Universität Stuttgart

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Institute: search.filters.UBSInstitut.Max-Planck-Institut für FestkörperforschungStart date: 2024

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Now showing 1 - 10 of 21
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    Exploring the growth of refractory metal and sapphire films by thermal laser epitaxy
    (2024) Majer, Lena N.; Mannhart, Jochen (Prof. Dr.)
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    On the thermal dimorphy of the strontium perrhenate Sr[ReO4]2
    (2024) Conrad, Maurice; Bette, Sebastian; Dinnebier, Robert E.; Schleid, Thomas
    Hygroscopic single crystals of a new hexagonal high‐temperature modification of Sr[ReO4]2 were prepared from a melt of Sr[ReO4]2 ⋅ H2O and SrCl2 ⋅ 6 H2O. The structure analysis of the obtained crystals by X‐ray diffraction revealed that the title compound crystallizes in the ThCd[MoO4]3‐type structure with the hexagonal space group P63/m and the lattice parameters a=1023.81(7) pm and c=646.92(4) pm (c/a=0.632) for Z=2 in its quenchable high‐temperature form. Two crystallographically independent Sr2+ cations are coordinated by oxygen atoms forming either octahedra or tricapped trigonal prisms, whereas the Re7+ cations are found in the centers of discrete tetrahedral meta‐perrhenate units [ReO4]-. Temperature‐dependent in‐situ PXRD studies of dry powder samples of Sr[ReO4]2 exhibited its thermal dimorphy with a phase‐transition temperature at 500-550 °C from literature‐known m‐Sr[ReO4]2 into the newly discovered h‐Sr[ReO4]2 (hexagonal).
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    Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids
    (2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, Sabine
    Covalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.
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    An atomic‐scale vector network analyzer
    (2024) Baumann, Susanne; McMurtrie, Gregory; Hänze, Max; Betz, Nicolaj; Arnhold, Lukas; Malavolti, Luigi; Loth, Sebastian
    Electronic devices have been ever‐shrinking toward atomic dimensions and have reached operation frequencies in the GHz range, thereby outperforming most conventional test equipment, such as vector network analyzers (VNA). Here the capabilities of a VNA on the atomic scale in a scanning tunneling microscope are implemented. Nonlinearities present in the voltage‐current characteristic of atoms and nanostructures for phase‐resolved microwave spectroscopy with unprecedented spatial resolution at GHz frequencies are exploited. The amplitude and phase response up to 9.3 GHz is determined, which permits accurate de‐embedding of the transmission line and application of distortion‐corrected waveforms in the tunnel junction itself. This enables quantitative characterization of the complex‐valued admittance of individual magnetic iron atoms which show a lowpass response with a magnetic‐field‐tunable cutoff frequency.
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    Insights into the first multi-transition-metal containing Ruddlesden-Popper-type cathode for all-solid-state fluoride ion batteries
    (2024) Vanita, Vanita; Waidha, Aamir Iqbal; Vasala, Sami; Puphal, Pascal; Schoch, Roland; Glatzel, Pieter; Bauer, Matthias; Clemens, Oliver
    Promising cathode materials for fluoride-ion batteries (FIBs) are 3d transition metal containing oxides with Ruddlesden-Popper-type structure. So far, the multi-elemental compositions have not been investigated, but it could alternate the electrochemical performance similar to what has been found for cathode materials for lithium-ion batteries. In this study, we investigate RP type La2Ni0.75Co0.25O4.08 as an intercalation-based active cathode material for all-solid-state FIBs. We determine the structural changes of La2Ni0.75Co0.25O4.08 during fluoride intercalation/de-intercalation by ex situ X-ray diffraction, which showed that F- insertion leads to transformation of the parent phase to three different phases. Changes in the Ni and Co oxidation states and coordination environment were examined by X-ray absorption spectroscopy and magnetic measurements in order to understand the complex reaction behaviour of the phases in detail, showing that the two transition metals behave differently in the charging and discharging process. Under optimized operating conditions, a cycle life of 120 cycles at a critical cut-off capacity of 40 mA h g-1 against Pb/PbF2 was obtained, which is one of the highest observed for intercalation electrode materials in FIBs so far. The average coulombic efficiencies ranged from 85% to 90%. Thus, La2Ni0.75Co0.25O4.08 could be a promising candidate for cycling-stable high-energy cathode materials for all-solid-state FIBs.
