For materials synthesized with f-electron elements, the interaction between f-electrons and conduction electrons often leads to interesting physics. As the temperature is lowered, the f-electrons can hybridize with the conduction electrons in a process known as the Kondo effect. In a Kondo lattice material, the screening may become coherent at the so-called coherence temperature T*. The resulting material is often metallic, containing heavy bands with effective masses many times larger than the free electron mass. In some cases, the development of coherence leads to a filled heavy-electron band where the chemical potential lies within the hybridization gap, resulting in insulating behavior.
CeAgBi2 is an antiferromagnetic compound (TN = 6.4 K) belonging to the former (metallic) case. The close energy scales of the Kondo coherence, antiferromagnetism, and crystal field levels results in complex physical properties. Transport measurements reveal a coupling between the different magnetic phases and Hall resistivity. As the field is increased, the antiferromagnetic transition temperature is suppressed to zero Kelvin. Typically, this is expected to result in a quantum critical point. However, due to strange transport behavior in the paramagnetic regime, the usual signatures of quantum criticality are hidden.
SmB6 is a Kondo insulator due to the fact that the hybridization results in the opening of a gap. However, as the temperature is further lowered, the resistance saturates. Originally believed to be due to in-gap conduction states in the bulk, the true reason for the resistance saturation is a robust conducting surface state. Several theories predict that the surface state is a result of SmB6 belonging to a class of materials known as topological insulators. However, direct imaging of the spin-momentum locking of the surface states indicative of a topological insulator has proved elusive. Through transport and magnetic measurements, indirect evidence of the nature of the conducting surface state is presented.