Novel Metallic Phase Emerging from Quantum Criticality: We Couldn��t Observe Electrons at Low Energies

The emergence of non-Fermi liquids near metallic quantum criticality is an essential feature in the system of strongly correlated electrons, where electron quasiparticles decay into other types of collective excitations. As a result, novel metallic phases appear without electrons, showing various exotic non-Fermi liquid physics beyond the Landau Fermi-liquid theory. In this review, we discuss what electrons disintegrate into, which allows us to classify theories for quantum critical metals into the Hertz-Moriya-Millis, Kondo breakdown, and local quantum criticality scenarios. We discuss both the goodness and the weakness of each theoretical framework, recalling experimental aspects for heavy-fermion quantum criticality.