Quantum phase transitions in correlated metals

The project focuses on quantum phase transitions in correlated metals. Correlated metals are intermetallic compounds which exhibit strongly renormalized Fermi liquid properties. Because of the huge enhancement of the effective mass they are also termed heavy-fermion compounds. Due to strong hybridisation effects, heavy-fermion systems can easily be driven to an antiferromagnetic or ferromagnetic quantum phase transition by using moderate mechanical or chemical pressure. In the T → 0 quantum regime, thermal fluctuations are absent, and the transition is driven wholly by quantum fluctuations. At the quantum critical point the standard Fermi liquid theory for metals breaks down and new forms of matter emerge.

Further reading: Evidence for a ferromagnetic quantum critical point in URhGe.

Topological insulators & superconductors

As regards charge transport, materials are normally divided into two groups: conductors and insulators. Surprisingly, a few years ago a new class of materials was discovered which have the extraordinary property that they are insulating in the bulk, but conducting on the surface. In these materials, termed topological insulators, the surface states are protected by topology and consequently scattering processes are absent, which makes them potential candidates for applications in spintronics. Moreover, some topological insulators can quite easily be transformed into a superconductor. Topological superconductors are predicted to host Majorana fermions, which in turn may form a platform for quantum computation. In this project we focus on magnetotransport of surface states in the highly-bulk insulating material system (Bi,Sb)2(Te,Se)3, as well as on unconventional superconductivity in candidate topological superconductors such as CuxBi2Se3, YPtBi and ErPdBi.

Further reading: Superconductivity in the doped topological insulator CuxBi2Se3 under pressure.

Unconventional superconductivity in correlated metals

In a ferromagnetic metal superconductivity should not exist, since ferromagnetic order impedes phonon-mediated pairing of electrons in singlet states. But in a twist of nature in the correlated metal UCoGe superconductivity and ferromagnetism coexist. Such an unconventional superconducting state calls for an exotic explanation: on the verge of magnetism critical magnetic fluctuations mediate superconductivity by pairing the electrons in triplet states. In this project we investigate the magnetic and superconducting properties of UCoGe.

Further reading: Superconductivity at the border of weak itinerant ferromagnetism in UCoGe.