Exotic Ground States in New Materials

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The ground states of most magnetic materials, and all magnetic materials which are well understood, are characterized by the appearance of long range order and conventional elementary excitations, such as spin waves. We are specifically interested in families of new magnetic materials, where this isn’t the case; that is where a disordered low temperature phase can persist due to the presence of very strong spin fluctuations. This is a very topical area of current condensed matter research as it is widely believed that if a conventional ordered state can be suppressed at low temperatures, it opens a window for an exotic ground state to be manifest, perhaps with qualitatively new and remarkable physical properties. Strong spin fluctuations can be encouraged in three ways in magnetic materials: magnetic moments can decorate local geometries (typically based on triangles and tetrahedra) in solids which are incompatible with a simple ordered state – this is referred to as geometrical frustration; S=1/2 and effective S=1/2 magnetic moments, which support extreme quantum fluctuations, can be made to decorate the lattice; and finally the magnetic lattice can be effectively two or one dimensional due to a three dimensional crystal structure which is made of a stacking of magnetic planes, or a bunching together of magnetic chains. That is, we can use geometrical frustration, quantum fluctuations or reduced dimensionality to enhance spin fluctuations and thereby destroy or weaken the tendency for a magnetic material to display conventional order at low temperatures. This proposal seeks to explore new magnetic materials which do precisely this. By carrying out advanced characterization, primarily with new neutron and x-ray scattering techniques we aim to elucidate the nature of the exotic ground states which ensue in several such materials, such that the basic principles that underlie the novel phenomena they display can be fully understood. This proposal seeks to carry out programs of neutron scattering measurements on several families of exotic magnetic materials that we will grow in pristine and single crystal form. Over the last six years, there has been a very dramatic renaissance in the instrumentation and capabilities available to carry out forefront inelastic neutron scattering. This has occurred mostly through the development of new time-of-flight neutron techniques at the Spallation Neutron Source, and ISIS, but also through new reactor-based techniques at NIST, the ILL, and elsewhere. The net result is that the nature of the information which neutron scattering can access relevant to the static and dynamic behaviour of new materials is qualitatively different and much more comprehensive than what was possible six years ago. This proposal seeks to apply this new infrastructure and these new techniques to several topical problems related to exotic magnetic ground states in new materials. A crucial component and starting point for the themes of this proposal is a program of sophisticated materials preparation and crystal growth which will provide large and pristine single crystals of the new magnetic materials. The resulting single crystal samples will be the subject of these forefront neutron and x-ray scattering studies. We are exceptionally well positioned, both with respect to our capacity to produce single crystals of interest, and with respect to access and expertise associated with the new neutron scattering infrastructure, to make real progress in this very topical area of condensed matter physics.