Kondo Insulator Research Articles

DMFT Reveals the Non-Hermitian Topology and Fermi Arcs in Heavy-Fermion Systems.

When a strongly correlated system supports well-defined quasiparticles, it allows for an elegant one-body effective description within the non-Hermitian topological theory. While the microscopic many-body Hamiltonian of a closed system remains Hermitian, the one-body quasiparticle Hamiltonian is non-Hermitian due to the finite quasiparticle lifetime. We use such a non-Hermitian description in the heavy-fermion two-dimensional systems with the momentum-dependent hybridization to reveal a fascinating phenomenon which can be directly probed by the spectroscopic measurements, the bulk "Fermi arcs." Starting from a simple two-band model, we first combine the phenomenological approach with the perturbation theory to show the existence of the Fermi arcs and reveal their connection to the topological exceptional points, special points in the Brillouin zone where the Hamiltonian is nondiagonalizable. The appearance of such points necessarily requires that the electrons belonging to different orbitals have different lifetimes. This requirement is naturally satisfied in the heavy-fermion systems, where the itinerant c electrons experience much weaker interaction than the localized f electrons. We then utilize the dynamical mean field theory to numerically calculate the spectral function and confirm our findings. We show that the concept of the exceptional points in the non-Hermitian quasiparticle Hamiltonians is a powerful tool for predicting new phenomena in strongly correlated electron systems.

Fine details of crystal structure and atomic vibrations in YbB12 with a metal-insulator transition.

The crystal structure of single-crystal Kondo insulator YbB12 was studied at nine temperatures in the range 85-293 K based on X-ray diffraction data. Very weak Jahn-Teller distortions of the cubic lattice were detected at all temperatures, but did not require a revision of the structural model. Heat capacity and electrical conductivity of YbB12 single crystals were studied in the temperature range 1.9-300 K. It is found that both the structural parameters and the indicated physical properties have some specific features in the temperature range 120-160 K. The unit cell of YbB12 contracts when cooled below 160 K and expands at around 120 K. The temperature dependences of the equivalent atomic displacement parameters Ueq(T) are no longer monotonic around 140 K and should be modeled by two Einstein curves for Yb and two Debye curves for boron atoms above and below this temperature. As follows from the temperature behavior of the specific heat, coupled oscillations of Yb ions in a double-well potential lead to the appearance of a charge gap in the density of states and gradual deterioration in conductive properties of the crystal below 150 K. This metal-insulator phase transition is accompanied by a kink in the Ueq(T) curves and changes in the unit-cell values.

Author Correction: Imaging emergent heavy Dirac fermions of a topological Kondo insulator

Author Correction: Imaging emergent heavy Dirac fermions of a topological Kondo insulator

Metallic islands in the Kondo insulator SmB6

The authors study the effects of magnetic impurities in a Kondo insulator and demonstrate that the Kondo insulating state is destroyed locally around impurities.

Tunable Kondo Resonance at a Pristine Two-Dimensional Dirac Semimetal on a Kondo Insulator.

The proximity of two different materials leads to an intricate coupling of quasiparticles so that an unprecedented electronic state is often realized at the interface. Here, we demonstrate a resonance-type many-body ground state in graphene, a nonmagnetic two-dimensional Dirac semimetal, when grown on SmB6, a Kondo insulator, via thermal decomposition of fullerene molecules. This ground state is typically observed in three-dimensional magnetic materials with correlated electrons. Above the characteristic Kondo temperature of the substrate, the electron band structure of pristine graphene remains almost intact. As temperature decreases, however, the Dirac Fermions of graphene become hybridized with the Sm 4f states. Remarkable enhancement of the hybridization and Kondo resonance is observed with further cooling and increasing charge-carrier density of graphene, evidencing the Kondo screening of the Sm 4f local magnetic moment by the conduction electrons of graphene at the interface. These findings manifest the realization of the Kondo effect in graphene by the proximity of SmB6 that is tuned by the temperature and charge-carrier density of graphene.

