In an exciting development, SmB6, a Kondo insulator has been shown to exhibit bulk quantum oscillations. We propose that SmB6 is a bulk scalar Majorana Fermi Liquid} (MFL) with a finite gap for charge and spin excitations. In their study of Kondo insulators in 1993, Coleman, Miranda and Tsvelik (CMT) envisaged such a remarkable possibility, using a mean field ansatz. We generalize CMT theory to non-zero magnetic fields and show a counter intuitive result that the scalar Majorana fermi liquid, while remaining electrically insulating, responds to external magnetic field and exhibits Landau diamagnetism and quantum oscillations. Physics of an emergent compactified 2-channel Kondo lattice physics that is behind formation of the novel scalar Majorana fermi liquid phase is discussed. It is also suggested that a known resistivity saturation in SmB6 as well as a new strong deviation of quantum oscillation amplitude from Lifshitz-Kosevich formula in SmB6 at low temperatures are due to a coherent fluctuation of charge of a neutral scalar Majorana fermion. Possible presence of 2-dimensional Majorana fermion excitations in surfaces of SmB6, and other Topological Kondo Insulators (TKI) is pointed out.
The discovery of topological insulators in non-interacting electron systems has motivated the community to search such topological states of matter in correlated electrons both theoretically and experimentally. In this paper we investigate a phase diagram for a topological Kondo insulating system, where an emergent "spin"-dependent Kondo effect gives rise to an inversion for heavy-fermion bands, responsible for a topological Kondo insulator. Resorting to the U(1) slave-boson mean-field analysis, we uncover an additional phase transition inside the Kondo insulating state in two dimensions, whic…
Kondo insulators are strongly correlated system in which a clean insulating gap emerges only at very low temperature due to many-body effects involving localized $f$-electrons. However, certain Kondo insulators, like SmB$_6$ and Ce${_3}$Bi${_4}$Pt${_3}$, display metallic behaviors at extremely low temperatures, that have defied current understanding. Recent advances in topological effects in materials has raised the attention on the protected surface states in these "topological Kondo insulators" as a potential resolution to some of the puzzling behaviors. Here we resolve these puzzles via a d…
The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a high-quality single crystal. In the Kondo-insulating regime realized at low temperature, the spectra exhibit a spin-gap structure with two sharp, dispersive, in-gap excitations at E = 14.5 and approximately 20 meV. The lower mode is shown to be associated with short-range correlations near the antiferromagnetic wave vector q0 = (1/2, 1/2, 1/2). Its properties are in overall agreement with those expected for a "spin exciton'' branch in an indirect hybridization gap semiconductor.
Using real-space dynamical mean field quantum Monte Carlo simulations, we study the optical conductivity of two-dimensional topological Kondo insulating states. We consider model parameters which allow us to consider mixed valence and local moment regimes. The real space resolution inherent to our approach reveals a renormalization of the hybridization gap as one approaches the edge. Low energy transport is dominated by the helical edge state and the corresponding Drude weight scales as the coherence scale of the heavy fermion state. The concomitant renormalization of the edge state velocity l…
The Kondo effect, an eminent manifestation of many-body physics in condensed matter, is traditionally explained as exchange scattering of conduction electrons on a spinful impurity in a metal. The resulting screening of the impurity's local moment by the electron Fermi sea is characterized by a Kondo temperature $T_K$, below which the system enters a non-perturbative strongly-coupled regime. In recent years, this effect has found its realizations beyond the bulk-metal paradigm in many other itinerant-electron systems, such as quantum dots in semiconductor heterostructures and in nanomaterials,…
We used the inverted resistance method to extend the bulk resistivity of SmB$_{6}$ to a regime where the surface conduction overwhelms the bulk. Remarkably, the bulk resistivity shows an intrinsic thermally activated behavior that changes ten orders of magnitude, suggesting that it is an ideal insulator that is immune to disorder. Non-stoichiometrically-grown SmB$_{6}$ samples also show an almost identical thermally activated behavior. At low temperatures, however, these samples show a mysterious high bulk resistivity plateau, which may arise from extended defect conduction in a 3D TI.
