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Department of Physics

The Cavendish Laboratory

Phase diagram for the ferromagnet UGe2Exotic States on the Edge of Magnetism and Multi-Wavevector Quantum States

The crossover from ferromagnetism or anti-ferromagnetism to paramagnetism as a function of a 'quantum' control parameter such as pressure has turned out to be a much more subtle and interesting problem than was originally imagined in the early days of the field.

On the border of long-range magnetic order one expects several quantum states of more or less exotic nature to be nearly degenerate. Minute metallurgical or environmental changes can induce transitions between these states and give rise to complex behaviour that, in extreme cases, could be called 'adaptive'. Research has focused on the investigation of the temperature-pressure phase diagram on the border of magnetism in relatively simple stoichiometric compounds of high purity in which well-defined forms of quantum order might be observed.

One such example is the ferromagnet UGe2. Upon applying hydrostatic pressure in a pressure cell, the magnetic transition temperature TCurie, is suppressed towards zero, revealing a superconducting state at low temperatures. Superconductivity in a ferromagnetically ordered system is exotic and rare, and was thought impossible for many decades. In this material there is also evidence that electrons pair up in an unconventional way, with spins aligned in the same direction forming an unusual 'spin-triplet' state.

Phase diagram of MnSi showing T3/2 dependenceNon-local Marginal Fermi Liquids

For the last fifty years understanding how the interactions between electrons affect low-temperature properties in metals has been based on the Landau theory of a Fermi Liquid. Recently, however, there have been an increasingly large number of samples in which the predictions of the Fermi liquid theory appear to be violated. The group is providing evidence for the existence of a particular kind of non-Fermi liquid state in a metal with long-range spin-spin interactions at low temperatures - the theoretically predicted non-local marginal Fermi liquid.

The marginal Fermi liquid provides a link between the Fermi liquid and more exotic non-Fermi liquid states that are of growing interest in condensed matter physics. The idea of a marginal Fermi liquid has also arisen in other context, in particular in the study of relativistic plasmas and quark-gluon matter.