Intrinsic Bulk Quantum Oscillations in a Bulk Unconventional Insulator SmB6.

SummaryThe finding of bulk quantum oscillations in the Kondo insulator SmB6 proved a considerable surprise. Subsequent measurements of bulk quantum oscillations in other correlated insulators including YbB12 lent support to our discovery of a class of bulk unconventional insulators that host bulk quantum oscillations. Here we perform a series of experiments to examine evidence for the intrinsic character of bulk quantum oscillations in floating zone-grown single crystals of SmB6 that have been the subject of our quantum oscillation studies. We present results of thermodynamic, transport, and composition analysis experiments on pristine floating zone-grown single crystals of SmB6 and compare quantum oscillations with metallic LaB6 and elemental aluminum. These results establish the intrinsic origin of quantum oscillations from the insulating bulk of floating zone-grown SmB6. The similarity of the Fermi surface in insulating SmB6 with the conduction-electron Fermi surface in metallic hexaborides is at the heart of a theoretical mystery.

Topological Phase and Strong Correlation in Rare-Earth Hexaborides XB6 (X = La, Ce, Pr, Nd, Pm, Sm, Eu).

The rare-earth hexaboride SmB6, known as the topological Kondo insulator, has attracted tremendous attention in recent years. It was revealed that the topological phase of SmB6 is insensitive to the value of on-site Coulomb interactions (Hubbard U), indicating that the topological phase in SmB6 is robust against strong correlations. On the contrary, the isostructural YbB6 displays a sensitivity to the Hubbard U value. As U increases, YbB6 transforms from topological Kondo insulator to trivial insulator, showing the weak robustness of the topological phase of YbB6 against U. Consequently, the dependence of the topological phase on Hubbard U is a crucial issue in the rare-earth hexaboride family. In this work, we investigate the structural and electronic properties of rare-earth hexaboride compounds through first-principles calculations based on density functional theory. By taking the strong correlations into consideration using a wide range of on-site U values, we study the evolution of the topological phases in rare-earth hexaboride (XB6, X = La, Ce, Pr, Nd, Pm, Sm, Eu). Unlike YbB6, the topological trends in all the examples of XB6 studied in this work are insensitive to the U values. We conclude that in addition to the well-known SmB6, PmB6, NdB6 and EuB6 are also topologically nontrivial compounds, whereas LaB6, CeB6 and PrB6 are topologically trivial metal.

Crystal Growth by the Floating Zone Method of Ce-Substituted Crystals of the Topological Kondo Insulator SmB6

SmB6 is a mixed valence topological Kondo insulator. To investigate the effect of substituting Sm with magnetic Ce ions on the physical properties of samarium hexaboride, Ce-substituted SmB6 crystals were grown by the floating zone method for the first time as large, good quality single crystal boules. The crystal growth conditions are reported. Structural, magnetic and transport properties of single crystals of Sm1−xCexB6 (x=0.05, 0.10 and 0.20) were investigated using X-ray diffraction techniques, electrical resistivity and magnetisation measurements. Phase composition analysis of the powder X-ray diffraction data collected on the as-grown boules revealed that the main phase was that of the parent compound, SmB6. Substitution of Sm ions with magnetic Ce ions does not lead to long-range magnetic ordering in the Sm1−xCexB6 crystals. The substitution with 5% Ce and above suppresses the cross-over from bulk conductivity at high temperatures to surface-only conductivity at low temperatures.

