I first met Neil Ashcroft when Robert Shelton and I picked him up near Cologne, Germany, in early August 1981 to drive to a high-pressure conference that Robert and I had co-organized in Bad Honnef. Neil was already well known through his graduate-level textbook Solid State Physics written together with David Mermin. A number of the leading scientists in Condensed Matter Physics, particularly in physics at high pressures, attended the conference.

Besides contributing greatly to the scientific excitement at the Bad Honnef conference, Neil revealed his love of classical music by coming up to me after a live concert of Schumann Lieder in the conference venue, pointing out that the concert was really special and how much he enjoyed it. I really appreciated his sincere gesture.

Neil was not only one of the world's leading scientists in Theoretical Condensed Matter Physics, but he was also a truly great communicator and warm, personable human being who thrived on interacting with students and colleagues alike. It was thus no accident that he had a particular love for the biennial Gordon Conferences on Research at High Pressure where much time is set aside for informal discussions and activities. He attended many of these conferences, relishing the informal atmosphere seen particularly clearly in this photo from the 2006 GRC Conference ably chaired by former AIRAPT President Reinhard Boehler.

In December 2006 Neil was kind enough to visit Washington University in St. Louis where his former postdoc Anders Carlsson and I resided. His colloquium presentation was simple but profound, placed at a level accessible to everyone in the audience. Here is the abstract for this colloquium.

Superconductivity, Close to its Centennial

N. W. Ashcroft (Cornell University)

Superconductivity had quite explosive beginnings, illustrating with one of the most remarkable of all physical phenomena, the important role that chance plays in discovery. The importance of superconductivity, especially for the energy sector, was already recognized by Kamerlingh Onnes, in whose laboratory the effect was discovered nearly a century ago. The concept of electron pairing, central to superconducting order, owes its origin to the earliest claim of the sighting of high temperature superconductivity, in fact in a very light element system rife with polarizable constituents. The high motional energy scales in light element systems make them attractive for superconductivity from the standpoint of the electron phonon-electron pairing mechanism, but among the elements themselves, and under normal conditions, it is well known that the superconducting transition temperatures (T_c) are quite limited. However from the latecomer superconductor MgB₂ (T_c ~40 K) and also from the modern class of cuprate superconductors it is now clear that for the light elements in combination the situation can change radically.

Structures admitting of a large number of electrons per cell (and a corresponding proliferation of occupied electron energy bands) can be especially favorable for superconductivity, and not only from the standpoint of electron-phonon coupling. Accordingly it is of interest to examine the prospects for superconductivity, beyond the cuprate class and particularly for light element systems (and also beyond standard conditions), these including metallic states containing the lightest of all elements, namely hydrogen.

On March 18, 1987 Neil Ashcroft and Brian Maple co-chaired a special evening session at the March Meeting of the American Physical Society in New York City. This session, later called the Woodstock of Physics, reported recent developments with the high-T_c cuprates, in particular the famous YBa₂Cu₃O₇ or simply 123-compound with T_c = 92 K discovered by Ching-Wu (Paul) Chu's group at the University of Houston. This overflow session began at roughly 8 pm in the evening and lasted until 4-5 am the next morning.

Neil had a particular appreciation for investigations of matter under high pressure. For his many contributions to this field he received in 1992 the coveted Bridgman Award, the highest distinction that AIRAPT can bestow.

In a highly regarded paper in 1999 Jeffrey Neaton and Neil pointed out that lithium, the lightest metal, might become superconducting and even insulating under sufficiently high pressure. Struzhkin et al in the US and Shimizu et al in Japan soon confirmed superconductivity in lithium, but it was not until the classic experiments of student Shanti Deemyad that the fascinating superconducting phase diagram of lithium was accurately mapped out to very high hydrostatic pressures.

In 1968 Neil brought great excitement to the field of superconductivity by predicting that under extreme pressures pure hydrogen would become a metal that would likely superconduct at near-ambient temperatures. After experiments to ever higher pressures failed to detect superconductivity in pure hydrogen, Neil showed in 2004 that hydrogen-rich compounds, the hydrides and superhydrides, should exhibit high-temperature superconductivity, but at considerably lower pressures than for pure hydrogen. It must have been a source of great satisfaction for Neil that several groups worldwide have recently reported superconductivity in superhydrides at temperatures approaching and even equaling ambient.

Neil Ashcroft leaves behind a unique legacy as having not only made enormous contributions to Condensed Matter Physics, particularly matter at extreme densities, both also, through his brilliant physical insight and marvelous communication skills, by bringing home the excitement and appreciation of physics research to future generations of scientists throughout the world.

We are very privileged that he walked among us for so many years.