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

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.

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The pseudogap in high-temperature superconductors: an experimental survey

When asked to explain the importance of the discovery of conducting polymers, I offer two basic answers: first they did not (could not?) exist, and second, that they offer a unique combination of properties not available from any other known materials. The first expresses an intellectual challenge; the second expresses a promise for utility in a wide variety of applications.

Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials (Nobel Lecture).

The demand for ever-increasing density of information storage and speed of manipulation has triggered an intense search for ways to control the magnetization of a medium by means other than magnetic fields. Recent experiments on laser-induced demagnetization and spin reorientation use ultrafast lasers as a means to manipulate magnetization, accessing timescales of a picosecond or less. However, in all these cases the observed magnetic excitation is the result of optical absorption followed by a rapid temperature increase. This thermal origin of spin excitation considerably limits potential applications because the repetition frequency is limited by the cooling time. Here we demonstrate that circularly polarized femtosecond laser pulses can be used to non-thermally excite and coherently control the spin dynamics in magnets by way of the inverse Faraday effect. Such a photomagnetic interaction is instantaneous and is limited in time by the pulse width (approximately 200 fs in our experiment). Our finding thus reveals an alternative mechanism of ultrafast coherent spin control, and offers prospects for applications of ultrafast lasers in magnetic devices.

Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses.

Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.

https://iopscience.iop.org/article/10.1088/0034-4885/58/11/003/pdf

The flux-line lattice in superconductors

REBa2Cu3Oy superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T c due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa2Cu3Oy with fine Y2BaCuO5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1-x , REx )2Cu3Oy phase in a good superconductive matrix.

https://iopscience.iop.org/article/10.1143/JJAP.33.L715/pdf

Flux pinning in melt-grown NdBa2Cu3Oy and SmBa2Cu3Oy superconductors

We have studied the superconducting properties of Nd(Ba1-xNdx)2Cu3O7-δ (Nd123, x≈0.1) single crystals grown by the traveling-solvent floating-zone method under 0.1% O2 in Ar atmosphere. An anomalous peak effect in the magnetization hysteresis (M-B) loop is observed in the Nd123 single crystals as well as in the Nd123 bulk crystals prepared by the oxygen-controlled-melt-growth (OCMG) method. The critical current density (JC) of the Nd123 single crystals is 70,600 A/cm2 in 1.0T at 77K for the applied field perpendicular to the ab-plane. The existence of Nd and Ba composition variation in the matrix of Nd123 crystals is observed by an analytical TEM equipped with a field-emission gun. The high Jc value of the Nd123 single crystals in the applied field is explained by the field-induced pinning centers caused by the Nd-Ba substitutions in the Nd123 matrix.

High critical‐current density of Nd(Ba,Nd)2Cu3O7−δ single crystals

An extremely large levitation force of as high as 30 N at a height of 1 mm was achieved in Ag-doped YBaCuO fabricated by the Melt-Powder-Melt-Growth process using a repulsive force against a 0.4 T rare-earth magnet at 77 K. The combination of a large Jc value and large shielding current loop is the source of such a large levitation force.

https://iopscience.iop.org/article/10.1143/JJAP.29.L1991/pdf

Large Levitation Force due to Flux Pinning in YBaCuO Superconductors Fabricated by Melt-Powder-Melt-Growth Process

A compact waveguide optical isolator whose input and output light are TE or TM mode has been realized by making adjacent nonreciprocal and reciprocal TE‐TM mode converters in a (BiGdLu)3(FeGa)5O12 film. Designed configurations of the magnetization directions were produced by making use of local laser annealing of the film grown on a Gd3Ga5O12 substrate of which the face normal is tilted from the [111] axis. The laser‐annealed and nonannealed regions worked as the nonreciprocal and reciprocal mode converters, respectively. The observed isolation agreed with theoretical calculation. An isolation ratio of 12.5 dB was achieved at a wavelength λ=1.15 μm.

Waveguide magneto‐optic isolator fabricated by laser annealing

Highly flat, clean, and stable surfaces of as-prepared Nd1Ba2Cu3Oy (Nd123) single crystals have been studied using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV-STM/STS) system. The results of large area STM scans together with the results of STS measurements, magnetization loops measured at several temperatures below Tc, and critical current density measurements reveal the presence of a new magnetic flux pinning center in Nd123 single crystals prepared in a reduced oxygen partial pressure. This pinning center is more conductive on the surface than the matrix and has a size ranging from several to several tens of nanometers as seen by STM. Similar pinning center may be introduced to other high temperature superconductors to increase their critical current densities in an applied magnetic field.

Naoki Koshizuka

Papers

Flux pinning in melt-grown NdBa2Cu3Oy and SmBa2Cu3Oy superconductors

High critical‐current density of Nd(Ba,Nd)2Cu3O7−δ single crystals

Large Levitation Force due to Flux Pinning in YBaCuO Superconductors Fabricated by Melt-Powder-Melt-Growth Process

Waveguide magneto‐optic isolator fabricated by laser annealing

Evidence of a new magnetic flux pining center in Nd1Ba2Cu3Oy single crystals