To explore the doping dependence of the recently discovered charge density wave (CDW) order in YBa2Cu3Oy, we present a bulk-sensitive high-energy x-ray study for several oxygen concentrations, including strongly underdoped YBa2Cu3O6.44. Combined with previous data around the so-called 1/8 doping, we show that bulk CDW order exists at least for hole concentrations (p) in the CuO2 planes of 0.078 <~ p <~ 0.132. This implies that CDW order exists in close vicinity to the quantum critical point for spin density wave (SDW) order. In contrast to the pseudogap temperature T*, the onset temperature of CDW order decreases with underdoping to T_CDW ~ 90K in YBa2Cu3O6.44. Together with a weakened order parameter this suggests a competition between CDW and SDW orders. In addition, the CDW order in YBa2Cu3O6.44 shows the same type of competition with superconductivity as a function of temperature and magnetic field as samples closer to p = 1/8. At low p the CDW incommensurability continues the previously reported linear increasing trend with underdoping. In the entire doping range the in-plane correlation length of the CDW order in b-axis direction depends only very weakly on the hole concentration, and appears independent of the type and correlation length of the oxygen-chain order. The onset temperature of the CDW order is remarkably close to a temperature T^dagger that marks the maximum of 1/(T_1T) in planar 63^Cu NQR/NMR experiments, potentially indicating a response of the spin dynamics to the formation of the CDW. Our discussion of these findings includes a detailed comparison to the charge stripe order in La2-xBaxCuO4.
The effect of a magnetic field on the charge stripe order in La(2-x)Ba(x)CuO(4) has been studied by means of high energy (100 keV) x-ray diffraction for charge carrier concentrations ranging from strongly underdoped to optimally doped. We find that charge stripe order can be significantly enhanced by a magnetic field applied along the c-axis, but only at temperatures and dopings where it coexists with bulk superconductivity at zero field. The field also increases stripe correlations between the planes, which can result in an enhanced frustration of the interlayer Josephson coupling. Close to the famous x=1/8 compound, where zero field stripe order is pronounced and bulk superconductivity is suppressed, charge stripe order is independent of a magnetic field. The results imply that static stripe order and three-dimensionally coherent superconductivity are competing ground states.
The optical properties the high-temperature superconductor La{2-x}Ba{x}CuO{4} have been measured over a wide frequency and temperature range for light polarized in the a-b planes and along the c axis. Three different Ba concentrations have been examined, x=0.095 with a critical temperature T_c=32 K, x=0.125 with T_c ~ 2.4 K, and x=0.145 with T_c ~ 24 K. The in-plane behavior of the optical conductivity for these materials at high temperature is described by a Drude-like response with a scattering rate that decreases with temperature. Below T_c in the x=0.095 and 0.145 materials there is a clear signature of the formation of a SC state in the optical properties allowing the strength of the condensate (rho_{s0}) and the penetration depth to be determined. In the anomalous 1/8 phase, some spectral weight shifts from lower to higher frequency (above 300 cm^{-1}) on cooling below the spin-ordering temperature T_{so} ~ 42 K, associated with the onset of spin-stripe order; we discuss alternative interpretations in terms of a conventional density-wave gap versus the response to pair-density-wave SC. The disappearance of the low-frequency spectral weight at low temperature may indicate the formation of a 2D SC state below the Berezinskii-Kosterlitz-Thouless transition at about 16 K prior to the onset of bulk 3D SC. The two dopings for which a SC response is observed both fall on the universal scaling line rho_{s0}/8 ~ 4.4 sigma_{dc} T_c. The optical properties for light polarized along the c axis reveal an insulating character dominated by lattice vibrations, superimposed on a weak electronic background. In the x=0.095 and 0.145 materials a Josephson plasma edge is observed in the reflectance below T_c. No Josephson plasma edge is observed in the 1/8 phase, suggesting that the presence of charge and spin order frustrates the formation of a supercurrent and bulk 3D SC.
The correlations between stripe order, superconductivity, and crystal structure in La(2-x)Ba(x)CuO(4) single crystals have been studied by means of x-ray and neutron diffraction as well as static magnetization measurements. The derived phase diagram shows that charge stripe order (CO) coexists with bulk superconductivity in a broad range of doping around x=1/8, although the CO order parameter falls off quickly for x<>1/8. Except for x=0.155, the onset of CO always coincides with the transition between the orthorhombic and the tetragonal low temperature structures. The CO transition evolves from a sharp drop at low x to a more gradual transition at higher x, eventually falling below the structural phase boundary for optimum doping. With respect to the interlayer CO correlations, we find no qualitative change of the stripe stacking order as a function of doping, and in-plane and out-of-plane correlations disappear simultaneously at the transition. Similarly to the CO, the spin stripe order (SO) is also most pronounced at x=1/8. Truly static SO sets in below the CO and coincides with the first appearance of in-plane superconducting correlations at temperatures significantly above the bulk transition to superconductivity (SC). Indications that bulk SC causes a reduction of the spin or charge stripe order could not be identified. We argue that CO is the dominant order that is compatible with SC pairing but competes with SC phase coherence. Comparing our results with data from the literature, we find good agreement if all results are plotted as a function of x instead of the nominal x, where x represents an estimate of the actual Ba content, extracted from the doping dependence of the structural transition between the orthorhombic phase and the tetragonal high-temperature phase.
We present new x-ray and neutron scattering measurements of stripe order in La(1.875)Ba(0.125)CuO(4), along with low-field susceptibility, thermal conductivity, and specific heat data. We compare these with previously reported results for resistivity and thermopower. Temperature-dependent features indicating transitions (or crossovers) are correlated among the various experimental quantities. Taking into account recent spectroscopic studies, we argue that the most likely interpretation of the complete collection of results is that an unusual form of two-dimensional superconducting correlations appears together with the onset of spin-stripe order. Recent theoretical proposals for a sinusoidally-modulated superconducting state compatible with stripe order provide an intriguing explanation of our results and motivate further experimental tests. We also discuss evidence for one-dimensional pairing correlations that appear together with the charge order. With regard to the overall phenomenology, we consider the degree to which similar behavior may have been observed in other cuprates, and describe possible connections to various puzzling phenomena in cuprate superconductors.
The electronic interlayer transport of the lightly doped antiferromagnet La1.79Eu0.2Sr0.01CuO4 has been studied by means of magneto-resistance measurements. The central problem addressed concerns the differences between the electronic interlayer coupling in the tetragonal low-temperature (LTT) phase and the orthorhombic low-temperature (LTO) phase. The key observation is that the spin-flip induced drop in the c-axis magneto-resistance of the LTO phase, which is characteristic for pure La2-xSrxCuO4, dramatically decreases in the LTT phase. The results show that the transition from orthorhombic to tetragonal symmetry and from collinear to non-collinear antiferromagnetic spin structure eliminates the strain dependent anisotropic interlayer hopping as well as the concomitant spin-valve type transport channel. Implications for the stripe ordered LTT phase of La2-xBaxCuO4 are briefly discussed.
