We present angle-resolved photoemission spectroscopy data on moderately underdoped ${\mathrm{La}}_{1.855}{\mathrm{Sr}}_{0.145}{\mathrm{CuO}}_{4}$ at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple $d$-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample ${\mathrm{La}}_{1.895}{\mathrm{Sr}}_{0.105}{\mathrm{CuO}}_{4}$ also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.
We have investigated the electronic states in quasi-one-dimensional CuO chains by microprobe angle resolved photoemission spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale ($\ensuremath{\sim}40\text{ }\text{ }\mathrm{meV}$), temperature dependence, and behavior with Zn-doping] are identical to those of the bosonic mode observed in ${\mathrm{CuO}}_{2}$ planes of high-temperature superconductors, indicating they have a common origin.
Unlike the widely studied $s$-type two-gap superconductor ${\mathrm{MgB}}_{2}$, the chemically similar compounds ${\mathrm{ZrB}}_{2}$ and ${\mathrm{HfB}}_{2}$ do not superconduct above 1 K. Yet it has been shown that small amounts of self or extrinsic doping (in particular with vanadium), can induce superconductivity in these materials. Based on results of different macroscopic and microscopic measurements, including magnetometry, nuclear magnetic resonance (NMR), resistivity, and muon-spin rotation (${\ensuremath{\mu}}^{+}\mathrm{SR}$), we present a comparative study of ${\mathrm{Zr}}_{0.96}{\mathrm{V}}_{0.04}{\mathrm{B}}_{2}$ and ${\mathrm{Hf}}_{0.97}{\mathrm{V}}_{0.03}{\mathrm{B}}_{2}$. Their key magnetic and superconducting features are determined and the results are considered within the theoretical framework of multiband superconductivity proposed for ${\mathrm{MgB}}_{2}$. Detailed Fermi surface (FS) and electronic structure calculations reveal the difference between ${\mathrm{MgB}}_{2}$ and transition-metal diborides.
Angle-resolved photoemission data in the superconducting state of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ show a kink in the dispersion along the zone diagonal, which is related via a Kramers-Kr\"onig analysis to a drop in the low energy scattering rate. As one moves towards $(\ensuremath{\pi},0)$, this kink evolves into a spectral dip. The occurrence of these anomalies in the dispersion and line shape throughout the zone indicates the presence of a new energy scale in the superconducting state.
We report a study of the normal- and superconducting-state electronic properties of the centrosymmetric compound SrPt3P via 31P nuclear-magnetic-resonance (NMR) and magnetometry investigations. Essential features such as a sharp drop of the Knight shift at T < Tc and an exponential decrease of the NMR spin-lattice relaxation ratio 1/(T1T) below Tc are consistent with an s-wave electron pairing in SrPt3P, although a direct confirmation in the form of a Hebel-Slichter-type peak is lacking. Normal-state NMR data at T < 50 K indicate conventional features of the conduction electrons, typical of simple metals such as lithium or silver. Our data are finally compared with available NMR results for the noncentrosymmetric superconductors LaPt$_3$Si and CePt$_3$Si, which adopt similar crystal structures.
We report an x-ray diffraction study on the charge-density-wave (CDW) LaTe$_3$ and CeTe$_3$ compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave-vector, and provide direct evidence for a pressure-induced quenching of the CDW phase. We observe subtle differences between the chemical and mechanical compression of the lattice. We account for these with a scenario where the effective dimensionality in these CDW systems is dependent on the type of lattice compression and has a direct impact on the degree of Fermi surface nesting and on the strength of fluctuation effects.
We examine the momentum and energy dependence of the scattering rate of the high temperature cuprate superconductors using angle resolved photoemission spectroscopy. The scattering rate is of the form a + bw around the Fermi surface for under and optimal doping. The inelastic coefficient "b" is found to be isotropic. The elastic term, "a", however, is found to be highly anisotropic for under and optimally doped samples, with an anisotropy which correlates with that of the pseudogap. This is contrasted with heavily overdoped samples, which show an isotropic scattering rate and an absence of the pseudogap above T_c. We find this to be a generic property for both single and double layer compounds.
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