Sebastian has recently joined Physics and is setting up his lab where he will seek to answer the question: what kind of novel and potentially functional states of matter can we discover in materials in the vicinity of quantum phase transitions? His experiments are typically conducted at extreme conditions of ultra-high pressure and sub-Kelvin temperatures.  Sebastian joined UEA from the University of Cambridge where he completed his PhD and a postdoc in the Department of Physics before moving to a postdoctural position in the Department of Earth Sciences.

I teach the 3rd year physics labs PHY-6003Y Advanced Physics Laboratory and from the 2022/23 academic year I will be module organiser and a lecturer on the second year core physics module PHY-5004B Heat, Atoms and Solids.

High pressure, low temperatures, XRD and neutron scattering

Quantum Phase Transitions

Quantum phase transitions and in particular quantum critical points, have been key concepts in many of the most important discoveries in condensed matter physics: high Tc cuprates, skyrmions, magnetically mediated superconductivity etc… The same degeneracy that makes such systems unstable to novel emergent phenomena also promises new functionalities.

Breakdown of the Mott insulator in 2D

Substantial interest lies in tuning Mott insulators towards their metallization transition. The simple Hubbard model traditionally used to describe such systems only yields solutions in the limiting metallic and insulating cases. Tuning the parameters of the system to an intermediate state accesses physics that is not yet fully understood. Additionally, many unconventional superconducting materials are low dimensional and lie in close proximity to antiferromagnetic Mott insulator phases in their phase diagrams, and theoretical calculations suggest that these states have a strengthening influence on the formation of superconductivity. Tunable (for instance, through pressure) antiferromagnetic two-dimensional Mott insulators then provide a rich and clean environment to probe the core mechanisms of several unsolved problems in condensed matter physics.

Quantum critical paraelectrics

Whilst it may seem counter-intuitive to look at an insulator in order to understand the strange metallic state seen on the ‘hidden’ order side of the metallic magnet quantum critical point phase diagram that is exactly what can be done. SrTiO₃ is one of the most studied and well characterised materials in history. The melting of the long range ordered ferroelectric state by fluctuations was established a long time ago but there was scepticism about the idea of a quantum critical paraelectric due to some strong theoretical arguments. Despite these objections the field was established in 2014 showing that these arguments must be flawed at least in this specific case. Since then, by making careful pressure measurements I have shown the nature of the quantum paraelectric state (analogous with the quantum paramagnetic ‘strange’ metallic state) and discovered a low temperature state that gives us hope of explaining the order-by-disorder susceptibility peak ubiquitous in the extremely diverse field of quantum criticality.