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 properties of the Y1−xPrxBa2Cu3O7 (YPrBCO) system are reviewed. These include superconducting, normal state, structural, chemical, optical, magnetic, and thermal properties. The destruction of superconductivity with Pr doping is discussed in view of possible models such as hole filling, localization, magnetic pair-breaking, and the role of hybridization. Applications to electronic devices using YBCO/PrBCO/YBCO multilayers are also reviewed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400017489

A review of the superconducting and normal state properties of Y1−xPrxBa2Cu3O7

The discovery of superconductors with high critical temperatures has led to a considerable effort to fabricate Josephson junctions operating at temperatures approaching, or even exceeding, 77 K for both scientific investigations and potential applications. Superconductor - normal - superconductor (SNS) devices, with noble or oxide metals as normal interlayers, are perhaps the most widely explored high- junction type at present. Although demonstrations of individual high- SNS devices exhibiting excellent current - voltage characteristics, high critical current - resistance products, and low noise behaviour have been made, reproducible devices suitable for electronic applications are elusive. It is therefore important to ask how well these nominally SNS high- junctions are understood. We review the available data, with emphasis on junction critical currents, and conclude that there is little evidence supporting a conventional proximity effect interpretation in the majority of reported high- devices. The strongest candidates for SNS behaviour are junctions in which N is a superconductor above its transition temperature. We discuss the present experimental and theoretical understanding of SNS junctions with emphasis on the implications for future research and development of these devices.

https://iopscience.iop.org/article/10.1088/0953-2048/9/4/001/pdf

Stationary properties of high-critical-temperature proximity effect Josephson junctions

The traditional distinction between tunnel and highly transmissive barriers does not currently hold for high critical temperature superconducting Josephson junctions, both because of complicated materials issues and the intrinsic properties of high temperature superconductors (HTS). An intermediate regime, typical of both artificial superconductor–barrier–superconductor structures and of grain boundaries, spans several orders of magnitude in the critical current density and specific resistivity. The physics taking place at HTS surfaces and interfaces is rich, primarily because of phenomena associated with d-wave order parameter (OP) symmetry. These phenomena include Andreev bound states, the presence of the second harmonic in the critical current versus phase relation, a doubly degenerate state, time reversal symmetry breaking and the possible presence of an imaginary component of the OP. All these effects are regulated by a series of transport mechanisms, whose rules of interplay and relative activation are unknown. Some transport mechanisms probably have common roots, which are not completely clear and possibly related to the intrinsic nature of high-TC superconductivity. The d-wave OP symmetry gives unique properties to HTS weak links, which do not have any analogy with systems based on other superconductors. Even if the HTS structures are not optimal, compared with low critical temperature superconductor Josephson junctions, the state of the art allows the realization of weak links with unexpectedly high quality quantum properties, which open interesting perspectives for the future. The observation of macroscopic quantum tunnelling and the qubit proposals represent significant achievements in this direction. In this review we attempt to encompass all the above aspects, attached to a solid experimental basis of junction concepts and basic properties, along with a flexible phenomenological background, which collects ideas on the Josephson effect in the presence of d-wave pairing for different types of barriers.

http://iopscience.iop.org/article/10.1088/0034-4885/68/11/R03/pdf

Weak links in high critical temperature superconductors

Josephson junctions of the SNS type have been fabricated in the edge and trilayer junction geometries using YBa2Cu3O7−δ/CaRuO3/YBa2Cu3O7−δ. The conducting oxide CaRuO3 was chosen because it grows epitaxially on YBa2Cu3O7−δ and its resistivity shows metallic behavior. Current‐voltage characteristics display behavior with significant deviations from the resistivity shunted junction model. The dependence of critical currents and junction resistances on the CaRuO3 thickness suggest that the interface resistance between YBa2Cu3O7−δ and CaRuO3 plays an important role in determining the critical parameters of the junctions. A dc SQUID functioning at 77 K has been demonstrated with this type of junction.

