Cornelius Krellner

Spin and charge fluctuations are key for understanding the properties of materials with strongly correlated electrons, but their coupling to the atomic crystal lattice is often assumed to be weak and perturbative. In this work, we demonstrate that at tricritical points (TCPs), that are associated with electronic correlations, a coupling to crystal elasticity is generically nonperturbative. It will modify the tricritical universality class leading to elastic tricriticality , characterized by a compressibility that diverges in a universal manner with the correlation length. To test this scenario experimentally, we use single crystals of EuPd ₂ (Si _{1− x} Ge _x ) ₂ where, depending on x and pressure, magnetic order collapses due to strong valence fluctuations. First, we confirm the presence of a TCP for x ≈ 0.1 by using complementary probes, in particular, measurements of relative length changes under control of temperature and He-gas pressure. Second, by studying the elastic response, we uncover clear signatures of elastic tricriticality, notably a drastically enhanced compressibility.

In the heavy-fermion metal YbRh 2 Si 2 , quantum criticality at a suppressed antiferromagnetic order is governed by the interplay of local magnetic moments and itinerant conduction electrons. We demonstrate how this can be investigated by a new experimental approach that enables the observation of electron spin resonance (ESR) across a broad range of frequencies and fields at very low temperatures. This allowed us to cover a large part of the phase diagram from the paramagnetic Fermi-liquid phase to the phase with antiferromagnetic order and including the quantum-critical regime. Both the ESR g factor and the linewidth present distinct behaviors in these three regimes, providing further insight into the physics across a quantum critical point. Notably, when cooling down at a field directly toward the quantum critical point, both g factor and linewidth continuously decrease. Furthermore, we observe a very good matching of the g -factor behavior upon field tuning and temperature tuning toward the quantum-critical point. We analyze and discuss the results in the context of present theories on ESR in strongly correlated electron systems.

Across regular phase transitions, systems remain in thermal equilibrium. However, when a system is driven far from equilibrium, non-Hermitian phase transitions may arise where the dynamical behavior—rather than steady properties—undergoes a qualitative change at a critical, so-called exceptional point. We experimentally realize a non-Hermitian phase transition in a bulk condensed-matter system. Optical excitation creates charge carriers in ferromagnetic europium monoxide. In a temperature-dependent interplay with the Hermitian transition to ferromagnetic order, a non-Hermitian change of the relaxation dynamics occurs, manifesting in our time-resolved reflection data as the transition from biexponential real to single-exponential complex decay. Our theory models this behavior and suggests that non-Hermitian phase transitions may generically emerge in bulk condensed matter.

We present the thermopower of EuCd 2 P 2 , a material which exhibits a large resistivity peak with significant magnetic field dependence in the temperature range of 10–25 K. In the same region, we observe a highly unusual behavior of the thermopower with two sign changes and giant extrema. The overall variation of the thermopower exceeds 4000 μ V / K and takes place in an extremely narrow temperature region of less than 5 K. The anomaly is suppressed completely in a small magnetic field of 0.5 T. We discuss this observation using a simple drift-diffusion picture and taking into account that the temperature gradient inducing the thermopower voltage is accompanied by a gradient of the electrical resistivity. Our simple estimation yields the correct magnitude, shape, and field dependence of the thermopower anomaly observed in EuCd 2 P 2 . These results open a new route to giant thermopower values via gradients of electronic properties.

Single crystals of EuPd 3 Si 2 were grown using a high-temperature EuPd-flux method. The material was structurally and chemically characterized by single-crystal x-ray diffraction, powder x-ray diffraction, Laue method and energy-dispersive x-ray spectroscopy. The structural analysis confirmed the orthorhombic crystal structure (space group I m m a ) but revealed differences in the lattice parameters and bond distances. The composition is close to the ideal 1:3:2 stoichiometry with an occupation of 7% of the Si sites by Pd. The heat capacity, electrical resistivity, and magnetic susceptibility show two magnetic transitions indicating magnetic ordering below T N1 = 61 K and a spin reorientation at T N 2 = 40 K. The orthorhombic material shows magnetic anisotropy, with anisotropy constants K 1 = 7.1 × 10 5 J m − 3 and K 2 = 4.2 × 10 5 J m − 3 , for field applied along the three main symmetry axes, which is summarized in the temperature-field phase diagrams. The susceptibility data hint to an alignment of the magnetic moments along [ 100 ] between T N1 and T N 2 . Below T N 2 the magnetic structure changes to an arrangement with moments canted away from [ 100 ] . The single crystals investigated in this study are suggested to show antiferromagnetic order below T N1 instead of ferromagnetism that sets in at higher T C1 = 78 K which might originate from certain differences in the structure, composition or defects that have an impact on the dominant coupling constants of the Ruderman–Kittel–Kasuya–Yosida interaction.

The interplay of strongly localized 4f electrons with itinerant spd valence states gives rise to a wide range of correlated phenomena and properties that place lanthanide materials at the focus of considerable research efforts. Beyond the bulk, their surfaces are of particular interest, where the reduced coordination, a modified crystal electric field, broken inversion symmetry in combination with strong spin–orbit coupling, and the emergence of surface states and resonances considerably reshape 4f-driven electronic and magnetic properties. This, in turn, enables novel functionalities of particular relevance for low-dimensional systems and their applications. This review summarizes how advances in photoelectron spectroscopies, together with improved crystal growth, have enabled detailed insights into bulk and surface phenomena of lanthanide-based crystals. After a brief overview of key developments from the 1970s to the 1990s, we discuss recent progress, focusing on systematic studies by the authors and collaborators that form a coherent line of research. These include the unveiling of k-resolved f–spd hybridization, the formation and evolution with temperature of f-derived Fermi surface in Kondo lattices, layer-dependent 4f magnetic anisotropy, the emergence of ferromagnetically ordered surfaces in systems with non-magnetic bulk ground state. This coherent line of research addresses core questions in the physics of 4f systems and opens opportunities for engineering novel lanthanide-based architectures, including heterostructures and supramolecular complexes with novel physical properties and functionalities.

Topological superconductors, characterized by spin-triplet Cooper pairing, are important for exploring unconventional pairing mechanisms and protected quantum states. Yet experimentally established odd-parity, spin-triplet superconductors remain scarce. Here we demonstrate that the heavy-fermion compound YbRh ₂ Si ₂ hosts distinct magnetic-field-tuned superconducting states, both Pauli limited and beyond this limit, revealed by high-resolution measurements of the complex electrical impedance. We also find that superconductivity is abruptly suppressed at the critical field associated with the primary antiferromagnetic transition of this compound. The onset of electro-nuclear spin density wave order enhances the superconductivity. We propose that this behaviour can be explained by the formation of a pair density wave that boosts a selected spin-triplet superconducting order parameter. Together, our findings indicate odd-parity superconductivity in YbRh ₂ Si ₂ and point to one of the superconducting states being the topological helical phase.