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Search for superconductivity in the hydrides of lanthanum
52
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
Vol° i, No. 2
ii. KINETICS OF OXIDE FILM GROWTH ON METAL CRYSTALS - II. HOMOGENOUS FIELD APPROXIMATIONS .* A.T. Fromhold, Jr. (Sandla Laboratory, Albuquerque, New Mexico, U.S.A.) Homogenous field approximations yield simple equations relating film thickness L(t) to time t of oxidation from coupled steady-state diffusion equations of ~ assuming equal ionic and electronic currents. High temperature parabolic equations are obtained with rate determined by transport of the low mobility species in the presence of the electric field created by the high mobility species; these parabolic equations have a modified rate constant k' due to a constant electrical potential difference ~ across the film. Both k' an4 ~ increase wlth(concentration)x (mobility) and ratio of boundary concentrations for the electronic species, and decrease for corresponding changes for the ionic species. For tunnelling or field emission of one species with rate determined by diffusion of the second species, the expression obtained is (l* ~t : e x p ( ~ t ) ) - ~6L(t)), w i t h ~ and ~ determined from emission and diffusion parameters. ~ i s equation is based on the intermediate temperature approximation that the surface charge is essentially independent of ~ t ) for tunnelling from the metal crystal through the film, to traps, ar~between traps in the oxide for film thicknesses less than some cri%ical thickness L .... . For limiting cases this equation reduces to Nth root ( ~ t ) ~ tl/N) and logarithm~'~t) ~ log (l+t/to)) laws. The final stage of growth occurs for ~ t ) > L c r i t and u~,,11y predominates at low te~eratures;~rate is controlled by t-nnelling or field emission in a thickness dependent field ( E ~ l / ~ t ) ) so that a limiting thickness L o~ usually results, with L ~ - Lcritequal to several monolayers only. For this region of growth, a logarithmic law and an exponential law ( ~ t ) ~ l -exp(- ~t)) result for FowlerNordheim behavior of the current with field, a modified exponential law (L(t) ~ 1 - exp -(L(t)/L~ ) - t/~) ) results for a linear dependence of tunnel current through the film on field, and a cubic law ( ~ t ) ~ tl/3) results for a linear dependence of tunnel current between traps in the oxide on field. Application of this work is made to published experimental data for copper monocrystals at l173OK and between 451 and 78oK. (Received 16 Januar~r 1963; in revised form 17 May 1963) 12. SEARCH FOR SUPERCONDUCTIVITY IN THE HYDRIDES OF LANTHANUM.** M.F. Mmrriam & D.S. Schreiber (University of California, La Jolla and Berkeley, California, U.S.A.) Met-]Sic lanthanum hydride samples having atomic ratios H/La of 1.8, 1.96, 2.03, 2.11, 2.15, and 2.36 are not superconducting at 1.1OK. LaH1.96 moreover is not superconducting at 0.33OK. Extensive N ~ measurements made previously by Schreiber on the same samples provide considerable information on their electronic structure. The lack of superconductivity is compatible with the density of states information obtained from the N ~ data, and is consistent with a split d-band model of the hydrides. It is also consistent with Matthias' empirical rules. (Received 20 MaT 1963) 13. Z BANDS IN EEl.*** N . Kleefstra (Compagnie G~n~rale de T.S.F., Paris, France). The osc~1]-tor strength of the F centre has been chemically determined as f = 0.93 0.11. From the transformation of F into Z 1 and ~ centres, the values of f of the * Work performed under the auspices of the United States Atomic Energy Co~mission. **Research supported in part by the U.S. Air Force Office of Scientific Research and the National Science Foundation. *** Part of the contents of a thesis by M. Kleefstra entitled "Recherches sur les Centres Color@s en KE1 Contamin~par Sr" (Groningen 1961).
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
Vol° i, No. 2
ii. KINETICS OF OXIDE FILM GROWTH ON METAL CRYSTALS - II. HOMOGENOUS FIELD APPROXIMATIONS .* A.T. Fromhold, Jr. (Sandla Laboratory, Albuquerque, New Mexico, U.S.A.) Homogenous field approximations yield simple equations relating film thickness L(t) to time t of oxidation from coupled steady-state diffusion equations of ~ assuming equal ionic and electronic currents. High temperature parabolic equations are obtained with rate determined by transport of the low mobility species in the presence of the electric field created by the high mobility species; these parabolic equations have a modified rate constant k' due to a constant electrical potential difference ~ across the film. Both k' an4 ~ increase wlth(concentration)x (mobility) and ratio of boundary concentrations for the electronic species, and decrease for corresponding changes for the ionic species. For tunnelling or field emission of one species with rate determined by diffusion of the second species, the expression obtained is (l* ~t : e x p ( ~ t ) ) - ~6L(t)), w i t h ~ and ~ determined from emission and diffusion parameters. ~ i s equation is based on the intermediate temperature approximation that the surface charge is essentially independent of ~ t ) for tunnelling from the metal crystal through the film, to traps, ar~between traps in the oxide for film thicknesses less than some cri%ical thickness L .... . For limiting cases this equation reduces to Nth root ( ~ t ) ~ tl/N) and logarithm~'~t) ~ log (l+t/to)) laws. The final stage of growth occurs for ~ t ) > L c r i t and u~,,11y predominates at low te~eratures;~rate is controlled by t-nnelling or field emission in a thickness dependent field ( E ~ l / ~ t ) ) so that a limiting thickness L o~ usually results, with L ~ - Lcritequal to several monolayers only. For this region of growth, a logarithmic law and an exponential law ( ~ t ) ~ l -exp(- ~t)) result for FowlerNordheim behavior of the current with field, a modified exponential law (L(t) ~ 1 - exp -(L(t)/L~ ) - t/~) ) results for a linear dependence of tunnel current through the film on field, and a cubic law ( ~ t ) ~ tl/3) results for a linear dependence of tunnel current between traps in the oxide on field. Application of this work is made to published experimental data for copper monocrystals at l173OK and between 451 and 78oK. (Received 16 Januar~r 1963; in revised form 17 May 1963) 12. SEARCH FOR SUPERCONDUCTIVITY IN THE HYDRIDES OF LANTHANUM.** M.F. Mmrriam & D.S. Schreiber (University of California, La Jolla and Berkeley, California, U.S.A.) Met-]Sic lanthanum hydride samples having atomic ratios H/La of 1.8, 1.96, 2.03, 2.11, 2.15, and 2.36 are not superconducting at 1.1OK. LaH1.96 moreover is not superconducting at 0.33OK. Extensive N ~ measurements made previously by Schreiber on the same samples provide considerable information on their electronic structure. The lack of superconductivity is compatible with the density of states information obtained from the N ~ data, and is consistent with a split d-band model of the hydrides. It is also consistent with Matthias' empirical rules. (Received 20 MaT 1963) 13. Z BANDS IN EEl.*** N . Kleefstra (Compagnie G~n~rale de T.S.F., Paris, France). The osc~1]-tor strength of the F centre has been chemically determined as f = 0.93 0.11. From the transformation of F into Z 1 and ~ centres, the values of f of the * Work performed under the auspices of the United States Atomic Energy Co~mission. **Research supported in part by the U.S. Air Force Office of Scientific Research and the National Science Foundation. *** Part of the contents of a thesis by M. Kleefstra entitled "Recherches sur les Centres Color@s en KE1 Contamin~par Sr" (Groningen 1961).