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Epitaxial MgO thin films have been grown on single crystal substrates of M-plane (100), A-plane (20), C-plane (0001) and R-plane (102) sapphire by laser ablation of a Mg metal target in a molecular oxygen atmosphere using 1064 nm radiation from a Nd:YAG laser. X-ray measurements indicate that the MgO grows epitaxially on all substrates, with its orientation dependent on the cut of the sapphire substrate. (110)-oriented MgO grows on M-plane sapphire, while (111)-oriented MgO films are found on both the A-plane and the C-plane sapphire. The orientation of MgO found on R-plane sapphire appears to be surface and temperature dependent. The principal growth orientation obtained on the R-plane is (100)-oriented MgO although (100) growth tilted 30° from the [11] Al2O3 direction can result. X-ray area mapping has been performed to determine the mosaicity, d value spread and strain present in the films. These data are compared with the in-plane orientation and the mismatch of the MgO and Al2O3 lattices in an attempt to relate the preferred orientation to the plane of the sapphire on which it is grown.

Thin films of YBa 2 Cu 3 O 7 (YBCO) have been grown on MgO buffer layers on LaAlO 3 and silicon substrates by laser ablation at substrate temperatures of 750°C in a 250 mTorr oxygen atmosphere. The films grown on MgO/LaAlO 3 are c -axis oriented and epitaxial, as shown by X-ray measurements. The films grown on MgO/Si have mixed a -, b -, and c -axis orientation. For YBCO/MgO/Si the presence of the MgO buffer layer cannot be detected by X-ray diffraction, suggesting a possible reaction between the Si and MgO at elevated temperatures. Pole figures show the YBCO growth on Si to be epitaxial with a significant in plane random component. Resistivity measurements indicate that the transition temperature of the YBCO/MgO/LaAlO 3 is 87 K, while the transition temperature for YBCO/MgO/Si is lower, 85 K, probably due to the poor epitaxy of the film.

We study the heavy quark potential in the SU(2) positive plaquette model using monopoles in the maximum abelian gauge, and vortices. Monopoles give a quantitative description of the string tension. Vortices approximately reproduce the entire heavy quark potential., Comment: 3 pages, latex, 2 postscript figures

We study the effect of vortices of different size on the long range heavy quark potential. Under certain simplifying assumptions, we find that the subdominant term in the potential at long range is controlled by the distribution of vortices of large size.

Measurements of the energy deposited in silicon surface-barrier detectors as a result of proton-induced nuclear reactions were carried out at the Harvard Cyclotron for protons with incident energies ranging from 50 to 158 MeV and detectors with thicknesses of 2.5, 4.2, 24.1, 100, and 200 ?m. The number of events in which a given threshold amount of energy is deposited in a 4.2 ?m detector varied with incident proton energy in a manner similar to previous measurements of the proton-induced soft-error cross section. The number of events in which at least a threshold amount of energy was deposited in the detector fell off in a near exponential manner with increasing threshold energy. The data were found to be in reasonable agreement with a computer simulation model developed in our laboratory. The model is used to illustrate how the mass spectra of the residual nuclear fragments shifts towards lower masses with increasing recoil energy. Lighter recoils have longer ranges and a greater chance of leaving a microscopically thin sensitive volume element before coming to their end of range.