Electric Quadrupole Orders in f-Electron Systems on the fcc Lattice

We theoretically showed that electric quadrupoles in some heavy fermion materials exhibit a very rich phase diagram including unique partially-ordered phases stabilized by an interaction specific to these systems.

Superconductivity in Heavy Fermion Materials

The heavy fermion materials have small superconducting transition temperature and large specific heat corresponding to large effective masses. In these materials the superconductivity co-exists with ferromagnetic or antiferromagnetic order at low temperature. It shows phenomena like magnetic instabilities, quantum critical points (QCP), non-fermi liquid (NFL) and unconventional superconductivity. By comparing the superconducting properties, phase diagram and effect of magnetic field and pressure of heavy fermions based on uranium, cerium, and praseodymium, the basic physics behind pairing mechanism can be imagined. This paper aims to present remarkable findings in superconductivity of various heavy fermion materials.

Quantum engineered Kondo lattices

Atomic manipulation techniques have provided a bottom-up approach to investigating the unconventional properties and complex phases of strongly correlated electron materials. By engineering artificial systems containing tens to thousands of atoms with tailored electronic or magnetic properties, it has become possible to explore how quantum many-body effects emerge as the size of a system is increased from the nanoscale to the mesoscale. Here we investigate both theoretically and experimentally the quantum engineering of nanoscale Kondo lattices – Kondo droplets – exemplifying nanoscopic replicas of heavy-fermion materials. We demonstrate that by changing a droplet’s real-space geometry, we can not only create coherently coupled Kondo droplets whose properties asymptotically approach those of a quantum-coherent Kondo lattice, but also markedly increase or decrease the droplet’s Kondo temperature. Furthermore we report on the discovery of a new quantum phenomenon – the Kondo echo – a signature of droplets containing Kondo holes functioning as direct probes of spatially extended, quantum-coherent Kondo cloud correlations.

Anisotropic Zeeman Splitting in YbNi4P2

The electronic structure of heavy-fermion materials is highly renormalised at low temperatures with localised moments contributing to the electronic excitation spectrum via the Kondo effect. Thus, heavy-fermion materials are very susceptible to Lifshitz transitions due to the small effective Fermi energy arising on parts of the renormalised Fermi surface. Here, we study Lifshitz transitions that have been discovered in YbNi_44P_22 in high magnetic fields. We measure the angular dependence of the critical fields necessary to induce a number of Lifshitz transitions and find it to follow a simple Zeeman-shift model with anisotropic g-factor. This highlights the coherent nature of the heavy quasiparticles forming a renormalised Fermi surface. We extract information on the orientation of the Fermi surface parts giving rise to the Lifshitz transitions and we determine the anisotropy of the effective gg-factor to be \eta \approx 3.8η≈3.8 in good agreement with the crystal field scheme of YbNi_44P_22.