Pd-capped gasochromic metal hydrides can be used as sensing layer in fiber optic hydrogen detectors. Using a sensing layer consisting of a 50 nm thick Mg₇₀Ti₃₀ film capped with a 30 nm Pd catalytic layer we demonstrate a drop in reflectance by a factor of 10, at hydrogen levels down to 10% of the lower flammability limit. The switching takes place within a few seconds, depending on the actual amount of hydrogen in the vicinity of the detector. We report on the sensitivity of the fiber optic detector for hydrogen, both in argon and oxygen rich environments. We further show that the sensing behavior in oxygen rich gas mixtures and in normal air can be improved by adding silver to the catalytic Pd cap layer.

By means of high-sensitivity capacitance torque magnetometers we have measured the superconducting current j_s and the dynamic magnetic-moment relaxation of YBa₂Cu₃O₇ and YBa₂Cu₄O₈ films of typically 100 nm thickness at temperatures between 2 K and T_c in magnetic fields up to 6 T. For the measurements of the dynamic relaxation rate Q≡d ln j_s/d ln (dB_e/dt) magnetic-field sweep rates were varied between 0.5 and 40 mT/s. At low fields (typically 0.5 T) the dynamical relaxation rate exhibits a plateau at Q≈0.06 in YBa₂Cu₃O₇ and 0.04 in YBa₂Cu₄O₈. At high fields (B_e=μ₀H_e≈ 6 T) the plateaus have completely disappeared and Q increases almost linearly with increasing temperature. At all fields a sharp increase up to Q≊1 is observed when the irreversibility line is approached. By means of the generalized inversion scheme (GIS), the j_s(T, B_e) and Q (T, B_e) data are used to determined the current dependent activation energy U (j, T, B_e) for thermally activted flux creep. Although the GIS does not make any a priori assumptions about the explicit functional dependences on T and j, the U(j, T=0, B_e) function derived from the experimental data by means of the GIS can remarkably well be described with the collective-creep interpolation formula U(j)=(U_c/μ)[(j_c/j)^μ-1] with μ≈0.6 for currents j>0.15j_c (T=0, B_e) where J_c(T=0, B_e) is the critical current at T=0, and wi U_c depending on B_e. At lower current densities U(j, T=0, B_e) does not diverge as j^-0.6 but shifts gradually to a weaker ln(j_c/j) dependence. At low temperatures the current and relaxation data cannot be explained in terms of a thermally activated flux-motion model. Quantum creep has an influence up to ∼13 K.