2018 Paul Dirac Medal and Prize

Professor John Chalker of University of Oxford for his pioneering, deep, and distinctive contributions to condensed-matter theory, particularly in the quantum Hall effect, and to geometrically frustrated magnets.

2018 Paul Dirac Medal and Prize John Chalker

Professor John Chalker is distinguished for his theoretical research on condensed matter, including quantum Hall systems, disordered conductors, and frustrated magnets. His work is known for both its depth and its utility, and has been highly influential to experimentalists and theorists alike. One of Chalker’s striking abilities is to initiate a bold new approach to a problem while also providing deep and lasting results.

Chalker has made seminal contributions to the theory of disordered electronic systems and Anderson localisation. His invention of the network (Chalker-Coddington) model for the quantum Hall plateau transition gave a way both to understand the fundamental physics of certain localisation transitions and to do useful and precise numerical simulations. It showed that details of Landau levels and wavefunctions, previously thought to be essential to the story, are in fact not. One subsequent generalisation proved particularly important – the extension to disordered superconductors by Chalker and collaborators constituted one of the earliest descriptions of topological states of matter.

John Chalker’s studies of the statistical physics of geometrically frustrated magnets have established much of the current understanding of their behaviour. This work has been extremely influential not only in its original classical setting, but also in the search for quantum topological order. While conventional magnets are a paradigm for conventional ordering, frustrated magnets avoid simple orders due to competing interactions that cannot be simultaneously satisfied.

Chalker’s work showed that strong frustration leads to a macroscopic ground-state degeneracy at the classical level, explaining simply the reluctance to order. Chalker then elucidated considerably how finite-temperature or quantum fluctuations can drive ordering via the ‘order-by-disorder’ phenomenon. He also extensively analysed the experimentally crucial aspect of weak degeneracy-breaking interactions as well as uncovering a remarkable new form of spin dynamics, with timescales set by temperature rather than interaction strength.

Chalker’s recent investigations of loop models have provided a unified viewpoint of disparate systems ranging from vortex lines in chaotic three-dimensional optical fields to fractionalised excitations at deconfined critical points of quantum magnets. This work has provided a first detailed understanding of these exceedingly subtle systems.

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