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

This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems---an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

How to detect fluctuating stripes in the high-temperature superconductors

Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.

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Scanning tunneling spectroscopy of high-temperature superconductors

Scanning tunneling spectroscopy of the high-Tc superconductor Bi2Sr2CaCu2O8+delta reveals weak, incommensurate, spatial modulations in the tunneling conductance. Images of these energy-dependent modulations are Fourier analyzed to yield the dispersion of their wavevectors. Comparison of the dispersions with photoemission spectroscopy data indicates that quasiparticle interference, due to elastic scattering between characteristic regions of momentum-space, provides a consistent explanation for the conductance modulations, without appeal to another order parameter. These results refocus attention on quasiparticle scattering processes as potential explanations for other incommensurate phenomena in the cuprates. The momentum-resolved tunneling spectroscopy demonstrated here also provides a new technique with which to study quasiparticles in correlated materials.

https://science.sciencemag.org/content/sci/297/5584/1148.full.pdf

Imaging Quasiparticle Interference in Bi2Sr2CaCu2O8+δ

The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging-scanning tunneling microscopy to study the electronic structure of a representative compound CaFe1.94Co0.06As2 in the "parent" state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter-iron-atom distance a(Fe-Fe) and aligned along the crystal a axis are observed. In contrast, the delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b axis only and are consistent with a nematic alpha2 band with an apparent band folding having wave vector q vector congruent with +/-2pi/8a(Fe-Fe) along the a axis. All these effects rotate through 90 degrees at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected.

https://science.sciencemag.org/content/sci/327/5962/181.full.pdf

Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1–xCox)2As2

We present scanning tunneling spectroscopy measurements of the CuO chain plane in YBa2Cu3O6+x, showing a similar to25 meV gap in the local density of states (LDOS) filled by numerous intragap resonances: intense peaks in LDOS spectra associated with one-d

Nanoscale one-dimensional scattering resonances in the CuO chains of YBa(2)Cu(3)O(6+x).

${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ exhibits properties that are characteristic of the parent compounds of the newly discovered iron (Fe)-based high-${T}_{C}$ superconductors. By combining real-space imaging of scanning tunneling microscopy and spectroscopy ($\mathrm{STM}+\mathrm{STS}$) with momentum-space quantitative low-energy electron diffraction (LEED), we have identified the surface plane of cleaved ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ crystals as the As terminated Fe-As layer---the plane where superconductivity occurs. LEED and $\mathrm{STM}+\mathrm{STS}$ data on the ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}(001)$ surface indicate an ordered arsenic (As) terminated metallic surface without reconstruction or lattice distortion. It is surprising that STM images the different Fe-As orbitals associated with the orthorhombic structure, but not the As atoms in the surface plane.

Surface Geometric and Electronic Structures of BaFe2As2(001)

Thin films of the superconductor Bi-Sr-Ca-Cu-O (BSCCO) have been prepared by thermal evaporation in an evaporator featuring an RF-excited oxygen plasma generator. Formation of the 2212 phase is obtained in situ, as confirmed by X-ray analysis. The films require postannealing, however, in order to exhibit a superconducting transition. Specifically, postanneals are required to obtain critical temperatures of up to 75 K; 1330 Pa is the minimum annealing pressure. The authors report the study of the superconducting properties as a function of oxygen annealing pressure as well as a characterization of the oxygen plasma. An investigation of the surface morphology was performed using a force microscopy (AFM) and a tunneling microscopy (STM). The latter clearly shows terraces and steps with a height of 1.5 nm, or multiples thereof, corresponding to one-half of the c-axis lattice constant of the 2212 compound. The AFM, on the other hand, shows a drastic difference between as-grown and annealed films, even when the former are sometimes insulating.

In-situ formation of BSCCO thin films by plasma assisted thermal evaporation

Guorong Li, Xiaobo He, Ang Li, Shuheng H. Pan, Jiandi Zhang, Rongying Jin, A. S. Sefat, M. A. McGuire, D. G. Mandrus, B. C. Sales, and E. W. Plummer Department of Physics & Astronomy, Louisiana State University, Baton Rouge, Louisiana 70802, USA Department of Physics & Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5002, USA Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA and Department of Materials Science & Engineering, The University of Tennessee, Knoxville, Tennessee 37996, USA

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Shuheng Pan

Papers

Nanoscale one-dimensional scattering resonances in the CuO chains of YBa(2)Cu(3)O(6+x).

Surface Geometric and Electronic Structures of BaFe2As2(001)

In-situ formation of BSCCO thin films by plasma assisted thermal evaporation

BaFe2As2 Surface Domains and Domain Walls: Mirroring the Bulk Spin Structure

Nanoscale one-dimensional scattering resonances in the CuO chains in YBa{sub 2}Cu{sub 3}O{sub 6+x}