### Browsing by Author "Dvorak, Marc"

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Item An Alternative to the Born Rule : Spectral Quantization(SPRINGER, 2023-06) Dvorak, Marc; Computational Electronic Structure Theory; Department of Applied PhysicsWe show that there is a hidden freedom in quantum many-body theory associated with overcompleteness of the time evolution through the single-particle subspace of a many-body system. To fix the freedom, an additional constraint is necessary. We argue that the appropriate constraint on the time evolution through the subspace is to quantize the propagation of entangled pairs of particles, represented by the single-particle spectral function, instead of individual particles. This solution method creates a surface that indicates the multiplicity of every solution to the inverse problem defined by matching the freedom to the constraint. Upon measurement, the system collapses nonlocally onto a single quantized solution. In addition to a combinatoric multiplicity, each solution acquires a multiplicity due to its stability when subject to a small variation in the microscopic degrees of freedom. Numerical calculations for a two-level system show that our theory improves upon standard theory in the description of non-quasiparticle spectral features. Our reinterpretation of quantum many-body theory is not based on the Born rule and offers a more faithful representation of experiments than current theory by modeling individual, quantized events with an explicit collapse model.Item Atomic and electronic structure of cesium lead triiodide surfaces(American Institute of Physics, 2021-02-21) Seidu, Azimatu; Dvorak, Marc; Rinke, Patrick; Li, Jingrui; Department of Applied Physics; Computational Electronic Structure TheoryThe (001) surface of the emerging photovoltaic material cesium lead triiodide (CsPbI3) is studied. Using first-principles methods, we investigate the atomic and electronic structure of cubic (α) and orthorhombic (γ) CsPbI3. For both phases, we find that CsI-termination is more stable than PbI2-termination. For the CsI-terminated surface, we then compute and analyze the surface phase diagram. We observe that surfaces with added or removed units of nonpolar CsI and PbI2 are most stable. The corresponding band structures reveal that the α phase exhibits surface states that derive from the conduction band. The surface reconstructions do not introduce new states in the bandgap of CsPbI3, but for the α phase, we find additional surface states at the conduction band edge.Item Computational study of the Kondo effect in a molecule/graphene system(2017-04-25) Kotilahti, Janne; Dvorak, Marc; Perustieteiden korkeakoulu; Alava, MikkoItem Coupled Yu-Shiba-Rusinov States in Molecular Dimers on NbSe2(2018-04-11) Kezilebieke, Shawulienu; Dvorak, Marc; Ojanen, Teemu; Liljeroth, Peter; Department of Applied Physics; Atomic Scale Physics; Theory of Quantum Matter (TQM)Magnetic impurities have a dramatic effect on superconductivity by breaking the time-reversal symmetry and inducing so-called Yu-Shiba-Rusinov (YSR) low energy bound states within the superconducting gap. The spatial extent of YSR states is greatly enhanced in two-dimensional (2D) systems, which should facilitate the formation of coupled states. Here, we observe YSR states on single cobalt phthalocyanine (CoPC) molecules on a 2D superconductor NbSe2 using low-temperature scanning tunneling microscopy (STM) and spectroscopy. We use STM lateral manipulation to create controlled CoPc dimers and demonstrate the formation of coupled YSR states. The experimental results are corroborated by theoretical analysis of the coupled states in lattice and continuum models.Item Dynamical configuration interaction: Quantum embedding that combines wave functions and Green's functions(American Physical Society, 2019-03-25) Dvorak, Marc; Rinke, Patrick; Department of Applied Physics; Computational Electronic Structure TheoryWe present the concept, derivation, and implementation of dynamical configuration interaction, a quantum embedding theory that combines Green's function methodology with the many-body wave function. In a strongly correlated active space, we use full configuration interaction (CI) to describe static correlation exactly. We add energy-dependent corrections to the CI Hamiltonian which, in principle, include all remaining correlations derived from the bath space surrounding the active space. Next, we replace the exact Hamiltonian in the bath with one of excitations defined over a correlated ground state. This transformation is naturally suited to the methodology of many-body Green's functions. In this space, we use a modified GW/Bethe-Salpeter equation procedure to calculate excitation energies. Combined with an estimate of the ground-state energy in the bath, we can efficiently compute the energy-dependent corrections, which correlate the full set of orbitals, for very low computational cost. We present dimer dissociation