Colloquium on June 30th, 2008

Piers Coleman
Center for Materials Theory, Rutgers University

Qu-transitions. Phase transitions in the quantum era

Physicists are often so awestruck by the lofty achievements of the past, we end up thinking all the big stuff has been done. Unfortunately, this blinds us to the revolutions ahead, even when experiment clearly reveals the fault lines in our understanding. This is as true today, as it was a hundred years ago.

We are still very much in the throws of the quantum revolution that began a hundred years ago. It took more than two centuries for the full impact of Newton and Gallileo to be felt by society: the Victorian era, with its clockwork view of reality, can be seen as the crowning peak of this epoch. Today, while the consequences of quantum physics on the atomic scale are well known, our understanding of its full ramifications are very much in a state of flux, in much the same way that classical physics was, more than a hundred years after Newton. Quantum gravity, quantum computers, qu-bits and quantum phase transitions, are all indications of this ongoing revolution. Nowhere is this more so, than in the evolution of our understanding of the collective properties of quantum matter.

Just fifty years ago, physicists were profoundly shaken by the discovery of universal power-law correlations at thermal second-order phase transitions. This was, in many ways, one of the last great hurrahs of the classical era. Today, our understanding of phase transitions has entered a new stage, with the discovery of quantum phase transitions: phase transitions at absolute zero driven by the violent jigglings of quantum zero-point motion. Quantum phase transtiions have been discovered in a wide range of materials, including ferromagnets, helium-3, ferro-electrics, heavy electron and high temperature superconductors. I'll introduce you to some of these examples, and explain how a quantum critical point is a kind of "black hole" in the materials phase diagram: a singularity at absolute zero that profoundly influnces wide swaths of the material phase diagram at finite temperature.

I'll talk about some of the radical ideas in this field and we'll discuss the idea of " avoided criticality" - the possibility that high temperature superconductivity nucleates about quantum critical points. Does the electron fall apart a quantum phase transition, and if so... what is the associated Goldstone mode? These are some of the questions we'll touch upon in this colloquium.