The strongly correlated system Ho11B12 with boron sublattice Jahn-Teller instability and nanoscale electronic phase separation (dynamic charge stripes) was studied in detail by inelastic neutron scattering (INS), magnetometry and heat capacity measur
ements at temperatures in the range 3-300 K. From the analysis of registered INS spectra, we determined parameters of the cubic crystal field at holmium sites, B4=- 0.333 meV and B6= -2.003 meV (in Stevens notations), with an unconventional large ratio B6/B4 pointing on the dominant role of conduction electrons in the formation of a crystal field potential. The molecular field in the antiferromagnetic state, Bloc = (1.75+- 0.1) T has been directly determined from the INS spectra together with short-range order effects detected in the paramagnetic state. A comparison of measured magnetization in diluted Lu0.99Ho0.01B12 and concentrated HoB12 single crystals showed a strong suppression of Ho magnetic moments by antiferromagnetic exchange interactions in holmium dodecaboride. To account explicitly for the short-range antiferromagnetic correlations, a self-consistent holmium dimer model was developed that allowed us to reproduce successfully field and temperature variations of the magnetization and heat capacity in the cage-glass phase of HoB12 in external magnetic fields.
SrTm$_2$O$_4$ has been investigated using heat capacity, magnetic susceptibility, magnetization in pulsed fields, and inelastic neutron scattering measurements. These results show that the system is highly anisotropic, has gapped low-energy dispersin
g magnetic excitations, and remains in a paramagnetic state down to 2K. Two theoretical crystal field models were used to describe the single-ion properties of SrTm$_2$O$_4$without any optimization procedures; a standard point-charge model and a Density Functional Theory (DFT) based model that uses Wannier functions. The DFT model was found to better describe the system at low energy by predicting a singlet ground state for one Tm site and a doublet for the second Tm site and anisotropy of second site Tm dominating the anisotropy of the system. Additionally, muon spin rotation/relaxation ($mu^+$psr) spectra reveal oscillations, typically a sign of long-range magnetic order. We attribute these observations to lattice distortion induced by muon implantation, causing renormalization of the gap size.
The research effort prompted by the prediction that SmB$_6$ could be the first topological Kondo insulator has produced a wealth of new results, though not all of these seem compatible. A major discrepancy exists between scanning tunneling microscopy
/ spectroscopy (STM/S) and angle-resolved photoemission spectroscopy (ARPES), because the two experimental methods suggest a very different number of terminations of the (100) surface with different properties. Here we tackle this issue in a combined STM/S and ARPES study. We find that two of the well-ordered topographies reported in earlier STM studies can be associated with the crystal terminations identified using photoemission. We further observe a reversal of the STM contrast with bias voltage for one of the topographies. We ascribe this result to a different energy dependence of Sm and B-derived states, and show that it can be used to obtain element specific images of SmB$_6$ and identify which topography belongs to which termination. We finally find STS results to support a modification of the low-energy electronic structure at the surface that has been proposed as the trivial origin of surface metallicity in this material.
Recent theoretical and experimental studies suggest that SmB$_6$ is the first topological Kondo insulator: A material in which the interaction between localized and itinerant electrons renders the bulk insulating at low temperature, while topological
surface states leave the surface metallic. While this would elegantly explain the materials puzzling conductivity, we find the experimentally observed candidates for both predicted topological surface states to be of trivial character instead: The surface state at $bar{Gamma}$ is very heavy and shallow with a mere $sim 2$ meV binding energy. It exhibits large Rashba splitting which excludes a topological nature. We further demonstrate that the other metallic surface state, located at $bar{X}$, is not an independent in-gap state as supposed previously, but part of a massive band with much higher binding energy (1.7 eV). We show that it remains metallic down to 1 K due to reduced hybridization with the energy-shifted surface 4$f$ level.
We investigate the phase diagram of TmB4, an Ising magnet on a frustrated Shastry-Sutherland lattice by neutron diffraction and magnetization experiments. At low temperature we find Neel order at low field, ferrimagnetic order at high field and an in
termediate phase with magnetization plateaus at fractional values M/Msat = 1/7, 1/8, 1/9 ... and spatial stripe structures. Using an effective S = 1/2 model and its equivalent two-dimensional (2D) fermion gas we suggest that the magnetic properties of TmB4 are related to the fractional quantum Hall effect of a 2D electron gas.
