The discoveries in quantum condensed matter during the 1980s led to a major resurgence in advances in theoretical techniques to deal with the new challenges. The sophistication of these theoretical methods and the intellectual questions they posed attracted a whole new cadre of young, budding condensed matter theorists. Many of these looked to Aspen as a place to be during summers for new ideas; with their regular participation, Aspen became a Mecca for theoretical discourse in the field.
In parallel with the explosion in the number and quality of upcoming young theorists in condensed matter and other fields, ACP was confronted with the need to change its governance. From a situation where a few volunteers worked for the Center in the 1960s, and Trustee-members were elected for unspecified terms, the number of potential members of the Advisory Board grew significantly by the 1980s. It became an honor to be asked to serve on the Board. To avoid having a Board that was too large and unwieldy, in 1989, the ACP formed a committee to address this problem. The committee, chaired by one of the newly appointed members (Ravin Bhatt), came up with a recommendation to limit the terms of members of the Board. All former life-members were asked to resign effective the summer of 1990, and many were re-elected with staggered terms ending in 1-5 years. For condensed matter, this change was not only healthy, but also necessary, given the large number of promising recruits who were young and energetic. They could be expected to carry on the efforts of their predecessors with enthusiasm, and hopefully with foresight as well. The rules were flexible enough to permit dedicated members to continue with an honorary status, which allowed them to participate as non-voting members of the governing body. Needless to say, the nineties saw many new condensed matter board members, including the first women members in the field — Susan Coppersmith, Catherine Kallin, Barbara Jones, Karin Rabe, and Premala Chandra.
Along with the new constitution, the ACP found itself urgently needing an upgrade of its facilities. Hilbert Hall, which had been built with a finite (short) timeframe, needed replacing. A campaign was organized, in which A. Douglas Stone, a Condensed Matter ACP Board Member, played a crucial role. The result was a much-improved facility, Smart Hall, with the attached new auditorium, the Flug Forum, several discussion areas, and many more offices than before. With the new facilities, the nineties also saw a significant rise in applications to ACP, which, in turn, led to an increased formalization of the procedure for selecting workshops, as well as selection of applicants, requiring greater diversity and priority for fresh, deserving applicants. All this worked well with the ever-increasing numbers of condensed matter theorists who were applying to attend ACP programs, both in the summer as well as in the winter season.
For condensed matter physics, the nineties at Aspen, as elsewhere, could be called the decade of correlated electron systems. While there had been a very strong presence of correlated systems physics at Aspen from the first days of condensed matter, e.g. helium (60s-70s), and Heavy Fermions (70s-80s), the issue rose to unprecedented prominence with the discovery of high temperature superconductivity in the cuprates. Condensed matter physicists all over the world were attracted to the promise of high temperature superconductivity, as bees to honey. Superconductivity at room temperature would lead to a revolution in technology comparable to the semiconductor revolution of the 1950s.
At Aspen, six summer programs and four winter workshops dealt with various aspects of the subject, with titles like Quantum Criticality, Non-Fermi Liquids, Quantum Magnetism, High-Temperature Superconductivity, Strongly Correlated Metals, Strong Interactions in Low Dimensions, etc. In addition, strong interaction physics in the quantum Hall regime was featured in two of the summers. The development of theoretical methods to treat strong electron-electron correlations in quantum matter continued to be the major theme. Among the new ideas that emerged and were explored in some detail in the context of strongly correlated materials were Dynamic Mean Field Theory, and slave boson and fermion methods. For the fractional quantum Hall problem, a new method for rationalizing the hierarchy of states by mapping the fractional QH problem to an integer QH problem of composite fermions was developed by Jainendra Jain, and was extensively discussed at the program in 1993, which he attended. Novel variational wavefunctions were proposed for non-Abelian FQH states. New calculational tools were developed to explore beyond the inspired variational wavefunction approach. These involved Chern-Simons Field Theory methods coupled with the Landau-Ginzburg approach, to study fluctuation effects for quantum Hall phases, and transitions between them. Field-theoretic methods had become an essential part of the condensed matter theorist’s toolkit. Communication and interactions with field theorists became more common, and ACP provided an ideal environment for such interactions and collaborations. In 1993 summer, a program on Integrable Quantum Field Theories was organized, involving participants from several disciplines, including condensed matter.
As ACP participants represent various areas of theoretical physics, it is natural that often discussions result in unexpected collaborations. A case in point is the collaboration between Nick Read and Greg Moore that began at ACP in the summer of 1989 and resulted in an oft-cited path-breaking paper (1991) on exotic non-Abelian quantum Hall states. This is an example of an advance that happens without a particular workshop, and which is a result of having physicists with different expertise talking together at ACP.