We study higher-spin ($S \geq 1$) generalization of the one-dimensional Kondo-Heisenberg model, in which the local spin-$S$ moments of the Kondo lattice model interact with each other via the antiferromagnetic Heisenberg interaction ($J_{\text{H}}$), by analytical and numerical methods. The strong-coupling (i.e., large Kondo-coupling) expansion maps out an insulating phase at half-filling whose magnetic correlation depends on the parity of $2S$ as well as a ferromagnetic metallic phase which dominates the strong-coupling region at generic fillings. Then, we carried out the Density-Matrix Renor…
Low-energy properties of the one-dimensional Kondo lattice model are investigated by using bosonization and renormalization group methods. The formation of the spin and charge excitation gaps at half-filling is discussed in detail both for antiferromagnetic and ferromagnetic Kondo couplings. Also, the effect of the interaction between conduction electrons on the gap formation is studied both for the Kondo insulating phase and for the Haldane gap phase. It is found that while the repulsive interaction changes the dependence of the excitation gap on the Kondo exchange coupling, the attractive in…
We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound SmB6, a prototype Kondo insulator. Temperature dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at ~T*=20 K, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20K) indicates the emergence of a surface state with an effective electron mass of 0.1me. Conductivity dynamics following optical excitation are also measured and interpreted using Rothwarf-Taylor (…
The predicted interplay between Kondo physics and non-trivial topology in SmB$_{6}$ has stimulated many experimental reports, some of which are in apparent contradiction. The origin of the dispute may lie on the fragility of the Kondo insulating phase in the presence of Sm vacancies (Kondo holes) and/or natural impurities, such as Gd$^{3+}$. In this work, we locally investigated this fragility for Al-flux grown Sm$_{1-x}$Gd$_{x}$B$_{6}$ single crystals (0 $\leq$ $x$ $\leq$ 0.02) by combining electron spin resonance (ESR) and complementary bulk measurements. The Gd$^{3+}$ ESR spectra in a highl…
We show that the inclusion of nonlocal correlation effects in a variational wave function for the ground state of a topological Anderson lattice Hamiltonian is capable of describing both topologically trivial insulating phases and nontrivial ones characterized by an indirect gap, as well as its closure at the transition into a metallic phase. The method, though applied to an oversimplified model, thus captures the metallic and insulating states that are indeed observed in a variety of Kondo semiconductors, while accounting for topologically nontrivial band structures.
We investigate the necessary conditions for the emergence of complex, noncoplanar magnetic configurations in a Kondo lattice model with classical local moments on the geometrically frustrated Shastry-Sutherland lattice and their evolution in an external magnetic field. We demonstrate that topologically nontrivial spin textures, including a new canted flux state, with nonzero scalar chirality arise dynamically from realistic short-range interactions. Our results establish that a finite Dzyaloshinskii-Moriya (DM) interaction is necessary for the emergence of these novel magnetic states when the …
Antiferromagnetic topological insulator (AFTI) is a topological matter that breaks time-reversal symmetry. Since its proposal, explorations of AFTI in strong-correlated systems are still lacking. In this paper, we show for the first time that a novel AFTI phase can be realized in three-dimensional topological Kondo insulator (TKI). In a wide parameter region, the ground states of TKI undergo a second-order transition to antiferromagnetic insulating phases which conserve a combined symmetry of time reversal and a lattice translation, allowing us to derive a $\mathcal{Z}_2$-classification formul…
Kondo insulators have recently aroused great interest because they are promising materials that host a topological insulator state caused by the strong electron interactions. Moreover, recent observations of the quantum oscillations in the insulating state of Kondo insulators have come as a great surprise. Here, to investigate the surface electronic state of a prototype Kondo insulator YbB$_{12}$, we measured transport properties of single crystals and microstructures. In all samples, the temperature dependence of the electrical resistivity is insulating at high temperatures and the resistivit…
