The coherent {\it d}-wave superconducting gap in underdoped La2−x_{2-x}Srx_{x}CuO4_4 as studied by angle-resolved photoemission

We present angle-resolved photoemission spectroscopy (ARPES) data on moderately underdoped La$_{1.855}$Sr$_{0.145}$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 {\it 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 La$_{1.895}$Sr$_{0.105}$CuO$_4$ also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.Comment: Accepted by Phys. Rev. Let

Origins of large critical temperature variations in single layer cuprates

We study the electronic structures of two single layer superconducting cuprates, Tl$_2$Ba$_2$CuO$_{6+\delta}$ (Tl2201) and (Bi$_{1.35}$Pb$_{0.85}$)(Sr$_{1.47}$La$_{0.38}$)CuO$_{6+\delta}$ (Bi2201) which have very different maximum critical temperatures (90K and 35K respectively) using Angular Resolved Photoemission Spectroscopy (ARPES). We are able to identify two main differences in their electronic properties. First, the shadow band that is present in double layer and low T$_{c,max}$ single layer cuprates is absent in Tl2201. Recent studies have linked the shadow band to structural distortions in the lattice and the absence of these in Tl2201 may be a contributing factor in its T$_{c,max}$.Second, Tl2201's Fermi surface (FS) contains long straight parallel regions near the anti-node, while in Bi2201 the anti-nodal region is much more rounded. Since the size of the superconducting gap is largest in the anti-nodal region, differences in the band dispersion at the anti-node may play a significant role in the pairing and therefore affect the maximum transition temperature.Comment: 6 pages, 5 figures,1 tabl

Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8

We have investigated the electronic states in quasi one dimensional (1D) 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 (~40 meV), temperature dependence and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high temperature superconductors, indicating they have a common origin.Comment: accepted in Phys. Rev. Let

Anisotropic quasiparticle scattering rates in slightly underdoped to optimally doped high-temperature \LSCO\ superconductors

An angle-resolved photoemission study of the scattering rate in the superconducting phase of the high-temperature superconductor \LSCO\ with $x=0.145$ and $x=0.17$, as a function of binding energy and momentum, is presented. We observe that the scattering rate scales linearly with binding energy up to the high-energy scale $E_1\sim0.4$ eV. The scattering rate is found to be strongly anisotropic, with a minimum along the (0,0)-($\pi,\pi$) direction. A possible connection to a quantum-critical point is discussed.Comment: Final version published in PR

Field-induced soft-mode quantum phase transition in La1.855_{1.855}Sr0.145_{0.145}CuO4_{4}

Inelastic neutron-scattering experiments on the high-temperature superconductor La$_{1.855}$Sr$_{0.145}$CuO$_{4}$ reveal a magnetic excitation gap $\Delta$ that decreases continuously upon application of a magnetic field perpendicular to the CuO$_2$ planes. The gap vanishes at the critical field required to induce long-range incommensurate antiferromagnetic order, providing compelling evidence for a field-induced soft-mode driven quantum phase transition

When low- and high-energy electronic responses meet in cuprate superconductors

The existence of coherent quasiparticles near the Fermi energy in the low temperature state of high-temperature superconductors has been well established by angle-resolved photoemission spectroscopy (ARPES). This technique directly probes the momentum-resolved electronic excitation spectrum of the CuO$_2$ planes. We present a study of close to optimally doped La$_{1.83}$Sr$_{0.17}$CuO$_4$ in the superconducting state and report an abrupt change in the quasiparticle spectral function, as we follow the dispersion of the ARPES signal from the Fermi energy up to 0.6 eV. The interruption in the quasiparticle dispersion separates coherent quasiparticle peaks at low energies from broad incoherent excitations at high energies. We find that the boundary between these low-energy and high-energy features exhibits a cosine-shaped momentum dependence, reminiscent of the superconducting d-wave gap. Further intriguing similarities between characteristics of the incoherent excitations and quasiparticle properties (lifetime, Fermi arcs) suggest a close relation between the electronic response at high and low energies in cuprate superconductors

Utilisation de la diffusion des neutrons dans l’étude des mécanismes microscopiques à l’origine de la supraconductivité

La supraconductivité non conventionnelle est certainement l’un des phénoménes les plus remarquables associés à la physique des systémes dits à fortes corrélations électroniques. Dans ces systémes, les degrés de liberté du spin et de la charge de l’électron sont trés fortement couplés, de sorte que le déplacement des charges est contraint par l’environnement magnétique formé par leurs spins. La diffusion inélastique des neutrons est l’unique sonde expérimentale permettant de photographier l’état magnétique de ces systémes dans tout l’espace réciproque et de suivre son évolution sur une échelle d’énergie parfaitement adaptée à celle de la supraconductivité. A travers l’exemple des cuprates supraconducteurs haute température critique, nous montrons comment la diffusion inélastique des neutrons apporte des informations uniques permettant de comprendre l’organisation électronique de ces systèmes à l’échelle microscopique et les mécanismes mise en jeu dans l’apparition de la supraconductivté

Anomalous asymmetry in the Fermi surface of the high-temperature superconductor YBa2Cu4O8 revealed by angle-resolved photoemission spectroscopy

We use microprobe angle-resolved photoemission spectroscopy to study the Fermi surface and band dispersion of the CuO2 planes in the high-temperature superconductor, YBa2Cu4O8. We find a strong in-plane asymmetry of the electronic structure between directions along a and b axes. The saddle point of the antibonding band lies at a significantly higher energy in the a direction (π,0) than the b direction (0,π), whereas the bonding band displays the opposite behavior. We demonstrate that the abnormal band shape is due to a strong asymmetry of the bilayer band splitting, likely caused by a nontrivial hybridization between the planes and chains. This asymmetry has an important implication for interpreting key properties of the Y-Ba-Cu-O family, especially the superconducting gap, transport, and results of inelastic neutron scattering