Muon-spin-rotation studies of the temperature dependence of the magnetic penetration depth in the YBa2Cu3Ox family and related compounds

A systematic muon-spin-rotation (+SR) study is presented of the temperature dependence of the London penetration depth in sintered powder samples of the YBa2Cu3Ox system and related compounds. The in-plane penetration depth ab is estimated from the SR depolarization rate of Bi2Sr2CaCu2O8, YBa2Cu4O8, and a series of samples of the YBa2Cu3Ox family, respectively. It is found that not only the low-temperature value ab(0), but also the temperature behavior ab(T) is specific to each compound. The form of ab(T) can be well characterized by a simple power law. In particular, the YBa2Cu3Ox family shows a systematic variation of the form of ab(T) with the oxygen content x which points to a varying coupling strength, whereas ab(0) as a function of x suggests a positive charge transfer into the CuO2 planes with increasing oxygen doping. Furthermore, our data is consistent with an empirical ansatz which has been proposed in the framework of a Bose-gas picture of high-temperature superconductivity. As a consequence, the pressure and the isotope coefficients can be extracted from the SR depolarization rate and compared to direct measurements of these quantities, showing good agreement. Moreover, in the Bose-gas picture the variation of ab(T) in the YBa2Cu3Ox family may be interpreted as a crossover from a dense (high-Tc) to a dilute (low-Tc) system of weakly interacting local pairs.