Quantum Critical Electron Systems: The Uncharted Sign Worlds
J. Zaanen
Phases of classical matter, such as solids and liquids, are
ruled by emergence principles that are well understood. Although
the same principles govern forms of quantum matter that have
no secrets for physicists, such as the superfluids, having to
deal with fermions and the associated Fermi sign problem shatters
this analogy. This Perspective addresses the Fermion sign problem
and describes experiments on metals undergoing quantum phase
transitions exhibiting scale-invariant electronic behavior,
a description of which is at odds with established quantum theory.
Instituut-Lorentz for Theoretical Physics, Leiden University, Leiden 2333 CA, Netherlands. E-mail: jan{at}lorentz.leidenuniv.nl