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Theories of the origin of superconductivity in cuprates are dependent on an understanding of their normal state which exhibits various competing orders. Transport and thermodynamic measurements on La$_{2-x}$Sr$_x$CuO$_4$ show signatures of a quantum critical point, including a peak in the electronic specific heat $C$ versus doping p, near the doping p*, where the pseudogap collapses. The fundamental nature of the fluctuations associated with this peak is unclear. Here we use inelastic neutron scattering to show that close to $T_c$ and near p*, there are low-energy collective spin excitations with characteristic energies $\approx$ 5 meV. The correlation length of the spin fluctuations does not diverge in spite of the low energy scale and we conclude that the underlying quantum criticality is not due to antiferromagnetism but most likely to a collapse of the pseudogap. We show that the large specific heat near p* can be understood in terms of collective spin fluctuations. The spin fluctuations we measure exist across the superconducting phase diagram and may be related to the strange metal behaviour observed in overdoped cuprates.

A comprehensive angle-resolved photoemission spectroscopy study of the band structure in single-layer cuprates is presented with the aim of uncovering universal trends across different materials. Five different hole- and electron-overdoped cuprate superconductors (${\mathrm{La}}_{1.59}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.21}{\mathrm{CuO}}_{4}$, ${\mathrm{La}}_{1.77}{\mathrm{Sr}}_{0.23}{\mathrm{CuO}}_{4}$, ${\mathrm{Bi}}_{1.74}{\mathrm{Pb}}_{0.38}{\mathrm{Sr}}_{1.88}{\mathrm{CuO}}_{6+{\delta}}$, ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6+{\delta}}$, and ${\mathrm{Pr}}_{1.15}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4}$) have been studied with special focus on the bands with a predominately $d$-orbital character. Using a light polarization analysis, the ${e}_{g}$ and ${t}_{2g}$ bands are identified across these materials. A clear correlation between the ${d}_{3{z}^{2}{-}{r}^{2}}$ band energy and the apical oxygen distance ${d}_{\mathrm{A}}$ is demonstrated. Moreover, the compound dependence of the ${d}_{{x}^{2}{-}{y}^{2}}$ band bottom and the ${t}_{2g}$ band top is revealed. A direct comparison to density functional theory (DFT) calculations employing hybrid exchange-correlation functionals demonstrates excellent agreement. We thus conclude that the DFT methodology can be used to describe the global band structure of overdoped single-layer cuprates on both the hole- and electron-doped side." name="description" />

We present a soft x-ray angle-resolved photoemission spectroscopy study of the overdoped high-temperature superconductors La$_{2-x}$Sr$_x$CuO$_4$ and La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$. In-plane and out-of-plane components of the Fermi surface are mapped by varying the photoemission angle and the incident photon energy. No $k_z$ dispersion is observed along the nodal direction, whereas a significant antinodal $k_z$ dispersion is identified. Based on a tight-binding parametrization, we discuss the implications for the density of states near the van-Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La$_{2-x}$Sr$_x$CuO$_4$ can not be assigned to the van-Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.

We present a resonant inelastic x-ray scattering (RIXS) study of spin and charge excitations in overdoped La1.77Sr0.23CuO4 along two high-symmetry directions. The line shape of these excitations is analyzed and they are shown to be highly overdamped. Their spectral weight and damping are found to be strongly momentum dependent. Qualitative agreement between these observations and a calculated random-phase approximation susceptibility is obtained for this overdoped compound, implying that a significant contribution to the RIXS signal stems from a continuum of charge excitations. Furthermore, this suggests that the spin excitations in the overdoped regime can be captured qualitatively by an itinerant picture. Our calculations also predict a low-energy spin-excitation branch to exist along the nodal direction near the zone center. With the energy resolution of the present experiment, this branch is not resolvable, but we show that the next generation of high-resolution spectrometers will be able to test this prediction.

Nodal angle-resolved photoemission spectra taken on overdoped La1.77Sr0.23CuO4 are presented and analyzed. It is proven that the low-energy excitations are true Landau Fermi-liquid quasiparticles. We show that momentum and energy distribution curves can be analyzed self-consistently without quantitative knowledge of the bare band dispersion. Finally, by imposing Kramers-Kronig consistency on the self-energy Sigma, insight into the quasiparticle residue is gained. We conclude by comparing our results to quasiparticle properties extracted from thermodynamic, magnetoresistance, and high-field quantum oscillation experiments on overdoped Tl2Ba2CuO6+delta.

International audience; High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki–Wood's relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present.

High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki-Wood's relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present.

We use small-angle neutron scattering to study the superconducting vortex lattice in La2−xSrxCuO4 as a function of doping and magnetic field. We show that near optimally doping the vortex lattice coordination and the superconducting coherence length ξ are controlled by a Van Hove singularity crossing the Fermi level near the Brillouin zone boundary. The vortex lattice properties change dramatically as a spin-density-wave instability is approached upon underdoping. The Bragg glass paradigm provides a good description of this regime and suggests that spin-density-wave order acts as a source of disorder on the vortex lattice.

