Previous work in our group demonstrated that for the cuprates
superconductors in the underdoped region: _{} where *T*_{C} is
the superconductor transition temperature, *J*_{f} a magnetic
energy scale for a given cuprates family, and *n*_{s} is the
superconducting carrier density. However there is still no clear experimental
confirmation that *J*_{f} is independent of doping for a given
family, or of a correlation between the maximum transition temperature *T*_{c}^{max}
and the Néel temperature *T*_{N}.

In the first part of the work I wanted to verify that the
energy scale *J*_{f} is unique to each family for high temperature
superconductors [HTSC], and is constant throughout all the doping range in the
family. For this I performed NQR measurements, thus determining doping levels,
on YBCO samples with various oxygen contents. I found that *T*_{C}
is proportional to the NQR resonance frequency, and this in tern is
proportional to *n*_{s}. From that _{} hence *J*_{f}
must be a constant for the YBCO family.

In the second part of the work I investigated the correlation
between *T*_{C}^{max} and *T*_{N} at very low
doping, for a given family. For this purpose I used the CLBLCO families of
superconductors and applied zero field muon spin relaxation measurements to
determine both the spin glass and Néel transition temperatures *T*_{g}
and *T*_{N}. This allowed me to construct the phase diagram of the
CLBLCO system. Using this phase diagram I showed that the family with the
higher value of *T*_{N} in the undoped parent compound, and
hence a higher value of the magnetic energy scale *J*_{f}, has
indeed the higher value of *T*_{C}^{max} in the
superconducting region.