Controllable synthesis of Gd-doped SmB6 nanobelt arrays for modulating their surface transport behaviors

Samarium hexaboride (SmB6) nanobelts are believed as better platforms to study the interaction between strongly correlated electron states and topological states than their bulk topological Kondo insulator (TKI) counterparts because they have more abundant surface states. To date, there are few reports focused on the modulation of surface transport behaviors of SmB6 nanobelts because of the absence of high-quality controllable synthesis and doping techniques, despite which is very essential for their future applications. In this paper, the undoped and Gd-doped SmB6 nanobelt arrays have been successfully fabricated on the Si substrate by a one-step co-evaporation method. Both of the undoped and Gd-doped nanobelts were indexed as single-crystalline cubic structures with a growth direction of [110]. The results show that the low-temperature resistivity of the Gd-doped SmB6 nanobelts becomes unsaturated when the magnetic Gd content is increased to about 2.0 at.%. Different from the undoped SmB6 nanobelts, the extraordinary non-linear behaviors of the Hall resistivity–temperature curves are firstly found in the Gd-doped SmB6 nanobelts, attributing to the anomalous Hall effect (AHE). Both the breaking of the surface time-reversal symmetry of the SmB6 nanobelts and the resultant magnetism by the Gd doping are suggested to be responsible for the occurrence of the AHE. Our study may shed light on comprehending the interaction between correlated electron states and topological surface states in TKI nanostructures and modulating their surface transport behaviors for future quantum information applications.

Effect of Rashba Impurities on Surface State of a Topological Kondo Insulator

In this communication, we report surface state, with Rashba impurities, of a generic topological Kondo insulator (TKI) system by performing a mean-field theoretic (MFT) calculation within the framework of slave-boson protocol. The surface metallicity together with bulk insulation is found to require very strong f-electron localization. The possibility of intra-band as well as inter-band unconventional plasmons exists for the surface state spectrum. The paramountcy of the bulk metallicity, and, in the presence of the Rashba impurities, the TKI surface comprising of ‘helical liquids’ are the important outcomes of the present communication. The access to the gapless Dirac spectrum leads to spin-plasmons with the usual wave vector dependence q1/2. The Rashba coupling does not impair the Kondo screening and does not affect the quantum critical point (QCP) for the bulk.

Fragility of the Kondo insulating gap against disorder: Relevance to recent puzzles in topological Kondo insulators

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 different route, by showing that the emergent Kondo insulating scale is extremely vulnerable against moderate degree of disorder, such that the gap is filled with a small number of states. Therefore, the real samples are probably never truly insulating and this in turn compromises the essential building block of topological considerations. Our results suggest strongly that systems like the Slater insulators would be a more promising direction to extend the realm of topology to strongly correlated systems.

Universal signatures of Majorana-like quasiparticles in strongly correlated Landau–Fermi liquids

Motivated by recent experiments in the Kitaev honeycomb lattice, Kondo insulators, and the ‘Luttinger’s theorem-violating’ Fermi liquid phase of the underdoped cuprates, we extend the theoretical machinery of Landau–Fermi liquid theory to a system of itinerant, interacting Majorana-like particles. Building upon a previously introduced model of ‘nearly self-conjugate’ fermionic polarons, a Landau–Majorana kinetic equation is introduced to describe the collective modes and Fermi surface instabilities in a fluid of particles whose fermionic degrees of freedom obey the Majorana reality condition. At large screening, we show that the Landau–Majorana liquid harbors a Lifshitz transition for specific values of the driving frequency. Moreover, we find the dispersion of the zero sound collective mode in such a system, showing that there exists a specific limit where the Landau–Majorana liquid harbors a stability against Pomeranchuk deformations unseen in the conventional Landau–Fermi liquid. With these results, our work paves the way for possible extensions of the Landau quasiparticle paradigm to nontrivial metallic phases of matter.

