Interfacial properties of Ce<Subscript>0.75</Subscript>Zr<Subscript>0.25</Subscript>O<Subscript>2</Subscript> supported noble metals (Pd,Pt) from first principles

First-principles electronic structure calculations of noble metals (NM=Pd, Pt)/Ce_0.75Zr_0.25O₂ systems are presented. It is found that: the NM adatoms do not prefer to stay at the atop or the bridge sites of the cations (Ce and Zr), but prefer to be adsorbed at or around the anion sites. The most preferable adsorption sites for both the Pd and Pt adatoms are the O-bridge sites neighboring the Zr dopant. The Pt adatom show much stronger interaction with the Ce_0.75Zr_0.25O₂(111) surface than does the Pd adatom. The interactions of the NM/Ce_0.75Zr_0.25O₂(111) interfaces are stronger than those of the corresponding NM/ceria(111) interfaces. There are some metal induced gap states (MIGS) appeared in the gaps of the NM/Ce_0.75Zr_0.25O₂(111) interfaces, which are important to catalytic properties of the NM/Ce_0.75Zr_0.25O₂(111) catalysts.     

Magnetic interactions of cold atoms with anisotropic conductors

We analyze atom-surface magnetic interactions on atom chips where the magnetic trapping potentials are produced by current carrying wires made of electrically anisotropic materials. We discuss a theory for time dependent fluctuations of the magnetic potential, arising from thermal noise originating from the surface. It is shown that using materials with a large electrical anisotropy results in a considerable reduction of heating and decoherence rates of ultra-cold atoms trapped near the surface, of up to several orders of magnitude. The trap loss rate due to spin flips is expected to be significantly reduced upon cooling the surface to low temperatures. In addition, the electrical anisotropy significantly suppresses the amplitude of static spatial potential corrugations due to current scattering within imperfect wires. Also the shape of the corrugation pattern depends on the electrical anisotropy: the preferred angle of the scattered current wave fronts can be varied over a wide range. Materials, fabrication, and experimental issues are discussed, and specific candidate materials are suggested.     

Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses

Europium-doped lead germanate and lead fluorogermanate glasses are studied by using differential thermal analysis, X-ray diffraction, photoluminescence and fluorescence lifetimes measurements of the ⁵D_j, j = 0, 1, 2 levels. PbF₂ addition increases the thermal stability of the lead germanate glass, while Eu³⁺ ions promote the crystallization of β-PbF₂:Eu³⁺ nano-crystals embedded in a glassy matrix. In the lead fluorogermanate glasses, Eu³⁺ ions exhibit a strong affinity for F⁻ ions although oxygen ions are much more numerous. It appears that luminescence concentration quenching is not important, while cross relaxation is very efficient in the glasses. The results allow to propose for these glasses a molecular model in which small fluorine rich island, incorporating the Eu³⁺ ions in low symmetry sites, are separated from each other by chains of germanate (GeO₄)⁴⁻ ions linked together.     

Pseudomorphic growth of thick n-type AlxGa1−xN layers on low-defect-density bulk AlN substrates for UV LED applications

Recently it has been discovered that when growing Al_xGa_1−xN on low-defect-density bulk AlN substrates pseudomorphic layers can be achieved with a thickness far exceeding the critical thickness as given by the Matthews and Blakeslee model. For instance, the critical thickness of an Al_xGa_1−xN layer (with x=0.6) is about 40 nm thick. However we have been able to grow layers with this composition that are pseudomorphic with a thickness exceeding the critical thickness by more than an order of magnitude. This work defines the limits of pseudomorphic growth on low defect density, bulk AlN substrates to obtain low defect density, high-power UV LEDs.     

