Part of current research of the Theory Group is concerned with quantum degenerate, strongly-correlated matter, where interactions play as big a role as quantum statistics - examples include the electron liquid in high-temperature superconductors.
Advances in experimental methods are leading to the discovery in such systems of new, sometimes poorly understood states of matter, often associated with quantum criticality. The ISIS Theory Group is contributing to establish what types of such "hidden order" may exist and identify their experimental signatures.
Recent research by the group in this area has focused on Fermi surface shape instabilities (see figures). These are novel forms of quantum order where the re-arrangement of electrons takes place in reciprocal space, rather than real space. They are very subtle phase transitions, which makes them interesting candidates for the "hidden order" detected in various itinerant systems such as the heavy fermion URu2Si2 and the quantum metamagnet Sr3Ru2O7. Our research has thrown new light on the microscopic mechanism that might lead to such instabilities. It has also shown that systems displaying the Pomeranchuk instability (whereby the Fermi surface distorts so as to break rotational symmetry) can also have other, even subtler phase transitions where the change in Fermi surface shape is topological.
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