We study the Fermi surface topology and quasiparticle properties in the heavy fermion compound YbNi4P2 at high magnetic fields using quantum oscillation measurements. We observe a large decrease of the quasiparticle mass with increasing field and demonstrate good qualitative agreement with the single-ion Kondo model. At the putative Lifshitz transition at 17 T, we observe a sudden change of quantum oscillation frequencies suggesting an abrupt change of the electronic structure and/or quasiparticle characteristics. Our results demonstrate the ability to tune the electronic structure and provide input for theoretical models of YbNi4P2 and correlated electron systems in high magnetic fields.

In the heavy-fermion metal YbRh2Si2, 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 towards 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 towards the quantum-critical point. We analyze and discuss the results in the context of present theories on ESR in strongly correlated electron systems.

We present the thermopower of EuCd2P2, 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 4 000 muV/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 EuCd2P2. These results open a new route to giant thermopower values via gradients of electronic properties.

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.

We report on the electron spin resonance (ESR) of Eu 2+ in Eu T 2 P 2 ( T =Mn, Zn, Cd) single crystals. The temperature dependencies of ESR linewidth and resonance shift show a similar behaviour when approaching the Eu-ordered state – a divergence towards T N , indicating the growing importance of magnetic correlations and the buildup of internal magnetic fields. For T=Mn an additional temperature scale of ≈ 47 K has considerable impact on linewidth, resonance field and intensity. This points to the presence of Mn magnetic correlations which yet were not reported.

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.

Y-kapellasite [Y_{3}Cu_{9}(OH)_{19}Cl_{8}], which hosts an original anisotropic kagome sublattice, is a promising candidate for studying elusive and complex correlated physics. It exhibits a theoretically predicted in-plane (1/3,1/3) magnetic order [Hering et al., npj Comput. Mater. 8, 1 (2022)2057-396010.1038/s41524-021-00689-0], but its magnetic interaction values place it close to a phase boundary to a spin liquid state [Chatterjee et al., Phys. Rev. B 107, 125156 (2023)PRBMDO2469-995010.1103/PhysRevB.107.125156]. Our μSR measurements under hydrostatic pressure demonstrate the complete suppression of static magnetism in favor of a fully dynamical ground state at 2.3 GPa. Complementary high-pressure x-ray and optical phonon measurements reveal a gradual reduction of the kagome anisotropy, enhancing magnetic frustration without structural transitions. Our results establish Y-kapellasite as a rare clean kagome model in which long-range order is suppressed by pressure-tuned frustration, the first fingerprint for the realization of a quantum spin liquid without strong disorder.

We report on the single crystal growth of Eu(Pd1-xAux)(2)Si-2, 0 < x <= 0.2, from a levitating Eu-rich melt using the Czochralski method. Our structural analysis of the samples confirms the ThCr2Si2-type structure as well as an increase of the room temperature a and c lattice parameters with increasing x. Chemical analysis reveals that, depending on the Au concentration, only about 25-35% of the amount of Au available in the initial melt is incorporated into the crystal structure, resulting in a decreasing substitution level for increasing x. Au substitution leads both to negative chemical pressure and the addition of electrons, which give rise to large changes in the valence crossover temperatures, as it was already observed for low substitution levels x. In contrast to previous studies, we do not find any signs of a first-order transition in samples with x(nom) = 0.1 or AFM order for higher x. Furthermore, we observe the formation of quaternary side phases for a higher amount of Au in the melt. In addition, cubic-mm-sized single crystals of EuPd2(Si1-xGex)(2) with x(nom) = 0.2 were grown. The analysis of the X-ray fluorescence revealed that the crystals exhibit a slight variation in the Ge content. Such tiny compositional changes can cause changes in the sample properties concerning variations of the crossover temperature or changes in the type of transition from crossover to magnetic order. Furthermore, we report on a new orthorhombic phase EuPd1.42Si1.27Ge0.31 that orders antiferromagnetically below 17 K.

Developing a framework that provides access to the momentum-resolved fine electronic structure of strongly correlated f-electron materials is central to understanding the microscopic mechanisms governing their low-temperature properties. In Ce- and Yb-based heavy-fermion compounds, subtle features arising from crystal-electric-field (CEF)-split 4f states and their hybridization with itinerant bands govern low-energy phenomena, but a detailed, momentum-resolved mapping of these spectral features has so far remained elusive. Here, we present a computational approach that accurately describes the momentum-dependent hybridization of CEF-split 4f states with itinerant electrons, exemplified for two canonical heavy-fermion materials, YbRh2Si2 and YbIr2Si2. The method is based on prior knowledge of the CEF parameters, the energies of the 4f states, and the valency, which must be determined experimentally. Our approach reproduces both surface- and bulk-related 4f hybrid bands, with the computed f-derived Fermi surface in excellent agreement with momentum-resolved photoemission data. It provides access to subtle f − spd hybrid features in regions of the Brillouin zone that are challenging to access experimentally. While demonstrated for Yb-based systems, this framework can be extended to other lanthanide and actinide compounds, providing a general tool to guide and complement experiments and reveal the complex f-electron spectral structure that governs f-driven phenomena in strongly correlated materials. Accessing the momentum-resolved electronic structure of strongly correlated f-electron materials is crucial for understanding their low-temperature properties. Here, the authors introduce a computational framework that accurately maps the hybridization of crystal-field-split 4f states with itinerant electrons, aligning well with experimental data and offering insights into complex f-electron interactions across various compounds.

