Femtosecond electron and spin dynamics probed by nonlinear optics

A theoretical calculation is performed for the ultrafast spin dynamics in nickel using an exact diagonalization method. The present theory mainly focuses on a situation where the intrinsic charge and spin dynamics is probed by the nonlinear (magneto-)optical responses on the femtosecond time scale, i.e. optical second harmonic generation (SHG) and the nonlinear magneto-optical Kerr effect (NOLIMOKE). It is found that the ultrafast charge and spin dynamics are observable on the time scale of 10 fs. The charge dynamics proceeds ahead of the spin dynamics, which indicates the existence of a spin memory time. The fast decay results from the loss of coherence in the initial excited state. Both the material specific and experimental parameters affect the dynamics. We find that the increase of exchange interaction mainly accelerates the spin dynamics rather than the charge dynamics. A reduction of the hopping integrals, such as present at interfaces, slows down the spin dynamics significantly. Besides, it is found that a spectrally broad excitation yields the intrinsic speed limit of the charge (SHG) and spin dynamics (NOLIMOKE) while a narrower width prolongs the dynamics. This magnetic interface dynamics then should become accessible to state of art time resolved nonlinear-optical experiments.Comment: 5 pages with 3 figures, to appear in Applied. Phys. B 68, (1999

Visible light driven photocatalysis mediated via ligand-to-metal charge transfer (LMCT): An alternative approach to solar activation of titania

Visible light harvesting or utilization through semiconductor photocatalysis is a key technology for solar chemical conversion processes. Although titania nanoparticles are popular as a base material of photocatalysis, the lack of visible light activity needs to be overcome. This mini-review is focused on an uncommon approach to visible light activation of titania: the ligand-to-metal charge transfer (LMCT) that takes place between TiO2 nanoparticles and surface adsorbates under visible light irradiation. We discuss a basic concept of photoinduced LMCT and the recent advances in LMCT-mediated visible light photocatalysis which has been applied in environmental remediation and solar energy conversion. Although the LMCT processes have been less investigated and limited in photocatalytic applications compared with other popular visible light activation methods such as impurity doping and dye sensitization, they provide lots of possibilities and flexibility in that a wide variety of organic or inorganic compounds can form surface complexes with TiO2 and introduce a new absorption band in the visible light region. The LMCT complexes may serve as a visible light sensitizer that initiates the photocatalytic conversion of various substrates or the self-degradation of the ligand complexes (usually pollutants) themselves. We summarized and discussed various LMCT photocatalytic systems and their characteristics. The LMCT-mediated activation of titania and other wide bandgap semiconductors has great potential to be developed as a more general method of solar energy utilization in photocatalytic systems. More systematic design and utilization of LMCT complexes on semiconductors are warranted to advance the solar-driven chemical conversion processes.open11144136Ysciescopu

Measurement of surface potential decay of corona-charged polymer films using the pulsed electroacoustic method

In this paper, the pulsed electroacoustic (PEA) technique that allows the determination of space charge in a dielectric material has been used to monitor the electrical potential decay of corona-charged polyethylene films of different thicknesses. To prevent possible disturbance on the surface charge during the PEA measurements, two thin polyethylene films were placed on both sides of the corona-charged sample. Charge profiles measured at different times were used to calculate the potential across the sample. The obtained potential decay was compared with the potential measured using the conventional method. A good agreement has been obtained. More importantly, the charge profile obtained using the PEA technique indicates that bipolar charge injection has taken place

NLTE study of scandium in the Sun

We investigate the formation of neutral and singly ionized scandium lines in the solar photospheres. The research is aimed derive solar $\log gf\epsilon_{\odot}$(Sc) values for scandium lines, which will later be used in differential abundance analyses of metal-poor stars. Extensive statistical equilibrium calculations were carried out for a model atom, which comprises 92 terms for \ion{Sc}{i} and 79 for \ion{Sc}{ii}. Photoionization cross-sections are assumed to be hydrogenic. Synthetic line profiles calculated from the level populations according to the NLTE departure coefficients were compared with the observed solar spectral atlas. Hyperfine structure (HFS) broadening is taken into account. The statistical equilibrium of scandium is dominated by a strong underpopulation of \ion{Sc}{i} caused by missing strong lines. It is nearly unaffected by the variation in interaction parameters and only marginally sensitive to the choice of the solar atmospheric model. Abundance determinations using the ODF model lead to a solar Sc abundance of between $\log\epsilon_\odot = 3.07$ and 3.13, depending on the choice of $f$ values. The long known difference between photospheric and meteoritic scandium abundances is confirmed for the experimental $f$-values.Comment: 10 pages, 6 figures, A&A accepte

Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components

We report on an RXTE/PCA observation of Aql X-1 during its outburst in March 1997 in which, immediately following a Type-I burst, the broad-band 2-10 keV flux decreased by about 10% and the kilohertz QPO frequency decreased from 813+-3 Hz to 776+-4 Hz. This change in kHz QPO frequency is much larger than expected from a simple extrapolation of a frequency-flux correlation established using data before the burst. Meanwhile a very low frequency noise (VLFN) component in the broad-band FFT power spectra with a fractional root-mean-square (rms) amplitude of 1.2% before the burst ceased to exist after the burst. All these changes were accompanied by a change in the energy spectral shape. If we characterize the energy spectra with a model composed of two blackbody (BB) components and a power law component, almost all the decrease in flux was in the two BB components. We attribute the two BB components to the contributions from a region very near the neutron star or even the neutron star itself and from the accretion disk, respectively.Comment: 12 pages with 4 figures, accepted for publication in ApJ Letters, typos corrected and references update

NMR Probing Spin Excitations in the Ring-Like Structure of a Two-Subband System

Resistively detected nuclear magnetic resonance (NMR) is observed inside the ring-like structure, with a quantized Hall conductance of 6e^2/h, in the phase diagram of a two subband electron system. The NMR signal persists up to 400 mK and is absent in other states with the same quantized Hall conductance. The nuclear spin-lattice relaxation time, T1, is found to decrease rapidly towards the ring center. These observations are consistent with the assertion of the ring-like region being a ferromagnetic state that is accompanied by collective spin excitations.Comment: 4 pages, 4 figure