New theory links two models of impurities in quantum matter
Heidelberg physicists say a new framework connects mobile and nearly fixed impurity behavior in dense quantum systems.
By Priya Raghavan · Science Reporter
3 min read
Physicists at Heidelberg University say they have developed a theory that connects two long-separated descriptions of how a single impurity behaves inside crowded quantum matter. The work could help researchers interpret experiments involving ultracold atomic gases, two-dimensional materials and new semiconductor systems, according to the university.
The study, by Xin Chen, Eugen Dizer, Emilio Ramos Rodríguez and Richard Schmidt, was published in Physical Review Letters under the title “Mass-Gap Description of Heavy Impurities in Fermi Gases.” Heidelberg University said the research was carried out through its STRUCTURES Cluster of Excellence and the ISOQUANT Collaborative Research Centre 1225.
Two limits of the same problem
The problem concerns an impurity placed in a Fermi sea, a many-particle system made up of fermions. Fermions include particles such as electrons, protons and neutrons, and their collective behavior underlies many areas of condensed-matter and nuclear physics.
One established model describes a mobile impurity that moves through the surrounding fermions while interacting with them. In that picture, the impurity becomes dressed by nearby particles and behaves as a combined object known as a Fermi polaron.
Heidelberg doctoral candidate Eugen Dizer said, according to the university, that this quasiparticle approach is a central tool for studying strongly interacting systems. The university cited ultracold atomic gases, solid-state materials and nuclear matter as examples where the concept is used.
A second model applies when the impurity is so heavy that it is treated as nearly fixed in place. In that limit, Heidelberg University said, Anderson’s orthogonality catastrophe describes a very different result: the impurity changes the surrounding fermions so strongly that their wave functions no longer resemble the original state.
That altered quantum background blocks the orderly collective motion needed for a quasiparticle description, according to the university. For decades, Heidelberg said, physicists lacked a common theory showing how the mobile-impurity and heavy-impurity pictures fit into one framework.
Small motion changes the outcome
The Heidelberg team used analytical methods to connect the two descriptions, the university said. The key point in its account is that even an extremely heavy impurity retains a small amount of motion as the surrounding quantum environment adjusts.
Those slight movements create an energy gap that permits quasiparticles to form, according to Heidelberg University. The framework also describes how quantum systems move between polaronic and molecular states, the university said.
Dizer, a member of Schmidt’s Quantum Matter Theory group at Heidelberg’s Institute for Theoretical Physics, said the framework explains quasiparticle formation in systems with very heavy impurities and links ideas that had long been treated separately, according to the university.
Schmidt said the theory offers a flexible description of quantum impurities across different spatial dimensions and interaction strengths, Heidelberg University reported. He also said the work is relevant to current experiments with ultracold atomic gases, two-dimensional materials and novel semiconductors.
This story draws on original reporting from ScienceDaily.