Search

The crystal structure of high-quality single crystals of YBa 2 Cu 3 O 6 + x grown in zirconia curcibles was studied systematically versus oxygen concentration by four-circle neutron diffraction at room temperature. The oxygen contents ranged from x = 0.18 (insulating, tetragonal phase) to x = 0.96 (superconducting, orthorhombic phase). In the orthorhombic phase, both twinned and detwinned samples were measured. A detailed discussion is presented of the systematic changes taking place in the atomic positions and Debye-Waller factors as the oxygen content is varied. Some qualitative highlights are that one can identify two distinct regions within the unit cell: the characteristics of the atoms near the CuO x basal plane are essentially driven by the effect of oxygen addition, whereas near the CuO 2 bilayer, the trends are more subtle and presumably dominated by charge transfer and bond-distance effects. Also, there is neither an abrupt change of the apical-oxygen position at the orthorhombic to tetragonal transition, nor is there any indication of depletion of the atomic sites except for the chain-site oxygen. The superconducting critical temperatures or Neel temperatures, as well as the existence of oxygen-ordered superstructures are reported.

We present measurements or Ortho-III phase correlations in an untwinned single crystal of YBa2Cu3O6.77 by neutron scattering and the novel method of hard (95 keV) X-ray scattering. The Ortho-III ordering is essentially two-dimensional, exhibiting Lorentzian peak shapes in the a-b plane. At room temperature, the correlation lengths deduced from the Lorentzian peak widths are about 5 unit cells (⋍20 A) along h and 19 unit cells (⋍74 A) along k, and are comparable to some correlation lengths measured for the Ortho-II phase. Upon heating, the superstructure intensity was found to decrease rapidly at temperatures above 50°C with about 10% of the original intensity remaining at 100°C and completely vanishing above 150°C. Such a low transition temperature explains why this phase is more difficult to observe than other superstructures, such as the Ortho-II.

We have measured the vapour pressure of oxygen in Y1Ba2Cu3Ox as a function of x at constant temperature between 450°C and650°C for pressures between 10 m Torr and 1000 Torr. From these vapour pressure curves we have determined ( ∂x ∂μ ) T (μ is the chemical potential), a thermodynamic response function analogous to the compressibility and specific heat. ( ∂x ∂μ ) T is a quantity which is sensitive to the oxygen configuration and which can be calculated by any candidate theory on the thermodynamics of oxygen. So, the measured ( ∂x ∂μ ) T curves should help in the search to determine the correct model for oxygen thermodynamics in Y1Ba2Cu3Ox. In this paper we use a simple quasi-chemical approximation with an empirical x and T dependent on-site energy to accurately fit the vapour pressure curves. This fit provides a fairly simple method to empirically reproduce the vapour pressure as a function of x and T, which should prove useful in materials preparation.

Magnetic ordering has been investigated by neutron scattering on an YBa 2 Cu 3 O 6+ x single crystal with x =0.1 and x =0.18, and an Al doped YBa 2 (Cu 2.86 Al 0.14 )O 6.25 crystal. For the undoped crystal an antiferromagnetic ordering transition (AFI) was observed at T N =410 K and 368 K (respectively for x=0.1 and x=0.18), but no second antiferromagnetic transition was found down to 2 K. In contrast, an antiferromagnetic reordering (AFII) was observed for the aluminum doped sample below T 2 =8 K. This shows the high sensitivity of this second phase to impurities and indicates that this reordering is not necessarily an intrinsic property of YBa 2 Cu 3 O 6+ x .

In YBa 2 Cu 3−x Al x O 6+δ single crystals antiferromagnetic AFII ordering has been observed below 18 K by neutron diffraction. The transition temperature T 2 to the AFI ordering increases with x and δ.

We have studied the magnetic ordering of the Cu and Pr ions in PrBa 2 Cu 3 O 6+x by neutron diffraction on single crystals with different oxygen contents. Two types of Cu ordering were observed, qualitatively similar to the anti-ferromagnetic phases reported in some studies of YBa 2 Cu 3 O 6+x . A third magnetic structure was observed below 15K, which we believe corresponds to the magnetic ordering of the Pr sub-lattice.

Neutron and synchrotron x-ray measurements of the Ortho-II superstructure reflections on a high quality single crystal of YBa 2 Cu 3 O 6.5 revealed that the intrinsic line shape is a Lorentzian to the power 5 2 . It is argued that such a line shape implies late-stage domain coarsening of a quenched system ordering in three dimensions (d=3) with a two component order parameter (n=2).

Muon Spin Rotation (μ + SR) studies of flux vortex structure in single crystal, crushed crystal, thick film, and sintered specimens of Bi 2 Sr 2 CaCu 2 O 8 are reported. Field-Cooled & Zero-Field-Cooled measurements indicate that the flux vortices in the mixed state of each material are highly mobile at temperatures above a pinning/freezing temperature T p c . Measurements on the thick film sample with H ext =10, 4.1, 2 and 1kG show that T p decreases with increasing field strength.

YBa2Cu3Ox is known to be antiferromagnetic (AFM) for x ≲ 6.45 but the details of its sublattice magnetization(s) are still controversial. A zero-field μ+SR investigation of the x- and T-dependence of the local fields at several muon sites reveals sharp changes at 10K and 80K for an oriented powder sample with x ≈ 6.0, but other x ≈ 6.0 samples do not show the same features. The direction of the local field at one μ+ site is shown to be along the ĉ-axis. For x < 6.0 the samples still show local spin freezing up to 300K, but coherent AFM order is lost even at 3K.

Resistivity, susceptibility and μ + SR measurements have been used to investigate the ( x , T ) phase diagram of YBa 2 Cu 3 O x . Carefully annealed samples are antiferromagnetic (AFM) insulators for 6.0 ≲ x ≲ 6.35 and nonmagnetic superconductors (SC) for 6.5 ≲ x ≲ 7.0; SC coexists with AFM or spin glass (SG) static magnetic order at low temperatures for 6.38 ≲ x ≲ 6.48. A sample with x = 6.462 is SC below 25 K and simultaneously magnetic below 10 K; the internal diamagnetism of the SC state is progressively reduced as local spin freezing sets in at low temperatures.

The internal magnetic fields in La 1.85 Sr 0.15 CuO 4 and YBa 2 Cu 3 O x have been investigated below 90 mK using muon spin rotation. In YBa 2 Cu 3 O x the magnetic order parameter remains constant with increasing x up until x =6.30 and then rapidly approaches zero just above x =6.44. We find little evidence for static or slowly fluctuating electronic moments in superconducting La 1.85 Sr 0.15 CuO 4 .