4$f$ crystal field ground state of the strongly correlated topological insulator SmB$_6$
Martin Sundermann, Hasan Yavaş, Kai Chen, Dae Jeong Kim, Zachary Fisk, Deepa Kasinathan, Maurits Wim Haverkort, peter Thalmeier, Andrea Severing, Liu Hao Tjeng
We investigated the crystal-electric field ground state of the 4$f$ manifold in the strongly correlated topological insulator SmB$_6$ using core level non-resonant inelastic x-ray scattering (NIXS). The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the $Γ_8$ quartet state of the Sm $f^5$ $J$=$5/2$ configuration governs the ground-state symmetry and hence the topological properties of SmB$_6$. Our findings contradict the results of density functional calculations reported so far.
We prove theoretically that certain strongly correlated Kondo insulators are topological crystalline insulators with nontrivial topology protected by crystal symmetries. In particular, we find that SmB$_6$ is such a material. In addition to a nontrivial Z$_2$ topological index protected by time reversal symmetry, SmB$_6$ also has nontrival mirror Chern numbers protected by mirror symmetries. On the $(100)$ surface of SmB$_6$, the nontrivial mirror Chern numbers do not generate additional surface states beyond those predicted by the Z$_2$ topological index. However, on the $(110)$ surface, two …
The finding of bulk quantum oscillations in the bulk Kondo insulator SmB$_6$, which has been proposed to be a correlated topological insulator, proved a considerable surprise. The subsequent measurement of bulk quantum oscillations in other correlated insulators including YbB$_{12}$ have lent support to our discovery of a class of unconventional insulators that are host to bulk quantum oscillations, of which SmB$_6$ was the first example. Here we perform a series of experiments to examine evidence for the intrinsic character of bulk quantum oscillations in floating zone-grown single crystals o…
For the strongly correlated topological insulator SmB6 we discuss the influence of a 2x1 reconstruction of the (001) surface on the topological surface states. Depending on microscopic details, the reconstruction can be a weak or a strong perturbation to the electronic states. While the former leads to a weak backfolding of surface bands only, the latter can modify the surface-state dispersion and lead to a Lifshitz transition. We analyze the quasiparticle interference signal: while this tends to be weak in models for SmB6 in the absence of surface reconstruction, we find that the 2x1 reconstr…
Topological Kondo insulators have been attracting great attention from the condensed-matter physics community due to their fascinating topological and strongly correlated properties. Here, we report angle-dependent c-axis magnetoresistance (MR) oscillations in a Kondo insulator, SmB6 single crystal, in a magnetic field of up to 13 T rotated in the ab-plane. Four-fold symmetric MR oscillations are first observed above 8 K, which result from the four-fold (C4) degeneracy of the bulk Fermi surface of SmB6. With decreasing temperature down to 2.3 K, the C4 symmetry of the MR oscillations gradually…
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…
We advert to the fact that presence of valence fluctuations (VFs) in semiconductors with in-gap impurity bands unconditionally leads to dynamical changes (fluctuations) of energies of localized impurity states. We provide arguments that in the impurity subnetwork consisting of centers having energy levels fluctuating around the Fermi energy there exist favorable conditions for hops from occupied states to empty states of less energy. Consequently, we propose original valence-fluctuation induced hopping mechanism as a new possibility to explain unusual metallic-like conduction of SmB$_6$ and ot…
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 present the results of a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator SmB6 using magnetization and muon-spin relaxation/rotation (muSR) measurements. The muSR measurements exhibit magnetic field fluctuations in SmB6 below 15 K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement betwe…
We have investigated the surface states of a potential mixed-valent topological insulator SmB$_6$ based on the first principles density functional theory slab band structure analysis. We have found that metallic surface states are formed in the bulk band gap region, providing evidence for the topological insulating nature of SmB$_6$. The obtained surface in-gap states are quite different from those in existing reports in that they are formed differently depending on the Sm or B$_6$ surface termination, and are composed of mainly Sm $4f$ state indicating the essentiality of including $f$ electr…
