Tongfei Shi

Structures and magnetic properties of wurtzite Zn1- xCoxO dilute magnetic semiconductor nanocomposites

Cobalt-doped ZnO dilute magnetic semiconductor nanocomposites Zn1−xCoxO with Co concentrations from 0.02 to 0.25 were prepared by the sol-gel method. The magnetic measurement shows paramagnetic behavior for all the samples. The structures of these composites were investigated by x-ray diffraction and fluorescence x-ray absorption fine structure spectroscopy. It is indicated that at low Co concentration (x⩽0.05), the Co atoms are incorporated into the ZnO lattice and located at the substitutional sites of the Zn atoms. At higher Co doping concentration (x⩾0.10), the secondary phase Co3O4 is precipitated. Correlating the magnetic behavior with the structural properties of the Zn1−xCoxO nanocomposites, the authors interpret the paramagnetism to be intrinsic in nature as a result of the low effective doping of Co in ZnO and the lack of oxygen vacancies.

Structures and magnetic properties of (Mn, N)-codoped ZnO thin films

Zn0.96Mn0.04O and Zn0.96Mn0.04O:N thin films with wurtzite structure were grown by inductively coupled plasma enhanced chemical vapor deposition method. Although both samples exhibit ferromagnetism at room temperature, the saturation magnetic moment (1.4μB∕Mn) of the Mn- and N-codoped sample is much larger than that (0.3μB∕Mn) of the N-free one. The x-ray absorption near edge structure analysis reveals that the codoped Mn and N impurities can be substitutionally incorporated into the ZnO host in the Zn0.96Mn0.04O:N thin film. The first-principles calculations suggest that the N substitution for the O site in Mn-doped ZnO can change the interaction of neighboring Mn–Mn pairs from antiferromagnetic to ferromagnetic, and accordingly the effective magnetic moment per Mn is greatly enhanced.

Lehmann-Symanzik-Zimmermann reduction approach to multiphoton scattering in coupled-resonator arrays

We present a quantum field theoretical approach based on the Lehmann-Symanzik-Zimmermann reduction for the multiphoton scattering process in a nanoarchitecture consisting of the coupled-resonator arrays (CRA), which are also coupled to some artificial atoms as a controlling quantum node. By making use of this approach, we find the bound states of a single photon for an elementary unit, the T-type CRA, and explicitly obtain its multiphoton scattering S matrix in various situations. We also use this method to calculate the multiphoton S matrices for the more complex quantum network constructed with main T-type CRAs, such as a H-type CRA waveguide.

Two-photon transport in a waveguide coupled to a cavity in a two-level system

We study two-photon effects for a cavity quantum electrodynamics system where a waveguide is coupled to a cavity embedded in a two-level system. The wave function of two-photon scattering is exactly solved by using the Lehmann-Symanzik-Zimmermann reduction. Our results about quantum statistical properties of the outgoing photons explicitly exhibit the photon blockade effects in the strong-coupling regime. These results agree with the observations of recent experiments.

The role of Zn interstitials in cobalt-doped ZnO diluted magnetic semiconductors

Co-doped ZnO samples Zn1−xCoxO and CoyZn1−yO were prepared by the sol-gel and magnetron sputtering methods, respectively. Although the Cou2009K-edge extended x-ray absorption fine structure spectra show that the doped Co ions are located at the Zn substitutional sites for both Zn1−xCoxO (x≤0.05) and CoyZn1−yO (y≤0.05) samples, magnetic measurements show paramagnetism in Zn1−xCoxO (x≤0.05) and high temperature ferromagnetism in CoyZn1−yO (y≤0.05). An experimental and numerical study of the Ou2009K-edge x-ray absorption near-edge structure spectra reveals that the incorporation of the Zn interstitials in the lattice is crucial to the appearance of high temperature ferromagnetism in CoyZn1−yO (y≤0.05) samples.

Origin of the high performance of perovskite solar cells with large grains

Due to excellent carrier transport characteristics, CH3NH3PbI3 film made of large single crystal grains is considered as a key to improve upon already remarkable perovskite solar cell (PSC) efficiency. We have used a simple and efficient solvent vapor annealing method to obtain CH3NH3PbI3 films with grain size over 1u2009μm. PSCs with different grain size films have been fabricated and verified the potential of large grains for improving solar cells performance. Moreover, the larger grain films have shown stronger light absorption ability and more photon-generated carriers under the same illumination. A detailed temperature-dependent PL study has indicated that it originates from larger radius and lower binding energy of donor-acceptor-pair (DAP) in larger grains, which makes the DAP is easily to be separated and difficult to be recombine.

Thickness-independent electron field emission from tetrahedral amorphous carbon films

Electron field emission properties of tetrahedral amorphous carbon films of different thicknesses have been studied. The experimental results indicate that there exists no close relationship between threshold electric field and film thickness. Different field emission models are used to examine the experimental results in order to explain the thickness-independent electron field emission properties.

Local environment of Er3+ in Er-doped Si nanoclusters embedded in SiO2 films

The local environment of Er3+ in heavily Er-doped (Er, 2.5 at. %) Si nanoclusters embedded in SiO2 films annealed at various temperatures was investigated by using the fluorescence-extended x-ray absorption fine structure spectroscopy. The results show that annealing caused a large effect on the local environment of Er3+ surrounded by O atoms and the 1.54 mu m photoluminescence intensity. The correlation between the local environment around Er3+ and the corresponding 1.54 mu m photoluminescence was discussed. (c) 2006 American Institute of Physics.

Singlet and triplet BCS pairs in a gas of two-species fermionic polar molecules

Taking into account the deformation of the Fermi surface, we investigate the spin-singlet and -triplet BCS pairings in a mixture of fermionic polar molecules with two different hyperfine states. In particular, we explore the relation between the critical temperatures and the Fock-exchange interaction. We also show that, by tuning short-range interaction between interspecies molecules, the singlet- and triplet-paired superfluids may coexist.

Electron field emission properties of as-deposited and nitrogen implanted tetrahedral amorphous carbon films

Abstract Tetrahedral amorphous carbon (ta-C) films have been prepared at ambient temperature by filtered arc deposition (FAD). Electron field emission properties of the ta-C films were studied using a parallel plate diode configuration. Research indicated that a threshold electric field of about 16 V / μm is required to initiate the electron emission from the as-grown ta-C film. An emission current of ∼20 μA was detected at an electric field of around 20 V / μm , following the Fowler–Nordheim (FN) field emission behavior. In order to investigate the effect of nitrogen on the field emission of ta-C, nitrogen ions were implanted into the ta-C films at an ion energy of 6 keV with ion dose varying from 1.7 to 20×10 15 atoms / cm 2 . It was found that the emission current and the threshold electric field are in close relationship with the ion dose. At the highest ion dose, the emission current increases to ∼80 μA. Correspondingly, the threshold electric field decreases from 16.7 to 14 V / μm . The FN behavior was also observed for the nitrogen implanted ta-C films. However, the degradation of the field emission properties is found when a lower dose of nitrogen is implanted into the films. The mechanism for the modification of field emission properties by nitrogen implantation is discussed.