The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of similar to 3.2 lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba2Cu3O6+x, with hole concentrations of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature (T-c); further cooling below T-c abruptly reverses the divergence of the charge correlations. In combination with earlier observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge density wave instability that competes with superconductivity

An understanding of the nature of superconductivity in cuprates has been hindered by the apparent diversity of intertwining electronic orders in these materials. Here we combine resonant X-ray scattering (REXS), scanning-tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) to observe a charge order that appears consistently in surface and bulk, as well as momentum and real space, with the Bi2Sr2−xLaxCuO 6+δ cuprate family. The observed wavevector rules out simple antinodal nesting in the single particle limit, but matches well with a phenomenological model of a many-body instability of the Fermi arcs. Combined with earlier observations in other cuprate families, these findings suggest the existence of a generic charge-ordered state in underdoped cuprates, and uncover its connection to the pseudogap regime. PACS numbers:Since the discovery of cuprate high-temperature superconductors, several unconventional phenomena have been observed in the region of the phase diagram located between the strongly localized Mott insulator at zero doping and the itinerant Fermi-liquid state that emerges beyond optimal doping [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The so-called 'pseudogap' opens at the temperature T * and obliterates the Fermi surface at the antinodes (AN) of the d-wave superconducting gap function, leaving behind disconnected "Fermi arcs" centered around the nodes. In addition, charge order has been observed on the surface of Bi-and Clbased compounds [4][5][6][7][8], in the bulk of La-based compounds [9][10][11], and most recently in YBa 2 Cu 3 O 6+δ (YBCO) [17][18][19][20], indicating this might be the leading instability in underdoped cuprates. The similarity between the observed charge ordering wavevector and the antinodal nesting vector of the hightemperature Fermi surface has prompted suggestions that a conventional Peierls-like charge-density-wave (CDW) might be responsible for the opening of the pseudogap [7,8,12,19]. We use complementary bulk/surface techniques to examine the validity of this scenario, and explore the connection between charge ordering and fermiology.By applying a suite of complementary tools to a single cuprate material, Bi 2 Sr 2−x La x CuO 6+δ (Bi2201), we reveal that the charge order in this system emerges just below T * , and that its wavevector corresponds to the Fermi arc tips rather than the antinodal nesting vector. We quantify the Fermi surface using ARPES, and we look for charge modulations along the Cu-O bond directions in both real-and reciprocalspace, using STM and REXS. The single-layer Bi2201 is well suited to this purpose owing to: (i) its two-dimensionality and high degree of crystallinity [22,23], and (ii) the possibility of probing the temperature evolution across T * , which is bettercharacterized [15,16] and more accessible than in bilayer sys-

Besides superconductivity, copper-oxide high-temperature superconductors are susceptible to other types of ordering. We used scanning tunneling microscopy and resonant elastic x-ray scattering measurements to establish the formation of charge ordering in the high-temperature superconductor Bi2Sr2CaCu2O(8+x). Depending on the hole concentration, the charge ordering in this system occurs with the same period as those found in Y-based or La-based cuprates and displays the analogous competition with superconductivity. These results indicate the similarity of charge organization competing with superconductivity across different families of cuprates. We observed this charge ordering to leave a distinct electron-hole asymmetric signature (and a broad resonance centered at +20 milli-electron volts) in spectroscopic measurements, indicating that it is likely related to the organization of holes in a doped Mott insulator.

