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CONDENSED MATTER
Inhomogeneous state of the electron system in the $\mathrm{Sr}_{14-x}\mathrm{Ca}_x\mathrm{Cu_{24}O_{41}}$ superconducting cuprate: $^{63}\mathrm{Cu}$, $^{17}\mathrm O$ NMR study
Yu. V. Piskunov,
V. V. Ogloblichev,
S. V. Verkhovskii Institute of Metal Physics, Ural Division of the Russian Academy of Sciences
Abstract:
Charge fluctuations in quasi-one-dimensional hole-doped Sr
14Cu
24O
41 (Ca-0) and Sr
2Ca
12Cu
24O
41 (Ca-12) spin-ladder cuprates have been studied with the use of
63Cu-
17O NMR. Spin-echo decay rates
17(1/
T 2) and
63(1/
T 2) have been measured in the temperature range of
T = 10–300 K. The variation of
17(1/
T 2) and
63(1/
T 2) 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/
T 2) and
63(1/
T 2) at temperatures of 25 and 50 K, respectively. This result indicates the presence of collective low-frequency (τ
c ~
T 2) charge density fluctuations in superconducting Ca-12 with an activation energy of
E A = 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 Sr
14 − x Ca
x Cu
24O
41 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.
PACS:
74.25.Jb,
74.72.Jt,
76.60.Es,
76.70.Fz Received: 01.11.2007
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