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Thermal Conductivity of SmB6 Under Uniaxial Strain


SmB6 is a mixed valent Kondo insulator with a conducting surface state and an insulating bulk at low temperatures, with this feature possibly being topological. Even though there is some idea of the nature of the surface state, the bulk of SmB6 has been generally ambiguous. This is due to several quantum oscillation and thermal conductivity studies having contradicting observations from each other. Two quantum oscillation studies of SmB6 have been noted to reach incompatible conclusions regarding the dimensionality of the Fermi surface. One of the studies have observed a 2D Fermi surface associated with the metallic surface while the other a 3D Fermi surface attributed with the insulating bulk. However, insulators do not have a Fermi surface, so if there was a 3D Fermi surface due to the insulating bulk, this would mean that the Fermi surface exists due to some charge neutral fermions. Thermal conductivity κ measurements allow probing of bulk states due to the measurement of the carried entropy by carriers regardless of their charge and spin. Three studies that measured the thermal conductivity of SmB6 have also not reached a unanimous conclusion, as one of the studies measured a non-negligible residual linear κ. The possible origin as to why these measurements do not agree with each other may be due to intrinsic differences of the sample. For example, the floating-zone grown SmB6 samples are known to have samarium deficiencies that may act as J = 5/2 magnetic impurities. Electrical transport measurement of SmB6 have demonstrated that the bulk and the surface changes dramatically with changes in pressure. Extending to this, our group have adopted a strain apparatus that uses tri-piezo technique in order to have controllable application of the mechanical strain for electrical measurements of SmB6, which it was discovered that surface conduction exists up to 200K with 0.7% tensile strain. We have developed a symmetrized design that utilizes the same uniaxial strain apparatus for our thermal conductivity measurements in order to simulate lattice distortion effects from samarium deficiencies present in floating-zone grown crystals, applying 0.27% uniaxial tensile strain to a flux-grown SmB6.

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