Muon spin relaxation measurements in isotropic molecular magnets (MM) with a spin value S ranging from 7/2 to 27/2 are used to determine the magnitude and origin of dephasing time tau(phi) of molecular magnets. It is found that tau(phi) approximately 10 nsec with no S or ligand dependence. This indicates a nuclear origin for the stochastic field. Since tau(phi) is a property of the environment, we argue that it is a number common to similar types of MM. Therefore, tau(phi) is shorter than the Zener and tunneling times of anisotropic MM such as Fe(8) or Mn(4) for standard laboratory sweep rates. Our findings call for a stochastic Landau-Zener theory in this particular case.

The conditions (temperature, pressure and time) for attaining equilibrium with oxygen for different YBCO materials are reported. The equilibrium oxygen content was used in order to determine more exact values for the enthalpies and entropies of interaction of YBa2Cu3Oy with oxygen. Internal equilibrium in YBCO is proposed and is able to explain the effects of ageing and low temperature annealing on Tc. Homogeneity and transitions to superconductivity of ceramics having equilibrium and non-equilibrium oxygen contents are discussed.

We developed an experimental method for measuring the intrinsic susceptibility χ of powder of cuprate superconductors in the zero field limit using a DC-magnetometer. The method is tested with lead spheres. Using this method we determine χ for a number of cuprate families as a function of doping. A universal linear (and not proportionality) relation between Tc and χ is found. We suggest possible explanations for this phenomenon.

Photons emitted by transition between the discrete levels of single molecular magnets might obey the elementary condition for Dicke's super-radiance. We investigate this possibility in the Fe 8 molecule where magnetization jumps are known to occur at discrete magnetic-field values. We found energy bursts each time the molecule undergoes a magnetization jump, confirming their quantum nature. A series of tests indicated that photons carry out the energy and that indeed these photons obey the elementary conditions for super-radiance.

We investigate the coupling between the magnetic and superconducting order parameters in an 8-m-long meander line ͑"wire"͒ made of a La 1.94 Sr 0.06 CuO 4 film with a cross section of 0.5ϫ 100 m 2 . The magnetic order parameter is determined using the low-energy muon spin relaxation technique. The superconducting order parameter is characterized by transport measurements and modified by high current density. We find that when the superconducting order parameter is suppressed by the current, the magnetic transition temperature, T m , increases. The extracted sign and magnitude of the Ginzburg-Landau coupling constant indicate that the two orders are repulsive, and that our system is located close to the border between first-and second-order phase transition.