On a winter morning in February 2025, amid the vast crowd huddled under a tangle of umbrellas in the pouring rain in Ein Ayala, Israel’s condensed matter physics community stands nearly fully assembled, shocked and struggling to say goodbye. Suddenly, the title “pillar of his community” does not seem like a cliché, but rather appropriate and even accurate. Indeed, Assa Auerbach was renowned for his groundbreaking contributions to the research of complex phenomena in quantum materials, earning him international recognition since his first steps as a young researcher at the very start of his career. His pioneering impact on science in Israel was truly revolutionary: his joining the Technion faculty in the early ‘90s (after a brief period at Boston University) was a defining event that shaped an entire generation of researchers who now hold a prominent place in the global community in the field.

Guest post by Prof. Efrat Shimshoni, Department of Physics, Bar-Ilan University

Assa began his career as a theoretical physicist during an exciting and tumultuous period at the forefront of condensed matter physics research, when several experimental discoveries challenged the paradigms that had previously dominated the study of many-particle quantum systems. Among the most notable was the discovery of superconductivity (electrical conduction without resistance) in non-metallic materials  that become superconductors at around 100 degrees Kelvin—nearly a hundred times higher than in metals. This observation, along with many other findings in complex materials, contradicted theoretical models that had successfully explained the properties of metals, including the superconductivity theory developed by Bardeen, Cooper, and Schrieffer (BCS) in the 1950s, which even earned them the Nobel Prize in 1972 [1].

These older theories were based on a central assumption that the interactions between electrons (the particles governing the properties of solid matter) are relatively weak compared to the electrons’ kinetic energy, and that they can be analyzed theoretically using approximation methods. This assumption indeed holds for most metals, for which the starting point for the theoretical model is an “ideal gas” of non-interacting electrons—that is, a system that is merely the sum of its parts. But this is the exception: a wide range of phenomena in materials cannot be explained from that starting point, which has compelled the community of theoreticians to develop new approaches. These echo the seminal 1972 paper by Nobel laureate Philip Anderson, who coined the phrase “more is different.” The guiding principle is that the collective behavior of a many-particle system cannot be predicted from the properties of a single particle, but is an emergent phenomenon arising from their mutual interactions. The key to constructing an effective and predictive model to describe such a system, in which there are strong correlations among countless fundamental building blocks, is to identify a limited number of collective variables. These variables take into account the aforementioned interactions and serve as “quasi-particles” that replace the microscopic particles [2].

Assa Auerbach was a pioneer among researchers who developed theoretical tools to translate these ideas into practical applications. A series of papers he published in collaboration with Dan Arovas (currently a professor at the University of California, San Diego) during their postdoctoral period at the University of Chicago paved the way for deciphering complex models describing quantum magnetic materials and predicting their behavior at low temperatures. The magnetic properties of these materials arise from the interaction between electrons located at adjacent sites in the crystal. This interaction tends to determine the relative orientation of their magnetic polarization, which stems from their spin—a fundamental property of quantum particles [3]. In principle, electronic spins are like tiny magnets, and at low temperatures, they are expected to settle into an ordered pattern consistent with their interactions. However, the quantum nature of spin undermines the ability to “freeze” into an ordered pattern, even at absolute zero: different components of the spin vector along the three spatial axes are subject to the uncertainty principle, meaning they cannot be determined simultaneously with absolute precision. As a result, magnetic fluctuations known as “spin waves” are observed in magnetic materials.

The work of Auerbach and Arovas succeeded in providing a theoretical understanding of these fluctuations by mapping spin systems to a model of quantum particles (Schwinger bosons) whose dynamics are governed by the interactions among the original electrons [4, 5]. The theoretical approach they developed contributed insights into magnetic materials and was later proven useful in solving a variety of problems in quantum systems with strong correlations, such as the mechanism underlying the formation of superconductivity. The textbook for graduate students published by Assa Auerbach in 1994 (Interacting Electrons and Quantum Magnetism), significant portions of which are based on methods he developed himself, became a bestseller and is featured on the shelf in almost every theoretical physicist’s office in the field of condensed matter [6].

Later in his career, Assa Auerbach left his mark on a wide range of topics at the forefront of research in the field, and in particular contributed to the understanding of strange and surprising conduction properties in complex materials. The phenomena he studied, such as electrical conductivity and thermal conductivity that exhibit anomalous dependence on temperature and magnetic and electric fields, illustrate in various ways the central role of emergent collective entities. To tackle the enormous technical challenges posed by theoretical research in this area, he developed sophisticated numerical and analytical methods that utilized physical insights and intuition to maximize their efficiency. In recent years, his main work focused on developing a kind of master formula for calculating conduction coefficients in many-particle quantum systems—a formula that obviates the need for prior assumptions—which are often unjustified—about the behavior of the constituent particles.

The most striking feature of his work over the years is the breaking of established conventions and a return to the fundamental principles of physics to form a fresh and creative perspective on problems. This is the scientific approach he instilled in an impressive cohort of research students, many of whom and their own students have become leading figures in the academic community of condensed matter physics.

In the last decade of his life, alongside his vigorous scientific activity, Assa dedicated his time to another project that made waves around the world: in collaboration with his brother-in-law, illustrator Richard Codor, he authored a graphic novel titled “Max the Demon,” aimed to deliver Maxwell’s demon paradox to the general public [7]. The plot of the novel presents a slightly unusual type of superhero, devoid of any special physical powers, but endowed with phenomenal computational ability combined with wisdom, curiosity, humor, and a love for humanity—everything needed to save the world from the rampant growth of entropy. It is hard to imagine a more fitting legacy for a scientist who served as an inspiration and mentor to all of us: students he supervised, theoreticians and experimentalists who collaborated with him on research, and many other colleagues who had the pleasure of passionately debating fundamental ideas in physics over a cup of coffee or a beer. May his memory be a blessing.

Source:

1. Previous article on BCS Theory
2. Philip Anderson’s paper “More is Different”
3. Previous article on quantum particle spin
4. Auerbach and Arovas's paper on quantum field theory for spin systems
5. Auerbach and Arovas's paper on spin dynamics in antiferromagnetic crystals
6. The book “Interacting Electrons and Quantum Magnetism”
7. The website for the book “Max the Demon”