Topological Kondo Insulator Research Articles

Generic Approach to Intrinsic Magnetic Second-Order Topological Insulators via Inverted p-d Orbitals.

The integration of intrinsically magnetic and topologically nontrivial two-dimensional materials holds tantalizing prospects for exotic quantum anomalous Hall insulators and magnetic second-order topological insulators (SOTIs). Compared with their well-studied nonmagnetic counterparts, the pursuit of intrinsic magnetic SOTIs remains limited. In this work, we address this gap by focusing on p-d orbitals inversion, a fundamental but often overlooked phenomena in the construction of topological materials. We begin by developing a theoretical framework to elucidate p-d orbital inversion through a combined density-functional theory calculation and Wannier downfolding. Subsequently we showcase the generality of this concept in realizing ferromagnetism SOTIs by identifying two real materials with distinct lattices: 1T-VS2 monolayer in a hexagonal lattice and CrAs monolayer in a square lattice. We further compare it with other mechanisms requiring spin-orbit coupling and explore the similarities to topological Kondo insulators. Our findings establish a generic pathway toward intrinsic magnetic SOTIs.

Evolution of surface conductivity in SmB6 under nonmagnetic (Yb2+) and magnetic (Eu2+) doping

Among of rare-earth (RE) hexaborides only two compounds SmB6 and YbB6 are discussed in literature to be members of a new class of 3D topological insulators. However, their ground states originate due to different physical mechanisms, including Kondo 4f-5d hybridization and 5d-2p band inversion, respectively. Here we report a comparative study of magnetotransport (resistivity and transverse magnetoresistance) measured on high quality single crystals of YbxSm1-xB6 and EuxSm1-xB6 solid solutions (x ≤ 0.05) at temperatures 1.7 − 300 K in magnetic fields up to 82 kOe. The choice of dopant was determined by the fact that the presence of magnetic/nonmagnetic (Eu2+/Yb2+) impurity in parent SmB6 matrix should lift/not lift the topological protection of surface states. Based on the two-gap paradigm the x-evolution of electron spectra in YbxSm1-xB6 and EuxSm1-xB6 was studied. Our data show that both the 4f lattice coherence (Eg) and the intrinsic gap (Ea) related to many-body states survive under RE doping at least for x ≈ 0.02 − 0.024. We also suggest that the point x(Eu) = 0.05 can be treated as an upper limit of the small gap closing in EuxSm1-xB6 materials. In YbxSm1-xB6 family a negative linear transverse magnetoresistance (TMR) was detected for the first time in the regime of surface conductivity (T < T∗ ≈ 5 K). The TMR anomaly at T∗ caused possibly by the topological protection of surface states in SmB6 is found to survive in Eu-doped compounds but disappears almost completely for Yb-doped compositions in the same fixed magnetic fields. This paradoxical observation is not consistent with general predictions of the topological Kondo insulator (TKI) model.

A microscopic Kondo lattice model for the heavy fermion antiferromagnet CeIn3

Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergence of a plethora of novel quantum matter states such as unconventional superconductivity, electronic-nematic states, hidden order and most recently topological states of matter such as topological Kondo insulators and Kondo semimetals and putative chiral superconductors. The outstanding challenge is that the archetypal Kondo lattice model that captures the underlying electronic dichotomy is notoriously difficult to solve for real materials. Here we show, using the prototypical strongly-correlated antiferromagnet CeIn3, that a multi-orbital periodic Anderson model embedded with input from ab initio bandstructure calculations can be reduced to a simple Kondo-Heisenberg model, which captures the magnetic interactions quantitatively. We validate this tractable Hamiltonian via high-resolution neutron spectroscopy that reproduces accurately the magnetic soft modes in CeIn3, which are believed to mediate unconventional superconductivity. Our study paves the way for a quantitative understanding of metallic quantum states such as unconventional superconductivity.

Multiple channel transport properties of topological surface states in SmB6 nanoribbons

SmB6, a topological Kondo insulator, possesses topologically protected surface states, in which carrier mobility is supposed to be high but is significantly varied in orders of magnitude dependent upon characterization methods. Herein, we performed magnetoresistance measurements on a Hall-bar device constructed on a highly uniform SmB6 nanoribbon with (001) surface grown by chemical vapor deposition. Unusual non-linear Hall resistance was observed at low temperature and explained with a two-carrier model. The two types of carriers were attributed to two types of Fermi pockets on the Brillouin zone of the SmB6 (001) surface. The Fermi pocket on Γ point provided high hole mobility up to 1.4×103 cm2/V s but low hole density at a temperature of 2.2 K. On the other hand, the Fermi pockets on X points supplied low electron mobility down to 1.8 cm2/V s but high electron density. The identification of the two types of pockets laid the foundation for the understanding of the transport via the topological surface states of the SmB6 (001) surface.

