The tetragonal compound UPt 2 Si 2 has been characterized as a moderately mass-enhanced system with an antiferromagnetic (AFM) ground state below T N = 32 K. Here, we present an extensive study of the behavior in high magnetic fields. We have performed pulsed field magnetization and static field resistivity measurements on single crystalline samples UPt 2 Si 2. Along the crystallographic a axis, at low temperatures, we find a metamagnetic-like transition in fields of the order 40 T, possibly indicating a first-order transition. Along the crystallographic c axis, in magnetic fields of B 24 T, we find distinct anomalies in both properties. From our analysis of the data we can distinguish new high-field phases above the AFM ground state. We discuss the emergence of these new phases in the context of Fermi surface effects and the possible occurrence of a Lifshitz or electronic topological transition, this in contrast to previous modelings of UPt 2 Si 2 based on crystal electric field effects.

We have measured the Hall effect, magnetotransport, and magnetostriction on the field-induced phases of single-crystalline UPt 2 Si 2 in magnetic fields up to 60 T at temperatures down to 50 mK, firmly establishing the phase diagram for magnetic fields B a and c axes. Moreover, for the B c axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium 5f electrons, we propose that these represent field-induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the proposed Lifshitz-type transition is of a discontinuous nature, as predicted for interacting electron systems.

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Recent studies have dealt with the electronic and magnetic ground state properties of the tetraboride material MnB 4 . So far, however, the ground state properties could not be established unambiguously. Therefore, here we present an experimental study on single-crystalline MnB 4 by means of resistivity and magnetization measurements. For this, we have developed a sample holder that allows four-point ac resistivity measurements on these very small (∼ 100 µm) samples. With our data we establish that the electronic ground state of MnB 4 is intrinsically that of a pseudo-gap system, in agreement with recent band structure calculations. Furthermore, we demonstrate that the material does neither show magnetic order nor a behavior arising from the vicinity to a magnetically ordered state, this way disproving previous claims.