P. Schleger

Thermodynamics of oxygen in Y1Ba2Cu3Ox between 450°C and 650°C

Abstract 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.

Site of the positive muon in YBa2Cu3O7

A combination of μSR and17O nuclear quadrupolar μLCR has been used to obtain detailed information on the muon site in YBa2Cu3O7. The rms magnitude of the nuclear dipolar fields in YBa2Cu3O7 enriched with 38%17O is about twice that in the natural16O sample. In addition the17O μLCR spectrum shows that the electric field gradient tensor at the oxygen closest to the muon is almost equal to that measured by NMR for the CuO2 plane sites O(2,3). These results suggest that the muon in fully oxygenated YBa2Cu3O7 is located at a single site 1.0 Å from an oxygen ion, probably O(2,3).

Magnetic ordering in YBa2Cu3Ox studied by μ+ spin rotation☆

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.

Coexistence of superconductivity and magnetic order in YBa2Cu3Ox

Abstract 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.

μ+Sr studies of flux mobility in the mixed state of Bi2Sr2CaCu2O8

Abstract 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.

μSR study of magnetism in La1.85Sr0.15CuO4 and YBa2Cu3Ox below 90 mK

Abstract 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 .