Despite belonging to the generalized field of correlated electrons, high-temperature superconductivity and FQHE were very different, as their historical development shows. The QH problem was clean, and the theoretical insight, though profound, came quickly. Further, as a result of strong experimental effort, most of the important theoretical issues got resolved relatively fast. In high Tc, by contrast, many properties seemed to depend on chemical details, there were competing effects, and the theoretical issues were far from clear. Descendent of the long appreciated, but never really solved, Mott problem of insulation by correlation, research in high Tc required many new insights. As a result, it was natural that the topic was involved in so many workshops at ACP, starting with the earliest one in 1988, where Anderson gave a detailed presentation of his Resonant Valence Bond idea, and many others gave competing proposals. Over the next two decades, many experimental findings emerged, and most workshops involved theorists and experimentalists working together to sort the wheat from the chaff, to work out the detailed phase diagram including new phenomena such as non-Fermi liquid (strange metal) behavior, and pseudogaps. While these workshops have helped the emergence of the consensus that strong electron correlations are a notable feature of the cuprate superconductors, the theoretical understanding of high Tc is still not at the level of the BCS theory for conventional superconductors.
The nineties also saw a rapid expansion in the winter workshops in condensed matter, to a level where they became an integral part of ACP’s activity in condensed matter. Board members Elihu Abrahams and Sudip Chakravarty took a leading role in this development, by identifying key people to be organizers (and twisting arms where necessary), and often by organizing the workshops themselves. By the end of the decade, the task became easier, and ACP members shared the task of being on the winter workshop committee. Winter workshops were organized in several areas, such as Highly Correlated Electron Systems (1990), Future Trends in Low Temperature Physics (1992), The Physics of Reduced Dimensionality Systems (1995), Defects in Soft Condensed Matter (1998), Quantum Criticality (1999) and Fundamental Physics of Ferroelectrics (2000). The format of these workshops, though closer to conferences because of the shorter (one-week) duration, was flexible enough to accommodate the latest developments because of the shorter time from conception to implementation. For many, ACP Winter Conferences became an annual must-attend affair — the winter counterpart of the Gordon Conferences. These one-week meetings illustrate the organizational flexibility of the ACP, which enables it to respond quickly to important new developments and to contribute to setting the agenda for future work. A case in point is the 1999 winter conference with the title “Quantum Criticality”. Probably this was the first time that a full conference with this title was organized. Since then this subject has blossomed to an extent that it is now widely discussed in many contexts, particularly for the heavy-fermion compounds and remarkably, in the language of AdS/CF(M)T. In the latter topic, since ACP participants come from both the condensed matter and string theory communities, it has played a unique and crucial role in facilitating a remarkable synergy between string theorists and condensed matter theorists.
Though high temperature superconductivity took a major portion of the ACP summer and winter workshops, other fields were also represented through the decade. Physics of disorder, which had figured prominently in the late seventies and early eighties, saw a program devoted to it in 1997 summer, entitled “Quantum Phase Transitions in Disordered Systems”, organized by Subir Sachdev, Myriam Sarachik and Peter Young. This program dealt with the expanded field emerging from the studies in the eighties on metal-insulator transition in disordered systems. It included, in addition, superconductor-insulator transitions, as well as phase transitions in disordered quantum magnets, e.g., quantum spin glasses, a natural evolution of thermal phase transitions in classical spin glasses.
A major revolution that took place during this era was the PC revolution, and following that, the development of the World Wide Web. With physicists increasingly relying on computers both for research and for communication of research, Aspen could no longer remain isolated from the rest of the world, where “one could work in peace with nature as one’s only companion”. Computational access became imperative, and with it ACP became more hospitable to physicists who used computers as a major tool in their research. In condensed matter, this meant, besides electronic structure physicists, those doing Monte Carlo, Numerical Renormalization methods (such as Density-Matrix-Renormalization-Group) and the like. While ACP had hosted programs in the eighties in both electronic structure (e.g., Physics of Clusters, organized by the late Michael Schluter in summer 1986) and in Monte Carlo methods (1987), the availability of computer connections (first wired, then wireless) in the nineties really made ACP attractive to all areas of condensed matter research. More of the attendees were involved in computational research at various levels than in previous decades. Several summer programs, e.g., Materials at High Pressure (1997), Physics of Insulators (1998), and Physics of Semiconductor Lasers (1990), and a number of winter workshops broadened the scope of condensed matter physics at ACP in this period.
