Superconducting and normal states in quantum materials

Materials discovery feeds scientific and technological progress. Quantum materials host collective phenomena that defy a semi-classical description, for example because they arise from strong correlations or involve topological order. The diversity of these collective phenomena, their reach into practicable temperature regions and their tunability enable new technologies. Foremost among them is superconductivity, a macroscopic quantum phenomenon with multiple applications ranging from powerful magnets used in MRI scanners, fusion reactors and particle accelerators to lightweight motors and generators, low-noise rf filters, low-power electronics, and quantum devices used in sensing or computing. In most superconductors, the required electronic interactions are produced by dynamic lattice distortions. Alternatively, these interactions can be caused by more complex quantum processes similar to those which give rise to magnetism. Such unconventional 'superconductivity without phonons' is associated with a rich range of properties, some of which are highly desirable, such as resilience to high magnetic fields, current densities or temperatures.

In this project, we investigate the drivers of the unusual superconducting and normal states in four material families, building on our recent breakthroughs and discoveries:
(i) iron-germanide superconductors YFe2Ge2 and LuFe2Ge2,
(ii) moderate heavy fermion compounds CeNi2Ge2 and CePd2Si2,
(iii) the high pressure Kondo lattice superconductor CeSb2,
(iv) quasiperiodic host-guest structures such as high pressure Bi, Sb and Ba.

YFe2Ge2 in family (i) and CeNi2Ge2 in family (ii) form close to the border of magnetism at low temperature but just on the paramagnetic side, whereas their isoelectronic sister materials LuFe2Ge2 and CePd2Si2 order magnetically. High pressure CeSb2 (iii) displays robust superconductivity at magnetic fields that appear too high to allow spin singlet Cooper pairs. The quasiperiodic materials (iv) can host a low frequency sliding mode which dramatically affects normal state properties and causes unusually strong electron-phonon coupling.

Because these materials differ in many details but also share common phenomenology, new insights will arise from studying them in one coherent programme. Fuelled by the clean, high quality samples that our recent crystal growth advances have produced, the programme leverages strong input from multiple project partners. These augment our local high field, high pressure measurements with specialised spectroscopic, thermodynamic and transport techniques.

Prominent theory support will examine experimental findings to answer key research questions concerning
(a) the role of soft modes, whether vibrational, magnetic or otherwise,
(b) the origin of non-Fermi liquid signatures in transport and the notion of Planckian dissipation in correlated metals,
(c) the nature and tunability of superconducting pairing interactions, and
(d) the nature and gap structure of the superconducting state itself.

These are hard but timely questions: 40 years after the discovery of the first unconventional superconductor, CeCu2Si2, the nature of its superconducting state is again under intense scrutiny, and the first oxide superconductor to be found outside the copper-oxide family, Sr2RuO4, is likewise hotly debated. The new superconductors listed above significantly widen the range of clean materials in which these fundamental questions can be studied effectively.

The resulting insights help guide the search for further new unconventional superconductors in the vast space of materials, and studying these new materials in turn produces new insights and more precise guiding principles. There is scope and need for improving the success rate of these searches by leveraging computer modelling, which will gather momentum as the programme unfolds, eventually leading the way to functional quantum materials with practically useful properties.