Unconventional free charge in the correlated semimetal Nd2Ir2O7

Nd₂Ir₂O₇ is a correlated semimetal with the pyrochlore structure, in which competing spin–orbit coupling and electron–electron interactions are believed to induce a time-reversal symmetry-broken Weyl semimetal phase characterized by pairs of topologically protected Dirac points at the Fermi energy^1–4. However, the emergent properties in these materials are far from clear, and exotic new states of matter have been conjectured^5–7. Here, we demonstrate optically that, at low temperatures, the free carrier spectral weight is proportional to T², where T is the temperature, as expected for massless Dirac electrons. However, we do not observe the corresponding T³ term in the specific heat. That the system is not in a Fermi liquid state is further corroborated by the charge carrier scattering rate approaching critical damping and the progressive opening of a correlation-induced gap at low temperatures. These observations cannot be reconciled within the framework of band theory of electron-like quasiparticles and point towards the effective decoupling of the charge transport from the single particle sector.