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Avraham Schiller

30 October 2013

Professor Avraham (Avi) Schiller of the Hebrew University of Jerusalem died of cancer on June 22, 2013. He was fifty years old. Avi is best known for his contributions to the theories of strongly-correlated electron systems, the Kondo effect, and magnetic impurities away from equilibrium.

Avi was born in Jerusalem on February 19, 1963. He attended the Hebrew University High School. After completing his military service in the Israel Defense Forces, Avi enrolled at the Hebrew University studying physics and computer science and received his B.Sc. degree in 1987. He then pursued his M.Sc. and Ph.D. degrees in physics at the Hebrew University under the supervision of Vilen Zevin, graduating in 1993. He took a postdoctoral position at the University of Florida, where he worked with Kevin Ingersent and Selman Hershfield, followed by a second postdoctoral term at the Ohio State University, where he worked with Daniel L. Cox and John W. Wilkins. Avi returned to the Hebrew University in 1998 as a faculty member at the Racah Institute of Physics.

Avi was one of the early pioneers of the theory of nonequilibrium transport in the strong-coupling limit. During his post-doc years in Florida, he wrote a remarkable paper on the exactly solvable nonequilibrium Kondo problem using bosonization techniques. At the time, many people still doubted that the Kondo effect could be experimentally detectable in mesoscopic devices, such as single-electron transistors and molecular junctions. Avi and his collaborators also advanced our understanding of Fano resonances in scanning tunneling microscopy of magnetic adatoms on a metallic surface and applied their theory to explain the mirror-images detected in quantum mirages.

In a series of papers Avi proposed and developed a theory for the systematic inclusion of 1/d corrections to the dynamical mean-field theory. These corrections are known to be of importance for phase transitions in correlated electron systems, such as heavy-fermions and high-temperature superconductors. In order to rigorously calculate the non-linear magnetic susceptibility in a two-channel model relevant for uranium based heavy-fermions, he derived a new quantum Monte Carlo approach for thermodynamic properties. This work preceded the closely related strong-coupling continuous time Monte Carlo scheme, which became popular about 10 years later.

After the strong-coupling limit became experimentally accessible in nano-devices by the pioneering work of David Goldhaber-Gordon, Avi shifted his attention to the rich physics in such junctions, exploring regimes where perturbative approaches usually fail. He and his collaborators began applying Wilson’s numerical renormalization-group approach to the Coulomb blockade in quantum boxes in order to reveal signatures of quantum phase transitions.

Avi possessed a deep grasp of nonequilibrium dynamics, and constantly looked for methods able to explore the new physics brought about when the system can no longer be described by conventional means. His early use of the Y-operators for steady state currents across a quantum dot illustrates this drive. More recently, he developed the concept of duality for systems not in equilibrium. This enabled him to explore the interplay between phononic modes and fermionic degrees of freedom, which is necessary for the description of complex impurities when electric and thermal currents flow through them.

Utilizing his expertise in nonequilibrium physics, Avi and his collaborators were able to extend the numerical renormalization-group for treating real-time dynamics of quantum impurity problems out of equilibrium. To Avi’s own surprise, this work became his most widely recognized contribution.

The quest for rigorous solutions has been the major driving force throughout Avi’s scientific career. In seeking a deeper understanding of the underlying physics in complex problems he was often able to reach an admirable degree of clarity and simplicity. This rare combination of lucidity and profundity characterized not only his published scientific papers but was also present in his memorable talks. It was also felt by many colleagues and students in personal conversations.

Avi was an exceptional teacher. Last year he received the prestigious Michael Milken prize for excellence in teaching at the Hebrew University. His students fondly remember his ability to impart to them a deep appreciation of physics through his brilliant organization of the material. They single out his patience in answering questions (both in class and afterwards), his kindness to and empathy for every student, and the modesty animating his rapport with them. Despite his illness, Avi continued to teach the taxing Quantum Mechanics II course up to the end of the last semester. As numerous spontaneous letters of condolence that were received by his family from students after his death show, Avi will be remembered by generations of students who both enjoyed and benefitted from the courses he gave over the years.

We would like to highlight here a singular event, which exemplifies Avi's dedication and determination under the most difficult conditions. Between December 30 2001 and January 8, 2002, Avi served as the scientific director of the 19th Jerusalem Winter School in Theoretical Physics on "High temperature superconductivity and correlated electron systems", which took place at the Institute for Advanced Studies in the Givat Ram Campus of the Hebrew University. During the weeks before the starting date of the school, Israel was hit by several terror attacks, after which most of the invited speakers and many of the students cancelled their participation, jeopardizing the event. Avi did not give up – he reconceived the scientific program and quickly recruited a new cadre of invited speakers, who agreed to come to Jerusalem on short notice. The winter school was greatly successful, featuring excellent lecture series and lively discussions, stimulated by Avi's leadership and care. Those of us who participated in the event vividly remember the warm atmosphere and the sense of accomplishment in holding the event in spite of the adverse circumstances. In the following years Avi continued to be involved in the organization of scientific events, serving as a co-organizer of two workshops on quantum impurity physics and related topics, which took place in the Max Planck Institute for Complex Systems in Dresden in 2003 and 2007.

Avi had a keen enthusiasm and love for physics (even above his interest in soccer!). He was deeply concerned with the future of the Racah Institute (and of physics in general), and his advice was often sought out by both junior and senior colleagues. Avi was a family man, who will be deeply missed by his wife, Ruth and their three sons, Noam, Amit and Eran.

Yet, how can one describe the essence of Avi? His personality towered above all his considerable scientific achievements. The twinkle in his eye and his smile disclosed a man whose warmth and deep sense of humanity shone through and were evident to anyone after the briefest exchange, whether one talked physics or merely conversed with him on any topic, be it the cakes at the Moebius bakery in Dresden, the latest soccer match, politics, or above all, his pride in his wife and three sons. This sense of a harmony, decency and humor lifted him above the daily humdrum and made any contact with him memorable and rewarding.

He will be greatly missed and affectionately remembered by his many friends and colleagues in the physics community.

Submitted by:

Amnon Aharony

Frithjof Anders

Natan Andrei

Ofer Biham

Ora Entin-Wohlman

Joe Imry

Václav Špička

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