A benefit of setting up a broad collaboration is the opportunity to tackle problems that straddle different sub-fields of physics, using complementary materials and techniques. The aim of this group is to carry out investigation of strongly interacting electrons in ‘quantum critical’ materials. If a critical temperature of a continuous thermal phase transition is tuned towards T = 0 using a control parameter external to a many-body system, the result is a quantum critical point (QCP). At a QCP, the phase change is governed by quantum zero point fluctuations rather than the standard thermal ones, and entanglement is thought to be a key ingredient of the resultant quantum critical state.
The situation of fermionic quantum criticality in metals is particularly intriguing. For a range of temperatures above the QCP, there is a scale-invariant fluid of charge carriers whose properties are still poorly understood. Even the cornerstone concept of many-body quantum mechanics in metals, the fermion quasiparticle, is thought to break down. Making progress in this field requires a combination of experiments at extreme conditions (for example low temperature, high pressure and high magnetic field) with the development of new classes of theory. Empirically, the field continues to produce surprises. There is growing evidence, for example, that the dissipative scattering rate in quantum critical fluids is limited by a simple ratio of kB and . This is right at the boundaries of applicability of quantum physics, because dissipation in many-body systems gives time its arrow, while the theory of quantum mechanics is symmetric under time reversal. The same basic physics may well be at play not just in metals but in hydrodynamic fluids as disparate as the quark-gluon plasma and ultracold 6Li, providing potentially fruitful links with the physics of cold atoms.
The correlated electron groups at the Max Planck Institute for the Physics and Chemistry of Solids in Dresden and at the University of St Andrews share a strong reputation in the field and have complementary theoretical and experimental expertise. Working together, they have the potential for world-leading advances in our understanding of one of the most fundamental challenges in contemporary physics. On a 5 year timescale, the groups would benefit strongly from personnel interchange at all levels, and the consequent joint use of their complementary experimental facilities.