08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Item Open Access An atomic‐scale vector network analyzer(2024) Baumann, Susanne; McMurtrie, Gregory; Hänze, Max; Betz, Nicolaj; Arnhold, Lukas; Malavolti, Luigi; Loth, SebastianElectronic 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.Item Open Access Atomistic simulation of ultra-short pulsed laser ablation of Al : an extension for non-thermalized electrons and ballistic transport(2022) Eisfeld, Eugen; Förster, Daniel; Klein, Dominic; Roth, JohannesFor our model material aluminum, the influence of laser pulse duration in the range between 0.5 and 16 ps on the ablation depth is investigated in a computational study with a hybrid approach, combining molecular dynamics with the well known two-temperature model. A simple, yet expedient extension is proposed to account for the delayed thermalization as well as ballistic transport of the excited electrons. Comparing the simulated ablation depths to a series of our own experiments, the extension is found to considerably increase the predictive power of the model.Item Open Access Bell-state measurement exceeding 50% success probability with linear optics(2023) Bayerbach, Matthias J.; D’Aurelio, Simone E.; Loock, Peter van; Barz, StefanieItem Open Access Character of doped holes in Nd1-xSrxNiO2(2021) Plienbumrung, Tharathep; Schmid, Michael Thobias; Daghofer, Maria; Oleś, Andrzej M.We investigate charge distribution in the recently discovered high-𝑇𝑐 superconductors, layered nickelates. With increasing value of charge-transfer energy, we observe the expected crossover from the cuprate to the local triplet regime upon hole doping. We find that the 𝑑-𝑝 Coulomb interaction 𝑈𝑑𝑝 makes Zhang-Rice singlets less favorable, while the amplitude of local triplets at Ni ions is enhanced. By investigating the effective two-band model with orbitals of 𝑥2-𝑦2 and s symmetries we show that antiferromagnetic interactions dominate for electron doping. The screened interactions for the s band suggest the importance of rare-earth atoms in superconducting nickelates.Item Open Access Classical interaction potentials for diverse materials from ab initio data : a review of potfit(2015) Brommer, Peter; Kiselev, Alexander; Schopf, Daniel; Beck, Philipp; Roth, Johannes; Trebin, Hans-RainerForce matching is an established technique to generate effective potentials for molecular dynamics simulations from first-principles data. This method has been implemented in the open source code potfit. Here, we present a review of the method and describe the main features of the code. Particular emphasis is placed on the features added since the initial release: interactions represented by analytical functions, differential evolution as optimization method, and a greatly extended set of interaction models. Beyond the initially present pair and embedded-atom method potentials, potfit can now also optimize angular dependent potentials, charge and dipolar interactions, and electron-temperature-dependent potentials. We demonstrate the functionality of these interaction models using three example systems: phonons in type I clathrates, fracture of α-alumina, and laser-irradiated silicon.Item Open Access Correlations for computation and computation for correlations(2021) Demirel, Bülent; Weng, Weikai; Thalacker, Christopher; Hoban, Matty; Barz, StefanieQuantum correlations are central to the foundations of quantum physics and form the basis of quantum technologies. Here, our goal is to connect quantum correlations and computation: using quantum correlations as a resource for computation - and vice versa, using computation to test quantum correlations. We derive Bell-type inequalities that test the capacity of quantum states for computing Boolean functions within a specific model of computation and experimentally investigate them using 4-photon Greenberger-Horne-Zeilinger (GHZ) states. Furthermore, we show how the resource states can be used to specifically compute Boolean functions - which can be used to test and verify the non-classicality of the underlying quantum states. The connection between quantum correlation and computability shown here has applications in quantum technologies, and is important for networked computing being performed by measurements on distributed multipartite quantum states.Item Open Access Effective Kugel-Khomskii type models for d4 and d5 materials(2023) Strobel, Pascal; Daghofer, Maria (Prof. Dr.)Item Open Access Exciton Bose-Einstein condensation and topology in Van Vleck-type Mott insulators(2024) Aust, Friedemann; Daghofer, Maria (Prof. Dr.)Item Open Access Excitonic antiferromagnetism in two-dimensional t4 2g systems(2020) Feldmaier, Teresa; Daghofer, Maria (Prof. Dr.)Item Open Access Experimental anonymous conference key agreement using linear cluster states(2023) Rückle, Lukas; Budde, Jakob; Jong, Jarn de; Hahn, Frederik; Pappa, Anna; Barz, StefanieItem Open Access From classical to quantum stochastic resonance(2022) McMurtrie, Gregory; Loth, Sebastian (Prof. Dr.)The open quantum system presented by atomic spins on surfaces is a unique platform to investigate the interplay between stochastic and deterministic behavior. This work investigates this interplay at the transition from classical to quantum behavior in tailored magnetic nanostructures. The structures are assembled with Fe atoms on a Cu2N surface grown on Cu(100) by using atom manipulation with a cryogenic-temperature scanning tunneling microscope. The spin state of the structures can be resolved with a spin-polarized tip, allowing their dynamic response to be measured. The stochastic evolution of the spin states is brought into a resonant regime by means of either a modulated exchange field or a modulated voltage applied with the tip. Undergoing this stochastic resonance yields insight into how these structures interact with their environment, with clear signatures of classical, semi-classical and quantum behavior being observed. This work sets the stage for a new way of interacting with incoherently evolving spin systems, by synchronizing their dynamics, and tailoring their interaction with their environment.Item Open Access From ground state properties to high energy spectroscopy : extending the application of DMFT for correlated quantum materials(2020) Schmid, Michael; Daghofer, Maria (Prof. Dr.)Strongly correlated electron systems exhibit rich physical phenomena reaching from superconductivity, Kondo- and, Mott physics to novel magnetic phases, which lie beyond most single-particle approaches such as density functional theory (DFT) or static mean-field theory. For many transition metal oxides (TMOs) such as Ca2RuO4 or LiV2O4 this is often a result of the partially filled d shells, leading to many-body wave functions, which cannot expressed as a single-slater determinant. Moreover, within this compounds there is often no clear hierarchy of energy scales, e.g. strong spin-orbit coupling, Hund’s coupling, and crystal-field splitting, making the description with minimal models difficult. The breakdown of the single-particle picture triggered the development of numerous numerical methods (DMFT, DMRG, VCA, . . . ) within the last decades, all aimed at tackling the aforementioned phenomena with complementary approximations. One of the most prominent methods for describing real compounds has become dynamical mean-field theory (DMFT), which in many cases has proven to describe local electronic phenomena in good agreement with experimental results. In this thesis we perform state of the art DFT+DMFT calculations in its single shot approach to complement theoretical k-resolved one-particle spectral functions to neutron and x-ray diffraction experiments on Ca2RuO4 . In the experiment small DC currents were applied to a Ca2RuO4 single-crystal resulting in the stabilization of new nonequilibrium phases. Based on experimentally refined structures, DFT calculations are performed to extract a tight binding model by projecting the correlated t2g -subspace onto maximally localized Wannier orbitals. Within our DMFT calculations spin-orbit coupling (SOC) and the spherical invariant Coulomb interaction are added to calculate spectral functions. The results indicate a semimetalic state with partially gapped Fermi surface in the nonequilibrium phases with elongated RuO6 octahedra. Additionally, we extend the DFT+DMFT scheme by a discretization scheme to obtain core-level spectroscopy data, such as XAS or RIXS spectra. This concept is based on the discretization of the DMFT hybridization function to construct an Anderson impurity model of finite bath sites. The discretized model is then extended by the core levels and core-valence interaction. To include sufficiently large amounts of bath sites, despite using an exact diagonalization (ED) solver, we choose the natural orbital basis as the single particle basis of choice to compute RIXS and XAS spectra.Item Open Access 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, DennisIn 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.Item Open Access Laser ablation of covalent materials(2023) Klein, Dominic; Roth, Johannes (Apl. Prof. Dr.)Ultra-fast laser ablation is the process of material removal from solid surfaces by pulsed sub-picosecond laser irradiation. In contrast to longer pulse durations, ultra-fast laser ablation shows the distinguishing feature of the timescale of excitation