Universität Stuttgart
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Item Open Access Readout and control of an endofullerene electronic spin(2020) Pinto, Dinesh; Paone, Domenico; Kern, Bastian; Dierker, Tim; Wieczorek, René; Singha, Aparajita; Dasari, Durga; Finkler, Amit; Harneit, Wolfgang; Wrachtrup, Jörg; Kern, KlausAtomic spins for quantum technologies need to be individually addressed and positioned with nanoscale precision. C60 fullerene cages offer a robust packaging for atomic spins, while allowing in-situ physical positioning at the nanoscale. However, achieving single-spin level readout and control of endofullerenes has so far remained elusive. In this work, we demonstrate electron paramagnetic resonance on an encapsulated nitrogen spin (14N@C60) within a C60 matrix using a single near-surface nitrogen vacancy (NV) center in diamond at 4.7 K. Exploiting the strong magnetic dipolar interaction between the NV and endofullerene electronic spins, we demonstrate radio-frequency pulse controlled Rabi oscillations and measure spin-echos on an encapsulated spin. Modeling the results using second-order perturbation theory reveals an enhanced hyperfine interaction and zero-field splitting, possibly caused by surface adsorption on diamond. These results demonstrate the first step towards controlling single endofullerenes, and possibly building large-scale endofullerene quantum machines, which can be scaled using standard positioning or self-assembly methods.Item Open Access Binder-free V2O5 cathode for high energy density rechargeable aluminum-ion batteries(2020) Diem, Achim M.; Fenk, Bernhard; Bill, Joachim; Burghard, ZaklinaNowadays, research on electrochemical storage systems moves into the direction of post-lithium-ion batteries, such as aluminum-ion batteries, and the exploration of suitable materials for such batteries. Vanadium pentoxide (V2O5) is one of the most promising host materials for the intercalation of multivalent ions. Here, we report on the fabrication of a binder-free and self-supporting V2O5 micrometer-thick paper-like electrode material and its use as the cathode for rechargeable aluminum-ion batteries. The electrical conductivity of the cathode was significantly improved by a novel in-situ and self-limiting copper migration approach into the V2O5 structure. This process takes advantage of the dissolution of Cu by the ionic liquid-based electrolyte, as well as the presence of two different accommodation sites in the nanostructured V2O5 available for aluminum-ions and the migrated Cu. Furthermore, the advanced nanostructured cathode delivered a specific discharge capacity of up to ~170 mAh g-1 and the reversible intercalation of Al3+ for more than 500 cycles with a high Coulomb efficiency reaching nearly 100%. The binder-free concept results in an energy density of 74 Wh kg-1, which shows improved energy density in comparison to the so far published V2O5-based cathodes. Our results provide valuable insights for the future design and development of novel binder-free and self-supporting electrodes for rechargeable multivalent metal-ion batteries associating a high energy density, cycling stability, safety and low cost.Item Open Access Thin film growth and structural investigation of DyBa2Cu3O7-δ(2020) Putzky, Daniel; Keimer, Bernhard (Prof. Dr.)Item Open Access Automated parametric Rietveld refinement and its application to two dimensional X-ray powder diffraction experiments(2011) Rajiv, Paneerselvam; Joswig, Manfred (Prof. Dr.)Parametric Rietveld refinement has opened new possibilities to simultaneously refine multiple powder diffraction patterns collected in in situ 2D experiments; in that way the models of crystallographic variables that changes with external variables can be introduced into the refinement. The substitution of a variable with its model during the refinement has several advantages, including the improved precision of variables, direct extraction/refinement of some parameters from powder data which is otherwise impractical (e.g., activation energy), etc. The basic requirement for the realization of sequential/parametric refinements (or Whole Powder Pattern Fit-WPPF) in 2D X-ray powder diffraction (XRPD) is a robust software that handles the data and performs fast WPPF. This concern has been primarily addressed in this thesis with the help of a software, in combination with the existing total pattern analysis software (Topas). The developed software could considerably speedup and automate the sequential/parametric quantitative analysis of large number of 2D powder data, which is in general a monotonous and time consuming task. The software also provides routines that automatically determines the reconstructive phase transitions of samples from the 2D powder data and facilitates the independent refinements (or WPPFs) of the determined phases. Two practical scientific applications of parametric Rietveld refinement method have been demonstrated with the assistance of the developed program. The first application concerns the kinetic analysis of several polymorphs and polymorphs-additives mixtures of copper phthalocyanine (CuPC). The reaction rate constant and the order of reactions involving the phase transitions of various forms of CuPC were directly extracted from the isothermal experimental data by introducing the Johnson-Mehl-Avrami-Kolmogorov relation as a model of the phase fraction during the multi phase parametric Rietveld refinement. Parametric refinements could be successfully performed for most of the CuPC data collected in the experiment, however the convergence of some of the refinements showed a strong dependence on the reaction rate. In many cases, the precision of the refined parameters could be improved considerably when the data collected between the optimal time steps alone were used in the refinement. The second application demonstrates the feasibility of the parameterization of crystallite size with respect to the annealing time/temperature. Some