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Register a new userRotational Reconstruction of Sapphire (0001)
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The structure of the $(sqrt{31}times sqrt{31})Rpm9^circ$ reconstructed phase on sapphire (0001) surface is investigated by means of a simulation based on the energy minimization. The interaction between Al adatoms is described with the semi-empirical many-body Sutton-Chen potential, corrected for the charge transfer between the metallic overlayer and the substrate. The interactions between the Al adatoms and sapphire substrate are described with a simple three-dimensional potential field which has the hexagonal periodicity of sapphire surface. Our energy analysis gave evidence that the structure which is observed at room temperature is in fact a frozen high-temperature structure. In accordance with the X-ray scattering, a hexagonal domain pattern separated by domain walls has been found. The Al adatoms, distributed in two monolayers, are ordered and isomorphic to metallic Al(111) in the domains and disordered in the domain walls. The main reason for the rotational reconstruction is the lattice misfit between the metallic Al and sapphire.
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Interfaces of sapphire are of technological relevance as sapphire is used as a substrate in electronics, lasers, and Josephson junctions for quantum devices. In addition, its surface is potentially useful in catalysis. Using first principles calculations, we show that, unlike bulk sapphire which has inversion symmetry, the (0001) sapphire surface is piezoelectric. The inherent broken symmetry at the surface leads to a surface dipole and a significant response to imposed strain: the magnitude of the surface piezoelectricity is comparable to that of bulk piezoelectrics.
We report the results of x-ray scattering studies of AlN on c-plane sapphire during reactive radiofrequency magnetron sputtering. The sensitivity of in situ x-ray measurements allowed us to follow the structural evolution of strain and roughness from initial nucleation layers to fullyrelaxed AlN films. A growth rate transient was observed, consistent with the initial formation of non-coalesced islands with significant oxygen incorporation from the substrate. Following island coalescence, a steady state growth rate was seen with a continuous shift of the c and a lattice parameters towards the relaxed bulk values as growth progressed, with films reaching a fully relaxed state at thicknesses of about 30 nm.
Most spectroscopic methods for studying the electronic structure of metal surfaces have the disadvantage that either only occupied or only unoccupied states can be probed, and the signal is cut at the Fermi edge. This leads to significant uncertainties, when states are very close to the Fermi level. By performing low-temperature scanning tunneling spectroscopy and ab initio calculations, we study the surface-electronic structure of La(0001) and Lu(0001), and demonstrate that in this way detailed information on the surface-electronic structure very close to the Fermi energy can be derived with high accuracy.
Wurtzite (Ga,Mn)N films showing ferromagnetic behaviour at room temperature were successfully grown on sapphire(0001) substrates by molecular beam epitaxy using ammonia as nitrogen source. Magnetization measurements were carried out by a superconducting quantum interference device at the temperatures between 1.8K and 300K with magnetic field applied parallel to the film plane up to 7T. The magnetic-field dependence of magnetization of a (Ga,Mn)N film at 300K were ferromagnetic, while a GaN film showed Pauli paramagnetism like behaviour. The Curie temperatures of a (Ga,Mn)N film was estimated as 940K.
We introduce an approach to exploit the existence of multiple levels of description of a physical system to radically accelerate the determination of thermodynamic quantities. We first give a proof of principle of the method using two empirical interatomic potential functions. We then apply the technique to feed information from an interatomic potential into otherwise inaccessible quantum mechanical tight-binding calculations of the reconstruction of partial dislocations in silicon at finite temperature. With this approach, comprehensive ab initio studies at finite temperature will now be possible.
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