Samarium hexaboride (SmB6) has recently been considered to be a topological Kondo insulator (TKI), the first strongly correlated electron system to exhibit topological surface conduction states. In this contribution, results of electrical resistivity measurements between 80 K and 0.08 K of various SmB6 single crystalline samples are presented, analyzed and discussed. The received results imply that the residual conductivity of SmB6 below about 4 K is of non-activated (metallic-like) nature. It is shown that this metallic-like behavior can be attributed both to surface (2D) conduction states, a…
Compounds within the hexaboride class of materials exhibit a wide variety of interesting physical phenomena, including polaron formation and quadrupolar order. In particular, SmB$_6$ has recently drawn attention as it is considered a prototypical topological Kondo insulator. Evidence in favor of this concept, however, has proven experimentally difficult and controversial, partly because of the required temperatures and energy resolution. Here, a powerful tool is Scanning Tunneling Microscopy (STM) with its unique ability to give local, microscopic information that directly relates to the one-p…
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…
We have investigated the band structure and the band symmetry of mixed-valence insulator SmB$_{6}$ systematically within the density functional theory (DFT). We have analyzed the symmetries and characters of Sm $4f$ and $5d$ bands near the Fermi level ($E_F$) in terms of the non-relativistic real cubic bases as well as the relativistic complex bases incorporating the spin-orbit coupling and the cubic crystal field. Further, we have found that the semi-core band located at $\sim 15$ eV below $E_F$ has the mixed parity and thereby affects the parity eigenvalues of the special ${\bf k}$-points co…
A necessary element for the predicted topological state in Kondo insulator SmB$_6$ is the hybridization gap which opens in this compound at low temperatures. In this work, we present a comparative study of the in-gap density of states due to Sm vacancies by Raman scattering spectroscopy and heat capacity for samples where the number of Sm vacancies is equal to or below 1 %. We demonstrate that hybridization gap is very sensitive to the presence of Sm vacancies. At the amount of vacancies above 1 % the gap fills in with impurity states and low temperature heat capacity is enhanced.
SmB$_6$, known to be a Kondo insulator, has received intense scrutiny in recent years due to its paradoxical experimental signatures: while some quantities show an insulating behavior, others point to a metallic state. This has led to the conjecture that SmB$_6$ hosts nontrivial excitations within its bulk gap, and has spawned several theories to that effect. In principle, there exists an alternative possibility: the system is a metal but unusually with both metal- and insulator-like properties. Inspired by this possibility, I consider a minimal model of a Kondo insulator---a flat band hybridi…
The compound SmB$_6$ is a Kondo Insulator, where the lowest-energy bulk electronic excitations are spin excitons. It also has surface states that are subjected to strong spin-orbit coupling. It has been suggested that SmB$_6$ is also a topological insulator. Here we show that, despite the absence of time-reversal symmetry breaking and the presence of strong spin-orbit coupling, the chiral spin texture of the Weyl cone is not completely protected. In particular, we show that the spin-exciton mediated scattering produces features in the surface electronic spectrum at energies separated from the …
Taken together and viewed holistically, recent theory, low temperature (T) transport, photoelectron spectroscopy and quantum oscillation experiments have built a very strong case that the paradigmatic mixed valence insulator SmB6 is currently unique as a three-dimensional strongly correlated topological insulator (TI). As such, its many-body T-dependent bulk gap brings an extra richness to the physics beyond that of the weakly correlated TI materials. How will the robust, symmetry-protected TI surface states evolve as the gap closes with increasing T? For SmB6 exploiting this opportunity first…
The compound SmB$_6$ is the best established realization of a topological Kondo insulator, in which a topological insulator state is obtained through Kondo coherence. Recent studies have found evidence that the surface of SmB$_6$ hosts ferromagnetic domains, creating an intrinsic platform for unidirectional ballistic transport at the domain boundaries. Here, surface-sensitive X-ray absorption (XAS) and bulk-sensitive resonant inelastic X-ray scattering (RIXS) spectra are measured at the Sm N$_{4,5}$-edge, and used to evaluate electronic symmetries, excitations and temperature dependence near t…