Cooling and Trapping 88Sr Atoms with 461nm Laser

We report the experimental realization of a ^88Sr magneto-optical trap （MOT） operating at the wavelength of 461 nm. The MOT is loaded via a 32 cm long spin-flip type Zeeman slower which enhances the MOT population by a factor of 22. The total laser power available in our experiment is about 300mW. We have trapped 1.6 × 10^8 ^88 Sr atoms with a 679nm and 707nm repumping laser. The two repumping lasers enhance the trap population and trap lifetime by factors of 11 and 7, respectively. The ^88 Sr cloud has a temperature of about 2.3 mK, measured by recording the time evolution of the absorption signal.     

Growth and Annealing Study of Mg-Doped AlGaN and GaN/AlGaN Superlattices

Mg-doped AlGaN and GaN/AlGaN superlattices are grown by metalorganic chemical vapour deposition （MOCVD） Rapid thermal annealing （RTA） treatments are carried out on the samples. Hall and high resolution x-ray diffraction measurements are used to characterize the electrical and structural prosperities of the as-grown and annealed samples, respectively. The results of hall measurements show that after annealing, the Mg-doped AIGaN sample can not obtain the distinct hole concentration and can acquire a resistivity of 1.4 ×10^3 Ωcm. However, with the same annealing treatment, the GaN/AlGaN superlattice sample has a hole concentration of 1.7 × 10^17 cm-3 and a resistivity of 5.6Ωcm. The piezoelectric field in the GaN/AlGaN superlattices improves the activation efficiency of Mg acceptors, which leads to higher hole concentration and lower p-type resistivity.     

Growth of multiwalled carbon nanotube arrays by chemical vapour deposition over iron catalyst and the effect of growth parameters

Multiwalled carbon nanotube (CNT) arrays were grown by catalytic thermal decomposition of acetylene, over Fe-catalyst deposited on Si-wafer in the temperature range 700-750 °C. The growth parameters were optimized to obtain dense arrays of multiwalled CNTs of uniform diameter. The vertical cross-section of the grown nanotube arrays reveals a quasi-vertical alignment of the nanotubes. The effect of varying the thickness of the catalyst layer and the effect of increasing the growth duration on the morphology and distribution of the grown nanotubes were studied. A scotch-tape test to check the strength of adhesion of the grown CNTs to the Si-substrate surface reveals a strong adhesion between the grown nanotubes and the substrate surface. Transmission electron microscopy analysis of the grown CNTs shows that the grown CNTs are multiwalled nanotubes with a bamboo structure, and follow the base-growth mechanism.     

MOCVD Growth and Characterization of Epitaxial AlxGa1-xN Films

We study the growth of AlxGa1-x N epilayers on （0001） sapphire by low-pressure MOCVD, using a lowtemperature AIN buffer. By varying the input flow rates of trimethylgallium （TMGa）, we obtain crack-free AlGaN films in the whole range of composition. A linear relationship between gas and solid Al content is observed. The structural properties of the layers （x =0- 1） are investigated by x-ray diffraction, atomic force microscopy （AFM） and scanning electron microscopy （SEM）. It is found that a two-direction growth appears along the c-axis and the （1011） directions for x ≥ 0.45. From the results of Raman spectroscopy, we suggest that the compressive stain and the lack of mobility orAl adatoms can induce the formation of （1011） grains.     

Three-Step Growth Optimization of AlN Epilayers by MOCVD

A three-step growth process is developed for depositing high-quality aluminium-nitride （AlN） epilayers on （001） sapphire by low pressure metalorganic chemical vapour deposition （LP-MOCVD）. We adopt a low temperature （LT） A1N nucleation layer （NL）, and two high temperature （HT） A1N layers with different V/Ⅲ ratios. Our results reveal that the optimal NL temperature is 840-880℃, and there exists a proper growth switching from low to high V/Ⅲ ratio for further reducing threading dislocations （TDs）. The screw-type TD density of the optimized AIN film is just 7.86×10^6 cm^-2, about three orders lower than its edge-type one of 2×10^9 cm^-2 estimated by high-resolution x-ray diffraction （HRXRD） and cross-sectional transmission electron microscopy （TEM）.