Colloquium: Heavy-electron quantum criticality and single-particle spectroscopy

Quantum critical behavior occurs at low temperatures where quantum fluctuations become more important than thermal fluctuations. Of particular current interest is unconventional quantum criticality, which lacks a classical thermal counterpart. Angle resolved photoemission and scanning tunneling microscopy are experimental techniques that measure single-particle excitations, and recently been used to probe strongly correlated metals near quantum critical points Here, how the single-particle properties reflect the unconventional quantum criticality is discussed, and the prospect for further measurements to unearth new physical behavior.