Quantum Phase Transitions in Spin Systems

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9623181 Sachdev The primary objective in this grant is to undertake a systematic theoretical study of the finite temperate properties of quantum spin systems. The project brings together a wide variety of problems from apparently diverse systems. For example the cuprate compounds, also known as high Tc superconductors, at low doping levels manifest complicated interplay between antiferromagnetic fluctuations and quenched disorder. NMR in Quantum Hall effect systems shows novel spin correlations. Apart from these principal examples, there are properties of rare earth intermetallics where the magnetic ground states are critically influenced by disorder or competing energy scales. Finally there are anisotropic materials where spin exchange and conduction occur primarily along one direction. All of this work will be carried out in the framework of a quantum field theory, derived from a continuum description of the spins. %%% The storm generated by the discovery of high Tc superconductivity led, rather early, to the solution of an old problem, that of quantum antiferromagnets. The PI was responsible for some of the major developments and is now proposing to extend that work to a variety of other problems, specifically involving finite temperature effects. The applications are in lightly doped compounds (which show how magnetism is destroyed by doping) and their temperature dependent properties. ***