Possible surface magnetism in the topological Kondo insulator candidate FeSi

Possible surface magnetism in the topological Kondo insulator candidate FeSi

Unraveling the unique response behaviors of ultrathin transition metal dichalcogenides to external excitations

Designing novel transition metal dichalcogenides (TMDCs) are highly important for achieving superior performances in advanced optoelectrical devices of broad applications. Although many TMDCs materials have been synthesized and studied, the response behaviors of TMDCs under external excitations still lack in-depth investigations. To address this challenge, we have established a library of TMDCs including 56 types of materials and revealed their response behaviors to the external electric fields and strains as well as the possibility to drive phase transitions under the external excitations. We have explored the thermodynamic stability, band structures, electronic structures, and optical properties of these TMDCs. Under the external electric field, the gradual change of energy results in the conversion from semiconductors to metals, which enables the construction of heterojunctions while the Landau phase transitions are absent. In comparison, the external strain not only lowers the overall symmetry but also induces strong p-π hybridizations to realize unique electronic properties. In particular, Dirac cones are noticed in the 1H-TcTe2 within 45–100 GPa, supporting a novel quasi heavy-fermion topological Kondo insulator induced by the phase transition under the external strains for the first time in TMDCs. This work offers fundamental knowledge to discover novel TMDCs candidates with superior optoelectrical properties.

Kondo interaction in FeTe and its potential role in the magnetic order

Finding d-electron heavy fermion states has been an important topic as the diversity in d-electron materials can lead to many exotic Kondo effect-related phenomena or new states of matter such as correlation-driven topological Kondo insulator. Yet, obtaining direct spectroscopic evidence for a d-electron heavy fermion system has been elusive to date. Here, we report the observation of Kondo lattice behavior in an antiferromagnetic metal, FeTe, via angle-resolved photoemission spectroscopy, scanning tunneling spectroscopy and transport property measurements. The Kondo lattice behavior is represented by the emergence of a sharp quasiparticle and Fano-type tunneling spectra at low temperatures. The transport property measurements confirm the low-temperature Fermi liquid behavior and reveal successive coherent-incoherent crossover upon increasing temperature. We interpret the Kondo lattice behavior as a result of hybridization between localized Fe 3dxy and itinerant Te 5pz orbitals. Our observations strongly suggest unusual cooperation between Kondo lattice behavior and long-range magnetic order.

Extraordinary bulk-insulating behavior in the strongly correlated materials FeSi and FeSb2

4f 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 interiors. To this end, we have investigated the electrical transport of the 3d-based correlated insulators FeSi and FeSb2, which have exhibited enough similarities to their f electron cousins to warrant investigation. By using a double-sided Corbino disk transport geometry, we show unambiguous evidence of surface conductance in both of these Fe-based materials. In addition, by using a four-terminal Corbino inverted resistance technique, we extract the bulk resistivity as a function of temperature. Similar to topological Kondo insulator SmB6, the bulk resistivity of FeSi and FeSb2 is confirmed to exponentially increase by up to 9 orders of magnitude from room temperature to the lowest accessible temperature. This demonstrates that these materials are excellent bulk insulators, providing an ideal platform for studying correlated 2D physics.

Origin of negative thermal expansion in strongly correlated f− electron systems

We have investigated the origin of the negative-thermal-expansion (NTE) phenomena observed in various strongly correlated $\mathit{f}\text{\ensuremath{-}}$electron systems, including Ce, Sm, and Yb rare-earth compounds and actinide element Pu. We have thoroughly surveyed existing nonmagnetic $\mathit{f}\text{\ensuremath{-}}$electron NTE materials and identified that the temperature ($T$)-induced valence transition plays an essential role in realizing the NTE in Sm- and Yb-based mixed-valence (MV) systems. We have discussed the contrasting thermal-expansion behaviors between (Sm, Yb)-based MV systems and their electron-hole symmetric counterparts (Ce, Eu)-based MV systems that exhibit positive-thermal-expansion (PTE) phenomena. We have also clarified the origin of intriguing thermal expansion behavior of Pu, which, upon cooling, reveals not only the NTE, but also the strong PTE. Finally, we have predicted the possible existence of an additional NTE feature in topological-Kondo-insulator (TKI) candidates of ${\mathrm{SmB}}_{6}$, g-SmS, and ${\mathrm{YbB}}_{12}$ in the low-$T$ regime where the TKI nature is expected to emerge.

Visualizing the atomic-scale origin of metallic behavior in Kondo insulators.

