We report the electronic structure of the iron-chalcogenide superconductor, Fe 1.04 ͑Te 0.66 Se 0.34 ͒, obtained with high-resolution angle-resolved photoemission spectroscopy and density-functional calculations. In photoemission measurements, various photon energies and polarizations are exploited to study the Fermi surface topology and symmetry properties of the bands. The measured band structure and their symmetry characters qualitatively agree with our density-functional theory calculations of Fe͑Te 0.66 Se 0.34 ͒, although the band structure is renormalized by about a factor of three. We find that the electronic structures of this iron chalcogenides and the iron pnictides have many aspects in common; however, significant differences exist near the ⌫ point. For Fe 1.04 Te 0.66 Se 0.34 , there are clearly separated three bands with distinct even or odd symmetry that cross the Fermi energy ͑E F ͒ near the zone center, which contribute to three holelike Fermi surfaces. Especially, both experiments and calculations show a holelike elliptical Fermi surface at the zone center. Moreover, no sign of spin density wave was observed in the electronic structure and susceptibility measurements of this compound.

The magnetic properties in the parent compounds are often intimately related to the microscopic mechanism of superconductivity. Here we report the first direct measurements on the electronic structure of a parent compound of the newly discovered iron-based superconductor, BaFe2As2, which provides a foundation for further studies. We show that the energy of the spin density wave in BaFe2As2 is mainly lowered through exotic exchange splitting of the band structure.

In the present photoemission study of a cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta, we discovered a large scale dispersion of the lowest band, which unexpectedly follows the band structure calculation very well. Similar behavior observed in blue bronze and the Mott insulator Ca2CuO2Cl2 suggests that the origin of hopping-dominated dispersion in an overdoped cuprate might be quite complicated. A giant kink in the dispersion is observed, and the complete self-energy containing all interaction information is extracted for a doped cuprate. These results recovered significant missing pieces in our current understanding of the electronic structure of cuprates.

Angle resolved photoemission spectroscopy study is reported on a high quality optimally doped Bi2Sr1.6La0.4CuO 6+δ high-Tc superconductor. In the antinodal region with maximal d-wave gap, the symbolic superconducting coherence peak, which has been widely observed in multi-CuO2-layer cuprate superconductors, is unambiguously observed in a single layer system. The associated peakdip separation is just about 19 meV, which is much smaller than its counterparts in multi-layered compounds, but correlates with the energy scales of spin excitations in single layer cuprates.In search of the mechanism of high temperature superconductivity, one central issue is whether there are certain bosons that play the critical mediating role of phonons in conventional BCS superconductor. Specifically, if there were such bosons, what would they be? So far, signatures of electron-boson interactions have been identified in single particle excitations measured by angle resolved photoemission spectroscopy (ARPES). For example, in the so-called nodal region, where the d-wave superconducting gap diminishes, a characteristic kink [1,2] was discovered in the dispersion of various cuprate superconductors. However, the nature of the corresponding boson, particularly whether it is due to lattice or spin excitations, was intensively debated.For multi-CuO 2 -layer cuprate superconductors, such as Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi2223) and YBa 2 Cu 3 O 7−δ (YBCO) [3,4,5,6,7], a sharp peak emerges out of the normal state broad spectrum when temperature is lowered below the superconducting transition temperature (T c ), forming a so-called "peak-dip-hump"(PDH) structure (see Fig. 4c for illustration) in the antinodal region, where d-wave superconducting gap is at its maximum. This sharp peak, also termed as superconducting coherence peak (SCP), contains rich information about superconductivity. Its position reflects the maximal pairing strength, while its intensity grows with decreasing temperature, illustrating the gain of coherence through the development of superconducting condensate or superfluid [4,8,9]. The PDH structure has been regarded as an evidence for interactions with a bosonic mode [10,11,12,13,14,15,16,17,18]. However, since this mode energy (∼ 35 meV) coincides with the energy of certain oxygen phonon and spin excitations near (π, π), scenarios based on both kinds of bosons have been put forward.In spite of its importance, an unambiguous observation of SCP or two-component spectrum in the antinodal region of single layer compounds is so far lacking [6,19,20,21,22]. In this letter, we report the discovery of the antinodal SCP in an optimally doped single layer Bi 2 Sr 2 CuO 6+δ (Bi2201). We found the peak-dip distance to be about 19 meV, suggesting the possibility of a bosonic mode in Bi2201 with a much lower energy than that in Bi2212. This low energy mode seems to correlate with the superconducting gap and the energy scale of spin excitations observed in various single layer compounds.Bi 2 Sr 1.6 La 0.4 ...

