At our institute, in collaboration with the groups of Seunghyun Khim and Manuel Brando, we found a new heavy fermion superconductor having two phases in the magnetic field-temperature phase diagram. Using thermodynamic probes, we establish the highly anisotropic critical field phase diagram. The superconducting state appearing at low fields is characterised by an enhanced and anisotropic Pauli limit. Furthermore, a c-axis field drives a transition to a second superconducting state that seems to be purely orbitally limited with a critical field as high as 14 T, remarkable in a material whose transition temperature is 0.26 K. In spite of the fact that CeRh₂As₂ is globally centrosymmetric, local inversion-symmetry breaking at the Ce sites would naturally enable Rashba spin-orbit coupling to dominate the critical fields and drive a transition from even to odd-parity superconductivity. [more]

Metallic delafossites are materials which show intriguing properties revealed over the past years. This includes ultra-high conductivity, unconventional magnetism, and the potential to host strongly spin-orbit coupled states at their surfaces and interfaces. Structurally, they are layered materials with triangular in-plane lattices of transition metal atoms. This results in a quasi two-dimensional Fermi surface and very anisotropic transport properties. [more]

Weyl semimetals are materials where valence and conduction bands cross in single points, the Weyl nodes. When the Fermi energy is near these nodes, the electrons effectively behave as relativistic Weyl fermions with a linear energy dispersion and a given chirality. We are searching for experimental signatures of Weyl fermions in bulk probes.   [more]

In the area of topological semimetals, our main objective is to find bulk experimental evidence for Weyl fermions such as the chiral anomaly in the longitudinal magnetoresistance. Measurements of the latter are highly non-trivial, when the anisotropy of the conductivity changes with magnetic field. We investigate this classical effect in the proposed Weyl semimetals and were able to show that the field concentrates the current to a narrow path through the sample in commonly used experimental conditions. After adapting the measurement conditions and ensuring homogeneous currents, we show that the longitudinal magnetoresistance can be understood in terms of orbital magnetoresistance and weak localisation in Weyl semimetals. We cannot confirm the presence of the chiral anomaly in these materials. [more]

The thermopower of a material measures the electrical field induced by a temperature gradient in that material. A special contribution is expected in materials with a temperature-dependent valence as we observed in EuIr₂Si₂ and EuNi₂P₂.
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