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Register a new userBulk transport paths through defects in floating zone and Al flux grown SmB$_6$
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We investigate the roles of disorder on low-temperature transport in SmB$_6$ crystals grown by both the Al flux and floating zone methods. We used the inverted resistance method with Corbino geometry to investigate whether low-temperature variations in the standard resistance plateau arises from a surface or a bulk channel in floating zone samples. The results show significant sample-dependent residual bulk conduction, in contrast to smaller amounts of residual bulk conduction previously observed in Al flux grown samples with Sm vacancies. We consider hopping in an activated impurity band as a possible source for the observed bulk conduction, but it is unlikely that the large residual bulk conduction seen in floating zone samples is solely due to Sm vacancies. We therefore propose that one-dimensional defects, or dislocations, contribute as well. Using chemical etching, we find evidence for dislocations in both flux and floating zone samples, with higher dislocation density in floating zone samples than in Al flux grown samples. In addition to the possibility of transport through one-dimensional dislocations, we also discuss our results in the context of recent theoretical models of SmB$_6$.
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Recent theoretical and experimental findings suggest that 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 other studies have ascribed this feature to impurity phases, sample vacancies, and surface reconstructions. Given the typical methods used to prepare SmB6 single crystals, the flux and floating zone procedures, these ascriptions should not be taken lightly. Here, we demonstrate how compositional variations and observable amounts of impurity phases in SmB6 crystals grown by floating zone and flux affect the physical properties. From neutron diffraction and X-ray computed tomography experiments, we observe that a 154Sm11B6 crystal prepared using aluminum flux contains co-crystallized, epitaxial aluminum. A large, nearly stoichiometric crystal of SmB6 was successfully grown using the float-zone technique; upon continuing the zone melting, samarium vacancies are introduced. These samarium vacancies drastically alter the resistance and plateauing magnitude of the low temperature resistance compared to stoichiometric SmB6. These results highlight that small presences of impurity phases and compositional variations must be considered when collecting and analyzing physical property data of SmB6. Finally, a more accurate samarium-154 coherent neutron scattering length value, 8.9(1) fm, is reported.
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 of SmB$_6$ that have been the subject of our quantum oscillation studies thus far. We present results of experiments including chemical composition analysis, magnetisation, thermal conductivity, electrical transport, and heat capacity on floating zone-grown single crystals of SmB$_6$, and a series of quantum oscillation experiments as a function of magnetic field, temperature, and magnetic field-orientation on single crystals of floating-zone grown SmB$_6$, LaB$_6$, and elemental Aluminium. Results of these experimental studies establish the intrinsic origin of quantum oscillations from the bulk of pristine floating zone-grown single crystals of SmB$_6$. The origin of the underlying bulk Fermi surface that bears close similarity with the unhybridised Fermi surface in metallic hexaborides despite the bulk insulating character of SmB$_6$ is thus at the heart of a theoretical mystery.
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 and may indicate an inhomogeneous Sm valence at the surface of SmB$_6$. In addition, atomically resolved topographies on a (110) surface are discussed.
We present results of Scanning Tunneling Microscopy and Spectroscopy (STS) measurements on the Kondo insulator SmB$_6$. The vast majority of surface areas investigated was reconstructed but, infrequently, also patches of varying size of non-reconstructed, Sm- or B-terminated surfaces were found. On the smallest patches, clear indications for the hybridization gap and inter-multiplet transitions were observed. On non-reconstructed surface areas large enough for coherent co-tunneling we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure.
Recent quantum oscillation experiments on SmB$_6$ pose a paradox, for while the angular dependence of the oscillation frequencies suggest a 3D bulk Fermi surface, SmB$_6$ remains robustly insulating to very high magnetic fields. Moreover, a sudden low temperature upturn in the amplitude of the oscillations raises the possibility of quantum criticality. Here we discuss recently proposed mechanisms for this effect, contrasting bulk and surface scenarios. We argue that topological surface states permit us to reconcile the various data with bulk transport and spectroscopy measurements, interpreting the low temperature upturn in the quantum oscillation amplitudes as a result of surface Kondo breakdown and the high frequency oscillations as large topologically protected orbits around the X point. We discuss various predictions that can be used to test this theory.
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