Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi2Sr2CaCu2O8+delta. This gap exists only in underdoped samples and closes around the doping level at which the superconducting transition temperature Tc is a maximum. The momentum dependence and magnitude of the gap closely resemble those of the dx2-y2 gap observed in the superconducting state. This observation is consistent with results from several other experimental techniques, which also indicate the presence of a gap in the normal state. Some possible theoretical explanations for this effect are reviewed.

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Magnetic topological insulators provide an important materials platform to explore emergent quantum phenomena such as the quantized anomalous Hall (QAH) effect, Majorana modes and the axion insulator state, etc. Recently, MnBi2Te4 was discovered to be the first material realization of a van der Waals (vdW) antiferromagnetic topological insulator (TI). In the two-dimensional (2D) limit, at a record high temperature of 4.5 K, MnBi2Te4 manifests the QAH effect in the forced ferromagnetic state above 12 T. To realize the QAH effect at lower fields, it is essential to search for magnetic TIs with lower saturation fields. By realizing a bulk vdW material MnBi4Te7 with alternating [MnBi2Te4]and [Bi2Te3] layers, we suggest that it is ferromagnetic in plane but antiferromagnetic along the c axis with a small out-of-plane saturation field of ~ 0.22 T at 2 K. Our angle-resolved photoemission spectroscopy and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Therefore, MnBi4Te7 provides a new material platform to investigate emergent topological phenomena associated with the QAH effect at much lower magnetic fields in its 2D limit.

The k-dependent electronic structure of the low temperature ferromagnetic state of La 1.2 Sr 1.8 Mn 2 O 7 was measured using angle-resolved photoemission spectroscopy and calculated using the local spin density approximation (LSDA). The measured near-Fermi energy states display E vs k and symmetry relationships which agree relatively well with the LSDA prediction through much of the Brillouin zone, and the locus of lowest energy excitations matches the predicted large Fermi surface quite well. However, the spectral features are too broad to be well described as Fermi-liquid-like quasiparticles, and they are strongly suppressed from the Fermi energy, i.e., there is a pseudogap in the excitation spectrum. We discuss the spectral properties in terms of strong coupling to a local effect such as a lattice distortion. [S0031-9007(98)06522-3]

Dessau, Shen, and Marshall Reply: In our Letter, we reported on high energy-resolution angle-resolved photoemission (ARPES) data taken from three Bi2212 samples [1]. All of these samples showed a gap anisotropy in the a-b plane which is very much larger than in conventional superconductors. This was the main point of our paper, and precluded the possibility of the isotropic ^wave gap A^ =A^o. We further compared our data to the k-space dependences that one would expect from other more complicated forms of the order parameter, including a simple form for the extended ^--wave gap A^ex -[QOs{kxa)-^cos{kya)] and the d^i^yi wave gap A^ --[cos(kxa) -cos(kya)], as well as other suggestions in the literature such as the mixed symmetry order parameter s + id. The direction of the gap anisotropy Uiat our measurement displayed-maximum near the M point (/r,0) and minimum near the V-XiV) zone diagonal -was consistent with the d^i-yi and s+id gaps, but was inconsistent with the simplest extended ^--wave gap (e.g.

Over the last several years there have been great improvements in the energy resolution and detection efficiency of angle-resolved photoemission spectroscopy. These improvements have made it possible to discover a number of fascinating features in the electronic structure of the high transition temperature (T(c)) superconductors: apparently bandlike Fermi surfaces, flat-band saddle points, and nested Fermi surface sections. Recent work suggests that these features, previously thought explainable only by one-electron band theory, may be better understood with a many-body approach. Furthermore, other properties of the high-T(c) superconductors, which are difficult to understand with band theory, are well described using a many-body picture. Angle-resolved photoemission spectroscopy has also been used to investigate the nature of the superconducting pairing state, revealing an anisotropic gap consistent with a d-wave order parameter and fueling the current debate over s-wave versus d-wave superconductivity.

We used high resolution angle-resolved photoemission spectroscopy to reveal the Fermi surface and key transport parameters of the metallic state of the layered Colossal Magnetoresistive (CMR) oxide La 1.2 Sr 1.8 Mn 2 O 7 . With these parameters the calculated inplane conductivity is nearly one order of magnitude larger than the measured DC conductivity. This discrepancy can be accounted for by including the pseudogap which removes at least 90% of the spectral weight at the Fermi energy. Key to the pseudogap and many other properties are the parallel straight Fermi surface sections which are highly susceptible to nesting instabilities. These nesting instabilities produce nanoscale fluctuating charge/orbital modulations which cooperate with Jahn-Teller distortions and compete with the electron itinerancy favored by double exchange.3

A Fermi arc 1,2 is a disconnected segment of a Fermi surface observed in the pseudogap phase 3,4 of cuprate superconductors. This simple description belies the fundamental inconsistency in the physics of Fermi arcs, specifically that such segments violate the topological integrity of the band 5 . Efforts to resolve this contradiction of experiment and theory have focused on connecting the ends of the Fermi arc back on itself to form a pocket, with limited and controversial success 6-9 . Here we show the Fermi arc, although composed of real spectral weight, lacks the quasiparticles to be a true Fermi surface 5 . To reach this conclusion we developed a new photoemission-based technique that directly probes the interplay of pair-forming and pair-breaking processes with unprecedented precision. We find the spectral weight composing the Fermi arc is shifted from the gap edge to the Fermi energy by pair-breaking processes 10 . Although real, this weight does not form a true Fermi surface, because the quasiparticles, although significantly broadened, remain at the gap edge. This non-quasiparticle weight may account for much of the unexplained behaviour of the pseudogap phase of the cuprates.