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    Probing self-diffusion of guest molecules in a covalent organic framework : simulation and experiment
    (2024) Grunenberg, Lars; Keßler, Christopher; Teh, Tiong Wei; Schuldt, Robin; Heck, Fabian; Kästner, Johannes; Groß, Joachim; Hansen, Niels; Lotsch, Bettina V.
    Covalent organic frameworks (COFs) are a class of porous materials whose sorption properties have so far been studied primarily by physisorption. Quantifying the self-diffusion of guest molecules inside their nanometer-sized pores allows for a better understanding of confinement effects or transport limitations and is thus essential for various applications ranging from molecular separation to catalysis. Using a combination of pulsed field gradient nuclear magnetic resonance measurements and molecular dynamics simulations, we have studied the self-diffusion of acetonitrile and chloroform in the 1D pore channels of two imine-linked COFs (PI-3-COF) with different levels of crystallinity and porosity. The higher crystallinity and porosity sample exhibited anisotropic diffusion for MeCN parallel to the pore direction, with a diffusion coefficient of Dpar = 6.1(3) × 10-10 m2 s-1 at 300 K, indicating 1D transport and a 7.4-fold reduction in self-diffusion compared to the bulk liquid. This finding aligns with molecular dynamics simulations predicting 5.4-fold reduction, assuming an offset-stacked COF layer arrangement. In the low-porosity sample, more frequent diffusion barriers result in isotropic, yet significantly reduced diffusivities (DB = 1.4(1) × 10-11 m2 s-1). Diffusion coefficients for chloroform at 300 K in the pores of the high- (Dpar = 1.1(2) × 10-10 m2 s-1) and low-porosity (DB = 4.5(1) × 10-12 m2 s-1) samples reproduce these trends. Our multimodal study thus highlights the significant influence of real structure effects such as stacking faults and grain boundaries on the long-range diffusivity of molecular guest species while suggesting efficient intracrystalline transport at short diffusion times.
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    Resonant inelastic X-ray scattering study of Kitaev spin liquid candidates
    (2024) Yang, Zichen; Keimer, Bernhard (Prof. Dr.)
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    Novel stochastic methods in electronic structure theory and their application
    (2024) Weser, Oskar; Alavi, Ali (Prof. Dr.)
    This cumulative thesis is based on four publications that describe newly developed methods within full CI quantum Monte-Carlo (FCIQMC) and their application to challenging systems of biochemical interest. FCIQMC is a well parallelised, stochastic electronic structure method that enables the correlation of many electrons by taking advantage of the sparsity of the wave function expressed in a finite basis. The new methods are about: a.) an improved excitation generator, that benefits the performance of FCIQMC, in particular for localised orbital bases, b.) fine-grained control over the structure of the wave function by developing a stochastic version of the generalized active space (GAS) method, including its mean-field optimised variant GASSCF, and c.) targeting states of desired spin in a basis of Slater determinants (SDs). The improved excitation generator is an extension of the precomputed heat bath (PCHB) strategy with more effective sampling of double excitations and a novel approach for non-uniform sampling of single excitations. The non-uniform sampling of single excitations relies on spatially decaying integrals and matrix elements. An overall efficiency gain by a factor of two to four, as measured by variance reduction per wall-clock time, is shown. In the GAS method the active space is partitioned into multiple disjoint subspaces. A full configuration interaction (CI) expansion is generated for each subspace, while the interspace excitations are restricted using chemically motivated constraints on the occupation numbers per subspace. Within FCIQMC these constraints are efficiently encoded in precomputed probability distributions which removes nearly all runtime overheads of GAS. Stochastic GAS reduced density matrices (RDMs) are stochastically sampled, allowing orbital relaxations via stochastic GASSCF, and direct evaluation of properties that can be extracted from density matrices, such as the spin expectation value. Restricted active space (RAS) or other truncated wave function schemes are special cases of the GAS strategy, thus they are promptly