Motivated by the high sensitivity to Fermi surface topology and scattering mechanisms in magneto-thermoelectric transport, we have measured the thermopower and Nernst effect on the (011)-plane of the proposed topological Kondo insulator SmB$_6$. These experiments, together with electrical resistivity and Hall effect measurements, demonstrate that the (011)-plane also harbors a metallic surface with the effective mass in the order of 10-10$^2$ $m_0$. The surface and bulk conductances are well distinguished in these measurements and are categorized into metallic and non-degenerate semiconducting…
Whether SmB6 is a topological insulator remains hotly debated. Our density functional theory plus dynamical mean field theory calculations are in excellent agreement with a large range of experiments, from the 4f5.5 intermediate valency to x-ray and photoemission spectra. Using the pole extended (PE) Hamiltonian, which fully captures the self-energy, we show that SmB6 is a strongly correlated topological insulator, albeit not a Kondo insulator. The PE Hamiltonian is proved to be topological (in)equivalent to the topological Hamiltonian for (non-)local self-energies. The topological surface sta…
Several technical issues and challenges are identified and investigated for the planar tunneling spectroscopy of the topological Kondo insulator SmB$_6$. Contrasting behaviors of the tunnel junctions prepared in two different ways are analyzed and explained in detail. The conventional approach based on an AlO$_\text{x}$ tunnel barrier results in unsatisfactory results due to the inter-diffusion between SmB$_6$ and deposited Al. On the contrary, plasma oxidation of SmB$_6$ crystals produces high-quality tunnel barriers on both (001) and (011) surfaces. Resultant conductance spectra are highly r…
Samarium hexaboride (SmB$_6$), a well-known Kondo insulator in which the insulating bulk arises from strong electron correlations, has recently attracted great attention owing to increasing evidence for its topological nature, thereby harboring protected surface states. However, corroborative spectroscopic evidence is still lacking, unlike in the weakly correlated counterparts, including Bi$_2$Se$_3$. Here we report results from planar tunneling that unveil the detailed spectroscopic properties of SmB$_6$. The tunneling conductance obtained on the (001) and (011) single crystal surfaces reveal…
Samarium hexaboride (SmB$_6$) is the first strongly correlated material with a recognized non-trivial band-structure topology. Its electron correlations are seen by inelastic neutron scattering as a coherent collective excitation at the energy of 14 meV. Here we calculate the spectrum of this mode using a perturbative slave boson method. Our starting point is the recently constructed Anderson model that properly captures the band-structure topology of SmB$_6$. Most self-consistent renormalization effects are captured by a few phenomenological parameters whose values are fitted to match the cal…
Kondo insulators (KIs) are strongly correlated materials in which the interactions between 4f and conduction electrons lead to a hybridization gap opening at low temperature 1-2. SmB6 is a typical KI, but its resistivity does not diverge at low temperatures, which was attributed to some in-gap states 3-10. However after several decades of research, the nature and origin of the in-gap states remain unclear. Recent band calculation and transport measurements suggest that the in-gap states could actually be ascribed to topological surface states. SmB6 thus might be the first realization of topolo…
Using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observ…
The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator-metal transition, where theory predicts a Fermi surface associated with neutral low energy excitations. Here we reveal another route to experimentally realise a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB$_6$ positioned close to the insulator-metal transition. We present experimental signatures down to low temperatures ($\ll 1$ K) ass…
The Kondo insulator SmB6 has long been known to exhibit low temperature (T < 10K) transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the (001)-surface of this material. The low-lying states are found to form ele…
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…
Starting from the kp method in combination with first-principles calculations, we systematically derive the effective Hamiltonians that capture the low energy band structures of recently discovered topological Kondo insulator SmB6. Using these effective Hamiltonians we can obtain both the energy dispersion and the spin texture of the topological surface states, which can be detected by further experiments.
