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In heavy-fermion compounds, the dual character of $f$ electrons underlies their rich and often exotic properties like fragile heavy quasipartilces, variety of magnetic orders and unconventional superconductivity. 5$f$-electron actinide…
Inhomogeneous electronic states resulting from entangled spin, charge, and lattice degrees of freedom are hallmarks of strongly correlated electron materials; such behavior has been observed in many classes of d-electron materials,…
We review theoretical concepts and models for materials with strongly correlated d- or f electrons. We discuss low-energy effective models and the renormalized band method for Ce-based Kondo lattice systems. They are applied to the analysis…
Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB$_{6}$ may be a realization of a topological Kondo insulator. We have performed an angle-resolved photoemission spectroscopy study on…
This article reviews recent theoretical and experimental work on a new class of topological material - topological Kondo insulators, which develop through the interplay of strong correlations and spin-orbit interactions. The history of…
4$f$ electron-based topological Kondo insulators have long been researched for their potential to conduct electric current via protected surface states, while simultaneously exhibiting unusually robust insulating behavior in their…
We review the cubic rare-earth boride series which displays diverse electronic states like localized 4f electron multiplets split by the crystal electric field (CEF), itinerant heavy-fermion quasiparticle bands of the Kondo lattice as well…
As an exemplary Kondo insulator, SmB6 has been studied for several decades; however, direct evidence for the development of the Kondo coherent state and the evolution of the electronic structure in the material has not been obtained due to…
We analyze the impact of strong correlations on the surface states of a three-dimensional topological Kondo insulator. An important observation is that correlations are strongly increased at the surface, which leads to an energetical…
Heavy-fermion or Kondo lattice materials are considered to be typical strongly correlated systems, for which mean-field approximations have shown that the Coulomb interaction increases the effective mass and narrows the band gap. In this…
Heavy fermion materials have recently attracted attention for their potential to combine topological protection with strongly correlated electron physics. To date, the ideas of topological protection have been restricted to the heavy…
When interacting electrons are confined to low-dimensions, the electron-electron correlation effect is enhanced dramatically, which often drives the system into exhibiting behaviors that are otherwise highly improbable. Superconductivity…
SmB6 is a strongly correlated material that has been attributed as a topological insulator and a Kondo insulator. Recent studies have found the topological surface states and low temperature insulating character to be profoundly robust…
We investigate the competition between the spin-orbit interaction of itinerant electrons and their Kondo coupling with local moments densely distributed on the honeycomb lattice. We find that the model at half-filling displays a quantum…
A Kondo lattice is often electrically insulating at low temperatures. However, several recent experiments have detected signatures of bulk metallicity within this Kondo insulating phase. Here we visualize the real-space charge landscape…
Samarium hexaboride (SmB$_6$), a representative Kondo insulator, has been characterized recently as a likely topological insulator. It is also a material with strong electron correlations, evident by the temperature dependence of its…
The Kondo lattice mode, as one of the most fundamental models in condensed matter physics, has been employed to describe a wide range of quantum materials such as heavy fermions, transition metal dichalcogenides and two-dimensional Moire…
The heavy quasiparticle bands in Kondo materials which originate in the hybridization of f- and conduction electrons exhibit numerous, sometimes coexisting, broken symmetry phases. Most notable are unconventional superconductivity,…
Heavy-fermion family exhibits fascinating and often puzzling properties due to the presence of open-shell $f$ ions and the complexity of the associated charge, orbital, and spin degrees of freedom. SmB$_6 $ is a prototypical heavy-fermion…
In Kondo insulators the many-body Kondo lattice effect drives the formation of bands containing heavy charge carriers with a hybridization gap, leading to insulating properties. These renormalized bands can host non-trivial topologies…