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. In this study, we visualized the real-space charge landscape within a Kondo lattice with atomic resolution using a scanning tunneling microscope. We discovered nanometer-scale puddles of metallic conduction electrons centered around uranium-site substitutions in the heavy-fermion compound uranium ruthenium silicide (URu2Si2) and around samarium-site defects in the topological Kondo insulator samarium hexaboride (SmB6). These defects disturbed the Kondo screening cloud, leaving behind a fingerprint of the metallic parent state. Our results suggest that the three-dimensional quantum oscillations measured in SmB6 arise from Kondo-lattice defects, although we cannot exclude other explanations. Our imaging technique could enable the development of atomic-scale charge sensors using heavy-fermion probes.

Hourglass-type bulk Ni 3d band and Ce 4f Kondo resonance states in the potential topological Kondo semimetal CeNiSn via angle-resolved photoemission spectroscopy

The electronic structure of CeNiSn, a potential topological Kondo insulator and a Dirac nodal-loop semimetal, is investigated here employing temperature-dependent angle-resolved photoemission spectroscopy (ARPES). Dispersive states crossing the Fermi level (${E}_{\text{F}}$) exhibit general agreement with theoretical predictions of bulk Ni 3$d$ bands and show consistency with a topological hourglass-type crossing of bulk bands below ${E}_{\text{F}}$. Flat Ce 4$f$ states at ${E}_{\text{F}}$ exhibit a Kondo resonance temperature dependence consistent with bulk transport. This work demonstrates the importance of coherent Kondo states in determining the topological properties of CeNiSn.

Observation of the Superconducting Proximity Effect from Surface States in SmB_{6}/YB_{6} Thin Film Heterostructures via Terahertz Spectroscopy.

The ac conduction of epitaxially grown SmB_{6} thin films and superconducting heterostructures of SmB_{6}/YB_{6} are investigated via time-domain terahertz spectroscopy. A two-channel model of thickness-dependent bulk states and thickness-independent surface states accurately describes the measured conductance of bare SmB_{6} thin films, demonstrating the presence of surface states in SmB_{6}. While the observed reductions in the simultaneously measured superconducting gap, transition temperature, and superfluid density of SmB_{6}/YB_{6} heterostructures relative to bare YB_{6} indicate the penetration of proximity-induced superconductivity into the SmB_{6} overlayer; the corresponding SmB_{6}-thickness independence between different heterostructures indicates that the induced superconductivity is predominantly confined to the interface surface state of the SmB_{6}. This study demonstrates the ability of terahertz spectroscopy to probe proximity-induced superconductivity at an interface buried within a heterostructure, and our results show that SmB_{6} behaves as a predominantly insulating bulk surrounded by conducting surface states in both the normal and induced-superconducting states in both terahertz and dc responses, which is consistent with the topological Kondo insulator picture.

Probing FeSi, a d-electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T, magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a "topological Kondo insulator" (TKI). The overall behavior of the magnetoresistanceof FeSi at temperatures above and below the onset temperature TS = 19 K of the CSS is similar to that of SmB6. The two energy gaps, inferred from the ρ(T) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of TS. Several studies of ρ(T) under pressure on SmB6 reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which TS vanishes, although the energy gaps in SmB6 initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at TS ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at TS ≈ 4.5 K for SmB6.

Collective and Quasi-Local Modes in the Optical Spectra of YB6 and YbB6 Hexaborides with Jahn–Teller Structural Instability

The broad-band reflection spectra of YB6 and YbB6 hexaborides with Jahn–Teller instability of the boron cage have been measured at room temperature. An optical conductivity analysis has revealed, along with the Drude electronic components, heavily overdamped collective modes, which are notable in YB6 for high dielectric contributions, Δε = 2000–5700. The fraction of nonequilibrium charge carriers in YB6, which is at the boundary of structural instability in the hexaboride family, reaches 85–90%, whereas this fraction in doped YbB6 semiconductor is not higher than 25%. It has been shown that unlike the predictions of the topological Kondo insulator model, the surface “metallization” in Yb2+B6 crystals can be explained by additional doping of a surface layer with Yb3+ ions.

Solvable periodic Anderson model with infinite-range Hatsugai-Kohmoto interaction: Ground-states and beyond

In this paper we introduce a solvable two-orbital/band model with infinite-range Hatsugai-Kohmoto interaction, which serves as a modified periodic Anderson model. Its solvability results from strict locality in momentum space, and is valid for arbitrary lattice geometry and electron filling. Case study on a one-dimension ($1D$) chain shows that the ground-states have Luttinger theorem-violating non-Fermi-liquid-like metallic state, hybridization-driven insulator and interaction-driven featureless Mott insulator. The involved quantum phase transition between metallic and insulating states belongs to the universality of Lifshitz transition, i.e. change of topology of Fermi surface or band structure. Further investigation on $2D$ square lattice indicates its similarity with the $1D$ case, thus the findings in the latter may be generic for all spatial dimensions. We hope the present model or its modification may be useful for understanding novel quantum states in $f$-electron compounds, particularly the topological Kondo insulator SmB$_{6}$ and YbB$_{12}$.

