using a variety of analytical techniques, the Heidelberg team showed how mobile and nearly fixed impurity models can be integrated within a single theoretical framework.
Small movements reveal lost connections
“The theoretical framework we have developed explains how quasiparticles appear in systems with very heavy impurities, and brings together two paradigms that have long been treated separately,” explains Eugen Deiser, a member of the Quantum Matter Theory Working Group led by Professor Richard Schmidt.
Researchers have discovered that even very heavy impurities are not completely immobile. These impurities still move slightly as the surrounding environment adjusts. These small movements create energy gaps that allow quasiparticles to emerge from regions that would otherwise remain highly correlated quantum backgrounds.
The new framework naturally also explains how quantum systems transition between so-called polaron and molecular states.
Quantum materials and implications for future experiments
Professor Schmidt says the new theory provides a versatile way to describe quantum impurities across different spatial dimensions and different interactions.
“Our research not only advances the theoretical understanding of quantum impurities, but also has direct implications for ongoing experiments with ultracold atomic gases, two-dimensional materials and new semiconductors,” added the Heidelberg physicist.
This research was carried out through the STRUCTURES Cluster of Excellence and ISOQUANT Collaborative Research Center 1225 at Heidelberg University. The results were published in a magazine. physical review letter.
