Microwave spectroscopy of unconventional metals, superconductors and insulators

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Society is currently reaping the benefits of the revolution in semiconductor electronics that occurred after the quantum behaviour of noninteracting electrons was understood in the early 20th century. It is the hope of many researchers around the world that there will be a second revolution in electronic materials when we understand a far more difficult quantum problem - the behaviour of systems of strongly interacting electrons. At this stage the challenges are fundamental: to understand how the important electronic properties emerge in systems of interacting electrons, and to harness these materials so that they produce the desired properties in a controlled way. Many of these materials display unusual forms of superconductivity, a state of matter in which all electrical resistance vanishes. Examples include the cuprate high temperature superconductors; the so-called "heavy fermion" materials, in which electrons appear to be 100 to 1000 times heavier than normal; and the recently discovered iron-based superconductors. At present, the mechanism responsible for superconductivity is not well understood in any of these materials. In addition, the nonsuperconducting states are unusual metals that pose some of the greatest challenges to our understanding of electrons in solids. Our research group has set up a novel facility for carrying out microwave spectroscopy of these materials that is the only one of its kind in the world. Measurements can be performed at temperatures down to 0.05 K, in magnetic fields up to 9 tesla, and at frequencies up to 25 GHz. Microwave fields are used to accelerate the electrons in order to probe their inertial response; and to study their collisions with impurities and with one another. Using this information we are able to understand the structure and dynamics of these novel electronic states. We are also developing new experimental tools, including systems for detecting and quantifying subtle signs of time-reversal-symmetry breaking, an effect that has important applications in magnetism, semiconductors, unconventional superconductors, and so-called topological insulators - materials that are electrically insulating in the bulk, but form robust and highly conducting surface states.