Unconventional superconductivity

Levitating superconductor
Superconductivity, where electricity flows without any resistance, is a dramatic example of the collective quantum mechanical behaviour of electrons in materials.

It has the capacity to revolutionize current and future technologies, although, at this point in time, widespread commercial use is unfeasible because it remains a property that is only exhibited at low temperatures. This provides such a significant impediment to its use that society has yet to harness its real potential.

Our goal is to develop a deeper understanding of this phenomena. Examples of specific knowledge that need to be accrued include: How can the superconducting transition temperature be increased? What possible mechanisms can be used to couple electrons to form Cooper pairs? Are there alternative routes to superconducting states beyond forming Cooper pairs of electrons?

Such challenging questions are difficult to answer directly. However, they may be addressed by assembling information assimilated from interpreting measurements of the fundamental properties of novel superconducting materials. Ultimately, this knowledge can then be used to help to engineer the next generation of superconducting materials with, for example, elevated transition temperatures.

Quantum critical phenomena

Phase diagram for a quantum critcial point
Nowhere are the conventional paradigms of condensed matter theory challenged more than in systems whose properties are governed by quantum critical phenomena.

For example, the central assumption of Landau’s Fermi-liquid theory is that electrons in metals should behave as a weakly interacting collection of electronic quasiparticles. One situation in which the properties of materials cannot be explained using this approximation is in the proximity of a quantum critical point.

A quantum critical point occurs when a non-thermal control parameter is used to suppress a phase transition to the absolute zero of temperature. At this point the phase transition is driven by quantum fluctuations due to the Heisenberg uncertainty principle as opposed to thermal fluctuations. In practice, the non-thermal control parameter is pressure (either externally or chemically applied) or a magnetic field. A dramatic feature of this apparent T= 0 K phenomenon is that the presence of the quantum critical point leaves its fingerprints on the behaviour of the system at temperatures many decades higher. As a consequence, quantum criticality has been suggested to be the underlying physics driving the properties of such diverse systems as the high temperature cuprate superconductors or even the universe itself, with the whole of existence sitting on the verge of a second order phase transition.

In the absence of a microscopic theory to describe behaviour at a quantum critical point, a picture of the phenomenology is starting to emerge, although many challenges remain. In particular it is crucial to be able to distinguish the effects of intrinsic physics from disorder. Many of the materials that have been studied, especially in the chemical pressure studies, are non-stoichiometric, and as such are prone to such ambiguities. Our research program ensures excellence by using magnetic field as the tuning parameter and concentrating on well-characterized samples. One can also study the effects of a quantum phase transition in the absence of itinerant electrons using the transverse field Ising model. The transverse magnetic field suppresses the magnetic ordering temperature to zero kelvin, whereby the phase transition between the order and disordered phases is controlled by quantum fluctuations.

Quantum magnetism

Spin-ice lattice with spins in a 2-in, 2-out configuration
In the case of novel magnetic systems, the research is primarily driven by looking for new and emergent physics resulting from geometrical frustration of the magnetic moments. The near equivalence in energy of many magnetic states in the presence of frustration makes the theoretical prediction of magnetic behaviour an extremely difficult task and hence experiment plays a pivotal role in this area of research. One type of phase of particular current interest is that of a spin liquid, an insulating magnetic phase that has no long-range magnetic order. Our objective is to experimentally identify spin liquid states, with the distinct possibility of uncovering qualitatively new states of matter in these complex systems.

Another interesting state is that adopted by the "spin-ice" materials (Ho2Ti2O7 and Dy2Ti2O7). The spin configuration imposed by frustration leads to a residual entropy analogous to that seen in (water) ice. The collective excitations of this magnetic system are also thought to have the properties of magnetic monopoles. Our research aims to take advantage of this unique opportunity by studying the transport properties of these infamous elementary excitations.