Electrons were no longer electrons near quantum criticality

One of missions in condensed matter physics is to search novel quantum matter, where electrons are no longer electrons at low temperatures. In this colloquium, we introduce how electrons decay into more basic but not fundamental, emergent excitations in various systems of strongly correlated electrons. First, we look around several examples for electron fractionalization and feel that these phenomena are rather ubiquitous near quantum criticality. Second, we discuss why it is difficult to understand non-Fermi liquid physics near quantum critical points. When correlations of electrons are enhanced, electrons become localized and Mott insulators appear. Third, we present what happens when quantum criticality meets the Mott metal-insulator transition. In particular, we introduce two competing scenarios for Mott quantum criticality, where one is to extend the Kondo effect and the other is to generalize the concept of Luttinger liquid. In this talk, we feel universality from variety, characterized by "emergence" in condensed matter physics.