. However, as a neutron is neutral, it does not detect charge but rather its associated lattice distortion 7 , so it is not known whether the stripes involve ordering of the doped holes. Here we present a study of the charge order in LBCO with resonant soft X-ray scattering (RSXS). We observe giant resonances near the Fermi level as well as near the correlated gap 8,9 , demonstrating significant modulation in both the doped-hole density and the 'Mottness', or the degree to which the system resembles a Mott insulator 10 . The peak-to-trough amplitude of the valence modulation is estimated to be 0.063 holes, which suggests 11 an integrated area of 0.59 holes under a single stripe, close to the expected 0.5 for half-filled stripes.The charge/spin superstructure in LNSCO 5 and LBCO 6 appears only in the low-temperature tetragonal (LTT) phase, is most stable at x = 1/8 and coincides with an anomalous suppression of the critical temperature T c (ref. 12). This phase is frequently interpreted as (quasi) static stripes that have been pinned by the LTT distortion. The charge reflections observed with neutron scattering are weak (∼6 times less intense than the magnetic reflections) as neutrons only detect the lattice distortion, which was estimated to be only about 0.004Å (ref. 7). However, one assumes that the hole modulation itself is significant. We point out, however, that the spin-density wave in elemental Cr also exhibits half-wavelength charge reflections that are weaker by about a factor of 4.1 and represent a distortion of similar size 13 . So, in the neutron Bragg peaks alone there is no clear difference between the phenomenon in LNSCO and a simple spin-density wave. To determine whether the doped holes are actually involved we have studied LBCO with RSXS near the O K (1s → 2p) and Cu L 3/2 (2p 3/2 → 3d x2−y2 ) edges, which provide direct sensitivity to valence electron ordering [14][15][16][17][18][19][20] .Single crystals of La 2−x Ba x CuO 4 with x = 1/8 were grown by the floating-zone method 21 . The sample used in this study had T c = 2.5 K indicating suppressed superconductivity and stabilized spin/charge order. The sample was cleaved in air revealing a surface with (0,0,1) orientation. RSXS measurements were performed on beam line X1B at the National Synchrotron Light Source, Brookhaven, using a 10-axis, ultrahigh-vacuumcompatible diffractometer. The sample was cooled with a He flow cryostat connected through Cu braids, providing a base temperature of 18 K. X-ray absorption spectra (XAS) were measured in situ in fluorescence yield mode at the O K and Cu L 3/2 edges and found to be consistent with previous studies 8 (see Figs 1, 3a). We will denote reciprocal space with Miller indices (H,K,L), which represent a momentum transfer Q = (2π/a H,2π/b K,2π/c L) where a = b = 3.788Å, c = 13.23Å. The incident X-ray polarization depends on Q but was approximately 60 • from the Cu-O bond for measurements at both edges.The O K XAS in the cuprates exhibits a mobile carrier peak (MCP) at 528.6 eV, corresponding to tran...

No abstract

No abstract

The unconventional normal-state properties of the cuprates are often discussed in terms of emergent electronic order that onsets below a putative critical doping of xc ≈ 0.19. Charge density wave (CDW) correlations represent one such order; however, experimental evidence for such order generally spans a limited range of doping that falls short of the critical value xc, leading to questions regarding its essential relevance. Here, we use X-ray diffraction to demonstrate that CDW correlations in La2−xSrxCuO4 persist up to a doping of at least x = 0.21. The correlations show strong changes through the superconducting transition, but no obvious discontinuity through xc ≈ 0.19, despite changes in Fermi surface topology and electronic transport at this doping. These results demonstrate the interaction between CDWs and superconductivity even in overdoped cuprates and prompt a reconsideration of the role of CDW correlations in the high-temperature cuprate phase diagram.

High resolution polar Kerr effect measurements were performed on La1.875Ba0.125CuO4 single crystals revealing that a finite Kerr signal is measured below an onset temperature TK that coincides with the charge ordering transition temperature TCO. We further show that the sign of the Kerr signal cannot be trained with the magnetic field, is found to be the same on opposite sides of the same crystal, and is odd with respect to strain in the diagonal direction of the unit cell. These observations are consistent with a chiral "gyrotropic" order above Tc for La1.875Ba0.125CuO4; similarities to other cuprates suggest that it is a universal property in the pseudogap regime.