We use inelastic neutron scattering to study the temperature dependence of the low-energy spin excitations in single crystals of superconducting FeTe${}_{0.6}$Se${}_{0.4}$ (${T}_{c}=14$ K). In the low-temperature superconducting state, the imaginary part of the dynamic susceptibility at the electron and hole Fermi-surfaces nesting wave vector $Q=(0.5,0.5)$, ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(Q,\ensuremath{\omega})$, has a small spin gap, a two-dimensional neutron spin resonance above the spin gap, and increases linearly with increasing $\ensuremath{\hbar}\ensuremath{\omega}$ for energies above the resonance. While the intensity of the resonance decreases like an order parameter with increasing temperature and disappears at temperature slightly above ${T}_{c}$, the energy of the mode is weakly temperature dependent and vanishes concurrently above ${T}_{c}$. This suggests that in spite of its similarities with the resonance in electron-doped superconducting BaFe${}_{2\ensuremath{-}x}$(Co,Ni)${}_{x}$As${}_{2}$, the mode in FeTe${}_{0.6}$Se${}_{0.4}$ is not directly associated with the superconducting electronic gap.

We use neutron scattering to show that spin waves in the iron chalcogenide ${\mathrm{Fe}}_{1.05}\mathrm{Te}$ display novel dispersion clearly different from both the first principles density functional calculations and recent observations in the related iron pnictide ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$. By fitting to a Heisenberg Hamiltonian, we find that although the nearest-neighbor exchange couplings in the two systems are quite different, their next-nearest-neighbor (NNN) couplings are similar. This suggests that superconductivity in the pnictides and chalcogenides share a common magnetic origin that is intimately associated with the NNN magnetic coupling between the irons.

A systematic angle-resolved photoemission study of the electronic structure of La2−xSrxCuO4 in a wide doping range is presented in this paper. In addition to the main energy band, we observed a weaker additional band, the (π, π) folded band, which shows unusual doping dependence. The appearance of the folded band suggests that a Fermi surface reconstruction is doping dependent and could already occur at zero magnetic field., New Journal of Physics, 12, ISSN:1367-2630

We use neutron scattering to show that spin waves in the iron chalcogenide Fe(1.05)Te display novel dispersion clearly different from both the first principles density functional calculations and recent observations in the related iron pnictide CaFe(2)As(2). By fitting to a Heisenberg Hamiltonian, we find that although the nearest-neighbor exchange couplings in the two systems are quite different, their next-nearest-neighbor (NNN) couplings are similar. This suggests that superconductivity in the pnictides and chalcogenides share a common magnetic origin that is intimately associated with the NNN magnetic coupling between the irons.

U.S. DOE BES[DE-FG02-05ER46202]; U.S. DOE, Division of Scientific User Facilities; CAS; U.S. DOE[DE-AC02-06CH11357]; DOE BES EPSCoR[DE-FG02-08ER46528]

We use inelastic neutron scattering to measure the magnetic excitations in underdoped La2-xSrxCuO4 (x=0.085, T_{c}=22 K) for large energy (5

The presence or absence of a quantum critical point and its location in the phase diagram of high-temperature superconductors have been subjects of intense scrutiny. Clear evidence for quantum criticality, particularly in the transport properties, has proved elusive because the important low-temperature region is masked by the onset of superconductivity. We present measurements of the low-temperature in-plane resistivity of several highly doped La 2– x Sr x CuO 4 single crystals in which the superconductivity had been stripped away by using high magnetic fields. In contrast to other quantum critical systems, the resistivity varies linearly with temperature over a wide doping range with a gradient that scales monotonically with the superconducting transition temperature. It is maximal at a critical doping level ( p c ) ∼ 0.19 at which superconductivity is most robust. Moreover, its value at p c corresponds to the onset of quasi-particle incoherence along specific momentum directions, implying that the interaction that first promotes high-temperature superconductivity may ultimately destroy the very quasi-particle states involved in the superconducting pairing.

We use inelastic neutron scattering to measure the magnetic excitations in underdoped La2-xSrxCuO4 (x=0.085, T_{c}=22 K) for large energy (5E200 meV) and temperature (5T300 K) ranges. At low T, the response is highly structured in E and q, with peaks in the local susceptibility at 15 and 50 meV and a four-peaked structure in q for E approximately 185 meV. Raising T from 30 K greater, similarT_{c} to 300 K causes the structure in chi;{''}(q,omega) present for E70 meV to disappear. It is replaced with a strongly damped response. We attribute this change to the disappearance of the pseudogap with increasing temperature.

We report a detailed inelastic neutron scattering study of the collective magnetic excitations of overdoped superconducting ${\mathrm{La}}_{1.78}{\mathrm{Sr}}_{0.22}{\mathrm{CuO}}_{4}$ for the energy range 0--160 meV. Our measurements show that overdoping suppresses the strong response present for optimally doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ which is peaked near 50 meV. The remaining response is peaked at incommensurate wave vectors for all energies investigated. We observe a strong high-frequency magnetic response for $E\ensuremath{\gtrsim}80\text{ }\text{ }\mathrm{meV}$ suggesting that significant antiferromagnetic exchange couplings persist well into the overdoped part of the cuprate phase diagram.

We report a detailed inelastic neutron scattering study of the collective magnetic excitations of overdoped superconducting La(1.78)Sr(0.22)CuO(4) for the energy range 0-160 meV. Our measurements show that overdoping suppresses the strong response present for optimally doped La(2-x)Sr(x)CuO(4) which is peaked near 50 meV. The remaining response is peaked at incommensurate wave vectors for all energies investigated. We observe a strong high-frequency magnetic response for E approximately80 meV suggesting that significant antiferromagnetic exchange couplings persist well into the overdoped part of the cuprate phase diagram.