Michael Merz

We investigate lattice dynamics in LaCoO 3 using inelastic neutron and x-ray scattering over T = 2 − 6 5 0 K , spanning the spin-state crossover at T 1 ≈ 1 0 0 K and the insulator-metal transition at T 2 ≈ 5 5 0 K . Comparison with quasiharmonic lattice-dynamical calculations helps reveal anomalous softening of a ≈ 1 0 − meV oxygen phonon, confined to the temperature interval T 1 ≤ T ≤ T 2 and localized in momentum space at q SSO = ( ½ , ½ , ½ ) c . This wave vector is characteristic of the spin-state-ordering pattern originally proposed by Goodenough [An interpretation of the magnetic properties of the perovskite-type mixed crystals La 1 − x Sr x CoO 3 − λ , ]. Our results provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co 3 + states characterized by q SSO = ( ½ , ½ , ½ ) c , demonstrating that phonons are sensitive probes of fluctuating spin-state correlations in LaCoO 3 .

High‐Ni layered oxides are among the most promising cathode materials for lithium‐ion batteries (LIBs) due to their high capacity. However, the reliance on Co, a scarce, expensive, and geographically constrained element, poses challenges to sustainability and large‐scale deployment. In this study, Ti is investigated as a cost‐effective and earth‐abundant substitute for Co in LiNi _1‐x Ti _x O ₂ (LNTO, x = 0.07 and 0.14), synthesized via a scalable co‐precipitation method. EXAFS analyses indicate that no obvious Jahn–Teller distortion is observed in LNTO‐1, while DFT calculations suggest that Ti substitution modulates the Ni─O electronic interaction and helps reduce the driving force for cooperative Jahn–Teller distortion within the interconnected NiO ₆ framework. In addition, it suppresses the detrimental H2–H3 phase transition and reduces internal mechanical stress during cycling. EXAFS and NEXAFS analyses further reveal that TiO ₆ octahedra act as compensatory structural units that accommodate lattice strain in both bulk and surface regions, thereby stabilizing the rhombohedral framework. As a result, the optimized LNTO‐1 ( x = 0.07) delivers a high initial discharge capacity of 197.5 mAh g ^{− 1} at 0.05 C and retains 80% capacity after 431 cycles at 0.5 C. This work provides strategic insights for the design of sustainable, high‐performance cobalt‐free Ni‐rich cathodes for next‐generation LIBs.

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 electronic ground states of transition-metal dichalcogenides are strongly shaped by reduced dimensionality, yet the properties of atomically thin layers remain difficult to probe due to their small size and environmental sensitivity. Here we demonstrate that controlled electrochemical intercalation of organic cations provides a robust bulk platform for accessing monolayer-like physics in NbSe ₂ . Intercalation of tetrapropylammonium and tetrabutylammonium expands the interlayer spacing by nearly a factor of two, electronically decoupling the NbSe ₂ layers while simultaneously introducing well-defined charge doping. Using a combination of Raman spectroscopy, scanning tunneling microscopy, X-ray diffraction, and photoemission, we uncover a pronounced enhancement of the charge-density-wave transition temperature to ~ 130 K together with a strong suppression of superconductivity, reproducing the phase diagram observed in exfoliated monolayers. The enhanced charge-density-wave order and reduced T _c arise from the combined effects of dimensionality reduction and electron injection, and are accompanied by distinct dip-hump anomalies in the tunneling spectra suggestive of collective mode excitations. Our results establish molecular intercalation as a powerful and scalable route for engineering competing orders in layered quantum materials.

Altermagnetism, a novel class of magnetic order, has recently attracted the scientific interest. Combining the favourable properties of ferromagnets, such as spin polarised conduction electrons, with the advantages of antiferromagnets, such as zero net magnetisation and high-frequency switching, it is a promising area of research. While the ground state properties of altermagnets have been studied, their dynamic properties, such as magnon dispersions, remain elusive. Understanding these properties includes the investigation of magnon excitations, which is essential for the development of spintronic and magnonic devices based on altermagnets. In this study, we measure the energy splitting of altermagnetic magnons with opposite chirality in the hexagonal altermagnet CrSb using a unique combination of angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. We selectively excite electronic and magnonic states with opposite spins and quantify their energy splitting. We find that the non-degeneracy of electronic and magnonic states exists away from high-symmetry directions in good agreement with recent ab initio calculations. Our experimental proof of chiral magnons demonstrates a new degree of freedom in magnetism that is both of fundamental importance and opens the door to the exploration of the non-reciprocal transport properties of these bosons in devices.

Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV ₃ Sb ₅ (A=K, Cs, Rb), raising fundamental questions about the mechanisms driving their intertwined phases. Here we combine high-resolution inelastic X-ray scattering with first-principles calculations to uncover the origin of CDW formation in CsV ₃ Sb ₅ . Guided by structure factor analysis, we identify a soft phonon mode along the reciprocal M - L direction, with the strongest effect at the L point, where the elastic scattering intensity also grows most rapidly upon cooling. First-principles calculations incorporating lattice anharmonicity and electron-phonon coupling reproduce these observations and establish a soft-mode instability at the L point as the driving mechanism of CDW formation. Despite the weakly first-order character of the transition, our results unambiguously demonstrate that the CDW in CsV ₃ Sb ₅ originates from a softened phonon, clarifying its microscopic origin and highlighting the central role of lattice dynamics in kagome metals.

We investigate the strain response of underdoped HgBa 2 CuO 4 + δ (Hg1201), by synchrotron X-ray diffraction and corresponding simulations of thermal diffuse scattering. The compression in the crystallographic a direction leads to relatively small expansion in the b and c directions, with Poisson ratios ν ba = 0.16 and ν ca = 0.11 , respectively. However, the Cu-O distance in the c direction exhibits a notable 0.9% increase at 1.1% a -axis compression. We further find strain-induced diffuse scattering that corresponds to a new type of two-dimensional charge correlation. Interestingly, this signal is insensitive to the onset of superconductivity and instead corresponds to a short-range, nearly commensurate modulation with a wave vector close to (0.5,0,0) and a correlation length of approximately 4 unit cells. It closely resembles the charge order theoretically predicted in the phase diagram of the spin-liquid model with resonating valence bonds on a square lattice.