Resonant X-ray diffraction studies on modulation structures of high temperature superconducting sample Sr<sub>2</sub>CuO<sub>3.4</sub>

Abstract

<sec>Sr<sub>2</sub>CuO<sub>3+<i>δ</i></sub> is cuprate, a high temperature superconducting (HTS) material that has a single copper oxide layer and a relatively high critical temperature. Its structure is simple and contains fewer atoms, but there are many modulation structures in it. A lot of studies have pointed out that the modulation structure is related to its superconductivity. In order to further study the relationship between modulated structure and superconductivity in Sr<sub>2</sub>CuO<sub>3.4</sub> HTS sample, a new explanation for the formation mechanism of modulation structure is proposed in this paper. </sec><sec>The synchrotron radiation resonant X-ray diffraction (RXD) technique is used to detect the variation of modulation structure near the absorption edge of Cu atom. Cu<sup>2+</sup>, Cu<sup>3+</sup> valence order is detected and used to explain the formation mechanism of modulation structure in Sr<sub>2</sub>CuO<sub>3.4</sub> high temperature superconducting sample. The energy values of incident light are selected to be 8.52, 8.95, 8.98, 9.05, 9.5, and 10.0 keV near the edge of Cu <i>K</i>. The energy resolution is about 1.5 eV. The detector used in the experiment is Mar165 CCD surface detector. The distance from the detector to the sample is about 315 mm. The two-dimensional diffraction pattern recorded by the CCD plane detector is processed by Fit2D software to obtain the diffraction integral intensity. In addition, the energy calibration for each of the copper foil samples is carried out prior to the start of the experiment and in the process of varying energy value.</sec><sec>The experimental results show that the Bragg diffraction peaks corresponding to <i>T</i><sub>c</sub> = 48 K and the modulation structures of <i>Fmmm</i> and <i>Pmmm</i> are visible and calibrated. The intensity of the corresponding (2/5, 4/5, 0) diffraction peak of <i>Fmmm</i> is energy-dependent near the Cu <i>K</i> edge and first increases and then decreases abruptly near the absorption edge. This indicates that a stable ordered arrangement structure of Cu<sup>2+</sup> and Cu<sup>3+</sup> is formed at this time. The weak diffraction signal of this ordered arrangement structure confirms the fact that the copper-O bonding is stronger.</sec><sec>The experiments indicate that oxygen vacancies occupy both the apical position and the CuO<sub>2</sub> plane. The ordering arrangement of oxygen vacancies results in the ordering of copper ions with variable valence. The Cu<sup>2+</sup>, Cu<sup>3+</sup> valence order is related to the superconductivity of Sr<sub>2</sub>CuO<sub>3.4</sub>.</sec>