We study the Kondo lattice model with the Heisenberg-type RKKY-exchange coupling among localized f-spins in the presence of a magnetic field. By means of an extended dynamical mean field theory combined with the non-crossing approximation, we investigate the one-particle spectral function and the dynamical spin correlation function in the Kondo insulating phase. It is shown that the magnetic field and the RKKY exchange interaction both cause the instability to the antiferromagnetic order with enhanced transverse spin fluctuations, which give rise to a strong renormalization of quasi-particles …
We study two different metal-insulating transitions possibly occurring in one-dimensional Kondo lattices. First, we show how doping the pure Kondo lattice model in the strong-coupling limit, results in a Pokrovsky-Talapov transition. This produces a conducting state with a charge susceptibility diverging as the inverse of the doping, that seems in agreement with numerical datas. Second, in the weak-coupling region, Kondo insulating transitions arise due to the consequent renormalization of the backward Kondo scattering. Here, the interplay between Kondo effect and electron-electron interaction…
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 within a Kondo lattice with atomic resolution using a scanning tunneling microscope. We discover nanometer-scale puddles of metallic conduction electrons centered around uranium-site substitutions in the heavy-fermion compound URu$_2$Si$_2$, and around samarium-site defects in the topological Kondo insulator SmB$_6$. These defects disturb the Kondo screening cl…
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 Kondo insulators is reviewed along with the theoretical models used to describe these heavy fermion compounds. The Fu-Kane method of topological classification of insulators is used to show that hybridization between the conduction electrons and localized f-electrons in these systems gives rise to interaction-induced topological insulating behavior. Finally, som…
SmB6 is a proposed topological Kondo insulator where the presence of topological nontriviality can be tuned by variations in the Sm valence. Experimentally, Sm valence can be changed by tuning stoichiometry of SmB6. We show that Raman scattering can detect vacancies lower than 1% of Sm sites in SmB6 crystal by probing the intensity of defect-induced scattering of the acoustic phonon branch at 10~meV. In the electronic Raman spectra of SmB6 at temperatures below 130~K, we observe features developing in A$_{1g}$ and E$_g$ symmetries at 100 and 41~meV which we assign to excitations between hybrid…
We demonstrate the importance of the Kondo effect in artificially created {\it f}-electron superlattices. We show that the Kondo effect does not only change the density of states of the {\it f}-electron layers, but is also the cause of pronounced resonances at the Fermi energy in the density of states of the non-interacting layers in the superlattice, which are between the {\it f}-electron layers. Remarkably, these resonances strongly depend on the structure of the superlattice; due to interference, the density of states at the Fermi energy can be strongly enhanced or even shows no changes at …
Topological insulators host spin-polarized surface states which robustly span the band gap and hold promise for novel applications. Recent theoretical predictions have suggested that topologically protected surface states may similarly span the hybridization gap in some strongly correlated heavy fermion materials, particularly SmB6. However, the process by which the Sm 4f electrons hybridize with the 5d electrons on the surface of SmB6, and the expected Fermi-level gap in the density of states out of which the predicted topological surface states must arise, have not been directly measured. We…
By using an extended slave-boson method, we draw a global phase diagram summarizing both magnetic phases and paramagnetic (PM) topological insulating phases (TI$_s$) in three-dimensional topological Kondo insulator (TKI). By including electron hopping (EH) up to third neighbor, we identify four strong topological insulating (STI) phases and two weak topological insulating (WTI) phases, then the PM phase diagrams characterizing topological transitions between these TI$_s$ are depicted as functions of EH, $f$-electron energy level and hybridization constant. We also find an insulator-metal trans…
Kondo insulators are particularly simple type of heavy electron material, where a filled band of heavy quasiparticles gives rise to a narrow band insulator. Starting with the Anderson lattice Hamiltonian, we develop a topological classification of emergent band structures for Kondo insulators and show that these materials may host three-dimensional topological insulating phases. We propose a general and practical prescription of calculating the Z_2 strong and weak topological insulator indices for various lattice structures. Experimental implications of the topological Kondo insulating behavio…