curves for H-2 and N-2 in good agreement with exact results. Additionally, excited states of N-2 and C-2 are in excellent agreement with benchmark theory and experiment. By combining the strengths of two disciplines, we achieve a balanced description of static and dynamic correlation in a fully ab initio, systematically improvable framework.Item Editorial: Many-Body Green’s Functions and the Bethe-Salpeter Equation in Chemistry: From Single Molecules to Complex Systems(FRONTIERS MEDIA SA, 2022-02-21) Dvorak, Marc; Baumeier, Björn; Golze, Dorothea; Leppert, Linn; Rinke, Patrick; Department of Applied Physics; Computational Electronic Structure Theory; Eindhoven University of Technology; University of TwenteItem Electronic Characterization of a Charge-Transfer Complex Monolayer on Graphene(AMERICAN CHEMICAL SOCIETY, 2021-06-22) Kumar, Avijit; Banerjee, Kaustuv; Ervasti, Mikko M.; Kezilebieke, Shawulienu; Dvorak, Marc; Rinke, Patrick; Harju, Ari; Liljeroth, Peter; Department of Applied Physics; Atomic Scale Physics; Computational Electronic Structure TheoryOrganic charge-transfer complexes (CTCs) formed by strong electron acceptor and strong electron donor molecules are known to exhibit exotic effects such as superconductivity and charge density waves. We present a low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) study of a two-dimensional (2D) monolayer CTC of tetrathiafulvalene (TTF) and fluorinated tetracyanoquinodimethane (F4TCNQ), self-assembled on the surface of oxygen-intercalated epitaxial graphene on Ir(111) (G/O/Ir(111)). We confirm the formation of the charge-transfer complex by dI/dV spectroscopy and direct imaging of the singly occupied molecular orbitals. High-resolution spectroscopy reveals a gap at zero bias, suggesting the formation of a correlated ground state at low temperatures. These results point to the possibility to realize and study correlated ground states in charge-transfer complex monolayers on weakly interacting surfaces.Item The GW Compendium: A Practical Guide to Theoretical Photoemission Spectroscopy(Frontiers Media S.A, 2019-07-09) Golze, Dorothea; Dvorak, Marc; Rinke, Patrick; Department of Applied Physics; Computational Electronic Structure TheoryThe GW approximation in electronic structure theory has become a widespread tool for predicting electronic excitations in chemical compounds and materials. In the realm of theoretical spectroscopy, the GW method provides access to charged excitations as measured in direct or inverse photoemission spectroscopy. The number of GW calculations in the past two decades has exploded with increased computing power and modern codes. The success of GW can be attributed to many factors: favorable scaling with respect to system size, a formal interpretation for charged excitation energies, the importance of dynamical screening in real systems, and its practical combination with other theories. In this review, we provide an overview of these formal and practical considerations. We expand, in detail, on the choices presented to the scientist performing GW calculations for the first time. We also give an introduction to the many-body theory behind GW, a review of modern applications like molecules and surfaces, and a perspective on methods which go beyond conventional GW calculations. This review addresses chemists, physicists and material scientists with an interest in theoretical spectroscopy. It is intended for newcomers to GW calculations but can also serve as an alternative perspective for experts and an up-to-date source of computational techniques.Item Improved One-Shot Total Energies from the Linearized GW Density Matrix(AMERICAN CHEMICAL SOCIETY, 2021-04-13) Bruneval, Fabien; Rodriguez-Mayorga, Mauricio; Rinke, Patrick; Dvorak, Marc; Department of Applied Physics; Computational Electronic Structure Theory; Université Paris-SaclayThe linearized GW density matrix (γGW) is an efficient method to improve the static portion of the self-energy compared to that of ordinary perturbative GW while keeping the single-shot simplicity of the calculation. Previous work has shown that γGW gives an improved Fock operator and total energy components that approach the self-consistent GW quality. Here, we test γGW for dimer dissociation for the first time by studying N2, LiH, and Be2. We also calculate a set of self-consistent GW results in identical basis sets for a direct and consistent comparison. γGW approaches self-consistent GW total energies for a starting point based on a high amount of exact exchange. We also compare the accuracy of different total energy functionals, which differ when evaluated with a non-self-consistent density or density matrix. While the errors in total energies among different functionals and starting points are small, the individual energy components show noticeable errors when compared to reference data. The energy component errors of γGW are smaller than functionals of the density and we suggest that the linearized GW density