of the data samples used in the kinetic analysis (CuPC) and the temperature dependent nanocrystalline TiO2 data were used in this demonstration. The success of the parameterization of crystallite size depended strongly on the quality of the data used, on the uniformity of the variation of the crystallite size with time/temperature and also on the correctness of the model that describes the crystallite size variation with time/temperature. This application in its present form is general; as such it can be used for stabilizing other variables during parametric refinement.Item Open Access First-principles thermodynamic study of oxygen vacancies in ABO3-type perovskites(2017) Arrigoni, Marco; Maier, Joachim (Prof. Dr.)Item Open Access Excitonic Fano resonances in Ta2NiSe5 and Ta2NiS5(2016) Larkin, Timofei I.; Keimer, Bernhard (Prof. Dr.)Item Open Access Spin-orbital entanglement and molecular orbital formation in 4d and 5d transition metal oxides(2020) Krajewska, Aleksandra; Takagi, Hidenori (Prof. Dr.)Item Open Access Raman scattering, magnetization and magnetotransport study of SrFeO3-delta, Sr3Fe2O7-delta and CaFeO3(2008) Damljanovic, Vladimir; Keimer, Bernhard (Prof. Dr.)In this thesis we have determined the Raman spectra as well as the magnetization, resistance and magnetoresistance of the compounds SrFeO3-delta, Sr3Fe2O7-delta and CaFeO3 as a function of temperature. These materials are interesting because they contain iron in the unusually high oxidation state +4, which has the same electroncic configuration as the Mn3+ ion in LaMnO3, a material that shows the giant magnetoresistance effect when doped with calcium or strontium. A novel aspect of the work described in this thesis is that it was performed on single crystals with controlled oxygen stoichiometry. In the compound SrFeO3-delta, delta can vary continuously in the range 0 to 0.5. The materialexhibits the following crystal structures due to oxygen vacancy ordering: cubic (delta=0), tetragonal (delta=0.125), orthorhombic (delta=0.25) or brownmillerite (delta=0.5). For other values of delta the material is a mixture of those phases. The cubic phase has the ideal cubic perovskite structure. In this thesis we describe the preparation of nearly stoichiometric SrFeO3-delta with delta<0.05. The Raman spectrum of a sample annealed under 5kbar of pure oxygen showed no phonon modes, as expected from a group-theoretical analysis of the ideal perovskite structure. The Mößbauer spectra on this sample shows that it contains 5.4% of the tetragonal phase. In another crystal annealed at oxygen pressure 40kbar Mößbauer spectra did not show any sign of additional phases, confirming that the sample is fully stoichiometric. In addition to the experiments we have performed lattice dynamics calculations for the ideal composition SrFeO3.00 in order to assign the phonon modes observed in infra-red experiments. The calculation accurately reproduces all frequencies observed. We have also measured the Raman spectra of the tetragonal phase in the temperature range 13K to 300K. While only three peaks can be resolved at room temperature, additional modes appear in the spectrum below the charge-ordering transition at 70K. This confirms that the crystal structure changes below this temperature. We have also measured the Raman spectra of the orthorhombic phase in the temperature range 6K to 475K. The paremeter delta in Sr3Fe2O7-delta can vary continuously between 0 and 1. We have measured the temperature dependence of the magnetization for the magnetic field along high symmetry axes of the crystal. We have also performed neutron diffraction measurements demonstrating that the magnetic moments are ordered in a helical structure. The resistivity and the magnetoresistance were measured in the range 10K to 300K. Finally we have measured the Raman spectra of the same sample in the temperature range 15K to 440K. In order to assign the observed modes, we have performed lattice dynamics calculations based on the published crystal structure of Sr3Fe2O7. The CaFeO3 compound has an orthorhombic crystal structure above 290K, which changes to monoclinic below this temperature. Here we describe the preparation of stoichiometric CaFeO3 single crystals by high pressure oxygenation of as-grown CaFeO2.5 samples, using KClO4 as an oxygen source. The powder X-ray diffraction pattern after annealing shows that the oxygen enrichment was successful. No magnetoresistance was observed within the experimental error up to magnetic fields of 9T. We have also measured Raman spectra of this material in the temperature range 15K to 300K. In contrast to tetragonal SrFeO2.875 these spectra are unaffected by the charge-ordering transition at 290K within the experimental sensitivity.Item Open Access Proton transport mechanisms of phosphoric acid and related phosphorus oxoacid systems : a first principles molecular dynamics study(2012) Vilciauskas, Linas; Maier, Joachim (Prof. Dr.)Fundamental understanding of proton transport in hydrogen bonded systems on the molecular level remains a key problem in many areas of science ranging from electrochemical energy conversion to biological systems. Despite the enormous advances in the research of these processes, the ostensibly simplest case, proton transport in homogeneous bulk media at thermodynamic equilibrium, proved to be one of the most challenging and elusive. It is only through enormous theoretical and experimental efforts that clear mechanistic pictures of the transport of excess protonic charge defects in water have emerged. However, water has negligible intrinsic proton conductivity. By contrast, the class of compounds known as phosphorus oxoacids have some of the highest reported proton conductivities. In this work, the molecular level proton transport mechanisms in this family of proton conductors (H3PO4, H3PO3 and H3PO2) and some closely related systems (H3PO4-H2O mixtures) are investigated with the help of ab initio molecular dynamics