We propose the notion of spin-selective Kondo insulator, which provides a fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice model with antiferromagnetic coupling. This unveils a remarkable feature of the ferromagnetic metallic phase: the majority-spin conduction electrons show metallic- while the minority-spin electrons show insulating-behavior. The resulting Kondo gap in the minority spin sector, which is due to the cooperation of ferromagnetism and partial Kondo screening, evidences a dynamically-induced commensurability for a combination of minority-spin electro…
Inspired by the recent experimental evidence of antiferromagnetism and superconductivity coexistence in heavy fermion CeRhIn$_5$, we propose a fully microscopic approach based on the idea of real space pairing within the Anderson-Kondo lattice model. We present an overall phase diagram incorporating the emergence of a quantum critical point, where Kondo insulating (KI), antiferromagnetic (AF) and superconducting (SC) phases meet. We also obtain the Kondo insulating state with totally compensated magnetic moments as the parental state for the emerging SC phases. Furthermore, the coexistent (AF+…
A theory is presented which accounts for the dynamical generation of a hybridization gap with nodes in the Kondo insulating materials $CeNiSn$ and $CeRhSb$. We show that Hunds interactions acting on virtual $4f^2$ configurations of the cerium ion can act to dynamically select the shape of the cerium ion by generating a Weiss field which couples to the shape of the ion. In low symmetry crystals where the external crystal fields are negligible, this process selects a nodal Kondo semimetal state as the lowest energy configuration.
The ground-state phase diagram of a half-filled anisotropic Kondo lattice model is calculated within a mean-field theory. For small transverse exchange coupling $J_{\perp}<J_{\perp c1}$, the ground state shows an antiferromagnetic long-range order with finite staggered magnetizations of both localized spins and conduction electrons. When $J_{\perp}>J_{\perp c2}$, the long-range order is destroyed and the system is in a disordered Kondo singlet state with a hybridization gap. Both ground states can describe the low-temperature phases of Kondo insulating compounds. Between these two distinct pha…
Heavy fermion materials naturally combine strong spin-orbit interactions and electronic correlations. When there is precisely one conduction electron per impurity spin, the coherent heavy fermion state is insulating. This Kondo insulating state has recently been argued to belong to the class of quantum spin Hall states. Motivated by this conjecture and a very recent experimental realization of this state, we investigate a model for Kondo insulators with spin-orbit coupling. Using DMFT, we observe an interaction-driven transition between two distinct topological states, indicated by a closing o…
The recent rekindling of interest in the mixed valent Kondo insulator SmB$_{6}$ as candidate for a first correlated topological insulator has resulted in a wealth of new experimental observations. In particular, angle-resolved photoemission experiments have provided completely new insights into the formation of the low temperature Kondo insulating state starting from the high temperature correlated metal. Here, we report detailed temperature and energy dependent measurements of the optical constants of SmB$_6$ in order to provide a detailed study from the point of view of a bulk sensitive spec…
The Kondo lattice model of spin-1/2 local moments coupled to the conduction electrons at half-filling is studied for its orbital response to magnetic field on bipartite lattices. Through an effective charge dynamics, in a canonical representation of electrons that appropriately describes the Kondo insulating ground state, the magnetization is found to show de Haas-van Alphen oscillations from intermediate to weak Kondo coupling. These oscillations are ascribed to Lifshitz-like inversion of a dispersion of the gapped quasiparticles, whose chemical potential surface is measured by the oscillatio…
Strongly correlated materials are characterized by the presence of electron-electron interactions in their electronic structure. They often have remarkable properties and transitions between competing phases of very different electronic and magnetic order. This thesis focuses on strongly correlated $f$-electron compounds containing Ce, Sm, and U. These materials exhibit a so-called heavy-fermion or Kondo-lattice behavior. They can become insulating due to hybridization effects (Kondo-insulator) or develop multipolar (hidden) order. Kondo insulators have recently been discussed in the context o…
