Copper NMR and NQR in YBa2Cu3Ox and GdBa2Cu3Ox withx between 6 and 7. A study of the oxygen ordering

Abstract

The NQR and NMR spectra of copper in polycrystalline samples of YBazCu30 ~ and GdBa2Cu30 ~ with different oxygen concentration x have been measured at 4.2 and 1.2 K. The different influence of Gd spins on the Cu(1) and Cu(2) sites was used to separate the contributions of both sites in the spectra. The quantitative analysis of the spectra in the region 6.3 < x < 6.75 allows the following conclusions about the ordering of the oxygen ions in the chains of the Cu(t) planes and about the magnetic state of the Cu(2) ions to be drawn, At about x = 6.5 oxygen orders in the ortho-II structure with every second chain filled. For higher x the coexistence of ortho-II and oxygen depleted ortho-I (all chains filled to about 80 %) gives the best description of the results. The average length of the Cu--O chains, n, is about 10 oxygen atoms per chain in the region of the 60 K plateau and decreases rapidly at lower x. The superconductivity disappears when n < 6. No signal of the Cu(2) sites is found in samples with T~ below the 60 K plateau due to the slowing down of the fluctuation tate of the electron spins of the Cu(2) ions. Magnetic order appears only in samples which do not become superconducting.

Ordering of oxygen in halogen treated 1-2-3 systems as studied by NQR

“…The separation of the Cu(2) and Cu(1) lines can be easily made in the GdBa 2 Cu3Ox compounds using the idea of the fluctuations of the rare earth moments [11]. The fluctuation of the nearly paramagnetic Gd moments above the Neel temperature TN = 2.2 K induces significant relaxation effects at the nearby nuclei in the Cu (2) layer.…”

“…We see that the treatment of iodine gives results very similar to the increase of the basal oxygen. In the compounds with intermediate oxygen concentration a sharp line at 24 MHz for 63 Cu was attributed to Cu(l) sites with three oxygen neighbors [11]. 63 Cu NQR spectrum of GdBa2Cu3 O6 , consists of one line at 30.1 MHz.…”

“…The characteristic NQR spectra of GdBaCuO. at different oxygen concentrations x are reported for 4.2 K [11]. In the high frequency region of the spectra in the samples with x below about 6.55 two more lines appear and the spectra exhibit three lines at the frequencies 31.2, 30.9 and 30.2 MHz.…”

“…It was shown that the Cu NQR spectra of the RBa 2 Cu3OX can be explained within the model of the ortho-II structure where in the samples with oxygen concentration x> 6.59 every "empty" chain is situated between two intact chains, and in the samples with x < 6.59 two or more neighboring chains appear to be empty [10,11]. The comparatively narrow lines of Cu (1) show that the EFG values are determined only by the nearest coordination sphere and thus three configurations of the Cu(1) sites could be identified from the NQR spectra.…”

“…The study of EFGs is particularly important, because EFGs depend sensitively on the charge distribution. Moreover, the EFG depends on other contributions like valence state and on the number and position of the oxygen neighbors of the ion under inspection [10,11]. In this paper, we present Cu NQR results for iodinated RBa2Cu3Ox + IZ (R = Y, Gd) compounds.…”