We use inelastic neutron scattering to study the temperature dependence of magnetic excitations (for energies up to 100 meV) in the cuprate La1.875Ba0.125CuO4. This compound exhibits stripe order below a temperature of ∼50 K; previous measurements have shown that the magnetic excitations of the stripe-ordered phase have an hour-glass-like dispersion, with a saddle point at ∼ 50 meV. Here we compare measurements in the disordered phase taken at 65 and 300 K. At energies on the scale of kBT , there is substantial momentum-broadening of the signal, and the low-energy incommensurate features can no longer be resolved at 300 K. In contrast, there is remarkably little change in the scattered signal for energies greater than kBT . In fact, the momentum-integrated dynamic susceptibility is almost constant with temperature. We suggest that the continuity of higher-energy magnetic excitations is strong evidence for dynamic stripes in the high-temperature, disordered phase. We reconsider the nature of the magnetic dispersion, and we discuss the correspondences between the thermal evolution of magnetic stripe correlations with other electronic properties of the cuprates.

We performed an experimental study of the temperature and doping dependence of the energy-loss function of the bilayer and trilayer bismuth cuprates family. The primary aim is to obtain information on the energy stored in the Coulomb interaction between the conduction electrons, on the temperature dependence thereof, and on the change of Coulomb interaction when Cooper pairs are formed. We performed temperature-dependent ellipsometry measurements on several Bi 2 Sr 2 CaCu 2 O 8−x single crystals: underdoped with T c ¼ 60, 70, and 83 K; optimally doped with T c ¼ 91 K; overdoped with T c ¼ 84, 81, 70, and 58 K; as well as optimally doped Bi 2 Sr 2 Ca 2 Cu 3 O 10þx with T c ¼ 110 K. Our first observation is that, as the temperature drops through T c , the loss function in the range up to 2 eV displays a change of temperature dependence as compared to the temperature dependence in the normal state. This effect at-or close to-T c depends strongly on doping, with a sign change for weak overdoping. The size of the observed change in Coulomb energy, using an extrapolation with reasonable assumptions about its q dependence, is about the same size as the condensation energy that has been measured in these compounds. Our results therefore lend support to the notion that the Coulomb energy is an important factor for stabilizing the superconducting phase. Because of the restriction to small momentum, our observations do not exclude a possible significant contribution to the condensation energy of the Coulomb energy associated with the region of q around ðπ; πÞ.

We report Raman measurements on Bi 2 Sr 2 CaCu 2 O 8+␦ single crystals that allow us to quantitatively evaluate the doping dependence of the density of Cooper pairs in the superconducting state. We show that the drastic loss of Cooper pairs in the antinodal region as the doping level is reduced is concomitant with a deep alteration of the quasiparticles dynamic above T c and consistent with a pseudogap that competes with superconductivity. Our data also reveal that the overall density of Cooper pairs evolves with doping, distinctly from the superfluid density above the doping level p c = 0.2.One of the most challenging issues in cuprate superconductors is to understand the low-energy quasiparticles dynamics above and below the critical temperature T c as the Mott insulating state is approached by decreasing the doping level. 1,2 On the overdoped side, below T c , the d-wave superconducting gap develops with maximum values along the principal axes of the Brillouin zone, the antinodes and vanishes in the nodal regions, the diagonals of the Brillouin zone. 3 On the underdoped side, above T c and below T ‫ء‬ , the pseudogap occurs in the antinodal regions. 4 Recent advances in angle-resolved photoemission spectroscopy ͑ARPES͒, 5-7 scanning tunneling spectroscopy ͑STS͒, 8,9 -SR spectroscopy 10 and electronic Raman scattering ͑ERS͒ ͑Refs. 11-13͒ have brought strong experimental evidences that superconductivity remains robust at the nodes even at low doping level while superconductivity is deeply altered at the antinodes. This manifests itself in angleresolved photoemission spectroscopy ͑ARPES͒, by the suppression of the coherent spectral weight at the antinodes with underdoping. 7 In STS, this is signaled by a shrinkage of the Bogoliubov quasiparticles arcs around the nodes 9 and in ERS, by the disappearance of the pair breaking peak in the antinodal Raman response as the doping level is reduced. 11,[13][14][15] These observations raise the question of the influence of doping on the k-space dependence of the superconducting properties and the relationship between the pseudogap and superconductivity. Our aim here is to capture the doping evolution of the density of Cooper pairs in momentum space and to compare it with the superfluid density one. We also want to address whether a connection exists or not between the pseudogap and superconductivity.To achieve this goal, we have developed a careful and systematic experimental protocol which allows us to quantitatively compare the changes of the Raman spectra of Bi 2 Sr 2 CaCu 2 O 8+␦ ͑Bi-2212͒ compounds as a function of the doping levels. Using a simple relationship between the integrated superconducting Raman response and the density of Cooper pairs, we show that the density of Cooper pairs in the superconducting state strongly decreases with underdoping at the antinodes while it still sizeable in the nodal region even at low doping level. Simultaneously, our data reveal that in the normal state, the low-energy quasiparticle dynamics is deeply altered as the doping is red...