A fascinating type of symmetry-protected topological states of matter are topological Kondo insulators, where insulating behavior arises from Kondo screening of localized moments via conduction electrons, and non-trivial topology emerges from the structure of the hybridization between the local-moment and conduction bands. Here we study the physics of Kondo holes, i.e., missing local moments, in three-dimensional topological Kondo insulators, using a self-consistent real-space mean-field theory. Such Kondo holes quite generically induce in-gap states which, for Kondo holes at or near the surfa…
We study the Kondo lattice model which is modified by the Holstein term, involving both the Kondo exchange coupling and the electron-phonon coupling constants, characterized by $J$ and $g$, respectively. The model is solved by employing the dynamical mean-field theory in conjunction with exact diagonalization technique. A zero temperature phase diagram of symmetry unbroken states at half filling is mapped out which exhibits an interplay between the two interactions and accounts for both spin and charge fluctuations. When the Kondo exchange coupling is dominant the system is in Kondo insulator …
The Kondo effect in a Tomonaga-Luttinger liquid ($U<<t$) is studied by using the non-Abelian bosonization. The $q=2k_F$ enhanced spin fluctuations generate a special Kondo effect, for any sign of the exchange coupling $J_K$ ($\left|J_K\right|<<U$) with the impurity. Then, unlike in Fermi liquids ($U\to 0$), the presence of a $2k_F$-polarized screening cloud around the impurity favors the occurrence of irrelevant electronic operators with scaling dimension $d=3/2$: the thermodynamics is nearly this of the two-channel Kondo model in a Fermi liquid. The Mott insulating transition does not affect …
The half-filled Kondo lattice model is studied at low temperatures on simple cubic lattice using the self-consistent theory developed in Phys. Rev. B 96, 075115 (2017). It is found to have three distinct insulating phases in the temperature-hopping plane, namely, the strong coupling Kondo singlet (KS) phase, the inverted Kondo singlet (iKS) phase distinguished from the KS by inversion, and the antiferromagnetic (AFM) phase. The quasiparticle density of states across the inversion transition is noted to exhibit dimensional reduction, which can differentiate between the KS and iKS insulators in …
In the Anderson lattice model for a mixed-valent system, the $d-f$ hybridization can possess a $p$-wave symmetry. The strongly-correlated insulating phase in the mean-field approximation is shown to be a $p$-wave Bose condensate of excitons with a spontaneous lattice deformation. We study the equilibrium and linear response properties across the insulator-metal transition. Our theory supports the empirical correlation between the lattice deformation and the magnetic susceptibility and predicts measurable ultrasonic and high-frequency phonon behavior in mixed-valent semiconductors.
The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu$_4$Sn$_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $ΔE_g1/k_B \simeq 30$K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat…
Kondo insulators are emerging as a simplified setting to study both magnetic and metal-to-insulator quantum phase transitions. Here, we study a half-filled Kondo lattice model defined on a magnetically frustrated Shastry-Sutherland geometry. We determine a "global" phase diagram that features a variety of zero-temperature phases; these include Kondo-destroyed antiferromagnetic and paramagnetic metallic phases in addition to the Kondo-insulator phase. Our result provides the theoretical basis for understanding how applying pressure to a Kondo insulator can close its hybridization gap, liberate …
A bilayer system comprising a Kondo insulator coupled to a simple metal (KI-M) is considered. Employing the framework of dynamical mean field theory, the model system is shown to exhibit a surface of quantum critical points (QCPs), that separates a Kondo screened, Fermi liquid phase from a local moment, Mott insulating phase. The quantum critical nature of these Mott transitions is characterized by the vanishing of (a) the coherence scale on the Fermi liquid side, and (b) the Mott gap on the MI side. In contrast to the usual `large to small' Fermi surface (FS) QCPs in heavy fermion systems, th…