matrix is a route to improving total energy evaluations in the adiabatic connection framework.Item Observation of coexistence of Yu-Shiba-Rusinov states and spin-flip excitations(AMER CHEMICAL SOC, 2019-07-10) Shawulienu, Kezilebieke; Žitko, Rok; Dvorak, Marc; Ojanen, Teemu; Liljeroth, Peter; Department of Applied Physics; Atomic Scale Physics; Computational Electronic Structure Theory; Theory of Quantum Matter (TQM); Jožef Stefan InstituteWe investigate the spectral evolution in different metal phthalocyanine molecules on NbSe2 surface using scanning tunnelling microscopy (STM) as a function of the coupling with the substrate. For manganese phthalocyanine (MnPc), we demonstrate a smooth spectral crossover from Yu-Shiba-Rusinov (YSR) bound states to spin-flip excitations. This has not been observed previously and it is in contrast to simple theoretical expectations. We corroborate the experimental findings using numerical renormalization group calculations. Our results provide fundamental new insight on the behavior of atomic scale magnetic/SC hybrid systems, which is important, for example, for engineered topological superconductors and spin logic devices.Item Protective Coating Interfaces for Perovskite Solar Cell Materials: A First-Principles Study(AMERICAN CHEMICAL SOCIETY, 2022-03-16) Fangnon, Azimatu; Dvorak, Marc; Havu, Ville; Todorović, Milica; Li, Jingrui; Rinke, Patrick; Department of Applied Physics; Computational Electronic Structure Theory; University of Turku; Xi'an Jiaotong UniversityThe protection of halide perovskites is important for the performance and stability of emergent perovskite-based optoelectronic technologies. In this work, we investigate the potential inorganic protective coating materials ZnO, SrZrO3, and ZrO2 for the CsPbI3 perovskite. The optimal interface registries are identified with Bayesian optimization. We then use semilocal density functional theory (DFT) to determine the atomic structure at the interfaces of each coating material with the clean CsI-terminated surface and three reconstructed surface models with added PbI2 and CsI complexes. For the final structures, we explore the level alignment at the interface with hybrid DFT calculations. Our analysis of the level alignment at the coating-substrate interfaces reveals no detrimental mid-gap states but rather substrate-dependent valence and conduction band offsets. While ZnO and SrZrO3 act as insulators on CsPbI3, ZrO2 might be suitable as an electron transport layer with the right interface engineering.Item Quantum embedding theory in the screened Coulomb interaction: Combining configuration interaction with GW/BSE(American Physical Society, 2019-07-31) Dvorak, Marc; Golze, Dorothea; Rinke, Patrick; Department of Applied Physics; Computational Electronic Structure TheoryWe present a quantum embedding theory called dynamical configuration interaction (DCI) that combines wave function and Green's function theories. DCI captures static correlation in a correlated subspace with configuration interaction and couples to high-energy, dynamic correlation outside the subspace with many-body perturbation theory based on Green's functions. DCI takes the strengths of both theories to balance static and dynamic correlation in a single, fully ab initio embedding concept. The theory adds dynamic correlation around a fixed active space of orbitals with efficient O(N-5) scaling, while maintaining a multireference treatment of the active space. We show that treating high-energy correlation up to the GW and Bethe-Salpeter equation level is sufficient even for challenging multireference problems. Our theory treats ground and excited states on equal footing, and we compute the dissociation curve of N-2, the vertical excitation energies of small molecules, and the ionization spectrum of benzene in excellent agreement with high-level quantum chemistry methods and experiment.Item Surface reconstruction of tetragonal methylammonium lead triiodide(American Institute of Physics, 2021-11-01) Seidu, Azimatu; Dvorak, Marc; Järvi, Jari; Rinke, Patrick; Li, Jingrui; Department of Applied Physics; Computational Electronic Structure TheoryWe present a detailed first-principles analysis of the (001) surface of methylammonium lead triiodide (MAPbI3). With density functional theory, we investigate the atomic and electronic structure of the tetragonal (I4cm) phase of MAPbI3. We analyzed surface models with MAI-termination (MAI-T) and PbI2-termination (PbI2-T). For both terminations, we studied the clean surface and a series of surface reconstructions. We find that the clean MAI-T model is more stable than its counterpart, PbI2-T. For the MAI-T, reconstructions with added or removed units of nonpolar MAI and PbI2 are most stable. The corresponding band structures reveal surface states originating from the conduction band. Despite the presence of such additional surface states, our stable reconstructed surface models do not introduce new states within the bandgap.