Photonic band gaps can be generally classified into two main categories: Bragg gap or resonance gap. For the Bragg gap, a dielectric mirror formed by stacking two different types of dielectrics in alternative sequence is a very good representative. This kind of gap is formed from Bragg scattering arising from the periodicity. For the resonance gap, photonic crystals formed from placing metallic spheres periodically inside a dielectric is also a good representative. The kind of gap is formed from the resonance of a single sphere. In this work, we found that there is an additional mechanism in forming photonic band gap if we can also have a double negative (DNG) medium in designing photonic crystals.
Evanescent waves can play an important role in meta-materials. The most famous example is the Perfect Lens invented by J B Pendry. The lens, a flat slab of material with both permittivity and permeability being minus one, can attain a sub-wavelength resolution by amplifying the evanescent wave within the lens. However, for evanescent waves with bigger and bigger transverse wave vectors, it is expected that the microstructure starts to become important so that the physics deviates from the local effective medium description of the meta-material.
In order to investigate the effect of microstructure, a dipolar model is employed to model the metamaterial so that on one hand we can derive the local effective medium directly. On the other hand, a complete ( , ) dispersion diagram can be obtained with microstructure taken into consideration. As a result, the complete dispersion diagram converges to the local effective medium description only near the Brillouin zone center.
The structure, atomic and crystallographic, as well as the microstructure of a material, determine the physical properties of the material. The characterization of a material is therefore of considerable importance. A solid is an aggregate of randomly oriented grains of crystal. A crystal grain is a regular array of one or more types of atoms. The disruption of the arrangement of atoms in a crystal results in a crystal defect. Material characterization involves the identification of the atoms, their crystallographic structure and the microstructure of the target solid. An element is usually identified by its characteristic X-ray spectrum. Crystal structure is often determined by X-ray or electron diffraction. The characterization of a small grain, especially nanocrystalline grain, requires a probe of nanometer size. The transmission electron microscope (TEM) is the only instrument capable of providing chemical and structural (atomic and crystallographic) data for the characterization of a solid with high spatial resolution. By focusing a nanometer probe of electrons onto a small grain in a TEM, characteristic X-ray and diffraction pattern are generated. The detection and analysis of X-ray by means of Energy Dispersive Spectroscopy (EDS) in a TEM allows the identity and amount of the elements in the solid to be determined. Electron diffraction provides a means for the identification and determination of crystal orientation and structure. Imaging, the ultimate goal of microscopy, can be carried out in different ways depending on the occasion. Diffraction contrast imaging is usually used for the determination of crystal defects. High resolution TEM imaging is used for the observation of the atomic arrangement in the crystal.
Novel Properties of Metallic Nanoparticles and composites
Giant Hall Effect from embedded nanoparticles
At the Physics Department of HKUST, a group of scientists have discovered a large increase in Hall response in magnetic metal/insulator nanocomposites, Fe-SiO2, Ni-SiO2 and Co-SiO2, when the metal concentration is just enough to form a connected network (the percolation threshold). It has been also demonstrated by studying the non-magnetic Cu-SiO2nanocomposites that the Giant Hall effect can be ascribed to the local quantum interference effect (Phys. Rev. Lett. 86, 5562 (2001)).
Fig. 1Fig. 2
Oxidation/corrosion resistant nanoparticles of Fe and their ferrofluid
Passivated nanocrystals of iron prepared by gas condensation of plasma evaporated vapor in Tianjin University are remarkably resistant to further oxidation and corrosion. We have shown by electron diffraction and high resolution transmission electron microscopy that each nanocrystal of Fe is enclosed by a shell of single crystal-like epitaxial gamma-Fe2O3 oxide (Fig 1 and Fig. 2). The passive oxide shell is about 4 nm thick. This is thick enough to provide effective protection to the metal core at room temperature (Appl. Phys. Lett. 77, 3971 (2000)).
Ferrofluid has been fabricated with these corrosion resistant nanocrystals of iron. As shown in Fig. 3, when the ferrofluid is subjected to the non-uniform magnetic field of a NdFeB permanent magnet, it transforms into a solid with long and sharp spikes.
Fig. 3
Discovery of magnetic refrigeration materials
Anisotropic magnetocaloric effects have been observed experimentally for the first time by using the identical anisotropic magnetic Fe8 clusters, which has been intensively studied for the Resonant Quantum tunnelling of Spins. It is also interesting to examine the anisotropy effect using this model system, because the nano-structured magnetic materials usually show a strong magnetic anisotropy (Phys. Rev. Lett. 87, 157203 (2001). Due to the simple form of the Hamiltonian in the system, the dynamics of the magnetic moments can be strictly obtained using quantum mechanics. The quantum mechanical solution agrees excellently with the experimental results.
Rare-earth based, inter-metallic alloys have been synthesized and characterized for the search of magnetic refrigeration materials used for Active Magnetic Regenerator Magnetic Refrigerator (AMR). LaFe12-xSix alloys with a NaZn13 type cubic structure have been discovered to be a good material candidate for AMR in the temperature range -70 degrees Celsius to room temperature (Appl. Phys. Lett. 77, 3072 (2000)).
At temperatures below , the Meissner effect, superconducting gap and fluctuation supercurrent have been observed in 0.4 nm carbon nanotube / zeolite composites. The measured superconducting behaviors display smooth temperature variation due to fluctuations, with a mean-field .
The three separately measured superconducting behaviors can be consistently explained within a unified theoretical framework. The three effects are separately explained below.
, 
) dispersion diagram can be obtained with microstructure taken into consideration. As a result, the complete dispersion diagram converges to the local effective medium description only near the Brillouin zone center.
, the Meissner effect, superconducting gap and fluctuation supercurrent have been observed in 0.4 nm carbon nanotube / zeolite composites. The measured superconducting behaviors display smooth temperature variation due to fluctuations, with a mean-field 
.