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Register a new userA time-dependent scattering approach to core-level spectroscopies
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While new light sources allow for unprecedented resolution in experiments with X-rays, a theoretical understanding of the scattering cross-section lacks closure. In the particular case of strongly correlated electron systems, numerical techniques are quite limited, since conventional approaches rely on calculating a response function (Kramers-Heisenberg formula) that is obtained from a time-dependent perturbative analysis of scattering processes. This requires a knowledge of a full set of eigenstates in order to account for all intermediate processes away from equilibrium, limiting the applicability to small tractable systems. In this work, we present an alternative paradigm allowing to explicitly solving the time-dependent Schrodinger equation without the limitations of perturbation theory, a faithful simulation of all scattering processes taking place in actual experiments. We introduce the formalism and an application to Mott insulating Hubbard chains using the time-dependent density matrix renormalization group method, which does not require a priory knowledge of the eigenstates and thus, can be applied to very large systems with dozens of orbitals. Away from the ultra short lifetime limit we find signatures of spectral weight at low energies that can be explained in terms of gapless multi-spinon excitations. Our approach can readily be applied to systems out of equilibrium without modification.
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We introduce a real time version of the functional renormalization group which allows to study correlation effects on nonequilibrium transport through quantum dots. Our method is equally capable to address (i) the relaxation out of a nonequilibrium initial state into a (potentially) steady state driven by a bias voltage and (ii) the dynamics governed by an explicitly time-dependent Hamiltonian. All time regimes from transient to asymptotic can be tackled; the only approximation is the consistent truncation of the flow equations at a given order. As an application we investigate the relaxation dynamics of the interacting resonant level model which describes a fermionic quantum dot dominated by charge fluctuations. Moreover, we study decoherence and relaxation phenomena within the ohmic spin-boson model by mapping the latter to the interacting resonant level model.
Xclaim (x-ray core level atomic multiplets) is a graphical interface for the calculation of core-hole spectroscopy and ground state properties within a charge-transfer multiplet model taking into account a many-body hamiltonian with Coulomb, spin-orbit, crystal-field, and hybridization interactions. Using Hartree-Fock estimates for the Coulomb and spin-orbit interactions and ligand field parameters (crystal-field, hybridization and charge-transfer energy) the program can calculate x-ray absorption spectroscopy (XAS), x-ray photoemission spectroscopy (XPS), photoemission spectrospcy (PES) and inverse photoemission (IPES) for d- and f-valence metals and different absorption edges. The program runs in Linux, Windows and MacOS platforms.
We consider spectroscopies of strongly interacting atomic gases, and we propose a model for describing the coupling between quasiparticles and gapless phonon-like modes. Our model explains features in a wide range of different experiments in both fermionic and bosonic atom gases in various spectroscopic methods.
The real-space multiple-scattering (RSMS) approach is applied to model non-resonant inelastic scattering from deep core electron levels over a broad energy spectrum. This approach is applicable to aperiodic or periodic systems alike and incorporates ab initio, self-consistent electronic structure and final state effects. The approach generalizes to finite momentum transfer a method used extensively to model x-ray absorption spectra (XAS), and includes both near edge spectra and extended fine structure. The calculations can be used to analyze experimental results of inelastic scattering from core-electrons using either x-ray photons (NRIXS) or electrons (EELS). In the low momentum transfer region (the dipole limit), these inelastic loss spectra are proportional to those from XAS. Thus their analysis can provide similar information about the electronic and structural properties of a system. Results for finite momentum transfer yield additional information concerning monopole, quadrupole, and higher couplings. Our results are compared both with experiment and with other theoretical calculations.
Strongly correlated materials are characterized by the presence of electron-electron interactions in their electronic structure. They often have remarkable properties and transitions between competing phases of very different electronic and magnetic order. This thesis focuses on strongly correlated $f$-electron compounds containing Ce, Sm, and U. These materials exhibit a so-called heavy-fermion or Kondo-lattice behavior. They can become insulating due to hybridization effects (Kondo-insulator) or develop multipolar (hidden) order. Kondo insulators have recently been discussed in the context of strongly correlated topological insulators. This new aspect caused an enormous activity in the field of Kondo insulators, theoretically as well as experimentally. Multipolar order as well as the formation of a Kondo insulating state strongly depend on the symmetry of the $f$ states involved. Also the character of the surface states in a topological insulator is determined by the properties of the bulk states. Therefore the scope of this thesis has been to unveil the underlying symmetries of the bulk $f$ states. Here the compounds CeB$_6$, UO$_2$, and URu$_2$Si$_2$, which exhibit multipolar order, as well as the Kondo insulators (semimetals) SmB$_6$ and CeRu$_4$Sn$_6$ have been studied.
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