Elusive polaron 'dance' discovery solves decades-old quantum mystery

An international team of researchers from Kiel University and the DESY research center has resolved a decades-old quantum physics mystery by experimentally identifying a polaron—a quasiparticle formed by the strong coupling of an electron with atomic vibrations—in a rare-earth compound made of thulium, selenium, and tellurium (TmSe₁₋ₓTeₓ). This discovery explains why the material abruptly transitions from a metal to a perfect insulator when the tellurium content reaches about 30 percent, a phenomenon previously unexplained by its chemical composition. The polaron represents a “dance” between the electron and a distortion in the crystal lattice, which slows electron movement and causes the loss of electrical conductivity. The breakthrough came after years of high-resolution photoemission spectroscopy experiments at global synchrotron facilities, where a persistent but initially dismissed “small bump” signal was systematically investigated. By incorporating electron-phonon coupling into the periodic Anderson model, theorists and experimentalists achieved a perfect match between