Quantum oscillations have long been regarded as the manifestation of the Fermi surface in metals. However, they were recently observed in Kondo insulators. We examine the Kondo screening due to Landau levels in Kondo insulators. It is shown that even for large Kondo insulating gaps, appreciable amplitudes of quantum oscillations that are consistent with experimental observations are present both in magnetization and resistivity. Specifically, we show that due to the periodic alignment between the Landau levels in the conduction and the f-orbit electrons, the Kondo screening itself undergoes os…
Strongly correlated electron systems show many exotic properties such as unconventional superconductity, quantum criticality, and Kondo insulating behavior. In addition, the Kondo insulator SmB6 has been predicted theoretically to be a 3D topological insulator with a metallic surface state. We report here transport measurements on doped SmB6, which show that ~3% magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior, evidence that the metallic surface state is only destroyed when time reversal symmetry is broken. We find as well a quantum percolation limit of impurity c…
To gain further insight into the properties of interacting topological insulators, we study a 1 dimensional model of topological Kondo insulators which can be regarded as the strongly interacting limit of the Tamm-Shockley model. Treating the model in a large $N$ expansion, we find a number of competing ground-state solutions, including topological insulating and valence bond ground-states. One of the effects to emerge in our treatment is a reconstruction of the Kondo screening process near the boundary of the material ("Kondo band bending"). Near the boundary for localization into a valence b…
Samarium hexaboride is a topological Kondo insulator, with metallic surface states manifesting from its insulating band structure. Since the insulating bulk itself is driven by strong correlations, both the bulk and surface host compelling magnetic and electronic phenomena. We employed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Sm M$_{4,5}$ edges to measure surface and bulk magnetic properties of Sm$^{2+}$ and Sm$^{3+}$ within SmB$_6$. We observed anti-alignment to the applied field of the Sm$^{3+}$ magnetic dipole moment below $T = 75$ K and of the…
Non-abelian anyons are highly desired for topological quantum computation purposes, with Majorana fermions providing a promising route, particularly zero modes with non-trivial mutual statistics. Yet realizing Majorana zero modes in matter is a challenge, with various proposals in chiral superconductors, nanowires, and spin liquids, but no clear experimental examples. Heavy fermion materials have long been known to host Majorana fermions at two-channel Kondo impurity sites, however, these impurities cannot be moved adiabatically and generically occur in metals, where the absence of a gap remov…
Topological insulators are materials characterized by dissipationless, spin-polarized surface states resulting from non-trivial band topologies. Recent theoretical models and experiments suggest that SmB6 is the first topological Kondo insulator, in which the topologically non-trivial band structure results from electron-electron interactions via Kondo hybridization. Here, we report that the surface conductivity of SmB6 increases systematically with bulk carbon content. Further, addition of carbon is linked to an increase in n-type carriers, larger low temperature electronic contributions to t…
Some well-established examples of itinerant-electron ferromagnetism in one dimension occur in a Mott-insulating phase. We examine the consequences of doping a ferromagnetic insulator and cou- pling magnons to gapless charge fluctuations. Using a bosonization scheme for strongly interacting electrons, we derive an effective field theory for the magnon-holon interaction. When the magnon momentum matches the Fermi momentum of the holons, the backscattering of the magnon at low energies gives rise to a Kondo effect of a pseudospin defined from the chirality degree of freedom (right- or left-moving…
We report magnetic quantum oscillations measured using torque magnetisation in the Kondo insulator YbB$_{12}$ and discuss the potential origin of the underlying Fermi surface. Observed quantum oscillations as well as complementary quantities such as a finite linear specific heat capacity in YbB$_{12}$ exhibit similarities with the Kondo insulator SmB$_6$, yet also crucial differences. Small heavy Fermi sections are observed in YbB$_{12}$ with similarities to the neighbouring heavy fermion semimetallic Fermi surface, in contrast to large light Fermi surface sections in SmB$_6$ which are more si…
