Field Induced Quantum Ordering

The conventional theoretical description of the low temperature properties of different materials begins with identifying the quantum ground state and considering excitations from it. However, when a material is tuned to a Quantum Critical Point (QCP), where a continuous change between two ground states occurs, a new approach is required to properly account for local quantum fluctuations between these two competing macroscopic configurations. Such a point occurs, for example, when the critical temperature for a continuous phase transition to a primary state, such as anti-ferromagnetism, is driven to zero Kelvin by applying pressure. When the critical temperature is suppressed to zero, the critical fluctuations become quantum instead of thermal giving rise to unusual temperature dependences of the specific heat and other properties. Ultimately as the temperature is reduced in a clean material either a macroscopic quantum entanglement of the two competing ground states will occur or a completely new ground state will emerge. The latter process arguably underlies the formation of many recently observed states including high temperature superconductivity and unusual forms of superconductivity seen in several 'heavy fermion' antiferromagnets. This proposal concerns QCPs in ferromagnetic metals, such as URhGe and UGe2, where the conduction electrons are intimately involved in the magnetism. It builds on our recent discovery of new ground states that are both ferromagnetic and superconducting. Materials displaying ferromagnetic QCPs are particularly interesting to study because they can be continuously tuned to cross QCPs by applying magnetic field. Remarkably, in URhGe superconductivity is induced close to a QCP over a very large range of fields (from 8 to above 28 Tesla) at low temperature. Superconductivity over such a large field range with such a low superconducting transition temperature (0.4 K) is completely unknown in conventional 3-dimensional superconductors.Through experimental investigation of the magnetic transitions and quantum critical behaviour of different ferromagnetic materials, including URhGe and UGe2, we aim to show how such unusual forms of superconductivity can be brought about, and to look for novel behaviour relevant for future applications. To do this we have to understand, firstly, the natures of the competing magnetic ground states that give rise to the QCPs, then the states in which electrons are bound in pairs to give superconductivity, and finally how these paired states are brought about by fluctuations between the competing magnetic states. We also plan to investigate some potentially unusual properties of ferromagnetic superconductors. For example, to establish whether superconductivity is suppressed at magnetic domain walls to form devices known as Josephson junctions, which could potentially be used to construct a quantum computer. If such junctions do form at magnetic domain walls, it might be possible to create and modify their positions and interconnections as easily as writing and erasing data on a magnetic disc.The new superconducting states occur only in almost perfect crystals, so a significant effort has to be made to grow high quality crystals. For many of the materials we wish to investigate, large pressures (up to 100,000 times atmospheric pressure) must be applied to drive them to the point where QCPs can be reached with laboratory magnetic fields. To do this we will develop apparatus, using specially grown designer-diamonds, to align and squash the crystals to carry out the studies.