Prof. Bill Halperin and Prof. Jim Sauls Win Fritz London Memorial Prize

February 20, 2017

The London Prize is the premier prize in the field of low-temperature physics.  It is awarded every 2 or 3 years, and previous winners include Lev Landau, John Bardeen, William Fairbank, Brian Josephson, David Thouless, Wolfgang Ketterle, and in 2014 to Michel Devoret, John Martinis, and Rob Schoelkopf.   Prof. Halperin and Prof. Sauls share the prize with Jeevak Parpia (Cornell).   To date, 12 winners of the London Prize also have the Nobel Prize in Physics.  They are cited by the committee for their pioneering work on the influence of disorder on the superfluidity of Helium.

Below, Prof. Sauls writes about his observations on their work.

"The work that Bill and I are cited for in the London Prize is based on a collaboration that began with the discovery in Bill's lab of a new state of matter in liquid Helium-Three (the light isotope of Helium) infused into a unique form of glass (SiO2) called silica aerogel. The latter is a remarkable material itself, a gossamer solid that is mostly empty space with a density that is only 1/100 th that of everyday glass. As for the Helium liquids they are arguably nature's gift to physics because in large part because they exist in the liquid state down to the absolute zero of temperature. This fact provides us with a liquid whose properties in aggregate are governed by the laws of quantum mechanics, i.e the Helium liquids are quantum liquids. One of the most remarkable states of matter are the superfluid phases of 3He, discovered in 1971 at Cornell by Doug Osheroff, David Lee and Bob Richardson. It was a landmark discovery as the superfluid phases are the realization of the theory of Bardeen, Cooper and Schrieffer in a quantum liquid. That discovery led to the Nobel prize to the discoverers, and to Anthony Leggett for the theory that led to their identification as BCS condensates.

The discovery of superfluidity of Helium confined within silica aerogel was unexpected. Indeed if you had asked me before 1995 I would have been very skeptical because the superfluid state of Helium-Three is very fragile and most theorists working on quantum liquids, particularly Helium-Three, likely would have said that the complex structure of a random solid like aerogel glass would destroy the quantum correlations that give rise to superfluidity in Helium-Three. So, the disocvery by Bill's group using Nuclear Magnetic Resonance spectroscopy and the independent discovery by Jeevak Parpia's group at Cornell using a mechanical oscillator to directly meassure the superfluid mass flow was a profound and unexpected disovery.

Bill Halperin's Lab and my theory group have had over the years a number of theory-experimental collaborations, and this award recognizes the progress we made in understanding the stability and sensitivity of the superfluid state even when the liquid has to "wind its way'' through a torturous and random maze of solid silica. Theoretically, the robustness of the quantum state in the presence of scattering by a random network of silica is based on the ability of the quantum correlations responsible for superfluidity to adjust to the large open regions in the random solid.

I will try an analogy which is a simplification but perhaps captures what is remarkable about the superfluid state of Helium-Three in a random solid.

Imagine the particles of Helium as cars on an 8-lane interstate. When the density cars gets large enough small changes in speed to avoid collisions - "near collisions'' lead to a slow down of the flow, eventually gridlock. That is highly viscous state of a normal liquid. But, if we could install in each particle a communication device to lock the speed and distance of each particle to its neighbors so that the motion is coherent among all its neighbors, and that coherence applies to each and every particle, then the stream can flow bumper to bumper without collisions, slow down or friction - that is the superfluid state.

Now imagine that there are randomly placed barriers through out the superhighway. A collision with a random barrier can be a disaster. The solution is to give up some of the particles, and localize them in the vicinity of every barrier, and equip these localized particles with the same communication device as the flowing particles. These "lookout particles'' protect the superflow by communicating the information about the random barriers to the flowing particles. The superfluidity is only destroyed when there is a sufficient density of barriers that it traps all the particles. So, that is my classical analogy of the quantum state of superfluidity in a random solid, and the "communication device'' is what is called phase coherence of quantum coherence in the quantum theory, and the "lookout'' particles are called Andreev bound states."