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

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

The discovery of high-temperature superconductivity in the copper oxides in 1986 triggered a huge amount of innovative scientific inquiry. In the almost three decades since, much has been learned about the novel forms of quantum matter that are exhibited in these strongly correlated electron systems. A qualitative understanding of the nature of the superconducting state itself has been achieved. However, unresolved issues include the astonishing complexity of the phase diagram, the unprecedented prominence of various forms of collective fluctuations, and the simplicity and insensitivity to material details of the 'normal' state at elevated temperatures.

/pdf/from-quantum-matter-to-high-temperature-superconductivity-in-3vq16xcye2.pdf

From quantum matter to high-temperature superconductivity in copper oxides

We present an experimental review of the nature of the pseudogap in the cuprate superconductors. Evidence from various experimental techniques points to a common phenomenology. The pseudogap is seen in all high-temperature superconductors and there is general agreement on the temperature and doping range where it exists. It is also becoming clear that the superconducting gap emerges from the normal state pseudogap. The d-wave nature of the order parameter holds for both the superconducting gap and the pseudogap. Although an extensive body of evidence is reviewed, a consensus on the origin of the pseudogap is as lacking as it is for the mechanism underlying high-temperature superconductivity.

/pdf/the-pseudogap-in-high-temperature-superconductors-an-22i3tjkdfn.pdf

The pseudogap in high-temperature superconductors: an experimental survey

The proximity effect at superconductor-ferromagnet interfaces produces damped oscillatory behavior of the Cooper pair wave function within the ferromagnetic medium. This is analogous to the inhomogeneous superconductivity, predicted long ago by Fulde and Ferrell (P. Fulde and R. A. Ferrell, 1964, ``Superconductivity in a strong spin-exchange field,'' Phys. Rev. 135, A550--A563), and by Larkin and Ovchinnikov (A. I. Larkin and Y. N. Ovchinnikov, 1964, ``Inhomogeneous state of superconductors,'' Zh. Eksp. Teor. Fiz. 47, 1136--1146 [Sov. Phys. JETP 20, 762--769 (1965)]), and sought by condensed-matter experimentalists ever since. This article offers a qualitative analysis of the proximity effect in the presence of an exchange field and then provides a description of the properties of superconductor-ferromagnet heterostructures. Special attention is paid to the striking nonmonotonic dependence of the critical temperature of multilayers and bilayers on the ferromagnetic layer thickness as well as to the conditions under which ``$\ensuremath{\pi}$'' Josephson junctions are realized. Recent progress in the preparation of high-quality hybrid systems has permitted the observation of many interesting experimental effects, which are also discussed. Finally, the author analyzes the phenomenon of domain-wall superconductivity and the influence of superconductivity on the magnetic structure in superconductor-ferromagnet bilayers.

Proximity effects in superconductor-ferromagnet heterostructures

We have investigated the magnetic and the superconducting properties of the hybrid ruthenate-cuprate compound ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ by means of zero-field muon-spin rotation (ZF-\ensuremath{\mu}SR) and dc magnetization measurements. The dc-magnetization data established that this material exhibits ferromagnetic order of the Ru moments [\ensuremath{\mu}(Ru)\ensuremath{\approx}1 ${\ensuremath{\mu}}_{B}]$ below ${T}_{C}=133$ K and becomes superconducting at a much lower temperature ${T}_{c}=16$ K. The ZF-\ensuremath{\mu}SR experiments indicate that the ferromagnetic phase is homogeneous on a microscopic scale and accounts for most of the sample volume. They also suggest that the magnetic order is not significantly modified at the onset of superconductivity.

