Stefan Hüfner

Bio: Stefan Hüfner is an academic researcher from Saarland University. The author has contributed to research in topics: Angle-resolved photoemission spectroscopy & Electronic band structure. The author has an hindex of 43, co-authored 212 publications receiving 7774 citations. Previous affiliations of Stefan Hüfner include ElringKlinger & ETH Zurich.

Photoelectron spectroscopy : principles and applications

Abstract: 1. Introduction and Basic Principles.- 2. Core Levels and Final States.- 3. Charge-Excitation Final States: Satellites.- 4. Continuous Satellites and Plasmon Satellites: XPS Photoemission in Nearly Free Electron Systems.- 5. Valence Orbitals in Simple Molecules and Insulating Solids.- 6. Photoemission of Valence Electrons from Metallic Solids in the One-Electron Approximation.- 7. Band Structure and Angular-Resolved Photoelectron Spectra.- 8. Surface States, Surface Effects.- 9. Inverse Photoelectron Spectroscopy.- 10. Spin-Polarized Photoelectron Spectroscopy.- 11. Photoelectron Diffraction.- A.1 Table of Binding Energies.- A.2 Surface and Bulk Brillouin Zones of the Three Low-Index Faces of a Face-Centered Cubic (fcc) Crystal Face.- A.3 Compilation of Work Functions.- References.

Direct measurements of the L -gap surface states on the (111) face of noble metals by photoelectron spectroscopy

Abstract: We present a comprehensive photoemission study of the L-gap surface states of the (111) surfaces of Cu, Ag, and Au by high-resolution angle-resolved photoelectron spectroscopy (PES). With an angular resolution of about $\ensuremath{\Delta}\ensuremath{\theta}=\ifmmode\pm\else\textpm\fi{}0.15\ifmmode^\circ\else\textdegree\fi{}$ and an energy resolution of $\ensuremath{\Delta}E\ensuremath{\approx}3.5\mathrm{meV},$ our data establish new values for the intrinsic lifetime broadening and the dispersion relation of these surface states. We compare our photoemission results to recently published theoretical calculations and measurements by scanning tunneling spectroscopy (STS). In the case of the Au(111) state, we observe particular qualitative discrepancies in comparison to the STS data: the PES data show a split dispersion and Fermi surface, possibly caused by a spin-orbit interaction of the surface state electrons.

Spin-orbit splitting of the L -gap surface state on Au(111) and Ag(111)

Abstract: We present high-resolution photoemission results on the L-gap surface state on Au(111) and Ag(111), in combination with fully relativistic density-functional calculations. In the case of Au(111), both experimental and theoretical results demonstrate that the lack of inversion symmetry at the surface leads to a considerable spin-orbit splitting of this surface state over the whole Fermi surface, whereas in the case of Ag(111) this splitting is far too small to be experimentally resolved in our data.

Photoemission spectroscopy—from early days to recent applications

Abstract: In this review we describe the development of photoemission spectroscopy (PES) from the first historic observations of the photoelectric effect by Hertz and Hallwachs to state-of-the-art experiments. We present several examples for the application of PES for chemical analysis of solids (ESCA), the determination of the valence band structure by angle-resolved photoemission (ARUPS), and the investigation of many-body effects, in particular by high-resolution PES on the meV-scale. Furthermore, we give a brief overview about the wide spectrum of experimental methods based on PES.

Electronic structure of 3d-transition-metal oxides: on-site Coulomb repulsion versus covalency

Abstract: We have performed photoemission and inverse photoemission experiments on a series of 3d-transition-metal oxides with formal ionic configuration from to . The photoemission core-level spectra are analysed in terms of a simple cluster model leading to estimates for the charge-transfer energy , the Coulomb correlation energy , and the hybridization strength V. It is found that the ratio of the correlation energy to the hybridization energy significantly decreases from the late to the early transition metal oxides. This trend is attributed mostly to the increasing number of empty d states in the early transition metals which enhances the effective metal-ligand hybridization. We also compare the experimental valence band spectra with densities of states (DOS) from band-structure calculations. The rather good agreement between the theoretical DOS and the measured single-particle excitation spectra of the early 3d-transition-metal oxides as opposed to the failure of the one-electron description for most of the late transition metal oxides supports the results of the cluster model analysis.

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Electronic excitations: density-functional versus many-body Green's-function approaches

Abstract: Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green’s-function equations, starting with a one-electron propagator and considering the electron-hole Green’s function for the response. Key ingredients are the electron’s self-energy S and the electron-hole interaction. A good approximation for S is obtained with Hedin’s GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of S. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green’s functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green’s functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress.

Angle-resolved photoemission studies of the cuprate superconductors

Abstract: The last decade witnessed significant progress in angle-resolved photoemission spectroscopy (ARPES) and its applications. Today, ARPES experiments with 2-meV energy resolution and $0.2\ifmmode^\circ\else\textdegree\fi{}$ angular resolution are a reality even for photoemission on solids. These technological advances and the improved sample quality have enabled ARPES to emerge as a leading tool in the investigation of the high-${T}_{c}$ superconductors. This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature. The low-energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and $d$-wave-like dispersion, evidence of electronic inhomogeneity and nanoscale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. Given the dynamic nature of the field, we chose to focus mainly on reviewing the experimental data, as on the experimental side a general consensus has been reached, whereas interpretations and related theoretical models can vary significantly. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides an overview of the scientific issues relevant to the investigation of the low-energy electronic structure by ARPES. The rest of the paper is devoted to the experimental results from the cuprates, and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self-energy, and collective modes. Within each topic, ARPES data from the various copper oxides are presented.

Plasmon-induced hot carrier science and technology

Abstract: The discovery of the photoelectric effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technology. In the early 1900s it played a critical role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of physical and chemical processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to electrically dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.