available by an appropriate choice of the GAS subspaces and corresponding constraints. The efficient implementation of the stochastic GAS method, using hybrid parallelisation, allowed e.g. uncontracted stochastic MRCISD calculations on the quintet - triplet spin gap in a Fe-porphyrin model complex, with up to 96 electrons and 159 orbitals and a large CAS(32, 34) active space reference wave function, greatly improving previous estimates. The spin-purification method allows targeting states of desired spin in SDs. This is achieved by using a modified Hamiltonian H' = H + J S², with a suitable J > 0 that artificially enforces anti-ferromagnetic order. While a basis of configuration state functions (CSFs) can target spin states by construction, there are conceptual and practical advantages of using a SD basis while ensuring the correct spin. It can be directly coupled with other rich theory and codes that are (not yet) available in a CSF basis; an incomplete list includes: transcorrelated Hamiltonians, tailored coupled cluster, or stochastic perturbation theory. In addition, while convergence with respect to walker number is usually faster for CSFs, the SD basis is numerically cheaper and allows more walkers for the same computational effort. A particular notable application of the new method is a trinuclear [Mn3O4] metal complex, serving as a biomimetic for the active centre of the oxygen evolving complex (OEC), with a non-monotonic spin ladder whose particular electronic features could be reproduced within a CAS(55, 38) model active space.
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    From topological nodal planes and multifold crossings in 3D to strong correlations in 2D
    (2024) Alpin, Kirill; Metzner, Walter (Prof. Dr.)
    Topological semimetals are hosts of numerous exotic phenomena, like Fermi arc surface states, the anomalous Hall effect, the quantized circular photogalvanic effect, the chiral anomaly, the anomalous Nernst effect and the anomalous thermal Hall effect. These phenomena are directly dependent on the presence and the properties of topological band crossings in the band structures of these compounds. Therefore, this thesis deals with classifying topological crossings in semimetals, deriving constraints on their topological charges, studying the properties and finding material realizations of more exotic band degeneracies, like nodal planes (NPs) and multifold degeneracies. The main results of this work includes an algorithm for fully classifying topological band structures of materials by DFT. This algorithm is used throughout this thesis for many different semimetals. The most detailed topological classification is presented for the B20 compound CoSi, shown to host topologically enforced NPs when including spin-orbit coupling (SOC). On the other hand, in ferromagnetic MnSi NPs are enforced by the space-group symmetries to be topological regardless if SOC is present or not, which we confirmed by a direct computation of symmetry eigenvalues. Tuning the magnetic moment, it is further possible to turn the presence of NPs on and off, leading to divergent Berry curvatures. By performing a complete topological classification of all multifold crossings in all space groups using an algorithm able to derive analytical low-energy Hamiltonians automatically, a 4-fold crossing was found to host an unusually high topological charge of +/-5. A material candidate is presented to feature this new topological phase at the Gamma point of BaAsPt. In two hexagonal space groups, SG 182 and SG 173, the concept of representation-enforced topological NPs have been introduced and material candidates were presented and classified using the above mentioned algorithms. One of these materials, NaCu5S3, exhibits a strikingly simple material band structure and surface DOS, resembling the ones produced by a tight-binding toy model. In chapter 7, we switch to physics of strongly correlated matter and examine the repulsive Hubbard model in 2D at large U. A unitary transformation is presented to map the interaction part of the Hubbard model to a single particle term. Applying this transformation to the whole Hubbard model results in a Hamiltonian of unconstrained fermions, which can then be mapped to a system of fermions interacting with spins. Integrating out the fermions using variational perturbation theory and replacing spins with classical spins reveals a phase transition at non-zero chemical potential to a phase with a finite d-wave order parameter.