The manifestation of Weyl fermions in strongly correlated electron systems is of particular interest. We report evidence for Weyl fermions in the heavy fermion semimetal YbPtBi from electronic structure calculations, angle-resolved photoemission spectroscopy, magnetotransport and calorimetric measurements. At elevated temperatures where 4f-electrons are localized, there are triply degenerate points, yielding Weyl nodes in applied magnetic fields. These are revealed by a contribution from the chiral anomaly in the magnetotransport, which at low temperatures becomes negligible due to the influence of electronic correlations. Instead, Weyl fermions are inferred from the topological Hall effect, which provides evidence for a Berry curvature, and a cubic temperature dependence of the specific heat, as expected from the linear dispersion near the Weyl nodes. The results suggest that YbPtBi is a Weyl heavy fermion semimetal, where the Kondo interaction renormalizes the bands hosting Weyl points. These findings open up an opportunity to explore the interplay between topology and strong electronic correlations.

Materials where the electronic bands have unusual topologies allow for the realisation of novel physics and have a wide range of potential applications. When two electronic bands with linear dispersions intersect at a point, the excitations could be described as Weyl fermions, which are massless particles with a particular chirality. Here we report evidence for the presence of Weyl fermions in the ferromagnetic state of the low-carrier density, strongly correlated Kondo lattice system CeSb, from electronic structure calculations and angle-dependent magnetoresistance measurements. When the applied magnetic field is parallel to the electric current, a pronounced negative magnetoresistance is observed within the ferromagnetic state, which is destroyed upon slightly rotating the field away. These results give evidence for CeSb belonging to a new class of Kondo lattice materials with Weyl fermions in the ferromagnetic state.

The topological nature of surface states in SmB6 is revealed using samples with magnetic substituents or magnetic tunneling tips.

We present a pressure study of the electrical resistivity, ac magnetic susceptibility and powder x-ray diffraction (XRD) of the recently discovered BiS2-based superconductor EuBiS2F. At ambient pressure, EuBiS2F shows an anomaly in the resistivity at around T0 ≈ 280 K and a superconducting transition at Tc ≈ 0.3 K. Upon applying hydrostatic pressure, there is little change in T0 but the amplitude of the resistive anomaly is suppressed, whereas there is a dramatic enhancement of Tc from 0.3 K to about 8.6 K at a critical pressure of pc ≈ 1.4 GPa. XRD measurements confirm that this enhancement of Tc coincides with a structural phase transition from a tetragonal phase (P 4/nmm) to a monoclinic phase (P 21/m), which is similar to that observed in isostructural LaO0.5F0.5BiS2. Our results suggest the presence of two different superconducting phases with distinct crystal structures in EuBiS2F, which may be a general property of this family of BiS2-based superconductors.

Here we report the evolution of bulk band structure and surface states in rare earth monobismuthides with partially filled f shell. Utilizing synchrotron-based photoemission spectroscopy, we determined the three-dimensional bulk band structure and identified the bulk band inversions near the X points, which, according to the topological theory, could give rise to nontrivial band topology with odd number of gapless topological surface states. Near the surface  point, no clear evidence for predicted gapless topological surface state is observed due to its strong hybridization with the bulk bands. Near the M point, the two surface states, due to projections from two inequivalent bulk band inversions, interact and give rise to two peculiar sets of gapped surface states. The bulk band inversions and corresponding surface states can be tuned substantially by varying rare earth elements, in good agreement with density-functional theory calculations assuming local f electrons. Our study therefore establishes rare earth mono-bismuthides as an interesting class of materials possessing tunable electronic properties and magnetism, providing a promising platform to search for novel properties in potentially correlated topological materials.