The electronic structure of a new charge-density-wave/ superconductor system, 1T-CuxTiSe2, has been studied by photoemission spectroscopy. A correlated semiconductor band structure is revealed for the undoped case. With Cu doping, the charge density wave is suppressed by the raising of the chemical potential, while the superconductivity is enhanced by the enhancement of the density of states. Moreover, the strong scattering at high doping might be responsible for the suppression of superconductivity in that regime. [5,6,7], whereas it rarely exists in 1T structured compounds.Recently, the discovery of superconductivity in 1T-Cu x TiSe 2 has further enriched the physics of TMD's [8]. The undoped 1T-TiSe 2 is a CDW material, whose mechanism remains controversial after decades of research. For example, some considered the CDW a band-type Jahn-Teller effect, where the electronic energy is lowered through structural distortion [9,10]. Some considered it a realization of the excitonic CDW mechanism proposed by Kohn in the 1960's [11,12]; but different models were proposed to interpret the electronic structure, depending on whether system was argued to be a semimetal, or a semiconductor [13,14]. With Cu doping, it was found that the CDW transition temperature quickly drops, similar to other M x TiSe 2 's (M=Fe,Mn,Ta,V and Nb) [15,16,17,18]. Meanwhile, the superconducting phase emerges from x ∼ 0.04 and reaches the maximal transition temperature of 4.3K at x ∼ 0.08, then decreases to 2.8K at x ∼ 0.10. Quite remarkably, this phase diagram resembles those of the cuprate and heavy fermion superconductors [19], except here the competing order of superconductivity is the charge order, instead of the antiferromagnetic spin order. The presence of this ubiquitous phase diagram in 1T-Cu x TiSe 2 calls for a detailed study of its electronic structure. In particular, the information retrieved might help resolve the controversy on the CDW mechanism for 1T-TiSe 2 .We studied 1T-Cu x TiSe 2 with high resolution angle resolved photoemission spectroscopy (ARPES). A correlated semiconductor band structure of the undoped system is evidently illustrated, resolving a long-standing controversy. Cu doping is found to effectively enhance the density of states around the Fermi energy (E F ), which explains the enhancement of superconductivity. On the other hand, severe inelastic scattering was observed near the solubility limit, corresponding to the drop of superconducting transition temperature in that regime. With increased doping, chemical potential is raised, and signs of the weakening electron-hole coupling is discovered, which is responsible for the suppression of the CDW. Our results indicate that the seeming "competition" between CDW and superconductivity in the phase diagram is a coincidence caused by different effects of doping in this 1T compound, in contrast to the 2H-TMD case [3].1T-Cu x TiSe 2 single crystals were prepared by the vapor-transport technique, with doping x = 0, 0. 015, 0.025, 0.055, 0.065 and 0.11 (accurate within ...

Topological insulators are electronic materials with an insulating bulk and conducting surface. However, due to free carriers in the bulk, the properties of the metallic surface are difficult to detect and characterize in most topological insulator materials. Recently, a new topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) was found, showing high bulk resistivities of 1–10 Ω.cm and greater contrast between the bulk and surface resistivities compared to other Bi-based topological insulators. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we present complex conductivity of BSTS single crystals, disentangling the surface and bulk contributions. We find that the Drude spectral weight is 1–2 orders of magnitude smaller than in other Bi-based topological insulators, and similar to that of Bi2Se3 thin films, suggesting a significant contribution of the topological surface states to the conductivity of the BSTS sample. Moreover, an impurity band is present about 30 meV below the Fermi level, and the surface and bulk carrier densities agree with those obtained from transport data. Furthermore, from the surface Drude contribution, we obtain a ~98% transmission through one surface layer — this is consistent with the transmission through single-layer or bilayer graphene, which shares a common Dirac-cone feature in the band structure.

The nature of the spin-density wave (SDW) and its relation with superconductivity are crucial issues in the newly discovered iron-pnictide superconductors. Particularly, it is unclear whether the superconducting phase and SDW are truly exclusive from each other. We here report splittings of the band structures in Sr1-xKxFe2As2 (x=0, 0.1, 0.18), and their unusual doping dependence. Our data on single crystalline samples prove that the SDW and superconductivity could coexist in iron pnictides.

NbSe2 is a prototypical charge-density-wave (CDW) material, whose mechanism remains mysterious so far. With angle resolved photoemission spectroscopy, we recovered the long-lost nesting condition over a large broken-honeycomb region in the Brillouin zone, which consists of six saddle band point regions with high density of states (DOS), and large regions away from Fermi surfaces with negligible DOS at the Fermi energy. We show that the major contributions to the CDW formation come from these barely occupied states rather than the saddle band points. Our findings not only resolve a long-standing puzzle, but also overthrow the conventional wisdom that CDW is dominated by regions with high DOS.