Square planar coordination, high temperature superconductors

High-temperature superconductors are based on copper, which has NQR-active nuclei, and NQR spectra can be obtained on these superconductors while they are in the superconducting state, which excludes magnetic fields. For the classic superconductor YBa2Cu306.9, two Cu NQR signals can be obtained. The square-planar-coordinated Cu(l) gives an NQR signal at 22.05 MHz, for which = 38.9 MHz... 

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