Fractional magnetization plateaus provide a sensitive probe of many-body spin states in frustrated quantum magnets, yet their microscopic origin in kagome antiferromagnets remains unresolved. This is particularly true of the mysterious 1/9 plateau, which is predicted by theory but infrequently observed in experiment. Here, we investigate this problem in the S = 1/2 anisotropic kagome antiferromagnet Y-kapellasite, Y_3Cu_9(OH)_19Cl_8, using pulsed-field magnetization measurements on single crystals and high-field ^35Cl NMR. We identify a hierarchy of field-induced fractional features, including 1/3 and 1/9 plateaus, as well as a weaker low-field feature. Analysis of the NMR spectra and the magnetic susceptibility across the 1/9 plateau demonstrate that it is accompanied by an ordered local spin configuration, a strong suppression of low-energy spin fluctuations and activated behavior, consistent with a gapped fractional state. These features differ from those in the only other material YCu_3(OH)_6Br_2[Br_1-y(OH)_y] in which this plateau is observed, implying a surprising robustness of the 1/9 state to the details of the underlying magnetism.

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 YbRh2Si2 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 YbRh2Si2 and point to one of the superconducting states being the topological helical phase.

Cerium diantimonide (CeSb2) is a layered heavy-fermion Kondo lattice material that hosts complex magnetism and pressure-induced superconductivity. The interpretation of its in-plane anisotropy has remained unsettled due to structural twinning, which superimposes orthogonal magnetic responses. Here we combine controlled crystal growth with magnetization and rotational magnetometry to disentangle the effects of twinning. Nearly untwinned high-quality single crystals reveal the intrinsic in-plane anisotropy: The in-plane easy axis saturates at Measy(4 T) approximate to 1.8 & micro;B/Ce, while the in-plane hard axis magnetization is strongly suppressed, nearly linear, and comparable to the out-of-plane response. These results resolve long-standing discrepancies in reported magnetic measurements, in which in-plane metamagnetic transition fields and saturation magnetization varied significantly across previous studies. Growth experiments demonstrate that avoiding the proposed alpha 3 structural transition-through Sb-rich flux and slower cooling-systematically reduces twinning. However, powder x-ray diffraction and differential thermal analysis measurements show no clear evidence of a distinct 3 phase. Our results establish a consistent magnetic phase diagram and provide essential constraints for crystal-electric field models, enabling a clearer understanding of the interplay between anisotropic magnetism and unconventional superconductivity in CeSb2.

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 $Imma$ ) 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_\textrm{N1} = $ 61 K and a spin reorientation at $T_{\textrm{N}\textrm{2}} = $ 40 K. The orthorhombic material shows magnetic anisotropy, with anisotropy constants $K_1 = 7.1 \times 10^5\,\text{J m}^{-3}$ and $K_2 = 4.2 \times 10^5\,\text{J m}^{-3}$ , 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 $T_\textrm{N1}$ and $T_{\textrm{N}\textrm{2}}$ . Below $T_{\textrm{N}\textrm{2}}$ 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 $T_\textrm{N1}$ instead of ferromagnetism that sets in at higher $T_\textrm{C1} = 78\,\textrm{K}$ 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 between magnetism and charge transport is central to understanding colossal magnetoresistance (CMR), a phenomenon well studied in ferromagnets. Recently, antiferromagnetic (AFM) EuCd2P2 has attracted considerable interest due to its remarkable CMR, for which magnetic fluctuations and the formation of ferromagnetic clusters have been proposed as key mechanisms. Here we provide direct evidence that these effects originate from the formation and percolation of magnetic polarons. We employ a complementary set of sensitive probes that allows for a direct comparison of electronic and magnetic properties on multiple time scales revealing pronounced electronic and magnetic phase separation below T* ≈ 2TN. These measurements indicate an inhomogeneous, percolating electronic system below T* and well above the magnetic ordering temperature TN = 11 K. In applied magnetic fields, the onset of the pronounced negative MR in the paramagnetic regime emerges at a universal critical magnetization. The characteristic size of the magnetic polarons near the percolation threshold is estimated to be ~6−10 nm. Our results establish dynamic polaron percolation within an AFM matrix as the microscopic origin of CMR in EuCd2P2, providing a unified framework for magnetotransport in Eu-based correlated semiconductors.

