Colloquium on April 18, 2011

Qimiao Si
Rice University

Quantum Criticality and Emergent Phases

Electronic materials exhibit diverse properties, some of which “merely” inspire our curiosity while others also change the way we live our lives. When electronic correlations are strong, the behavior of such materials becomes even richer but the theoretical description becomes notoriously difficult. The fundamental question is how the electrons are organized and, in particular, whether there are principles that are universal among the various classes of strongly correlated materials.

One such principle, which has come to prominence in recent years, is quantum criticality. It describes the collective fluctuations of matter undergoing a continuous transition between competing ground states. A particularly lively setting arises in correlated systems at the border of antiferromagnetism. Here, theoretical studies have brought out new classes of quantum critical points, which are supported by mounting experimental evidence. At the same time, experiments have signaled that quantum criticality nucleates novel phases, for which theoretical understandings are just beginning.

In this lecture, I will introduce the basics about quantum criticality including the motivations to study it, and use the prototype case of heavy fermion metals to summarize the recent developments. I will also discuss some of the open issues and future directions, including the interplay among antiferromagnetic quantum criticality, electronic localization and unconventional superconductivity.