Universität Stuttgart

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Institute: search.filters.UBSInstitut.Max-Planck-Institut für FestkörperforschungSubject: search.filters.subject.540Subject: search.filters.subject.530Start date: 2020

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Now showing 1 - 4 of 4
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    Development and application of embedded methods to strongly and weakly correlated systems
    (2022) Vitale, Eugenio; Alavi, Ali (Prof. Dr.)
    Coupled cluster (CC) theory is a popular and reliable tool in quantum chemistry due to its improvable hierarchy of methods able to rapidly converge to the full configuration interaction (FCI) limit in weakly correlated systems. Although it represents one of the most efficient single reference methods to treat many-body correlations with high accuracy and reliable outcomes, it yields qualitatively erroneous results when applied to strongly correlated systems. Within this thesis, the Distinguishable Cluster (DC) method (i.e., a small modification of CC amplitude equations able to qualitatively describe strongly correlated systems), is combined with FCI Quantum Monte Carlo (FCIQMC) in order to present a new tailored approach, the tailored DC (TDC), which is more accurate than the corresponding tailored CC and the pure DC. To demonstrate this, the method is first benchmarked with a variety of test cases and then further evaluated with computation of spin-state splittings in a few Fe(II) complexes. The systematic improvability of the TDC method is shown as the active space is increased. In the last part of the thesis, a further embedding scheme to treat strong correlation effects is evaluated. Specifically, the development and application of a screened Coulomb formalism is discussed. This simple approach inspired by Random Phase approximation (RPA) shows to be extremely efficient in the dissociation of one- and two-dimensional hydrogen systems.
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    Heteroepitaxial tuning of resonant forbidden reflections in a spinel
    (2024) Oka, Ryosuke; Kim, Minu; Wochner, Peter; Francoual, Sonia; Palstra, Thomas T. M.; Takagi, Hidenori; Huang, Dennis
    In resonant elastic X-ray scattering (REXS), low site symmetries in a crystal may be revealed through resonant Bragg reflections that are normally forbidden in conventional X-ray diffraction due to screw axes and/or glide planes. These resonant forbidden reflections have been observed in spinel compounds, but to better understand and utilize their connection to microscopic material parameters and possible charge and/or orbital ordering, a systematic study of their dependence on growth conditions and applied strain is desired. We performed REXS at the V K edge and examined the resonant forbidden (002) reflection in thin films of the spinel LiV2O4 grown on three substrates: MgAl2O4, SrTiO3, and MgO. The energy dependence of the (002) reflection shows a systematic evolution as epitaxial strain modifies the local anisotropy of the V site. More strikingly, the integrated intensity of the (002) reflection varies by more than an order of magnitude in films on different substrates. We speculate that the large variation in integrated intensity reflects the varying degree of antiphase domains that arise during the epitaxy.
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    Synthesis and optical properties of single‐crystalline phosphors Gd3In2Ga3O12:RE3+ (RE = Nd3+ and Ho3+) grown via the optical float zone method
    (2025) Yilmaz, Hasan; Kinik, Gülsüm; Isobe, Masahiko; Puphal, Pascal; Suta, Markus; Clemens, Oliver
    The continuous development of innovative optical materials with lanthanoid ions as activators has emerged as a modern sector of materials chemistry. The experience with the fabrication of single crystals with the optical float zone has motivated one to investigate the luminescence of Nd3+ and Ho3+ ions in the garnets (Gd3−xREx)In2Ga3O12 (RE = Nd and Ho, x = 0; 0.15–0.30). Upon usage of an Ar/O2 (80:20 ratio) atmosphere and application of an auxiliary pressure (6 bar) to suppress In2O3 evaporation, single‐crystalline domain sizes in the order of ≈6 × 6 × 1 mm3 are obtained. Structural analysis confirms the formation of a cubic garnet phase with space group Ia3¯d, with the substituents incorporated in accordance with Vegard's law. Backscattered electron imaging and energy‐dispersive X‐ray spectroscopy are conducted, demonstrating a homogeneous elemental distribution within the crystals. Photoluminescence studies are carried out, revealing the characteristic narrow‐line 4fn → 4fn transitions of Nd3+ and Ho3+, with decay times in the submillisecond range, suggesting non‐negligible cross‐relaxation effects are present. Despite this, the large nearest‐neighbor Gd-Gd distance (3.88 Å) in Gd3In2Ga3O12 and the low phonon cutoff energy (≈700 cm-1) are found to limit cross‐relaxation pathways, preserving significant photoluminescence brightness. These results highlight the potential of Gd3In2Ga3O12:RE3+ single crystals as promising candidates for advanced optical applications.
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    Anti-Zeno purification of spin baths by quantum probe measurements
    (2022) Dasari, Durga; Yang, Sen; Chakrabarti, Arnab; Finkler, Amit; Kurizki, Gershon; Wrachtrup, Jörg
    The quantum Zeno and anti-Zeno paradigms have thus far addressed the evolution control of a quantum system coupled to an immutable bath via non-selective measurements performed at appropriate intervals. We fundamentally modify these paradigms by introducing, theoretically and experimentally, the concept of controlling the bath state via selective measurements of the system (a qubit). We show that at intervals corresponding to the anti-Zeno regime of the system-bath exchange, a sequence of measurements has strongly correlated outcomes. These correlations can dramatically enhance the bath-state purity and yield a low-entropy steady state of the bath. The purified bath state persists long after the measurements are completed. Such purification enables the exploitation of spin baths as long-lived quantum memories or as quantum-enhanced sensors. The experiment involved a repeatedly probed defect center dephased by a nuclear spin bath in a diamond at low-temperature.