Karl W. Krämer

Photoluminescent materials have attracted a great attention due to their potential in remote temperature sensing, where conventional thermometers are not suitable, for instance in biological and medical applications. One of the popular techniques involves ratiometric thermometry, where the luminescence intensity ratio (LIR) between two thermally coupled transitions of a lanthanide ion is used for temperature readouts. However, aberrations in the optical collection system modify the LIR from setup to setup, preventing to have a standard to be shared from lab to lab. In this work, the thermometric parameters for upconverting (UC) lanthanide‐doped NaYF ₄ phosphors across a range of sizes (micron‐sized powders and nanoparticles) are obtained. More importantly, their absolute photoluminescence quantum yields (PLQY) are determined at different temperatures in a novel setup using a modified integrating sphere with a temperature‐controlled sample holder. Despite integrating sphere set‐up allowing for correction of aberrations arising from collection optical system, emission self‐absorption has a major effect on LIR.

Three new Cu(II) coordination polymers with 4,4′-bipyridine (bpy) were synthesized by hydrothermal reactions and their structures determined by single crystal X-ray diffraction. [Cu(bpy)₃(H₂O)₂](bpy)(PF₆)₂(H₂O)₃ (1) is built from bpy-bridged chains, [Cu(bpy)₂(H₂O)₂](bpy)(PF₆)₂(H₂O)₆ (2) from layers, and in [Cu(bpy)₂(NO₃)](bpy)(PF₆)₂(H₃O)(H₂O) (3) the layers are further connected by nitrate to a cuboid lattice. The magnetic properties of 3 are compared to [Cu(bpy)₂(H₂O)₂](SiF₆) (4) and [Cu(pyz)(bpy)(H₂O)₂](PF₆)₂ (5), where pyz = pyrazine. 3–5 are weakly coupled two-dimensional S = 1/2 antiferromagnetic Heisenberg lattices with 0.86 K < J < 1.47 K.

Although the search for quantum many-body phenomena in magnetic materials has a strong focus on highly frustrated systems, even unfrustrated quantum magnets show a multitude of unconventional phenomena in their spin excitation spectra. YbBr 3 is an excellent realization of the S = 1 / 2 antiferromagnetic Heisenberg model on the honeycomb lattice, and we have performed detailed spectroscopic experiments with both unpolarized and polarized neutrons at all applied magnetic fields up to saturation. We observe extensive excitation continua, which cause strong renormalization and the decay of single magnons at higher fields, while coherent features include field-induced “shadows” of the single magnons and the spectacular emergence of a rotonlike excitation. To guide and interpret our experiments, we performed systematic calculations by the method of cylinder matrix-product states that provide quantitative agreement with the neutron scattering data and a qualitative benchmark for the spectral signatures of strong quantum fluctuations even in the absence of magnetic frustration.

Supersymmetry is an algebraic property of a quantum Hamiltonian that, by giving every boson a fermionic superpartner and vice versa, may underpin physics beyond the Standard Model. Fractional bosonic and fermionic quasiparticles are familiar in condensed matter, as in the spin and charge excitations of the t-J model describing electron dynamics in one-dimensional materials, but this type of symmetry is almost unknown. However, the triplet excitations of a quantum spin ladder in an applied magnetic field provide a supersymmetric analogue of the t-J chain. Here we perform neutron spectroscopy on the spin-ladder compounds (C₅D₁₂N)₂CuBr₄ and (C₅D₁₂N)₂CuCl₄ over a range of applied fields and temperatures, and apply matrix-product-state methods to the ladder and equivalent chain models. From the momentum-resolved dynamics of a single charge-like excitation in a bath of fractional spins, we find essential differences in thermal broadening between the supersymmetric and non-supersymmetric sectors. The persistence of a strict zone-centre pole at all temperatures constitutes an observable consequence of supersymmetry that marks the beginning of supersymmetric studies in experimental condensed matter.

An effective, simple and convenient approach based on the interaction of tridentate triol-containing ligands and small Co(ii) isobutyrates enables the preparation of Co₁₆-based nanoclusters with a previously unknown mixed-valence {CoIII2CoII14} core.

Currently only Eu²⁺-based scintillators have approached the light yield needed to improve the 2% energy resolution at 662 keV of LaBr₃:Ce³⁺,Sr²⁺. Their major limitation, however, is the significant self-absorption due to Eu²⁺. CsCu₂I₃ is an interesting new small band gap scintillator. It is nonhygroscopic and nontoxic, melts congruently, and has an extremely low afterglow, a density of 5.01 g/cm³, and an effective atomic number of 50.6. It shows self-trapped exciton emission at room temperature. The large Stokes shift of this emission ensures that this material is not sensitive to self-absorption, tackling one of the major problems of Eu²⁺-based scintillators. An avalanche photo diode, whose optimal detection efficiency matches the 570 nm mean emission wavelength of CsCu₂I₃, was used to measure pulse height spectra. From the latter, a light yield of 36 000 photons/MeV and energy resolution of 4.82% were obtained. The scintillation proportionality of CsCu₂I₃ was found to be on par with that of SrI₂:Eu²⁺. Based on temperature-dependent emission and decay measurements, it was demonstrated that CsCu₂I₃ emission is already about 50% quenched at room temperature. Using temperature-dependent pulse height measurements, it is shown that the light yield can be increased up to 60 000 photons/MeV by cooling to 200 K, experimentally demonstrating the scintillation potential of CsCu₂I₃. Below this temperature, the light yield starts to decrease, which can be linked to the unusually large increase in the band gap energy of CsCu₂I₃.

Cs₄EuX₆ scintillators suffer from strong self-absorption and weak concentration quenching. Both can be avoided by Sm²⁺ doping. It improves the energy resolution, shortens the decay time and shifts the emission wavelength into the near-infrared.