First‐principles study of f‐orbital‐dependent band topology of topological rare earth hexaborides

AbstractThe rare earth hexaboride (RE)B6 series of compounds (RE = Ce, Pr, Nd, Pm, Sm, Eu, Gd) was investigated using the first‐principles approach for exploring new topological insulators. The calculated structural, electronic, and magnetic properties of the materials are discussed in detail. To account for better f‐electron correlation, the Hubbard correction was also tested to correct the Kondo states. The lattice appeared contracted for materials from CeB6 to GdB6, and charge is transferred from RE to boron. The inspection of band structure points to spd hybridization between RE‐5d and B‐2s, 2p states at Fermi level. The application of spin‐orbit coupling revealed the conversion in the parity, and an odd number of Dirac cones is found at Γ and X points without Kondo mechanism. PrB6, GdB6, and PmB6 revealed a dispersive conic region near the Fermi level at high symmetry points, which enclosed Kramer's degenerate points with surface states of opposite parity. These materials prove to be a strong topological Kondo insulator with an SmB6‐like cubic structure without involvement of f‐orbitals in the formation of a Dirac cone. The Hubbard U parameter determined from atomic calculation of RE f‐orbital, apart from the few cases when f‐states remain close to the Fermi level, revealed that the band structures are semimetallic in most cases.

Hybridization effect on the x-ray absorption spectra for actinide materials: Application to PuB4

Studying the local moment and 5$f$-electron occupations sheds insight into the electronic behavior in actinide materials. X-ray absorption spectroscopy (XAS) has been a powerful tool to reveal the valence electronic structure when assisted with theoretical calculations. However, the analysis currently taken in the community on the branching ratio of the XAS spectra generally does not account for the hybridization effects between local $f$-orbitals and conduction states. In this paper, we discuss an approach which employs the DFT+Gutzwiller rotationally-invariant slave boson (DFT+GRISB) method to obtain a local Hamiltonian for the single-impurity Anderson model (SIAM), and calculates the XAS spectra by the exact diagonalization (ED) method. A customized numerical routine was implemented for the ED XAS part of the calculation. By applying this technique to the recently discovered 5$f$-electron topological Kondo insulator PuB$_4$, we determined the signature of 5$f$-electronic correlation effects in the theoretical X-ray spectra. We found that the Pu 5$f$-6$d$ hybridization effect provides an extra channel to mix the $j=5/2$ and $7/2$ orbitals in the 5$f$ valence. As a consequence, the resulting electron occupation number and spin-orbit coupling strength deviate from the intermediate coupling regime.

Publisher Correction: Emergent mystery in the Kondo insulator samarium hexaboride

Publisher Correction: Emergent mystery in the Kondo insulator samarium hexaboride

Emergent mystery in the Kondo insulator samarium hexaboride

Samarium hexaboride (SmB6) is an example of a Kondo insulator, in which strong electron correlations cause a band gap to open. SmB6 hosts both a bulk insulating state and a conductive surface state. Within a Fermi-liquid framework, the strongly correlated ground-state electronic structure can be mapped to a simple state resembling a topological insulator. Although uncertainties remain, many experiments provide compelling evidence that the conductive surface states have a topological origin. However, the bulk behaviour is less well understood and some experiments indicate bulk in-gap states. This has inspired the development of many theories that predict the emergence of new bulk quantum phases beyond Landau’s Fermi-liquid model. We review the current progress on understanding both the surface and the bulk states, especially the experimental evidence for each. A mystery centres on the existence of the bulk in-gap states and why they appear in some experiments but not others. Adding to the mystery is why quantum oscillations in SmB6 appear only in magnetization but not in resistivity. We conclude by elaborating on three questions: why SmB6 is worth studying, what can be done to move forwards and what other correlated insulators could give additional insight. The Kondo insulator samarium hexaboride is the first experimentally demonstrated example of a strongly correlated topological insulator. This article reviews the topological theory and experimental evidence, including a mystery as to the origin of quantum oscillations and their relation to possible unconventional bulk in-gap states.