SmB6 was recently proposed to be both a strong topological insulator and a topological crystalline insulator. For this and related cubic topological Kondo insulators, we prove the existence of four different topological phases, distinguished by the sign of mirror Chern numbers. We characterize these phases in terms of simple observables, and we provide concrete tight-binding models for each phase. Based on theoretical and experimental results for SmB6 we conclude that it realizes the phase with C^+_{k_z=0}=+2, C^+_{k_z=pi}=+1, C^+_{k_x=k_y}=-1, and we propose a corresponding minimal model.
The true topological nature of the Kondo insulator SmB$_6$ remains to be unveiled. Our previous tunneling study not only found evidence for the existence of surface Dirac fermions, but it also uncovered that they inherently interact with the spin excitons, collective excitations in the bulk. We have extended such a spectroscopic investigation into crystals containing a Sm deficiency. The bulk hybridization gap is found to be insensitive to the deficiency up to 1% studied here, but the surface states in Sm-deficient crystals exhibit quite different temperature evolutions from those in stoichiom…
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 SmB$_6$. Angle-resolved photoelectron spectroscopy (ARPES) performed on the vicinal SmB$_6$(001)-$p$(2$\times$2) surface showed…
The thermal conductivity $κ$ of the Kondo insulator SmB$_6$ was measured at low temperature, down to 70 mK, in magnetic fields up to 15 T, on single crystals grown using both the floating-zone and the flux methods. The residual linear term $κ_0/T$ at $T \to 0$ is found to be zero in all samples, for all magnetic fields, in agreement with previous studies. There is therefore no clear evidence of fermionic heat carriers. In contrast to some prior data, we observe a large enhancement of $κ(T)$ with increasing field. The effect of field is anisotropic, depending on the relative orientation of fiel…
SmB6 is a promising candidate for topological Kondo insulator. In this letter, we report ultrafast carrier dynamics of SmB6. Two characteristic temperatures: T1=100 K and T2= 20 K are observed. T1 corresponds to the opening of the f-d hybridization gap revealed by an abrupt disappearance of terahertz f-band plasmon oscillations. Between T1 and T2, a phonon bottleneck effect dominates the photocarrier relaxation processes. Below T2, we observe the formation of in-gap states, which are strongly affected by optically injected hot electrons and the transient electron temperature change.
Topological Kondo insulators are a new class of topological insulators in which metallic surface states protected by topological invariants reside in the bulk band gap at low temperatures. Unlike other three-dimensional topological insulators, a truly insulating bulk state, which is critical for potential applications in next-generation electronic devices, is guaranteed by many-body effects in the topological Kondo insulator. Furthermore, the system has strong electron correlations that can serve as a testbed for interacting topological theories. This topical review focuses on recent advances …
The Kondo insulator SmB6 has long been known to exhibit low temperature transport anomalies whose origin is of great interest. Here we uniquely access the surface electronic structure of the anomalous transport regime by combining state-of-the-art laser- and synchrotron-based angle-resolved photoemission techniques. We observe clear in-gap states (up to 4 meV), whose temperature dependence is contingent upon the Kondo gap formation. In addition, our observed in-gap Fermi surface oddness tied with the Kramers' points topology, their coexistence with the two-dimensional transport anomaly in the …
Employing the $\mathbf{k}\cdot\mathbf{p}$ expansion for a family of tight-binding models for SmB$_6$, we analytically compute topological surface states on a generic $(lmn)$ surface. We show how the Dirac-cone spin structure depends on model ingredients and on the angle $θ$ between the surface normal and the main crystal axes. We apply the general theory to $(001)$, $(110)$, $(111)$, and $(210)$ surfaces, for which we provide concrete predictions for the spin pattern of surface states which we also compare with tight-binding results. As shown in previous work, the spin pattern on a $(001)$ sur…
SmB6 is one of the candidate compounds for topological Kondo insulators, a class of materials which combines a non-trivial topological band structure with strong electronic correlations. Here we employ a multiband tight-binding description, supplemented by a slave-particle approach to account for strong interactions, to theoretically study the surface-state signatures in scanning tunneling spectroscopy (STS) and quasiparticle interference (QPI). We discuss the spin structure of the three surface Dirac cones of SmB6 and provide concrete predictions for the energy and momentum dependence of the …
Here we propose the existence of a non-static ground state in the Kondo insulator SmB6, with a unique accompany-type valence fluctuation of Sm ions in its bulk. Whether SmB6 is a fashion of time crystal is an intriguing issue.