Kondo insulators are primary candidates in the search for strongly correlated topological quantum phases, which may host topological order, fractionalization, and non-Abelian statistics. Within some Kondo insulators, the hybridization gap is predicted to protect a nontrivial topological invariant and to harbor emergent heavy Dirac fermion surface modes. We use high-energy-resolution spectroscopic imaging in real and momentum space on the Kondo insulator, SmB$_6$. On cooling through $T^*_Δ\approx$ 35 K we observe the opening of an insulating gap that expands to $Δ\approx$ 10 meV at 2 K. Within …
A synergistic effect between strong electron correlation and spin-orbit interaction (SOI) has been theoretically predicted to result in a new topological state of quantum matter on Kondo insulators (KIs), so-called topological Kondo insulators (TKIs). One TKI candidate has been experimentally observed on the KI SmB6(001), and the origin of the surface states (SS) and the topological order of SmB6 has been actively discussed. Here, we show a metallic SS on the clean surface of another TKI candidate YbB12(001), using angle-resolved photoelectron spectroscopy. The SS showed temperature-dependent …
We report the results of high pressure x-ray diffraction, x-ray absorption, and electrical transport measurements of Kondo insulator Ce$_3$Bi$_4$Pt$_3$ up to 42 GPa, the highest pressure reached in the study of any Ce-based KI. We observe a smooth decrease in volume and movement toward intermediate Ce valence with pressure, both of which point to increased electron correlations. Despite this, temperature-dependent resistance data show the suppression of the interaction-driven ambient pressure insulating ground state. We also discuss potential ramifications of these results for the predicted to…
Kondo insulators are predicted to undergo an insulator-to-metal transition under applied magnetic field, yet the extremely high fields required to date have prohibited a comprehensive investigation of the nature of this transition. Here we show that Ce3Bi4Pd3 provides an ideal platform for this investigation, owing to the unusually small magnetic field of B ~ 11 T required to overcome its Kondo insulating gap. Above Bc, we find a magnetic field-induced Fermi liquid state whose characteristic energy scale T_FL collapses near Bc in a manner indicative of a magnetic field-tuned quantum critical p…
We obtain the rich phase diagrams in the Hubbard model on the triangular Kagomé lattice as a function of interaction, temperature and asymmetry, by combining the cellular dynamical mean-field theory with the continuous time quantum Monte Carlo method. The phase diagrams show the asymmetry separates the critical points in Mott transition of two sublattices on the triangular Kagomé lattice and produces two novel phases called plaquette insulator with an obvious gap and a gapless Kondo metal. When the Coulomb interaction is stronger than the critical value Uc, a short range paramagnetic insulatin…
Long known to have thermodynamic properties at odds with its insulating electrical transport, SmB6 has been the subject of great debate as it is unclear whether its unusual properties are related to the bulk or novel metallic surface states. We have observed a bulk moment-screening effect in nominally pure and Gd-doped SmB6 via heat capacity, magnetization, and resistivity measurements, and show this new Kondo-impurity like effect provides an unexpected but intuitive explanation for metal-like phenomena stemming from the strongly interacting host system. This affords a coherent understanding f…
The intermediate-valence compound SmB6 is a well-known Kondo insulator, in which hybridization of itinerant 5d electrons with localized 4f electrons leads to a transition from metallic to insulating behavior at low temperatures. Recent studies suggest that SmB6 is a topological insulator, with topological metallic surface states emerging from a fully insulating hybridized bulk band structure. Here we locally probe the bulk magnetic properties of pure and 0.5 % Fe-doped SmB6 by muon spin rotation/relaxation methods. Below 6 K the Fe impurity induces simultaneous changes in the bulk local magnet…
Kondo insulating materials lie outside the usual dichotomy of weakly versus correlated - band versus Mott - insulators. They are metallic at high temperatures but resemble band insulators at low temperatures because of the opening of an interaction induced band gap. The first discovered Kondo insulator (KI) SmB$_6$ has been predicted to form a topological KI (TKI) which mimics a topological insulator at low temperatures. However, since its discovery thermodynamic and transport anomalies have been observed that have defied a theoretical explanation. Enigmatic signatures of collective modes insi…
