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...

We here present how a self-consistent solution of the dynamical mean-field theory equations can be obtained using exact diagonalization of an Anderson impurity model with accuracies comparable to those found using renormalization group or quantum Monte Carlo methods. We show how one can solve a correlated quantum impurity coupled to several hundred uncorrelated bath sites, using a restricted active basis set. The number of bath sites determines the resolution of the obtained spectral function, which consists of peaks with an approximate spacing proportional to the bandwidth divided by the number of bath sites. The self-consistency cycle is performed on the real-frequency axis and expressed as numerical stable matrix operations. The same impurity solver has been used on ligand field and finite size cluster calculations and is capable of treating involved Hamiltonians including the full rotational invariant Coulomb interaction, spin-orbit coupling, and low-symmetry crystal fields. The proposed method allows for the calculation of a variety of correlation functions at little extra cost

A combined analysis of x-ray absorption and resonant reflectivity data was used to obtain the orbital polarization profiles of superlattices composed of four-unit-cell-thick layers of metallic LaNiO3 and layers of insulating RXO3 (R = La, Gd, Dy and X = Al, Ga, Sc), grown on substrates that impose either compressive or tensile strain. This superlattice geometry allowed us to partly separate the influence of epitaxial strain from interfacial effects controlled by the chemical composition of the insulating blocking layers. Our quantitative analysis reveal orbital polarizations up to 25 %. We further show that strain is the most effective control parameter, whereas the influence of the chemical composition of the blocking layers is comparatively small.