Quasiparticles Repel, Then Attract

W hen amobile impurity such as an electron interacts with a bath of bosons, it forms a quasiparticle—a polaron—whose properties are very different from those of the impurity itself. For example, in a superconductor, electron–phonon interactions generate polarons that attract one another (forming Cooper pairs) even though individual electrons are mutually repulsive. A general understanding of what dictates polarons’ properties and their resulting interactions remains elusive but is fundamental for finding ways to tune andmanipulate these quasiparticles. Addressing this problemwith experiments and theory, Li Bing Tan of the Swiss Federal Institute of Technology (ETH) in Zurich and her colleagues demonstrate a mechanism for modifying impurity interactions in a bosonic bath [1]. By changing the bath density, they turn repulsive interactions into attractive ones.