Physicists have developed a new theory integrating two areas of modern quantum physics, explaining the behavior of unusual particles within complex quantum environments known as many-body systems. This framework, created by researchers at the Institute for Theoretical Physics at Heidelberg University, describes how a particle can function as either freely moving or nearly fixed within a large collection of fermions, often referred to as a Fermi sea. The results are expected to influence ongoing experiments in quantum matter.
Quasiparticle Model and Impurity Behavior
In quantum many-body physics, the behavior of impurities (unusual electrons or atoms) surrounded by numerous other particles has been a subject of study. The quasiparticle model explains this by proposing that a single particle interacts with a Fermi sea, pulling nearby particles to form a combined entity called a Fermi polaron.
This entity behaves like a single particle, emerging from the shared motion of the impurity and its surroundings.
This concept is central to understanding strongly interacting systems, from ultracold gases to solid materials and nuclear matter.
Anderson's Orthogonality Catastrophe
A different scenario, Anderson's orthogonality catastrophe, occurs when an impurity is so heavy that its movement is minimal. Its presence significantly alters the surrounding system, causing fermion wave functions to change extensively and break down coordinated motion. Under these conditions, quasiparticles were not previously understood to form. A clear theory linking this extreme case with the mobile impurity picture was not available until now. The Heidelberg team connected these two descriptions within a single framework using analytical tools.
Connecting the Paradigms
The theoretical framework explains the emergence of quasiparticles in systems with extremely heavy impurities, linking two previously separate paradigms. A key insight is that even very heavy impurities are not perfectly static; they undergo small movements as their surroundings adjust. These slight shifts create an energy gap, enabling quasiparticle formation even in strongly correlated environments. The research also demonstrated that this process accounts for the transition from polaronic states to molecular quantum states.
Implications for Experiments
The new theory offers a flexible method for describing impurities across various dimensions and interaction types. It is relevant for ongoing experiments involving ultracold atomic gases, two-dimensional materials, and novel semiconductors.
The study was conducted as part of Heidelberg University's STRUCTURES Cluster of Excellence and the ISOQUANT Collaborative Research Centre 1225. The findings were published in Physical Review Letters.