Dynamic nuclear polarization ͑DNP͒ of 29 Si nuclei in isotopically controlled silicon single crystals with the 29 Si isotope abundance f 29Si varied from 1.2% to 99.2% is reported. It was found that both the DNP enhancement and 29 Si nuclear spin-lattice relaxation time under saturation of the electron paramagnetic resonance transitions of phosphorus donors increase with the decrease in the 29 Si abundance. A remarkably large steadystate DNP enhancement, E ss = 2680 which is comparable to the theoretical upper limit of 3310, has been achieved through the "resolved" solid effect that has been identified clearly in the f 29Si = 1.2% sample. The DNP enhancement depends not only on the 29 Si abundance but also on the electron spin-lattice relaxation time that can be controlled by temperature and/or illumination. The linewidth of 29 Si NMR spectra after DNP shows a linear dependence on f 29Si for f 29Si Յ 10% and changes to a square-root dependence for f 29Si Ն 50%. Comparison of experimentally determined nuclear polarization time with nuclear spin diffusion coefficients indicates that the rate of DNP is limited by the polarization transfer rather than by spin diffusion.

Optical detection of magnetic resonance (ODMR) and electron paramagnetic resonance (EPR) spectra are investigated in ZnO single crystals. The strong negative ODMR line with axial symmetry of the g-tensor around the c axis with g∥=2.0133±0.0001 and g⊥=2.0135±0.0001 is ascribed to Pb3+ ions. The strongest Pb3+ ODMR spectrum is observed under excitation with visible light from an Ar+ ion laser and below 40Hz of on/off microwave power modulation. The negative Pb3+ line, accompanied with effective mass (EM) donors over the spectral range of the photoluminescence (PL), shows spin dependent transfer of electrons from EM donors to Pb3+, competing with PL. The intensity of the Pb3+ EPR line as well as the EPR line of oxygen vacancy (VO+) investigated in samples irradiated with 2.5MeV electrons show a fast increase when the light is switched on and gradual decrease during illumination. The dependence of the growth and decay kinetics of the Pb3+ and VO+ EPR spectra on wavelength of light is analyzed by taking into account the double donor nature of the investigated centers and their energy level positions within the energy gap of ZnO.

Study of metal/ZnO based thin film ultraviolet photodetectors: The effect of induced charges on the dynamics of photoconductivity relaxationThe effect of persistent photoconductivity is observed in zinc oxide ͑ZnO͒ ceramics together with persistence of the electron paramagnetic resonance ͑EPR͒ spectra of defects and impurity centers in the samples. The spectral and time dependences of the EPR signals and microwave photoconductivity are investigated under excitation by light with different quantum energies below the ZnO band gap. The mechanisms of the persistent photoconductivity and coexistence of pnotoconductivity and optically induced EPR spectra after switching off the light are discussed. Based on the experimental results it is concluded that surface and intergranular conductivities dominate in ZnO ceramics.

Microwave losses due to conductivity and magnetic permeability are investigated in La 0.9 Ca 0.1 MnO 3 films illuminated with photons in the energy range of Eϭ0.5-2 eV. Growth of photoinduced ferromagnetic hysteresis is observed when the magnetic field is swept between Ϫ15 and 15 mT at temperatures below 60 K. The time required for saturation of the opening of the hysteresis loop depends on the photon energy having the minimum of Ϸ40 s at the illumination intensity Iϭ3.5ϫ10 14 photons/cm 2 s. Both photoinduced magnetization and the increase of microwave photoconductivity can be well explained with a model assuming that small ferromagnetic regions exist within an insulating ferromagnetic phase of the sample and that these regions are expanded by optically induced charge transfer between Jahn-Teller split e g states of neighboring Mn 3ϩ ions.

Low-field (6 − 110 mT) magnetic resonance of bismuth (Bi) donors in silicon has been observed by monitoring the change in photoconductivity induced by spin dependent recombination. The spectra at various resonance frequencies show signal intensity distributions drastically different from that observed in conventional electron paramagnetic resonance, attributed to different recombination rates for the forty possible combinations of spin states of a pair of a Bi donor and a paramagnetic recombination center. An excellent tunability of Bi excitation energy for the future coupling with superconducting flux qubits at low fields has been demonstrated.