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.

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

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.

https://arxiv.org/pdf/cond-mat/0208504.pdf

Angle-resolved photoemission studies of the cuprate superconductors

IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

On the theory of superconductivity

[Wu, Zhong-Shuai; Ren, Wencai; Xu, Li; Li, Feng; Cheng, Hui-Ming] Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, Shenyang 110016, Peoples R China.;Ren, WC (reprint author), Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, 72 Wenhua Rd, Shenyang 110016, Peoples R China;wcren@imr.ac.cn cheng@imr.ac.cn

Doped Graphene Sheets As Anode Materials with Superhigh Rate and Large Capacity for Lithium Ion Batteries

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Publisher's Note: Anomalous Quasiparticle Renormalization in Na 0.73 CoO 2 : Role of Interorbital Interactions and Magnetic Correlations [Phys. Rev. Lett. 99, 046403 (2007)]

Core-level photoelectron study of indium chains on Si(1 1 1) at 10 K

Plasmon dispersion in quasi-one-dimensional (TaSe4)2I and K0.3MoO3

The spectroscopic investigation of bulk samples of SWCNT is reviewed with examples taken from electron energy-loss spectroscopy in transmission, UV-vis and photoemission spectroscopy It is shown that, given the insight obtained from EELS studies, UV-vis spectroscopy is a powerful express tool for the characterization of the NT yield and diameter distribution in bulk SWCNT samples In addition, using photoemission spectroscopy, apart from showing the use of annealing in UHV to remove many of the contaminants commonly found in purified SWCNT material and determining the work function of pristine SWCNT to be 465±01 eV, we illustrate the similarity of the occupied electronic structure of SWCNT with that of graphite Open questions, however, remain, such as the appropriate model for the changes of the electronic structure upon intercalation and the reasons for the absence of the singularities in the occupied density of states in the photoemission spectra

The spectroscopic investigation of the optical and electronic properties of SWCNT

FIG. 3. !Color online" Momentum distribution of the photocurrent along the M-"-M direction measured in the second Brillouin zone as a function of excitation energy, showing several alterations of the high energy dispersion behavior. The color scale represents photoemission intensity integrated in a small binding energy window around 0.6 eV and normalized by the average intensity along each cut. The doubleheaded arrow marks the energy range covered by Fig. 2!c" in the original paper. PHYSICAL REVIEW B 79, 139901!E" !2009"

M. Knupfer

Papers

Publisher's Note: Anomalous Quasiparticle Renormalization in Na 0.73 CoO 2 : Role of Interorbital Interactions and Magnetic Correlations [Phys. Rev. Lett. 99, 046403 (2007)]

Core-level photoelectron study of indium chains on Si(1 1 1) at 10 K

Plasmon dispersion in quasi-one-dimensional (TaSe4)2I and K0.3MoO3

The spectroscopic investigation of the optical and electronic properties of SWCNT

Erratum: Excitation energy map of high-energy dispersion anomalies in cuprates