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We use Angle Resolved Photoemission Spectroscopy (ARPES) to study the relationship between the pseudogap, pairing and Fermi arcs in cuprates. High quality data measured over a wide range of dopings reveals a consistent picture of Fermiology and pairing in these materials. The pseudogap is due to an ordered state that competes with superconductivity rather than preformed pairs. Pairing does occur below T_pair ~ 150K and significantly above T_c, but well below T* and the doping dependence of this temperature scale is distinct from that of the pseudogap. The d-wave gap is present below T_pair, and its interplay with strong scattering creates “artificial” Fermi arcs for T_c ≤ T ≤ T_pair. However, above T_pair, the pseudogap exists only at the antipodal region. This leads to presence of real, gapless Fermi arcs close to the node. The length of these arcs remains constant up to T*, where the full Fermi surface is recovered. As a result, we demonstrate that these findings resolve a number of seemingly contradictory scenarios.

We use tunable laser-based angle-resolved photoemission spectroscopy to study the electronic structure of the multiband superconductor MgB₂. These results form the baseline for detailed studies of superconductivity in multiband systems. We find that the magnitude of the superconducting gap on both σ bands follows a BCS-like variation with temperature with Δ₀ ~ 7meV. Furthermore, the value of the gap is isotropic within experimental uncertainty and in agreement with a pure s-wave pairing symmetry. We observe in-gap states confined to k_F of the σ band that occur at some locations of the sample surface. As a result, the energy of this excitation, ~ 3 meV, was found to be somewhat larger than the previously reported gap on π Fermi sheet and therefore we cannot exclude the possibility of interband scattering as its origin.

We use tunable laser-based angle-resolved photoemission spectroscopy to study the electronic structure of the multiband superconductor MgB₂. These results form the baseline for detailed studies of superconductivity in multiband systems. We also find that the magnitude of the superconducting gap on both σ bands follows a BCS-like variation with temperature with Δ₀ ~ 7meV. The value of the gap is isotropic within experimental uncertainty and in agreement with a pure s-wave pairing symmetry. Furthermore, we observe in-gap states confined to k_F of the σ band that occur at some locations of the sample surface. The energy of this excitation, ~ 3 meV, is somewhat larger than the previously reported gap on π Fermi sheet and therefore we cannot exclude the possibility of interband scattering as its origin.

In this study, we use ultrahigh resolution, tunable, vacuum ultraviolet laser-based, angle-resolved photoemission spectroscopy (ARPES), temperature- and field-dependent resistivity, and thermoelectric power (TEP) measurements to study the electronic properties of WTe₂, a compound that manifests exceptionally large, temperature-dependent magnetoresistance. The Fermi surface consists of two pairs of electron and two pairs of hole pockets along the X–Γ–X direction. Using detailed ARPES temperature scans, we find a rare example of a temperature-induced Lifshitz transition at T≃160 K, associated with the complete disappearance of the hole pockets. Our electronic structure calculations show a clear and substantial shift of the chemical potential μ(T) due to the semimetal nature of this material driven by modest changes in temperature. This change of Fermi surface topology is also corroborated by the temperature dependence of the TEP that shows a change of slope at T≈175 K and a breakdown of Kohler’s rule in the 70–140 K range. Our results and the mechanisms driving the Lifshitz transition and transport anomalies are relevant to other systems, such as pnictides, 3D Dirac semimetals, and Weyl semimetals.

We use a tunable laser angle-resolved photoemission spectroscopy to study the electronic properties of the prototypical multiband BCS superconductor MgB₂. Our data reveal a strong renormalization of the dispersion (kink) at ~65meV, which is caused by the coupling of electrons to the E_2g phonon mode. In contrast to cuprates, the 65 meV kink in MgB₂ does not change significantly across T_c. More interestingly, we observe strong coupling to a second, lower energy collective mode at a binding energy of 10 meV. As a result, this excitation vanishes above T_c and is likely a signature of the elusive Leggett mode.

In tetragonal SrCo₂As₂single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe₂As₂ (A = Ca, Sr, Ba) parent compounds of high-T_c superconductors. This surprising discovery suggests that SrCo₂As₂ may also be a good parent compound for high-T_csuperconductivity. Here, structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity C_p, magnetic susceptibility χ, ⁷⁵As NMR and neutron diffraction measurements of SrCo₂As₂ crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient α_c is negative from 7 to 300 K, whereas α_a is positive over this T range. The ρ(T) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy E_F. The band calculations exhibit an extremely sharp peak in the density of states D(E_F) arising from a flat d_x²-y² band. A comparison of the Sommerfeld coefficient of the electronic specific heat with χ(T → 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo₂As₂ should be an itinerant ferromagnet, in conflict with the magnetization data. The χ(T) does have a large magnitude, but also exhibits a broad maximum at 115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we propose that metallic SrCo₂As₂ has a gapless quantum spin-liquid ground state.