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

THE superconducting state of a metal is characterized by a complex order parameter with an amplitude and a phase In the BCS-Eliashberg mean-field theory1, which is a very good approximation for conventional metals, the phase of the order parameter is un-important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition Here we argue that superconductors with low super-conducting carrier density (such as the organic and high-Tc oxide superconductors) are characterized by a relatively small phase 'stiffness9 and poor screening, both of which imply a significantly larger role for phase fluctuations As a consequence, in these mat-erials the transition to the superconducting state may not display typical mean-field behaviour, and phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties For some quasi-two-dimensional materials, notably underdoped high-temperature superconductors, the onset of long-range phase order controls the gross value of Tc as well as its systematic variation from one material to another

Importance of phase fluctuations in superconductors with small superfluid density

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

Many organic electronic devices exhibit switching behavior, and have therefore been proposed as the basis for a nonvolatile memory (NVM) technology. This Review summarizes the materials that have been used in switching devices, and describes the variety of device behavior observed in their charge-voltage (capacitive) or current-voltage (resistive) response. A critical summary of the proposed charge-transport mechanisms for resistive switching is given, focusing particularly on the role of filamentary conduction and of deliberately introduced or accidental nanoparticles. The reported device parameters (on-off ratio, on-state current, switching time, retention time, cycling endurance, and rectification) are compared with those that would be necessary for a viable memory technology.

Nonvolatile memory elements based on organic materials

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

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Electrodynamics of correlated electron materials

Muon-spin-relaxation and bulk measurements of the magnetic-field penetration depth suggest that the cuprate high-${\mathit{T}}_{\mathit{c}}$, bismuthate, organic Chevrel-phase, and heavy-fermion systems possibly belong to a unique group of superconductors characterized by high transition temperatures ${\mathit{T}}_{\mathit{c}}$ relative to the values of ${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$ (carrier density/effective mass). This feature distinguishes these exotic superconductors from ordinary BCS superconductors.

Basic similarities among cuprate, bismuthate, organic, Chevrel-phase, and heavy-fermion superconductors shown by penetration-depth measurements.

An ambient pressure superconductivity of (BEDT-TTF)2Cu(SCN)2 was observed by d.c. magnetic susceptibility and electrical conductivity measurements. The superconducting critical temperature is the highest (TC=10.4 K) among the organic superconductors so far obtained, even though the anion has a positional disorder in the crystal.

A New Ambient Pressure Organic Superconductor Based on BEDT-TTF with TC Higher than 10 K (TC=10.4 K)

The Physics and Chemistry of Organic Superconductors

We report the superconductivity with the onset temperature as high as 8 K in the organic conductor of β-(BEDT-TTF) 2 I 3 , bis-ethylenedithio otetrathiofulvalene-triiodide. The increased superconducting transition temperature was achieved by an application of a soft pressure, i.e. 1.3 kb. High value of the upper critical field, H c2 was observed. ( H c2 >13 kOe at 1.5 K along the c * -axis.)

Superconductivity with the Onset at 8 K in the Organic Conductor β-(BEDT-TTF)2I3 under Pressure

The temperature and pressure dependences of the dielectric constants have been measured on the mixed-stack organic charge-transfer crystal, tetrathiafulvalene-p-chloranil, in a frequency range from 100 Hz to 10 MHz. The dielectric response increases remarkably with decreasing temperature down to ${\mathit{T}}_{\mathit{c}}$=81 K, where the neutral-to-ionic (NI) phase transition occurs. On the other hand, with applied pressure at room temperature, the change in the dielectric response is less remarkable, whereas the dc conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{dc}}$ increases considerably. Such dynamical responses are explained by a Debye-type relaxation model. The strength of the dielectric response is closely correlated with both the relaxation time \ensuremath{\tau} and ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{dc}}$. These anomalous dielectric responses are attributed to the dynamics of the NI domain-wall pairs and ionic domains in the neutral lattice.

Gunzi Saito

Papers

Basic similarities among cuprate, bismuthate, organic, Chevrel-phase, and heavy-fermion superconductors shown by penetration-depth measurements.

A New Ambient Pressure Organic Superconductor Based on BEDT-TTF with TC Higher than 10 K (TC=10.4 K)

The Physics and Chemistry of Organic Superconductors

Superconductivity with the Onset at 8 K in the Organic Conductor β-(BEDT-TTF)2I3 under Pressure

Anomalous dielectric response in tetrathiafulvalene-p-chloranil as observed in temperature- and pressure-induced neutral-to-ionic phase transition.