C. Korn

INFLUENCE OF HYDROGEN ON THE 89Y NMR IN HXYBA2CU3O7

The ambient-temperature {sup 89}Y magnetic resonance line in H{sub {ital x}}YBa{sub 2}Cu{sub 3}O{sub 7} was measured at 14.707 MHz as a function of hydrogen concentration {ital x}. The Knight shift relative to an aqueous solution of YCl{sub 3} increases from {minus}100 ppm for {ital x}=0 to +115 ppm for {ital x}=1.82. The linewidth at half-amplitude is 1.2 kHz and increases slowly with {ital x} in the superconducting samples (0{lt}{ital x}{lt}0.74), then increases sharply in the normal samples. It was seen that, for a number of parameters, increasing the hydrogen content gives similar behavior to decreasing the oxygen concentration. Our {sup 89}Y NMR data and those of other investigators are analyzed and are consistent with previous evidence for local antiferromagnetic correlations in the superconducting compound.

Neutron diffraction study of hydrogen in α titanium

Abstract It has been determined, using neutron diffraction, that hydrogen dissolved in the α (h.c.p.) phase of titanium is distributed randomly among the tetrahedral sites.

Evidence of local magnetic order in the superconducting phase of hydrogen doped YBa2Cu3O7 using nuclear magnetic resonance

Abstract Nuclear magnetic resonance absorption traces were obtained for the 1H line in H0.11YBa2Cu3O7 as a function of temperature. The superconducting temperature Tc is 93K. A spectral splitting was observed which we attribute to local antiferromagnetism. The Neel temperature is approximately 320K.

Influence of hydrogen on 123 and 124 HTc superconductors studied by Cu NMR and NQR

Abstract The antiferromagnetic compounds produced by the reaction of the 123 and 124 HT c superconductors with hydrogen are studied by nuclear resonancee of copper. For the 123 compounds two reactions are possible, the reduction by hydrogen which leads to the normal antiferromagnetic YBa 2 Cu 3 O 6 , and the solution of hydrogen in YBa 2 Cu 3 O 7 which forms a different antiferromagnetic structure with hydrogen concentration of more than 2 H mol −1 . Both structures can be distinguished by the nuclear quadrupole resonance spectra of the Cu(1) sites and the nuclear magnetic resonance spectra in the hyperfine field of the antiferromagnetic state. By using samples containing Gd at the place of Y we can prove that only the Cu(2) sites carry permanent magnetic moments. In the hydrogen loaded YBa 2 Cu 4 O 8 the situation is similar with the antiferromagnetic order limited to the Cu(2) sites.

X-ray diffraction study of the influence of hydrogen on the crystallographic structure of HxYBa2Cu3O7−δ

Abstract X-ray diffraction was performed on powder samples of H x YBa 2 Cu 3 O 7−δ where δ≈0.07 and 0⩽ x ⩽1.52. A comparison is made with the crystallographic behavior upon oxygen extraction. The following features were found: (1) In the orthorhombic phase the a and b lattice parameters were consistent with the assumption that the addition of two hydrogen atoms is equivalent to the removal of one oxygen. (2) The orthorhombic-to-tetragonal transition takes place at x ≈0.8 (3) The a parameter in the tetragonal phase is larger when hydrogen is added than when oxygen is removed. (4) While c increase monotonically upon oxygen extraction, it decreases upon hydrogen addition both in the orthorhombic and tetragonal phases such that when the hydrogen concentration reaches x ≈1.0, then a = b = c 3 . (5) At x =1.0, the Cu(2) and Cu(1) planes are evenly spaced, c 3 apart. These phenomena are discussed in the context of the superconducting properties of the material upon oxygen removal or hydrogen addition. In addition, comparison between a pellet and a powder is made, which indicates that some surface oxygen may be removed upon hydriding.

Can halogen atoms be inserted into the YBCO system

Abstract We have measured the 19 F and 35 Cl NMR spectra of fluorine and chlorine doped YBCO. We show that the positions and widths of the resonance lines do not match any of the most probable decomposition products of the doped YBCO. In addition, we show that the temperature dependence of the 19 F linewidth follows the evolution of the vortex lattice in the superconducting phase. The 35 Cl NMR spectra are consistent with the fact that the Cl occupies the chain or apical oxygen site. ESR results show an increase in the ESR signal above a certain halogen concentration, which depends on the oxygen content. This is interpreted as an appearance of either a very small amount (of the order of 0.1%) of a new paramagnetic phase or a decrease in the grain size due to halogenation.

NMR study of the effect of hydrogen on YBa2Cu3O6

Abstract Previous studies have shown that when hydrogen is added to the superconductor YBa 2 Cu 3 O 7 (O 7 ), it affects many of its properties, as if chain oxygen were removed. Hence the effect of H on the material should be quite different when these oxygen atoms are missing. We have therefore studied the influence of H on YBa 2 Cu 3 O 6 (O 6 ). In contrast to the case of O 7 , the 89 Y NMR resonance frequency is independent of x ; instead a second line evolves that is shifted to a higher frequency with respect to the original line. The results confirm that hydrogen in O 6 qualitatively plays a different role than hydrogen in O 7 . The quadrupole interaction of 2 H in the material yields a coupling constant e 2 qQ h of about 250 kHz which indicates that the hydrogen forms an O-H bond. X-ray diffraction results are also discussed.

NMR study of 19F resonance above Tc in fluorinated YBa2Cu3Ox

Abstract The 19 F NMR spectra were obtained at 282.365 MHz, for a series of YBa2Cu3OxFy samples at a number of different temperatures, where y=0.1, 0.2 when x=7; y=0.10, 0.15, 0.29, 0.38 when x=6.7; and y=0.6, 0.98, 1.50 when x=6. These values of x span the fully oxygenated superconductor, the partially oxygenated superconductor, and the antiferromagnetic non-superconductor when the material is undoped. The spectra were obtained using a phase cycled echo. By measuring the echoes using a series of different times between pulses, we were able to resolve the spectra into two separate lines, representing two different F sites where the occupancy of one is much larger than the other, their ratio depending on the concentrations. The major site is attributed to the chain sites along a or b or both and the minor site is tentatively assigned to the apical position between the chain copper and the plain copper. The frequency shift of the major component is independent of both the oxygen and the fluorine concentration.

In-situ EPR study of room-temperature evolution of YBa2Cu3O6+x ceramics

Abstract Cu 2+ EPR has been used to study the process of room-temperature annealing in powdered YBa 2 Cu 3 O 6+ x (0 ≤ x ≤ 1) samples. The annealing and the consequent changes in the oxygen coordination of the Cu atoms in the basal plane do not affect the EPR signal. The results suggest that for both oxygen-deficient and oxygen-rich samples, the Cu 2+ EPR signal can arise only from defects unrelated to chain ordering. Samples exposed to ambient atmosphere exhibit growth of the EPR signal with time. The nature of this signal is discussed.

Non-exponential behavior of the nuclear magnetization in H-doped copper oxides.

Measurement of the spin lattice relaxation time in H-doped Y2BaCuO5 and YBa2Cu3O6 has revealed a power law growth of the nuclear magnetization M for short times following a saturation pulse sequence. Preliminary experimental results yield exponents of 1/2 and 1/3, depending on the amount of hydrogen present in the sample. We extend Blumbergs theory for relaxation due to paramagnetic centers via dipole-dipole interaction in the absence of spin diffusion to obtain M approximately tD/6 where D is the dimensionality of the system. This yields exponents of 1/2 and 1/3 for three- and two-dimensional systems, respectively. At long times the magnetization growth becomes exponential.