Pure as well as Ag- and Au-substituted YbInCu$_4$ single crystals were structurally and chemically characterized and investigated by means of heat capacity, magnetization, resistivity and ultrasonic measurements. We studied the influence of different compositions of the initial melt as well as of Au and Ag substitutions on the valence change and investigated whether this change occurs via a first-order phase transition or via crossover. We constructed a phase diagram of YbInCu$_4$ as a function of various substitutions and show that the position of the critical endpoint of the valence transition depends on the substituent and on the conditions under which the samples were grown. Multiple thermal cycles through the first-order transition lead to a significant modification of the physical properties which clearly demonstrated the influence of defects in substituted YbInCu$_4$.

We report on the successful single crystal growth of pure EuRh${_2}$Si${_2}$ and of Eu(Rh$_{1-x}$Co$_{x}$)$_2$Si$_2$ with $x\leq0.23$ by the flux method. Through Co substitution, EuRh$_2$Si$_2$ can be tuned from stable antiferromagnetism via a valence-transition state towards the valence-crossover regime. From magnetization measurements, we constructed a $B - T$ phase diagram for EuRh${_2}$Si${_2}$ comprising multiple magnetic phases and showing a sizable magnetic anisotropy within the basal plane of the tetragonal unit cell. This indicates a complex antiferromagnetic ground state for $x=0$. By applying positive chemical pressure through the substitution series Eu(Rh$_{1-x}$Co$_{x}$)$_2$Si$_2$, a sharp temperature-induced first-order phase transition is observed in magnetization, resistivity and heat capacity for 0.081 $\leq$ $x$ $\leq$ 0.119. The critical end point of this valence transition is located in the phase diagram in the vicinity of 0.119 $

Applying angle-resolved photoemission spectroscopy and density functional theory calculations, we present compelling spectroscopic evidence demonstrating the intertwining and mutual interaction between the Kondo and kagome sublattices in heavy-fermion intermetallic compound YbV_6Sn_6. We reveal the Yb 4f-derived states near the Fermi level, along with the presence of bulk kagome bands and topological surface states. We unveil strong interactions between the 4f and itinerant electrons, where the kagome bands hosting the Dirac fermions and van Hove singularities predominate. Such findings are well described using a c-f hybridization model. On the other hand, our systematic characterization of magnetic properties demonstrates an unusually enhanced antiferromagnetic ordering, where the kagome-derived van Hove singularities near E_F play a vital role in determining the unconventional nature of the Ruderman-Kittel-Kasuya-Yosida interaction and Kondo coupling. These unique kagome-state-mediated exchange interactions have never been reported before and could lead to a novel phase diagram and various quantum critical behaviors in YbV_6Sn_6 and its siblings. Our results not only expand the family of exotic quantum phases entangled with kagome structure to the strongly correlated regime, but also establish YbV_6Sn_6 as an unprecedented platform to explore unconventional many-body physics beyond the standard Kondo picture.

Topological superconductors are essential elements of the periodic table of topological quantum matter. However, the relevant odd-parity spin-triplet superconductors are rare. We report high-resolution measurements of the complex electrical impedance of YbRh$_2$Si$_2$ down to 0.4 mK, that reveal the presence of several superconducting states, suppressed differently by magnetic field, both Pauli-limited and beyond the Pauli limit. Superconductivity is abruptly switched off at the critical field of the primary antiferromagnetic order. The onset of electro-nuclear spin density wave order enhances the superconductivity, which we account for by the simultaneous formation of a spin-triplet pair density wave. Together these observations provide compelling evidence for odd-parity superconductivity, and its underpinning by antiferromagnetism, and allow us to identify the topological helical state.

Single crystals of EuPd_3Si_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 Imma) but revealed differences in the lattice parameters and bond distances in comparison to previous work by Sharma et al.. The composition is close to the ideal 1:3:2 stoichiometry with an occupation of 7

Similar to transitions in a range of correlated quantum materials, the valence transition exhibits a strong coupling to the crystal lattice, rendering it highly sensitive to stress tuning. In the present work, we determine the effect of uniaxial stress, which breaks the lattice symmetry, on the valence transition temperature and its crossover temperature in pure and Ag-substituted YbInCu 4 . Our key result is that hydrostatic stress is more effective in tuning this transition than uniaxial stress. Based on a symmetry decomposition of the stress-induced strains, we argue that this observation can be quantitatively understood, given that the valence transition is mostly sensitive to symmetric strains and thus volume changes of the lattice. These results support the notion that the valence transition can give rise to critical elasticity close to its critical endpoint.