Coexistence of ferromagnetism and superconductivity in the hybrid ruthenate-cuprate compound RuSr 2 GdCu 2 O 8 studied by muon spin rotation and dc magnetization

Neutron diffraction has been used to study the magnetic order in ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}.$ The Ru moments order antiferromagnetically at ${T}_{N}=136(2)$ K, coincident with the previously reported onset of ferromagnetism. Neighboring spins are antiparallel in all three directions, with a low T moment of $1.18(6){\ensuremath{\mu}}_{B}$ along the c axis. Our measurements put an upper limit of $\ensuremath{\sim}0.1{\ensuremath{\mu}}_{B}$ to any net zero-field moment, with fields exceeding $0.4T$ needed to induce a measurable magnetization. The Gd ions order independently at ${T}_{N}=2.50(2)$ K with the same spin configuration.

/pdf/antiferromagnetic-ordering-of-ru-and-gd-in-superconducting-152vlzpn9s.pdf

Antiferromagnetic ordering of Ru and Gd in superconducting RuSr 2 GdCu 2 O 8

The muon spin depolarization rate \ensuremath{\sigma} was measured in overdoped ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{3}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$. \ensuremath{\sigma}(T\ensuremath{\rightarrow}0) was found to decrease proportional to the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ as doping \ensuremath{\delta} is increased. In the framework of the clean-limit London model, \ensuremath{\sigma}(0)\ensuremath{\sim}${\ensuremath{\lambda}}^{\mathrm{\ensuremath{-}}2}$\ensuremath{\sim}${\mathit{n}}_{\mathit{s}}$/${\mathit{m}}^{\mathrm{*}}$, this implies that the depression of ${\mathit{T}}_{\mathit{c}}$ by overdoping is associated with a decrease of the superconducting condensate density ${\mathit{n}}_{\mathit{s}}$ in spite of the increasing normal-state carrier density. This can be largely accounted for in terms of strong pair breaking, which depresses both the condensate density and ${\mathit{T}}_{\mathit{c}}$ with increased doping.

Muon spin rotation study of the correlation between Tc and ns/m* in overdoped Tl2Ba2CuO6+ delta.

We report the observation of microscopically coexisting ferromagnetism and superconductivity in a hybrid ruthenocuprate RuSr/sub 2/GdCu/sub 2/O/sub 8/, with T/sub c//spl les/40 K. By means of various substituents it is established that the superconductivity originates in the CuO/sub 2/ planes and the ferromagnetism in the RuO/sub 2/ planes, as expected. Muon spin relaxation measurements show that the ferromagnetism, with Curie temperature T/sub M/=132 K, is microscopically uniform and originates from the entire sample bulk. This is probably the first confirmed example of uniform microscopic coexistence of superconductivity and atomic ferromagnetism. The material is determined from thermopower measurements to be an underdoped cuprate with a projected T/sub c,max/=90-100 K, typical of a two-layer cuprate. The oxygen isotope effect exponent of /spl alpha//sub Tc/=1.7 is the largest observed in the high-T/sub c/ cuprates. These materials are expected to provide a rich source of new physics and applications.

Coexisting ferromagnetism and superconductivity in hybrid rutheno-cuprate superconductors

The thermoelectric power $S(T)$ is studied for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ as a function of Zn substitution and hole concentration $p$. A strong enhancement in $S(T)$ is correlated with the smooth opening of an energy gap in the normal-state spectrum, as observed from NMR and heat capacity data, which appears to be responsible for the $p$-dependent variation in ${T}_{c}$. The data are consistent with the local suppression of this energy gap in the immediate neighborhood of a Zn atom. Studies of $S(T)$ provide a simple means to explore the energy scales associated with the superconducting phase diagram and give some key insights into the origins of cuprate superconductivity.

J. L. Tallon

Papers

Coexistence of ferromagnetism and superconductivity in the hybrid ruthenate-cuprate compound RuSr 2 GdCu 2 O 8 studied by muon spin rotation and dc magnetization

Antiferromagnetic ordering of Ru and Gd in superconducting RuSr 2 GdCu 2 O 8

Muon spin rotation study of the correlation between Tc and ns/m* in overdoped Tl2Ba2CuO6+ delta.

Coexisting ferromagnetism and superconductivity in hybrid rutheno-cuprate superconductors

Thermoelectric power: A simple, instructive probe of high-Tc superconductors.