Non-Fermi Liquid Behavior and Quantum Phase Transitions


May 12 23, 2003



The workshop's primary objective was to provide a status report on this rapidly developing subject. More specifically, the stated goal was: The workshop will address the mechanisms for non-Fermi liquid behavior in strongly correlated electron systems, with an emphasis on the extent to which the notion of quantum criticality plays a role. The materials to be covered in this context will include heavy fermion metals, as well as high Tc cuprates and other transition metal compounds. The theoretical issues raised by the experimental results will be extensively discussed.


We believe that the workshop did indeed live up to these and other aspects of our expectations.


In terms of participants, the workshop was successful on three accounts: a) Essentially all of the key people the organizers set out to invite did in the end come to Leiden; b) We made a conscious effort to invite junior people into the mix. Among the 45 participants, 15 are junior scientists (students, postdocs, assistant professors or equivalent); c) A balanced mixture of theorists (17) and experimentalists (26).


In terms of format, we made an effort to highlight the informal discussion part. There are 15 organized sessions of the workshop: we estimate that 41% of the time was devoted to formal presentations, 17% to questions and answers, and 42% to topical discussions on issues that are loosely tied up with the formal presentations. Almost all the participants commented that they truly enjoyed the topical discussions. The organizers believe that, the success of these topical discussions is due in part to the focused nature of the workshop subject.


The workshop covered some of the most exciting topical issues in the area of quantum criticality and non-Fermi liquid physics. These include magnetism and superconductivity (Maple and Sarrao), quantum critical heavy fermions in a broader context (von Loehneysen and Varma), non-Gaussian quantum critical metals (Coleman and Si), change of Fermi surface across quantum critical points (Paschen, Flouquet, and Norman), Field-driven quantum critical points (Stewart and Schofield), E/T scaling (Schroeder, Aronson and Osborn), the question of whether high temperature superconductors are quantum critical (van der Marel, Zaanen, and Chubukov), the effect of disorder near quantum critical points (MacLaughlin and Nakatsuji), quantum critical points in zero field (Gegenwart, Thompson, and de Visser), ferromagnetic quantum phase transitions (Pfleiderer and Belitz), quantum critical scaling and dynamics (Curro, Bernhoeft, and Kuechler), quantum criticality and non-Fermi liquids in geometrically frustrated systems (Georges and Tsvelik), quantum impurity and other models (Ingersent, Lavagna, and Sachdev), the effect of reduced dimensionality (Rosch and Pepin), and quantum critical physics in a broader context (Mydosh and Aeppli).


One aspect the organizers regretted somewhat is the lack of completely free time slots. There were only three half-days (not counting the last Friday of the second week) of the two-week period during which nothing was scheduled. Partly, this reflected the vitality of the subject: there are lots of new developments that are taking place in this area that the workshop is proud to feature. Partly, this was due to the fact that so much time was devoted to the organized "informal" topical discussions however, given the uniformly positive response the participants gave to this part of the workshop, we do not regret at all our decision.


This report would not be complete without an acknowledgement to the generous financial and organizational support of the Lorentz Center. We are particularly grateful to Yolande for her most cheerful assistance to the workshop. Many of the participants came to the Lorentz Center for the first time, and everyone left with the impression of how great a setting the Lorentz Center provides for workshops of this size.


Q. Si (Rice University Texas, USA)

G. Stewart (University of Florida, USA)

A. de Visser (University Amsterdam, The Netherlands)