SmB6 Single Crystals Research Articles

Surface conductivity in SmB6

We present a study of low temperature electron transport (resistivity, magnetoresistance and Hall coefficient) in SmB6 single crystals having different polar (100) and nonpolar (110) and (111) surfaces after mechanical polishing and chemical etching. The estimation of effective parameters for surface and bulk charge carriers allows us to conclude that surface conductivity is very sensitive to the method of surface treatment. The most pronounced change is observed for the polar (100) surface, for which the related Hall concentration of charge carriers at 2 K decreases more than by 2 orders of magnitude and the Hall mobility increases by a factor of 15 after etching these faces in diluted nitric acid. We suggest that the strong dependence of surface properties on the type of treatment may result from both topological protection, which is influenced by intrinsic defects or surface reconstruction, and band bending effects, which modulate the properties of the surface conduction layer in case of polar faces.

Dielectric and transient electrical response of SmB6 single crystals

We report results from an in–plane and out–of–plane impedance study on SmB6 single crystals, performed at low temperatures and over a wide frequency range. A universal equivalent circuit describes the dielectric behavior of this system across the transition, from surface to bulk dominated electrical conduction between 2 and 10 K. We identify the resistive, capacitive, and inductive contributions to the impedance. The equivalent inductance, obtained from fits to experimental data, drops drastically with increasing temperature, as the bulk starts to control electrical conduction. Self–sustained voltage oscillations, observed below 6 K across small crystals when biased with a dc current, point to a large SmB6 self–capacitance, likely brought by the surface states.

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.

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.

Structural symmetry changes in SmB6 - 2D correlation spectroscopy and principal component analysis

2D correlation analysis (2DCOS) and principal component analysis (PCA) have been performed on temperature-dependent Raman spectra from 12 K to 300 K of SmB6 single crystals grown from Boron with a purity of 99.9% (3N) or 99.9999% (6N). Significant differences in the Raman spectra of SmB6(3N) and SmB6(6N) are observed. The Raman active T2g, Eg, and A1g modes of SmB6(3N) are broader and shift to higher wavenumbers compared with those of SmB6(6N). The results of 2DCOS and PCA on temperature-dependent Raman spectra of SmB6(3N) and SmB6(6N) give us further information. We find that all the T2g, Eg, and A1g modes of SmB6(6N) are single bands, while those of SmB6(3N) show all doublet features. The appearance of multiple bands in Raman vibrational modes of SmB6(3N) sample implies that the structure of boron octahedra of the SmB6(3N) has lower symmetry than that of SmB6(6N). Our findings in 2DCOS and PCA of the Raman spectra strongly support the differences in the behavior of SmB6(3N) and SmB6(6N).

Bulk and surface electronic properties ofSmB6: A hard x-ray photoelectron spectroscopy study

We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single crystals. Using the multiplet-structure in the Sm 3d core level spectra, we determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5 K. Temperature dependent measurements revealed that the Sm valence gradually increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly observe that not only the J=0 but also the J=1 state of the Sm 4f 6 configuration becomes occupied at elevated temperatures. Making use of the polarization dependence, we were able to identify and extract the Sm 4f spectral weight of the bulk material. Finally, we revealed that the oxidized or chemically damaged surface region of the ex-situ polished SmB6 single crystal is surprisingly thin, about 1 nm only.

The electronic structure and thermionic emission property of single crystal SmB6

The optical floating zone method was employed to fabricate the Samarium hexaboride (SmB6), which were high-quality single crystals. The density function theory was used to calculate the electronic structures of the SmB6, which exhibits the large density of states near Fermi level. And the d electrons of Sm element contribute the electronic states near the Fermi level, which imply that the (100) crystal surface has a highest Sm concentration which provide a low work function. The thermionic emission test shows that the effective work function of the single crystal SmB6 on (100) crystal surface is 2.76eV. The SmB6 single crystals have potential application prospects as a hot cathode.

On the Chemistry and Physical Properties of Flux and Floating Zone Grown SmB6 Single Crystals.

Recent theoretical and experimental findings suggest the long-known but not well understood low temperature resistance plateau of SmB6 may originate from protected surface states arising from a topologically non-trivial bulk band structure having strong Kondo hybridization. Yet others have ascribed this feature to impurities, vacancies, and surface reconstructions. Given the typical methods used to prepare SmB6 single crystals, flux and floating-zone procedures, such ascriptions should not be taken lightly. We demonstrate how compositional variations and/or observable amounts of impurities in SmB6 crystals grown using both procedures affect the physical properties. From X-ray diffraction, neutron diffraction, and X-ray computed tomography experiments we observe that natural isotope containing (SmB6) and doubly isotope enriched (154Sm11B6) crystals prepared using aluminum flux contain co-crystallized, epitaxial aluminum. Further, a large, nearly stoichiometric crystal of SmB6 was successfully grown using the float-zone technique; upon continuing the zone melting, samarium vacancies were introduced. These samarium vacancies drastically alter the resistance and plateauing magnitude of the low temperature resistance compared to stoichiometric SmB6. These results highlight that impurities and compositional variations, even at low concentrations, must be considered when collecting/analyzing physical property data of SmB6. Finally, a more accurate samarium-154 coherent neutron scattering length, 8.9(1) fm, is reported.