Electrical detection of the inverse Edelstein effect on the surface of SmB6

We report the measurement of spin current induced charge accumulation, the inverse Edelstein effect (IEE), on the surface of a candidate topological Kondo insulator SmB6 single crystal. Robust surface conduction channel of SmB6 has been shown to exhibit large degree of spin-momentum locking, and spin polarized current through an external ferromagnetic contact induces the spin dependent charge accumulation on the surface of SmB6. The dependences of the IEE signal on the bias current, an external magnetic field direction and temperature are consistent with the anticlockwise spin texture for the surface band in SmB6 in the momentum space, and the direction and magnitude of the effect compared with the normal Edelstein signal are clearly explained by the Onsager reciprocal relation. Furthermore, we estimate spin-to-charge conversion efficiency, the IEE length, as 4.46 nm that is an order of magnitude larger than the efficiency found in other typical Rashba interfaces, implying that the Rashba contribution to the IEE signal could be small. Building upon existing reports on the surface charge and spin conduction nature on this material, our results provide additional evidence that the surface of SmB6 supports spin polarized conduction channel.

Superconducting Kondo phase in an orbitally separated bilayer

The nature of superconductivity in heavy-fermion materials is a subject under intense debate, and controlling this many-body state is central for its eventual understanding. Here, we examine how proximity effects may change this phenomenon, by investigating the effects of an additional metallic layer on the top of a Kondo-lattice, and allowing for pairing in the former. We analyze a bilayer Kondo Lattice Model with an on-site Hubbard interaction, $-U$, on the additional layer, using a mean-field approach. For $U=0$, we notice a drastic change in the density-of-states due to multiple-orbital singlet resonating combinations. It destroys the well-known Kondo insulator at half filling, leading to a metallic ground state, which, in turn, enhances antiferromagnetism through the polarization of the conduction electrons. For $U\neq 0$, a superconducting Kondo state sets in at zero temperature, with the occurrence of unconventional pairing amplitudes involving $f$-electrons. We establish that this remarkable feature is only possible due to the proximity effects of the additional layer. At finite temperatures we find that the critical superconducting temperature, $T_c$, decreases with the interlayer hybridization. We have also established that a zero temperature superconducting amplitude tracks $T_c$, which reminisces the BCS proportionality between the superconducting gap and $T_c$.

Metallic surface states in a correlated d-electron topological Kondo insulator candidate FeSb2

The resistance of a conventional insulator diverges as temperature approaches zero. The peculiar low-temperature resistivity saturation in the 4f Kondo insulator (KI) SmB6 has spurred proposals of a correlation-driven topological Kondo insulator (TKI) with exotic ground states. However, the scarcity of model TKI material families leaves difficulties in disentangling key ingredients from irrelevant details. Here we use angle-resolved photoemission spectroscopy (ARPES) to study FeSb2, a correlated d-electron KI candidate that also exhibits a low-temperature resistivity saturation. On the (010) surface, we find a rich assemblage of metallic states with two-dimensional dispersion. Measurements of the bulk band structure reveal band renormalization, a large temperature-dependent band shift, and flat spectral features along certain high-symmetry directions, providing spectroscopic evidence for strong correlations. Our observations suggest that exotic insulating states resembling those in SmB6 and YbB12 may also exist in systems with d instead of f electrons.

Magnetic impurities in Kondo insulators: An application to samarium hexaboride

Impurities and defects in Kondo insulators can have an unusual impact on dynamics that blends with effects of intrinsic electron correlations. Such crystal imperfections are difficult to avoid, and their consequences are incompletely understood. Here we study magnetic impurities in Kondo insulators via perturbation theory of the s-d Kondo impurity model adapted to small bandgap insulators. The calculated magnetization and specific heat agree with recent thermodynamic measurements in samarium hexaboride (SmB$_6$). This qualitative agreement supports the physical picture of multi-channel Kondo screening of local moments by electrons and holes involving both intrinsic and impurity bands. Specific heat is thermally activated in zero field by Kondo screening through sub-gap impurity bands and exhibits a characteristic upturn as the temperature is decreased. In contrast, magnetization obtains a dominant quantum correction from partial screening by virtual particle-hole pairs in intrinsic bands. We point out that magnetic impurities could impact de Haas-van Alphen quantum oscillations in SmB$_6$, through the effects of Landau quantization in intrinsic bands on the Kondo screening of impurity moments.