Topology and Quantum Criticality in Kondo Insulators

Quantum criticality has emerged as a key organizing principle in condensed matter. It is observed in systems as distinct as organic conductors, oxide interfaces, pnictide and cuprate superconductors, itinerant magnets, and heavy fermion metals. The latter have been most extensively investigated in this respect. Not only are the quantum critical fluctuations in these systems responsible for pronounced non-Fermi liquid behaviour and unconventional superconductivity but also does the analysis of the quantum critical properties reveal deep insight into the faith of the adjacent phases. Kondo insulators, the insulating sister compounds of heavy fermion metals, have received comparably little attention. This has changed since the recent claim that SmB6 is a topological Kondo insulator. Topological insulators, and in particular their strongly correlated Kondo versions, are of great fundamental interest as new phases of matter. In addition, their topologically protected conducting surface states might open new directions in electronic devices. TopQuantum aims at significantly advancing the understanding of the intriguing interplay between strong correlations, symmetry, and topology by driving carefully selected Kondo insulators with substitutions and/or pressure between different phases: insulating to metallic, paramagnetic to magnetic, topological to topologically trivial. The evolution of the physical properties across these transitions will be key to identify the nature of the accessed phases.

Topology and Quantum Criticality in Kondo Insulators Electrons in simple metals behave according to well established principles (the single particle Schrödinger equation). Their key application, as the carriers of electricity, draws on their electrical charge (the elementary charge). However, the electron possesses another degree of freedom, its spin (an elementary magnetic moment). Its exploitation for the processing, transportation, and storage of information is being actively pursued. A priori, the motion of the electron charge is independent of its spin state. However, under special conditions (in so-called Weyl semimetals), they can get locked to each other, i.e., the motion of an electron along one particular path may only be possible for a particular spin orientation. Such a state with "momentum - spin locking" has the potential to overcome bottlenecks in spin-based (quantum) applications. In the present project, by studying narrow-gap insulators stabilized by spin-dependent interactions between the electrons (Kondo insulators), a new state of matter, called Weyl-Kondo semimetal, was discovered by an international team of chemists and experimental and theoretical physicists. This new state has exotic properties. Its particles behave as "slow light" and show a giant transverse ("Hall") voltage in response to an electrical current that can usually only be generated by a magnetic field. Both properties can be switched off by temperature or magnetic field. These discoveries are of great fundamental interest and have potential for quantum applications.