A detailed phenomenology of low energy excitations is a crucial starting point for microscopic understanding of complex materials, such as the cuprate high-temperature superconductors. Because of its unique momentum-space discrimination, angle-resolved photoemission spectroscopy (ARPES) is ideally suited for this task in the cuprates, where emergent phases, particularly superconductivity and the pseudogap, have anisotropic gap structure in momentum space. We present a comprehensive doping-and temperaturedependence ARPES study of spectral gaps in Bi 2 Sr 2 CaCu 2 O 8+δ , covering much of the superconducting portion of the phase diagram. In the ground state, abrupt changes in near-nodal gap phenomenology give spectroscopic evidence for two potential quantum critical points, p = 0.19 for the pseudogap phase and p = 0.076 for another competing phase. Temperature dependence reveals that the pseudogap is not static below T c and exists p > 0.19 at higher temperatures. Our data imply a revised phase diagram that reconciles conflicting reports about the endpoint of the pseudogap in the literature, incorporates phase competition between the superconducting gap and pseudogap, and highlights distinct physics at the edge of the superconducting dome.quantum materials | correlated electrons | laser ARPES T he momentum-resolved nature of angle-resolved photoemission spectroscopy (ARPES) makes it a key probe of the cuprates, the interesting phases of which have anisotropic momentumspace structure (1-4): both the d-wave superconducting gap and the pseudogap above T c have a maximum at the antinode [AN, near (π, 0)] and are ungapped at the node, although the latter phase also exhibits an extended ungapped arc (5-8). Ordering phenomena often result in gapping of the quasiparticle spectrum, and distinct quantum states produce spectral gaps with characteristic temperature, doping, and momentum dependence. These phenomena were demonstrated by recent ARPES experiments that argued that the pseudogap is a distinct phase from superconductivity based on their unique phenomenology (8-15): the pseudogap dominates near the AN (8, 11), and its magnitude increases with underdoping (11, 12), whereas near-nodal (NN) gaps have a different doping dependence and can be attributed to superconductivity because they close at T c (8, 12). Previous measurements focused on AN or intermediate (IM) momenta, but laser-ARPES, with its superior resolution and enhanced statistics, allows for precise gap measurements near the node where the gap is smallest. Our work is unique in its attention to NN momenta using laser-ARPES, and we demonstrate, via a single technique, that three distinct quantum phases manifest in different NN phenomenology as a function of doping. ResultsGaps at parallel cuts were determined by fitting symmetrized energy distribution curves (EDCs) at k F to a minimal model (16).The Fermi wavevector, k F , is defined by the minimum gap locus. Example spectra, raw and symmetrized EDCs at k F , and fits are shown for UD92 (underdoped, T c = 92) ...

Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.

An ordered phase showing remarkable electronic anisotropy in proximity to the superconducting phase is now a hot issue in the field of high-transition-temperature superconductivity. As in the case of copper oxides, superconductivity in iron arsenides competes or coexists with such an ordered phase. Undoped and underdoped iron arsenides have a magnetostructural ordered phase exhibiting stripe-like antiferromagnetic spin order accompanied by an orthorhombic lattice distortion; both the spin order and lattice distortion break the tetragonal symmetry of crystals of these compounds. In this ordered state, anisotropy of in-plane electrical resistivity is anomalous and difficult to attribute simply to the spin order and/or the lattice distortion. Here, we present the anisotropic optical spectra measured on detwinned BaFe 2 As 2 crystals with light polarization parallel to the Fe planes. Pronounced anisotropy is observed in the spectra, persisting up to an unexpectedly high photon energy of about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below and slightly above the onset of the order. Detailed analysis of the optical spectra reveals an unprecedented electronic state in the ordered phase.anisotropic electronic state | iron pnictide | optical spectrum H igh-transition-temperature (high-T c ) superconductivity realized in both copper oxides and iron arsenides shares common features, namely, the superconducting phase is in close proximity to a symmetry-breaking phase and these phases coexist under certain circumstances, but apparently compete with each other. The close proximity suggests that our understanding of high-T c superconductivity will greatly improve once the nature of this proximate phase is revealed. The parent compounds of iron-arsenide superconductors, with BaFe 2 As 2 as a representative example, are unique metals that undergo a tetragonal-toorthorhombic structural phase transition at temperature T s with a shorter b axis and a longer a axis in the orthorhombic phase always accompanied by antiferromagnetic (AF) spin order at temperature T N . T N is equal to T s in some compounds (1-3) and slightly lower than T s in others (4). BaFe 2 As 2 exhibits stripe-like AF order in which Fe spins align antiferromagnetically in the a-axis direction in the Fe plane and ferromagnetically in the b-axis direction. Anisotropic electronic properties have been experimentally examined by various methods, such as neutron scattering (5), scanning tunneling microscopy (STM) (6), and angleresolved photoemission spectroscopy (ARPES) (7,8). These experiments suggest strong anisotropy of spin excitation and of the shape of Fermi surfaces. However, most of the experiments were performed on twinned crystals with randomly oriented domains, which inhibit the observation of genuine anisotropy.Recently, anisotropic resistivity has been measured on detwinned crystals (9, 10). The anisotropy of resistivity is quite anomalous in that the resistivity along the spin-ferromagnetic (FM) direction with a shorter b axis is...