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

Abstract

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.

Bulk Rotational Symmetry Breaking in Kondo Insulator SmB6

“…1(b), should be ∼2 mg. Since the weight of SmB 6 sample S5 is 12.9 mg, the amount of cocrystallized Al could be as large as 15 wt % (58.4 mol%), which is much larger than that revealed by an x-ray diffraction study [37]. Consequently, the dHvA patterns in Fig.…”

“…The existence of epitaxially oriented Al flakes in the fluxgrown SmB 6 single crystals has also been confirmed, with a percentage of 2-4 wt % [37]. However, the dHvA amplitude of single-crystalline aluminum is known to be Δτ ≃ 3.5 × 10 −7 N m at 4.2 K under B ¼ 2 T, for the strongest oscillation branch γ 5 in a sample with the mass of 45 mg [43].…”

“…The FZ growth are reported to induce a small portion of Sm vacancies in SmB 6 single crystal [35][36][37]. Subsequently, a slight difference on the Sm valence on the surface can be established, which in turn modifies the Kondo interaction near the surface [16,17].…”

Quantum spin fluctuations in the bulk insulating state of pure and Fe-doped SmB6

“…24 This difference is likely a result of Sm vacancies in the FZ crystals. 27 Stoichiometric quantities of Sm chunk, Fe powder and B powder as the reactants, and Al as the flux were carefully ground in a ratio of (Sm, Fe)B 6 :Al = 1:200. The starting materials were placed in an alumina crucible in a tube furnace and then pumped to high vacuum and purged with Ar three times.…”

Foreward for special issue of philosophical magazine on: topological correlated insulators and SmB6

“…The consequent use [67,68] of the Corbino geometry has corrected previous inferences from the Hall effect of carrier densities that are unphysically large for two-dimensional transport. Two different bulk sample preparation methods are common, flux growth and floating zone [69], and there is some sample dependence of measurement results, for example some studies finding that a true plateau resistivity in the latter occurs only below 0.5 K [70,71]. For the flux-grown samples, it is important to be sure that measured results are not affected by Al inclusions [47].…”