Crossover of Magnetoresistance from Fourfold to Twofold Symmetry in SmB6 Single Crystal, a Topological Kondo Insulator

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 weakens and C2 symmetry appears. This demonstrates a crossover from three-dimensional bulk states to two-dimensional surface states and implies the possible emergence of topological nematic states. Our experimental observations shed new light on the metallic surface states and nematic states in the Kondo insulator SmB6.

Ferromagnetic ordering of iron impurities in the valence-fluctuating semiconductor SmB6

The electron paramagnetic resonance (EPR) of the valence-fluctuating semiconductor SmB6 doped by 1 at % Fe is studied. The EPR measurements are performed on a SmB6 single crystal in a temperature range of 1.6–300.0 K. A number of resonance lines whose g factors indicate the presence of iron ions in the Fe0, Fe+, Fe2+, and Fe3+ states have been detected. The iron ions are ferromagnetically ordered below a Curie temperature T = 100 K, and this ordering can be caused by the exchange interaction of impurity ions due to matrix polarization (a similar mechanism is observed in PdFe alloys). This exchange interaction is estimated to be significantly higher than that in PdFe; this fact can result from a very high density of states in the narrow f band, which is characteristic of a valence-fluctuating material.

High Field Gap Closure in the Kondo Insulator SmB6

We have measured the longitudinal magnetoresistance of SmB6 single crystals in magnetic fields up to 145 T. The insulating gap closes at ≈80–90 T and above 100 T the magnetoresistance is positive and quadratic.

Low temperature magnetic properties of samarium hexaboride

Magnetization measurements to 2K and ac susceptibility measurements to 10mK have been performed on two intermediate valent SmB6 single crystals. The temperature dependence of susceptibility at temperatures above 50K follows the van Vleck behaviour based on the contribution of 4f6 and 4f55d1 configurations of Sm-ions, and is in agreement with the results obtained by other groups. Below 15K, however, the magnetization and susceptibility dependences seem to be strongly influenced by impurities in the samples.

ESR spectra of Er3+ in SmB6 single crystals: Dynamic Jahn-Teller effect in a mixed-valence compound.

In an ESR study on the rare-earth ion ${\mathrm{Er}}^{3+}$ in the intermediate-valence single crystal Sm${\mathrm{B}}_{6}$ a spectrum is found which is markedly different from those of comparable systems with integral valence. We analyze the unusual experimental results in terms of a dynamic Jahn-Teller effect in the ${\ensuremath{\Gamma}}_{8}$ cubic-crystal-field ground state. The characteristic features of our experimental data are well reproduced by the calculations, which suggest that the substitutional rare-earth ion ${\mathrm{Er}}^{3+}$ suffers enhanced electron-phonon coupling mediated by the valence-fluctuating host.

EPR on the Eu2+ions in SmB6

Using the EPR method at the frequency 1010 Hz and T=1.7-4.2K the authors have studied the local crystalline fields of the Eu2+ ions in SmB6 single crystal. The spectrum is described by the spin-Hamiltonian of cubic symmetry with the following parameters: b4=-45.7*10-4 cm-1; b6=0.021*10-4 cm-1; g=1.932+or-0.005; 151A=33.4*10-4 cm-1; 153A=14.9*10-4 cm-1. They believe that SmB6 is a paramagnet of the Van Vleck type.

The preparation and properties of CeB6, SmB6, and GdB6

Single crystals of CeB6, SmB6, and GdB6 have been prepared by a floating-zone technique under a pressurized gas atmosphere of Ar. A zone leveling technique has successfully been applied to prepare single crystals of GdB6, a compound which peritectically decomposes. The crystals so obtained were almost stoichiometric and had a low-impurity level. The measured work functions of (001) surfaces of CeB6 and GdB6 are 2.6±0.1 and 3.8±0.1 eV, respectively. The temperature dependence of the electrical resistivity of CeB6 resembles that of CeAl3. The temperature dependence of the resistivity of SmB6 single crystals is like that of a semiconductor and is similar to those reported for polycrystalline specimens. The resistivity has a very high saturation value of 70.7 Ω cm at low temperatures. A new phase transition of GdB6 was observed at about 7 K, which is indicated by hysteretic behavior in the resistivitiy versus temperature curve.