Towards the end of the nineties, advances in cold atom physics made it possible to address many topics of interest in condensed matter in these systems. As a direct consequence, ACP had a program entitled “Exploring Bose-Einstein Condensates” in the summer of 1999, the first of many programs devoted to this cross-disciplinary field between atomic physics and condensed matter. Another program the following year was entitled “Quantum Information and Computation”, exploring the interface between quantum physics, information theory and computation, a subject which has also grown substantially in the decade that followed.
The year 2000 began with a special Winter Workshop, entitled “Fifty years of Condensed Matter Physics” held in January 2000. This workshop celebrated the (up to that point) fifty-year involvement of Phil Anderson (“PWA”) with, and leadership of, the field of condensed matter physics. Given his strong connection with ACP, it was particularly appropriate that it was held as an Aspen Winter Workshop. Organized by Ravin Bhatt and Phuan Ong, two of Phil’s colleagues from Princeton, it was attended at full capacity for an Aspen winter workshop, with over a hundred attendees. Seminars covered a wide range of topics “of interest to PWA” (which pretty much covers all interesting aspects of condensed matter, and beyond, to other fields of physics, biology, and computer science). Most of the seminars were included in a volume entitled “More is Different: 50 years of Condensed Matter Physics” published by Princeton University Press; it gives an idea of the wide ranging influence that Anderson has had in condensed matter, both at Aspen and elsewhere.
The new millennium saw major discoveries in new materials including two-dimensional graphene, heavy-electron metals, new “high-temperature superconductors” such as the iron-based pnictides and oxychalcogenides, as well as the development of materials exhibiting unusual quantum states characterized by topological properties such as topological insulators and superconductors. Of course, the theoretical community responded to these developments with enthusiasm and continues to do so; this is reflected by the various summer and winter workshops that have been held on all these subjects. The advances in cold atom systems continued, and even accelerated, to include optical lattices, whereby actual many-body Hamiltonians encountered in real materials could be engineered. In parallel, manipulation of spins at the nanoscale became feasible, allowing one to truly manipulate quantum information, or qubits.
As in the past, ACP responded with swiftness to the community’s desire to have these new developments be part of the summer program and winter workshop agenda. Almost every summer, programs included some aspects of these new frontiers, e.g. Fundamental Issues in Quantum Gases (2001); Spins in Nanostructures (2001); Exploring the Interface Between Cold Atom and Condensed Matter Physics (2003); Coherence and Dissipation in Quantum Systems (2004); Ultracold Atomic Gases (2005); Interactions, Coherence and Control in Mesoscopic Systems (2006); Topological Phases and Applications to Information Processing (2007); Complexity, Disorder and Algorithms (2008); The Physics of Graphene (2008); Quantum Simulation and Computation with Cold Atoms and Molecules (2009); Low Dimensional Topological Matter (2010); Few- and Many-Body Physics in Cold Quantum Gases (2011); and Iron Pnictides and Beyond (2011). Many winter workshops were devoted to these new areas, such as Quantum Coherence and Dissipation (2002), Spins in Nanostructures (2004 and 2007) and Strong Correlations in Ultracold Fermi systems (2006).
In parallel, ACP continued to have both summer programs and winter workshops in its core areas of correlated and disordered systems, as well as in soft condensed matter. A program involving both communities was organized in the summer of 2002, entitled “Collective Phenomena in Disordered Insulators and Glassy Systems”, allowing the two groups to interact and learn from recent developments in each other’s field. New areas of research in soft matter such as Packing problems and Jamming were featured in summer programs in 2006 (Physical and Mathematical Aspects of Packing), 2007 (Jamming) and 2008 (Interfaces, Topological Defects and Flexible Packings). Summer programs in the past couple of years in soft matter have been on “New Perspectives on Strongly Correlated Electrostatics in Soft Matter” (2010) and “Fluctuations and Response in Granular Materials” (2011).
It is almost always the case that discussions at the ACP workshops in condensed matter physics result in the identification of research topics that offer great opportunities for the next years. For example, new ideas at the 2005 workshop on Ultracold Trapped Atomic Gases included collective modes in superfluids near Feshbach resonance and quantum spin dynamics in optical lattices. Similarly, the 2007 workshop on Topological Phases found topics for future emphasis; among them were theory of topological phases and transitions between them, structure of fractional quantum Hall states, non-Abelian topological order, anyon models, and characterizing topological order with entanglement entropy.
Clearly, the last two decades have seen an improvement in both the quality and quantity of the ACP programs in condensed matter (both summer and winter), and of the participants that contribute to its success. Thus, the numbers of publications in premier journals that acknowledge ACP continue to rise. (For example, during the first decade of the new century, the number of papers in only APS journals, Physical Review and Letters, which acknowledge ACP, was over a thousand, of which a majority was in condensed matter). Such progress augurs well for the future.