Breakdown of bulk-projected isotropy in surface electronic states of topological Kondo insulator SmB6(001)

The topology and spin-orbital polarization of two-dimensional (2D) surface electronic states have been extensively studied in this decade. One major interest in them is their close relationship with the parities of the bulk (3D) electronic states. In this context, the surface is often regarded as a simple truncation of the bulk crystal. Here we show breakdown of the bulk-related in-plane rotation symmetry in the topological surface states (TSSs) of the Kondo insulator SmB6. Angle-resolved photoelectron spectroscopy (ARPES) performed on the vicinal SmB6(001)-p(2 × 2) surface showed that TSSs are anisotropic and that the Fermi contour lacks the fourfold rotation symmetry maintained in the bulk. This result emphasizes the important role of the surface atomic structure even in TSSs. Moreover, it suggests that the engineering of surface atomic structure could provide a new pathway to tailor various properties among TSSs, such as anisotropic surface conductivity, nesting of surface Fermi contours, or the number and position of van Hove singularities in 2D reciprocal space.

Spin-selective tunneling from nanowires of the candidate topological Kondo insulator SmB6.

Incorporating relativistic physics into quantum tunneling can lead to exotic behavior such as perfect transmission through Klein tunneling. Here, we probed the tunneling properties of spin-momentum-locked relativistic fermions by designing and implementing a tunneling geometry that uses nanowires of the topological Kondo insulator candidate samarium hexaboride. The nanowires are attached to the end of scanning tunneling microscope tips and used to image the bicollinear stripe spin order in the antiferromagnet Fe1.03Te with a Neel temperature of about 50 kelvin. The antiferromagnetic stripes become invisible above 10 kelvin concomitant with the suppression of the topological surface states in the tip. We further demonstrate that the direction of spin polarization is tied to the tunneling direction. Our technique establishes samarium hexaboride nanowires as ideal conduits for spin-polarized currents.

Magnetoquantum oscillations in the specific heat of a topological Kondo insulator

Surprisingly, magnetoquantum oscillations (MQOs) characteristic of a metal with a Fermi surface have been observed in measurements of the topological Kondo insulator SmB6. As these MQO have only been observed in measurements of magnetic torque (dHvA) and not in measurements of magnetoresistance (SdH), a debate has arisen as to whether the MQO are an extrinsic effect arising from rare-earth impurities, defects, and/or aluminum inclusions or an intrinsic effect revealing the existence of charge-neutral excitations. We report here the first observation of MQO in the low-temperature specific heat of SmB6. The observed frequencies and their angular dependence for these flux-grown samples are consistent with previous results based on magnetic torque for SmB6 but the inferred effective masses are significantly larger than previously reported. Such oscillations can only be observed if the MQO are of bulk thermodynamic origin; the measured magnetic-field dependent oscillation amplitude and effective mass allow us to rule out suggestions of an extrinsic, aluminum inclusion-based origin for the MQO.

Persistence of correlation-driven surface states in SmB6 under pressure

The proposed topological Kondo insulator SmB$_{6}$ hosts a bulk Kondo hybridization gap that stems from strong electronic correlations and a metallic surface state whose effective mass remains disputed. Thermopower and scanning tunneling spectroscopy measurements argue for heavy surface states that also stem from strong correlations, whereas quantum oscillation and angle-resolved photoemission measurements reveal light effective masses that would be consistent with a Kondo breakdown scenario at the surface. Here we investigate the evolution of the surface state via electrical and thermoelectric transport measurements under hydrostatic pressure, a clean symmetry-preserving tuning parameter that suppresses the Kondo gap and increases the valence of Sm from 2.6+ towards a 3+ magnetic metallic state. Electrical resistivity measurements reveal that the surface carrier density increases with increasing pressure, whereas thermopower measurements show an unchanged Fermi energy under pressure. As a result, the effective mass of the surface state charge carriers linearly increases with pressure as the Sm valence approaches 3+. Our results are consistent with the presence of correlation-driven surface states in SmB$_{6}$ and suggest that the surface Kondo effect persists under pressure to 2 GPa.

Surface-state plasmons in topological Kondo insulator

Surface-state plasmons in topological Kondo insulator