We report the results of a comprehensive study of charge density wave (CDW) correlations in untwinned YBa2Cu3O6+x single crystals with 0.4 ≤ x ≤ 0.99 using Cu-L3 edge resonant x-ray scattering (RXS). Evidence of CDW formation is found for 0.45 ≤ x ≤ 0.93 (hole doping levels 0.086 p 0.163), but not for samples with x ≤ 0.44 (p 0.084) that exhibit incommensurate spin-density-wave order, and in slightly overdoped samples with x = 0.99 (p ∼ 0.19). This suggests the presence of two proximate zero-temperature CDW critical points at pc1 ∼ 0.08 and pc2 ∼ 0.18. Remarkably, pc2 is close to the doping level that is optimal for superconductivity. The CDW reflections are observed at incommensurate in-plane wave vectors (δa, 0) and (0, δ b ) with δa δ b . Both δa and δ b decrease linearly with increasing doping, in agreement with recent reports on Bibased high-Tc superconductors, but in sharp contrast to the behavior of the La2−x(Ba,Sr)xCuO4 family. The CDW intensity and correlation length exhibit maxima at p ∼ 0.12, coincident with a plateau in the superconducting transition temperature Tc. The onset temperature of the CDW reflections depends non-monotonically on p, with a maximum of ∼ 160 K for p ∼ 0.12. The RXS reflections exhibit a uniaxial intensity anisotropy. Whereas in strongly underdoped samples the reflections at (δa, 0) are much weaker than those at (0, δ b ), the anisotropy is minimal for p ∼ 0.12, and reversed close to optimal doping. We further observe a depression of CDW correlations upon cooling below Tc, and (for samples with p ≥ 0.09) an enhancement of the signal when an external magnetic field up to 6 T is applied in the superconducting state. For samples with p ∼ 0.08, where prior work has revealed a field-enhancement of incommensurate magnetic order, the RXS signal is field-independent. This supports a previously suggested scenario in which incommensurate charge and spin orders compete against each other, in addition to individually competing against superconductivity (Blanco-Canosa et al., Phys. Rev. Lett. 110, 187001 (2013)). We discuss the relationship of these results to prior observations of "stripe" order in La2−x(Ba,Sr)xCuO4, the "pseudogap" phenomenon, superconducting fluctuations, and quantum oscillations, as well as their implications for the mechanism of high-temperature superconductivity.

We have used resonant x-ray scattering to determine the momentum dependent charge correlations in YBa2Cu3O6.55 samples with highly ordered chain arrays of oxygen acceptors (ortho-II structure). The results reveal nearly critical, biaxial charge density wave (CDW) correlations at in-plane wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length are enhanced as superconductivity is weakened by an external magnetic field. Analogous experiments were carried out on a YBa2Cu3O6.6 crystal with a dilute concentration of spinless (Zn) impurities, which had earlier been shown to nucleate incommensurate magnetic order. Compared to pristine crystals with the same doping level, the CDW amplitude and correlation length were found to be strongly reduced. These results indicate a three-phase competition between spin-modulated, charge-modulated, and superconducting states in underdoped YBa2Cu3O 6+δ .PACS numbers: 74.20. Rp, 74.25.Gz, 74.25.Kc, 74.72.Bk High-temperature superconductivity in the cuprates arises from doping of charge carriers into Mott-insulators with antiferromagnetically ordered CuO 2 planes.[1] At sufficiently high density, the carriers screen out the random potential created by the donor or acceptor atoms and generate a uniform metallic state out of which superconductivity arises. In underdoped cuprates, however, the screening is less effective, and the role of materialsspecific disorder in the formation of the unusual spin and/or charge textures observed in this regime of the phase diagram has been a subject of long-standing debate. [2-16] Recent research on the stoichiometric underdoped compounds YBa 2 Cu 3 O 6.5 and YBa 2 Cu 4 O 8 has provided new perspectives for the resolution of the influence of disorder on the electronic phase behavior of the underdoped cuprates. In these materials, the oxygen acceptors are arranged in ordered chains rather than placed randomly in the crystal lattice, so that chemical and structural disorder is minimized.[17] The results of recent quantum oscillation experiments [18][19][20] in high magnetic fields indicate a reconstruction of the Fermi surface by a long-range electronic superstructure. [21][22][23] This discovery has sparked another intense debate on the nature of the high-field ordering and its relation to the "pseudogap" observed in these and other underdoped cuprates [24] above the superconducting transition temperature, T c , in the absence of external fields. The pseudogap, in turn, is intimately related to the superconducting gap, and an explanation of its origin is considered an essential element of any theory of high-temperature superconductivity.Whereas research on YBa 2 Cu 4 O 8 has been limited, because only small crystals are available and the doping level cannot be varied in a straightforward manner, YBa 2 Cu 3 O 6.5 is a member of the extensively studied YBa 2 Cu 3 O 6+δ (YBCO 6+δ , 123) family, where the concentration of mobile holes in the CuO 2 layer...