Inhomogeneous state of the electron system in the Sr14 − x Ca x Cu24O41 superconducting cuprate: 63Cu-17O NMR study

Abstract

Charge fluctuations in quasi-one-dimensional hole-doped Sr14Cu24O41 (Ca-0) and Sr2Ca12Cu24O41 (Ca-12) spin-ladder cuprates have been studied with the use of 63Cu-17O NMR. Spin-echo decay rates 17(1/T2) and 63(1/T2) have been measured in the temperature range of T = 10–300 K. The variation of 17(1/T2) and 63(1/T2) in the Ca-0 compound is monotonic in the entire temperature range of the NMR study. In the Ca-rich compound, pronounced peaks have been observed in the decay rates 17(1/T2) and 63(1/T2) at temperatures of 25 and 50 K, respectively. This result indicates the presence of collective low-frequency (τc ∼ T2) charge density fluctuations in superconducting Ca-12 with an activation energy of EA = 100(10) K. The fluctuations gradually slow down as the temperature decreases. The amplitude of the charge density fluctuations is only 0.01–0.02 hole per site. 17O-63Cu spin-echo double-resonance (SEDOR) experiments in Sr14 − xCaxCu24O41 oxides with x = 0 and 12 have been performed depending on the temperature and orientation of single crystals in a magnetic field. The constants of an indirect heteronuclear 17O-63Cu interaction of nuclear spins mediated by conduction electrons have been measured. The estimates of the indirect interaction constants for nearest neighbors O1-Cu and O2-Cu, as well as SEDOR experiments with selective excitation of separate sections of 17O and 63Cu NMR spectra, provide convincing evidence of the microscopically inhomogeneous spatial distribution of spin density developing in a crystal.