being below the timescale of consequent material heating. Excited charge carriers distribute the thermal energy over a larger volume than the optical penetration depth suggests, while the lattice remains in a cold state. Spatial energy distribution is followed by a fast carrier-lattice energy relaxation, which induces overheated and meta-stable states of matter. These meta-stable states are induced simultaneously in the laser-affected zone, forcing the material to relax in a variety of mechanisms, ranging from ultra-fast melting over hydrodynamic expansion to material ejection in a complex mixture of chunks, droplets or vapor. While a multitude of publications successfully study the laser irradiation induced material dynamics of metals, we investigate laser ablation of covalent materials. In contrast to metals, covalent materials show a band gap, excitation-dependent carrier heat conduction and strong excitation-dependent interatomic bonding strengths, rendering the theoretical description of such materials a difficult task. However, it also gives rise to a number of unique dynamics like non-thermal melting, Coulomb explosions and altered carrier heat conduction due to charge carrier confinement. In this work we choose silicon as our prototypical covalent material and perform molecular dynamics simulations of laser irradiated silicon, while applying an excitation-dependent interatomic potential. We present new parametrizations of the optical properties, as well as the extension of established charge carrier transport models for silicon, which are both tailored for the application on large scale massive multi-parallel high-performance computers. Finally we observe and characterize the novel and non-thermal ablation mechanics of laser irradiated silicon.Item Open Access Lasertreatment of Al-Cu materials(2023) Kümmel, SimonIn this work, the bond strength and stability of aluminium, copper and their alloys are investigated upon excitation using DFT calculations. In particular, free energy curves, elastic constants and phonon spectra are used to identify changes in the bond strength and the density of states at different degrees of excitation are used to explain the changes. We find nearly no change in bond strength in aluminium, a strong increase in bond strength in copper and bond hardening of certain modes in the AlCu alloys.Item Open Access MD simulations of 3D laser printing(2024) Schmid, JonasItem Open Access Measurements of entropic uncertainty relations in neutron optics(2020) Demirel, Bülent; Sponar, Stephan; Hasegawa, YujiThe emergence of the uncertainty principle has celebrated its 90th anniversary recently. For this occasion, the latest experimental results of uncertainty relations quantified in terms of Shannon entropies are presented, concentrating only on outcomes in neutron optics. The focus is on the type of measurement uncertainties that describe the inability to obtain the respective individual results from joint measurement statistics. For this purpose, the neutron spin of two non-commuting directions is analyzed. Two sub-categories of measurement uncertainty relations are considered: noise-noise and noise-disturbance uncertainty relations. In the first case, it will be shown that the lowest boundary can be obtained and the uncertainty relations be saturated by implementing a simple positive operator-valued measure (POVM). For the second category, an analysis for projective measurements is made and error correction procedures are presented.Item Open Access Molecular dynamics simulations of laser ablation in covalent materials(2017) Kiselev, Alexander; Roth, Johannes (Prof. Dr.)Item Open Access Molecular dynamics simulations of the laser ablation of silicon with the thermal spike model(2021) Klein, Dominic; Eisfeld, Eugen; Roth, JohannesThe purpose of this work is to model laser ablation of silicon on an atomistic scale in combination with a mesoscale model for the description of the electron-phonon interaction and an electron-temperature dependent interaction potential. The well-known continuum two-temperature model (TTM) for solids with highly excited electrons is extended from metals to silicon by explicitly taking charge carrier transport effects into account (nTTM). This is accomplished by the drift-diffusion limit of the Boltzmann-transport equation leading to the so called thermal-spike model (TSM). The model is further enhanced by extending the static modified Tersoff potential to a dynamical carrier excitation dependent interaction potential. We compare the TSM and nTTM with regard to physical correctness, numerical stability and applicability in the context of large-scale massive parallel high performance computing.