SmB6 has recently been predicted to be a Topological Kondo Insulator, the first strongly correlated heavy fermion material to exhibit topological surface states. High quality crystals are necessary to investigate the topological properties of this material. Single crystal growth of the rare earth hexaboride, SmB6, has been carried out by the floating zone technique using a high power xenon arc lamp image furnace. Large, high quality single-crystals are obtained by this technique. The crystals produced by the floating zone technique are free of contamination from flux materials and have been ch…
SmB$_6$, a so called Kondo insulator, is recently discussed as a candidate material for a strong topological insulator. We present detailed atomically resolved topographic information on the (001) surface from more than a dozen SmB$_6$ samples. Atomically flat, {\it in situ} cleaved surfaces often exhibit B- and Sm-terminated surfaces as well as reconstructed and non-reconstructed areas {\it coexisting} on different length scales. The terminations are unambiguously identified. In addition, electronic inhomogeneities are observed which likely result from the polar nature of the (001) surface an…
We propose the local density approximation (LDA)+Gutzwiller method incorporating Green's function scheme to study the topological physics of correlated materials from the first-principles. Applying this method to typical mixed valence materials SmB6, we found its non-trivial Z2 topology, indicating that SmB6 is a strongly correlated topological insulator (TI). The unique feature of this compound is that its surface states contain three Dirac cones in contrast to most known TIs.
Recently, evidence for a conducting surface state below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the conducting surface state and the bulk phase of FeSi were probed via electrical resistivity 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 to those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron…
Recently, the resistance saturation at low temperature in Kondo insulator SmB6, a long-standing puzzle in condensed matter physics, was proposed to originate from topological surface state. Here,we systematically studied the magnetoresistance of SmB6 at low temperature up to 55 Tesla. Both temperature- and angular-dependent magnetoresistances show a similar crossover behavior below 5 K. Furthermore, the angular-dependent magnetoresistance on different crystal face confirms a two-dimensional surface state as the origin of magnetoresistances crossover below 5K. Based on two-channels model consis…
A topological insulator (TI) is an unusual quantum state in which the insulating bulk is topologically distinct from vacuum, resulting in a unique metallic surface that is robust against time-reversal invariant perturbations. These surface transport properties, however, remains difficult to be isolated from the bulk in existing TI crystals (Bi2Se3, Bi2Te3 and Sb2Te3) due to impurity caused bulk conduction. We report in large crystals of topological Kondo insulator (TKI) candidate material SmB6 the thickness-independent surface Hall effects and non- local transport, which are robust against per…
Topological insulators are a class of materials with insulating bulk but protected conducting surfaces due to the combination of spin-orbit interactions and time-reversal symmetry. The surface states are topologically non-trivial and robust against non-magnetic backscattering, leading to interesting physics and potential quantum computing applications1, 2. Recently there has been a fast growing interest in samarium hexboride (SmB6), a Kondo insulator predicted to be the first example of a correlated topological insulator3, 4. Here we fabricated smooth thin films of nanocrystalline SmB6 films. …
We study the transport properties of the Kondo insulator SmB$_6$ with a specialized configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. We find that as the material is cooled below 4 K, it exhibits a crossover from bulk to surface conduction with a fully insulating bulk. We take the robustness and magnitude of the surface conductivity, as is manifest in the literature of SmB$_6$, to be strong evidence for the topological insulator metallic surface states recently predicted for this material.