simulations. In fact, neat liquid phosphoric acid has the highest intrinsic proton conductivity of any known substance. Apart from playing a central role in the structure and function of biological systems, systems containing phosphates/phosphonates are attracting an increasing interest as high-temperature electrolytes for emerging fuel cell applications. The results show that strong, mutually polarizable hydrogen bonds give rise to coupled proton motion and a pronounced protic dielectric response of the medium. This allows for the formation of extended, polarized hydrogen bonded (Grotthuss) chains, never truly observed in bulk hydrogen bonded systems. The results show that, in phosphoric acid such chains containing up to five consecutive hydrogen bonds can form. It is the interplay between these chains and a frustrated (there are more proton donor than acceptor sites) hydrogen bond network, which is found to lead to extremely high proton conductivity in phosphoric acid. This strongly contrasts to water, wherein the anomalously high rate of excess charge transport occurs not through extended chains but rather through local hydrogen bond rearrangements that drive individual proton transfer reactions. The mechanism proposed in this work, suggests that strong hydrogen bonding does not necessarily lead to protonic ordering and slow dynamics of the system, demonstrating that weak solvent coupling and sufficient degree of configurational disorder can lead to fast proton transport. Although, phosphonic and phosphinic acids possess even stronger hydrogen bonds, the stronger dipolar and dynamic backgrounds tend to oppose the formation of extended Grotthuss chains. Moreover, these systems do not have the same intrinsically frustrated hydrogen bond network (there are more proton acceptor than donor sites), thus hindering the solvent reorganization (depolarization). Nevertheless, the results show that the weak hydrogen bonded configurations, although not an intrinsic property of the hydrogen bond network, are still forming in a dynamical sense due to liquid disorder. The latter, together with the formation of polarized chains explain the high charge carrier concentrations and conductivities reported in these materials, especially in H3PO3, where they are only slightly lower than in the case of H3PO4. Apparently, proton transport in phosphoric acid is extremely susceptible to nearly all types of chemical perturbations. Apart from the severe conductivity reduction caused by the addition of bases, even the addition of acids leads to some decrease in conductivity. The only dopant that increases the conductivity of H3PO4 is water which, together with some condensation products is already present even in a nominally dry acid under the conditions of thermodynamic equilibrium. In fact, the severe increase in the conductivity of phosphoric acid upon dilution cannot be explained by simple hydrodynamic diffusion of hydronium ion, indicating that proton structural diffusion plays a major role in these systems as well. The results show that very similar molecular mechanisms are at play in phosphoric acid - water system as in neat oxoacid systems. The properties of hydrogen bonds even in 1:1 H3PO4 - H2O mixture are virtually identical to those of pure H3PO4, generally, showing no resemblance to liquid H2O. It is due to the strong and polarizable acid-water hydrogen bonds, that some degree of cooperativity can still be observed in the proton transport mechanism, although the solvent coupling in this case is much stronger due to the significantly different dielectric nature of the water phase. In addition to some vehicular contribution to proton conductivity, water also has some plasticizing effect, increasing the configurational disorder in the hydrogen bond network, therefore resulting in significantly higher conductivities observed in these systems.Item Open Access Renormalization group analysis of order parameter fluctuations in fermionic superfluids(2014) Obert, Benjamin; Metzner, Walter (Prof. Dr.)In this work fluctuation effects in two interacting fermion systems exhibiting fermionic s-wave superfluidity are analyzed with a modern renormalization group method. A description in terms of a fermion-boson theory allows an investigation of order parameter fluctuations already on the one-loop level. In the first project a quantum phase transition between a semimetal and a s-wave superfluid in a Dirac cone model is studied. The interplay between fermions and quantum critical fluctuations close to and at the quantum critical point at zero and finite temperatures are studied within a coupled fermion-boson theory. At the quantum critical point non-Fermi liquid and non-Gaussian behaviour emerge. Close to criticality several quantities as the susceptibility show a power law behaviour with critical exponents. We find an infinite correlation length in the entire semimetallic ground state also away from the quantum critical point. In the second project, the ground state of an s-wave fermionic superfluid is investigated. Here, the mutual interplay between fermions and order parameter fluctuations is studied, especially the impact of massless Goldstone fluctuations, which occur due to spontaneous breaking of the continuous U(1)-symmetry. Fermionic gap and bosonic order parameter are distinguished. Furthermore, the bosonic order parameter is decomposed in transverse and longitudinal fluctuations. The mixing between transverse and longitudinal fluctuations is included in our description. Within a simple truncation of the fermion-boson RG flow, we describe the fermion-boson theory for the first time in a consistent manner. Several singularities appear due the Goldstone fluctuations, which partially cancel due to symmetry. Our RG flow captures the correct infrared asymptotics of the system, where the collective excitations act as an interacting Bose gas. Lowest order Ward identities and the massless Goldstone mode are fulfilled in our truncation.