High Tc superconductor‐normal‐superconductor Josephson junctions using CaRuO3 as the metallic barrier

We report critical-current measurements in all high-${\mathit{T}}_{\mathit{c}}$ superconducting-normal-superconductor junctions using ${\mathrm{Y}}_{0.6}$${\mathrm{Pr}}_{0.4}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (with ${\mathit{T}}_{\mathit{c}\mathit{N}}$=40 K) as the normal metal. Above ${\mathit{T}}_{\mathit{c}\mathit{N}}$, we find a clear exponential dependence of ${\mathit{I}}_{\mathit{c}}$ on the thickness of the barrier which is characteristic of the proximity effect. The order-parameter decay length is about 120 \AA{} for Tg60 K, and it diverges as ${\mathit{T}}_{\mathit{c}\mathit{N}}$ is approached. We estimate that ${\ensuremath{\xi}}_{0}$ for this material is 80\ifmmode\pm\else\textpm\fi{}25 \AA{}.

Proximity effect in YBa2Cu3O7/Y0.6Pr0.4Ba2Cu3O7/YBa2Cu3O7 junctions

Thin‐film YBa2Cu3O7‐YBa2Cu3O7 edge junctions of 0.4×10 μm2 cross section were prepared in situ by a multistep laser ablation deposition process. The fabrication time was about 3 h and the yield of good devices was 50%. Typical junctions reached zero resistance at 72 K and had a critical current density Jc of 300 A/cm2 at 70 K. Their Jc as a function of temperature increased slowly with decreasing temperature down to 65 K and much faster below it. In the region of low Jc we observed suppression of the critical current by a magnetic field. Under microwave radiation clear Shapiro steps were observed whose magnitude versus the microwave field agreed qualitatively with the resistively shunted junction model of a current biased junction.

Properties of all YBa2Cu3O7 Josephson edge junctions prepared by in situ laser ablation deposition

We report measurements of the differential resistance of (YBa 2 Cu 3 O 7-δ )-normal-metal-(YBa 2 Cu 3 O 7-δ ) edge Josephson junctions. We interpret the observed features as arising from Andreev reflections, and deduce the temperature dependence and the anisotropy of the in-plane energy gap. We find 2Δb/kT c =4.2 and 2Δa/kT c =6.2. In addition, we discuss features at eV>2Δ, which appear at harmonics of the gap, and which we interpret as arising from the presence of correlated phase-slip centers near the junction

Measurements of the anisotropy and temperature dependence of the in-plane energy gap in YBa2Cu3O7- delta using Andreev reflections.

a‐b plane YBa2Cu3O7‐YBa2Cu3O7 (YBCO) edge junctions with improved properties were prepared by the use of SrTiO3 instead of BaF2 as an insulating layer in the c‐axis direction. The junctions were fabricated in an all in situ laser ablation deposition process, with the barriers formed by a plasma discharge in CF4 gas. Photolithographic rather than laser ablation patterning of the junctions was used, to avoid possible laser induced damage to the films. The normal resistance of the junction decreased with T for T≳40 K, and increased at lower temperatures, indicating that carrier localization takes place in the barrier. At 80 K, a pronounced diffraction pattern of the voltage across the junction versus magnetic field was observed. Critical currents persisted up to 84 K, and under microwave radiation many Shapiro steps were observed.

Characteristics of all YBa2Cu3O7 edge junctions operating above 80 K

Preparation and characterization of dc SQUIDs based on YBa2Cu3O7/Ag/YBa2Cu3O7 edge junctions is reported. The SQUIDs were prepared in situ by a multistep all laser ablation deposition process. Since the Ag on YBa2Cu3O7 was exposed to 730 °C in O2 ambient, the barrier resistance was quite high, ∼0.6 Ω. The SQUIDs showed voltage periodicity versus magnetic field H⊥ up to ∼80 K and had a maximum response of 30 μV/Φ0 at ∼70 K, where IcRN was 30 μV. The SQUID response versus H⊥ was superimposed on a diffractionlike pattern of its junctions, indicating an improved uniformity in the junction cross section. When the SQUID was exposed to microwave radiation, many pronounced Shapiro steps were observed in the I‐V curve, which modulated with the microwave power.

E. Aharoni

Papers

Proximity effect in YBa2Cu3O7/Y0.6Pr0.4Ba2Cu3O7/YBa2Cu3O7 junctions

Properties of all YBa2Cu3O7 Josephson edge junctions prepared by in situ laser ablation deposition

Measurements of the anisotropy and temperature dependence of the in-plane energy gap in YBa2Cu3O7- delta using Andreev reflections.

Characteristics of all YBa2Cu3O7 edge junctions operating above 80 K

dc SQUID made of Ag‐doped YBa2Cu3O7 a‐b plane‐edge junctions