I am a physicist specialising in the theory of condensed matter – the branch of physics that connects the microscopic world of subatomic particles, such as the electron, to the macroscopic world of human experience. These are very different realms: electrons obey the rules of Quantum Mechanics, and these rules are often at odds with our most deeply held intuitions, which are derived from everyday sensory experiences. For this reason I find particular fascination in quantum materials: substances where quantum properties “cross over” to the scale of everyday objects.
A case in point are superconductors. When cooled below their critical temperature, a macroscopic number of the electrons in a superconductor form so-called “Cooper pairs” that condense onto a single, shared quantum state, with profound consequences for observable properties. For instance, a superconducting device can be engineered to exist in a quantum superposition of two macroscopically distinct states – an ability normally reserved to elementary particles and which forms the basis of superconducting quantum computers currently under development.
While the superconductors used in present-day quantum computer prototypes are well understood, many others, called “unconventional superconductors”, do not conform to established theories. I have made contributions to this field, including the discovery, through theoretical analyses carried out in close collaboration with experimentalists, of new superconducting states with potentially useful properties. It is my conviction that quantum computing will one day become pervasive and that this will involve changing the material substrate underlying quantum technologies – much as our current, classical information technology jumped to the mainstream with the switch from vacuum tubes to semiconductors. My work on unconventional superconductors is in large part animated by this belief.
A particular focus has been broken time-reversal symmetry (BTRS), a phenomenon that indicates superconducting sates with unusual magnetic properties. My group-theoretical analyses of experimental data indicating BTRS in LaNiC2 and LaNiGa2 have been particularly influential: as of January 2022, my first three papers on these materials, published during 2009-2012, had accumulated >500 citations (Source: Google Scholar). This work led to many collaborations with experimental groups investigating these materials worldwide as well as other superconductors displaying this unusual phenomenon such as Re6Zr, Lu5Rh6Sn18, Zr3Ir, and even (very surprisingly) elemental Re. Several independent experiments have confirmed my predictions and the theory developed by my group for LaNiGa2 has been shown to describe the data quantitatively – a rare feat in the theory of unconventional superconductors where qualitative comparisons are often all one can hope for.
Another thrust of my work on quantum materials involves the study of strongly-correlated electron systems (SCES). These are systems whose collective behaviour cannot be described by reference to the individual constituent particles. For instance, in so called “spin ice” magnets Ho2Ti2O7 and Dy2Ti2O7 the atomic magnetic dipoles cooperate to create a state that is best described as a collection of effective magnetic monopoles – a type of particle which does not exist in nature as an isolated entity (or, at least, has never been observed). My work in this area has revealed the role played by quantum tunnelling in the motion of monopoles and has been used to argue for similar physics with much faster dynamics in other materials. I have investigated many other phases of SCES over the years, including “soft quantum matter” phases which are quantum analogues of smectic and nematic liquid crystals (including some well-known work on an exotic phase transition of electronic systems called the Pomeranchuk instability) and even the effect of the “small-world effect” of network theory on idealised conducting polymer chains.
Although I work as a theorist, I often do so in close collaboration not just with other theorists, but also with experimentalists. A prime example has been helping to interpret muon spin relaxation (μSR) experiments on superconductors. μSR is an accelerator-based technique where muons (an unstable elementary particle) are implanted in a sample and the radiation they emit as they decay is analysed, yielding atomic-scale information on the magnetic fields inside the material. I have also proposed ways to use neutron scattering to detect changes in the quantum entanglement between atomic magnetic moments in molecular quantum magnets (entanglement is the additional correlations between quantum objects that underlies Einstein’s “spooky action at a distance” and which constitutes the essential resource for quantum computation). Earlier on I made proposals to use ultra-cold atomic gases confined in crossed laser beams (called “optical lattices”) to realise analogues of various states of SCES.
I have often succeeded in explaining puzzling phenomena and on several occasions I have seen my predictions confirmed by independent experiments – one of the highest highs one can get as a thoeretical physicist. ;-)