A novel spectroscopy technique based on modulated photoconductivity measurements with varying illumination level has been applied to investigate the capture coefficients and the energy distribution of defect states in undoped amorphous silicon. From the experimental data, charged and neutral defect distributions are clearly resolved according to their own capture coefficients. The carrier capture process as well as the defect formation mechanism are both quantitatively discussed.

This paper presents a theory for spin pumping from finite-sized conductors at paramagnetic resonance. The spin generation and transport in the system exposed to a rotating magnetic field are analyzed by using the nonequilibrium Green's-function formalism, showing that a time-independent spin current is produced with no charge current when the electrochemical potentials in all leads attached to the system are identical. A physically transparent formulation for the spin pumping is derived in terms of the spin-current continuity equation with the source term due to the rotating field. The characteristics of spin-pumping systems are investigated by numerical calculation. In the ballistic transport regime, the pumped spin at resonance is augmented normally with the system size and the pumping intensity, insofar as the system contacts ideal leads that transport spin to the outside world and the pumping field is perturbatively weak. In the diffusive transport regime, the spin pumping is strongly enhanced with the degree of static disorder. The mechanism of enhancement is discussed in view of the weak localization.between the charge density operator charge ͑r , t͒ = † ͑r , t͒͑r , t͒ and the charge-current-density operator j charge ͑r , t͒ =Re † ͑r , t͒v͑r , t͒, while the spin-density opera-PHYSICAL REVIEW B 75, 205302 ͑2007͒

We show that the spin-orbit coupling in two-dimensional ͑2D͒ electron systems with a potential gradient parallel to the 2D plane exerts a spin-dependent transverse force on moving electrons while conserving their spins. Due to the spin conservation, the standard continuity equation holds between spin density and spin current. Using numerical calculations based on the nonequilibrium Green's function formalism, we demonstrate for crossed quantum wires with a harmonic confining potential in the ballistic limit that transverse spin separation and spin Hall current are generated in response to a longitudinal charge current.

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The adiabatic spin pumping from an interacting quantum dot driven by a rotating magnetic field is studied using the numerical renormalization-group method. From the spin-current continuity equation, we derive a general formalism for spin pumping, which proves that the rotating field generates a time-independent spin current. Following this formalism, the numerical calculation implemented in the adiabatic regime demonstrates that spin pumping is enhanced exponentially with Coulomb interaction. This mechanism is explained in terms of the Kondo resonance under the external field.