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We have used ion irradiation to damage the (001) surfaces of SmB6 single crystals to varying depths, and have measured the resistivity as a function of temperature for each depth of damage. We observe a reduction in the residual resistivity with increasing depth of damage. Our data are consistent with a model in which the surface state is not destroyed by the ion irradiation, but instead the damaged layer is poorly conducting and the initial surface state is reconstructed below the damage. This behavior is consistent with a surface state that is topologically protected.

We have investigated the low-temperature conducting state of two Kondo insulators, SmB6 and Ce3Bi4Pt3, which have been theoretically predicted to host topological surface states. Through comparison of the specific heat of as-grown and powdered single crystals of SmB6, we show that the residual term that is linear in temperature is not dominated by any surface state contribution, but rather is a bulk property. In Ce3Bi4Pt3, we find that the Hall coefficient is independent of sample thickness, which indicates that conduction at low temperatures is dominated by the bulk of the sample, and not by a surface state. The low-temperature resistivity of Ce3Bi4Pt3 is found to monotonically decrease with low concentrations of disorder introduced through ion irradiation. This is in contrast to SmB6, which is again indicative of the contrasting origins of the low-temperature conduction. In SmB6, we also show that the effect of low concentrations of irradiation damage of the surface with Fe+ ions is qualitatively consistent with damage with nonmagnetic ions. [ABSTRACT FROM AUTHOR]

We report the synthesis, crystal structure, and characterization by means of single-crystal x-ray diffraction, neutron powder diffraction, and magnetic, thermal, and transport measurements of the new heavy-fermion compounds Ce2MAl7Ge4 (M = Co, Ir, Ni, Pd). These compounds crystallize in a noncentrosymmetric tetragonal space group P421m, consisting of layers of square nets of Ce atoms separated by Ge-Al and M-Al-Ge blocks. Ce2CoAl7Ge4, Ce2IrAl7Ge4, and Ce2NiAl7Ge4 order magnetically below TM = 1.8, 1.6, and 0.8 K, respectively. There is no evidence of magnetic ordering in Ce2PdAl7Ge4 down to 0.4 K. The small amount of entropy released in the magnetic state of Ce2MAl7Ge4 (M = Co, Ir, Ni) and the reduced specific heat jump at TM suggest a strong Kondo interaction in these materials. Ce2PdAl7Ge4 shows non-Fermi liquid behavior, possibly due to the presence of a nearby quantum critical point. [ABSTRACT FROM AUTHOR]

Motivated by the search for design principles of rare-earth-free strong magnets, we present a study of electronic structure and magnetic properties of the ferromagnetic metal Fe3GeTe2 within the local-density approximation (LDA) of the density-functional theory, and its combination with dynamical mean-field theory (DMFT). To compare these calculations, we measure magnetic and thermodynamic properties as well as x-ray magnetic circular dichroism and the photoemission spectrum of single-crystal Fe3 GeTe2. We find that the experimentally determined Sommerfeld coefficient is enhanced by an order of magnitude with respect to the LDA value. This enhancement can be partially explained by LDA+DMFT. In addition, the inclusion of dynamical electronic correlation effects provides the experimentally observed magnetic moments, and the spectral density is in better agreement with photoemission data. These results establish the importance of electronic correlations in this ferromagnet. [ABSTRACT FROM AUTHOR]