Strongly correlated Kondo insulator SmB6 is known for its peculiar low temperature residual conduction, which has recently been demonstrated to arise from a robust metallic surface state, as predicted by the theory of topological Kondo insulator (TKI). Photoemission, quantum oscillation and magnetic doping experiments have provided evidence for the Dirac-like dispersion and topological protection. Questions arise as whether signatures of spin-momentum locking and electron interaction could be resolved in transport measurements. Here we report metallic conduction of surface state down to mK tem…
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…
The impact of non-magnetic and magnetic impurities on topological insulators is a central problem concerning their fundamental physics and possible novel spintronics and quantum computing applications. SmB$_6$, predicted to be a topological Kondo insulator, is considered a benchmark material. Using a spin-polarized tip in scanning tunneling spectroscopy destroys the signature peak of the topological surface state, revealing its spin texture. Further, combining local STS with macroscopic transport measurements on SmB$_6$ containing different substitutions enables us to investigate the effect of…
The temperature and thickness dependencies of the in-plane anisotropic magnetoresistance (AMR) of SmB$_6$ thin films are reported. We find that the AMR changes sign from negative ($ρ_{||}<ρ_{\perp}$) at high temperatures to positive ($ρ_{||}>ρ_{\perp}$) at low temperatures. The temperature, T$_s$, at which this sign change occurs, decreases with increasing film thickness $t$ and T$_s$ vanishes for $t$ $>$ 30 nm. We interpret our results in the framework of a competition between two components: a negative bulk contribution and a positive surface AMR.
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…
The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures, T$<5$ K. In this temperatures range, a plateau is observed in the dc resistivity, contrary to the exponential divergence expected for a gapped system. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator (TKI) and propose that the residual conductivity is due to topological surface states which reside within the Kondo gap. Since the TKI prediction many experiments have claimed to observe high mobility surface states within a perfectly insulat…
The Kondo insulator compound SmB6 has emerged as a strong candidate for the realization of a topologically nontrivial state in a strongly correlated system, a topological Kondo insulator, which can be a novel platform for investigating the interplay between nontrivial topology and emergent correlation driven phenomena in solid state systems. Electronic transport measurements on this material, however, so far showed only the robust surface dominated charge conduction at low temperatures, lacking evidence of its connection to the topological nature by showing, for example, spin polarization due …
Topological insulators give rise to exquisite electronic properties due to their spin-momentum locked Dirac-cone-like band structure. Recently, it has been suggested that the required opposite parities between valence and conduction band along with strong spin-orbit coupling can be realized in correlated materials. Particularly, SmB$_6$ has been proposed as candidate material for a topological Kondo insulator. By utilizing scanning tunneling microscopy and spectroscopy measurements down to 0.35 K, we observed several states within the hybridization gap of about $\pm$20 meV on well characterize…
The predicted interplay between Kondo physics and non-trivial topology in SmB$_{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…
In Kondo insulator samarium hexaboride SmB$_6$, strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 5 Kelvin, a behavior recently demonstrated to arise from the surface conductance. However, questions remain whether and where a topological surface state exists. Quantum oscillations have not been observed to map the Fermi surface. We solve the problem by resolving the Landau Level quantization and Fermi surface topology using torque magnetometry. The observed Fermi surface suggests a two dimen…
Incorporating relativistic physics into quantum tunneling can lead to exotic behavior such as perfect transmission via Klein tunneling. Here, we probe the tunneling properties of spin-momentum locked relativistic fermions by designing and implementing a tunneling geometry that utilizes nanowires of the topological Kondo insulator candidate, SmB6. 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 ~50 K. The antiferromagnetic stripes become invisible above 10 …
