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Register a new userX-Ray-Diffraction Study of Charge-Density-Waves and Oxygen-Ordering in YBa2Cu3O6+x Superconductor
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We report a temperature-dependent increase below 300 K of diffuse superlattice peaks corresponding to q_0 =(~2/5,0,0) in an under-doped YBa_2Cu_3O_6+x superconductor (x~0.63). These peaks reveal strong c-axis correlations involving the CuO_2 bilayers, show a non-uniform increase below ~220 K with a plateau for ~100-160 K, and appear to saturate in the superconducting phase. We interpret this unconventional T-dependence of the ``oxygen-ordering peaks as a manifestation of a charge density wave in the CuO_2 planes coupled to the oxygen-vacancy ordering.
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We show that the distribution of quantum oscillation frequencies observed over a broad range of magnetic field can be reconciled with the wavevectors of charge modulations found in nuclear magnetic resonance and resonant x-ray spectroscopy experiments in underdoped YBa2Cu3O6+x within a model of biaxial charge ordering occurring in a bilayer CuO2 planar system. Bilayer coupling introduces the possibility of different period modulations and quantum oscillation frequencies corresponding to each of the bonding and antibonding bands, which can be reconciled with recent experimental observations
Charge density wave (CDW) order has been shown to compete and coexist with superconductivity in underdoped cuprates. Theoretical proposals for the CDW order include an unconventional $d$-symmetry form factor CDW, evidence for which has emerged from measurements, including resonant soft x-ray scattering (RSXS) in YBa$_2$Cu$_3$O$_{6+x}$ (YBCO). Here, we revisit RSXS measurements of the CDW symmetry in YBCO, using a variation in the measurement geometry to provide enhanced sensitivity to orbital symmetry. We show that the $(0 0.31 L)$ CDW peak measured at the Cu $L$ edge is dominated by an $s$ form factor rather than a $d$ form factor as was reported previously. In addition, by measuring both $(0.31 0 L)$ and $(0 0.31 L)$ peaks, we identify a pronounced difference in the orbital symmetry of the CDW order along the $a$ and $b$ axes, with the CDW along the $a$ axis exhibiting orbital order in addition to charge order.
Striped phases in which spin and charge separate into different regions in the material have been proposed to account for the unusual properties of the high-$T_c$ cuprate superconductors. The driving force for a striped phase is the charge distribution, which self-organizes itself into linear regions. In the highest $T_c$ materials such regions are not static but fluctuate in time. Neutrons, having no charge, can not directly observe these fluctuations but they can be observed indirectly by their effect on the phonons. Neutron scattering measurements have been made using a specialized technique to study the phonon line shapes in four crystals with oxygen doping levels varying from highly underdoped to optimal doping. It is shown that fluctuating charge stripes exist over the whole doping range, and become visible below temperatures somewhat higher than the pseudogap temperature.
We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high temperature superconductor YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200 K in underdoped crystals. The temperature dependence of these new features indicates that they are associated with the incommensurate charge density wave state recently discovered using synchrotron x-ray scattering techniques on the same single crystals. Raman scattering has thus the potential to explore the evolution of this state under extreme conditions.
Competing density waves play an important role in the mystery of high-temperature superconductors. In spite of the large amount of experimental evidence, the fundamental question of whether these modulations represent charge or pairing density waves (CDWs or PDWs) is still debated. Here we present a method to answer this question using momentum and energy-resolved resonant X-ray scattering maps. Starting from a minimal model of density waves in superconductors, we identify distinctive signatures of incipient CDWs and PDWs. The generality of our approach is confirmed by a self-consistent solution of an extended Hubbard model with attractive interaction. By considering the available experimental data, we claim that the spatial modulations in cuprates have a predominant PDW character. Our work paves the way for using X-ray to identify competing and intertwined orders in superconducting materials.
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