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A LEED study of the (100) and (111) surfaces of the intermetallic compound FeTi
A403 Surface Science 122 (1982) 69-79 North-Holland Publishing Company
69
A L E E D S T U D Y O F T H E (100) A N D ( I l l ) S U R F A C E S THE INTERMETALLIC COMPOUND FeTi
OF
T.E. FELTER
Sandia National Laboratories, Livermore, California 94550, USA and S.A. S T E W A R D
a n d F.S. U R I B E
Lawrence Livermore National Laboratory, Livermore, California 94550, USA Received 7 May 1982; accepted for publication 20 July 1982 Two single crystals of FeTi, an intermetallic compound with a CsCI cubic structure, have been isolated from a bulk melt. After numerous cycles of concurrent argon bombardment and heating to at least 1200 K, clean and well-ordered surfaces appeared on both crystals, as determined by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The first crystal produced a hexagonal pattern after heating to 1400 K. The measured lattice parameter is a 0 =4.3-+0.2 A compared to 4.2 A,, which is the reported X-ray value for the (111) plane of FeTi. The second crystal became ordered at a lower temperature (1200 K) showing a square pattern corresponding to FeTi(100). The lattice parameter calculated from LEED was 3.1 -+ 0.15 A, again in good agreement with X-ray data of 3.0 ,~. Sulfur readily segregates to the surface above 1000 K and orders to produce a c(2x2) pattern on the (100) face. This is the first observation of an ordered overlayer on an intermetallic compound.
80 ON RESONANT J. H A L B R I T T E R
Surface Science 122 (1982) 80-98 North-Holland Publishing Company TUNNELING *
Surface Science Division, National Bureau of Standards, Washington, DC 20234, USA Received 19 February 1982; accepted for publication 26 July 1982 Localized electron states in oxides adjacent to metals hybridize with conduction electron states forming interface states, which at the localized site have an amplitude resonantly enhanced over the amplitude of the conduction electron states. The interface states mediate a continuous transition between the metal and the semiconducting or insulating oxide. Resonant tunneling via these interface states to an opposing metal surface can dominate over direct and intermediate-state tunneling. Resonant tunneling is obstructed by the correlation (Coulomb) energy which causes voltage, temperature and time dependencies. The obstruction increases with distance of the localized state from the metal and this increased obstruction causes the transition from resonant to intermediate-state tunneling. This corresponds to a space-wise metal-insulator transition. In oxides, like Nb2Os, the correlation energy is small and the hybridization is strong and thus resonant tunneling through localized states at the Fermi energy can account for various tunnel anomalies observed in the normal or superconducting state.
69
A L E E D S T U D Y O F T H E (100) A N D ( I l l ) S U R F A C E S THE INTERMETALLIC COMPOUND FeTi
OF
T.E. FELTER
Sandia National Laboratories, Livermore, California 94550, USA and S.A. S T E W A R D
a n d F.S. U R I B E
Lawrence Livermore National Laboratory, Livermore, California 94550, USA Received 7 May 1982; accepted for publication 20 July 1982 Two single crystals of FeTi, an intermetallic compound with a CsCI cubic structure, have been isolated from a bulk melt. After numerous cycles of concurrent argon bombardment and heating to at least 1200 K, clean and well-ordered surfaces appeared on both crystals, as determined by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The first crystal produced a hexagonal pattern after heating to 1400 K. The measured lattice parameter is a 0 =4.3-+0.2 A compared to 4.2 A,, which is the reported X-ray value for the (111) plane of FeTi. The second crystal became ordered at a lower temperature (1200 K) showing a square pattern corresponding to FeTi(100). The lattice parameter calculated from LEED was 3.1 -+ 0.15 A, again in good agreement with X-ray data of 3.0 ,~. Sulfur readily segregates to the surface above 1000 K and orders to produce a c(2x2) pattern on the (100) face. This is the first observation of an ordered overlayer on an intermetallic compound.
80 ON RESONANT J. H A L B R I T T E R
Surface Science 122 (1982) 80-98 North-Holland Publishing Company TUNNELING *
Surface Science Division, National Bureau of Standards, Washington, DC 20234, USA Received 19 February 1982; accepted for publication 26 July 1982 Localized electron states in oxides adjacent to metals hybridize with conduction electron states forming interface states, which at the localized site have an amplitude resonantly enhanced over the amplitude of the conduction electron states. The interface states mediate a continuous transition between the metal and the semiconducting or insulating oxide. Resonant tunneling via these interface states to an opposing metal surface can dominate over direct and intermediate-state tunneling. Resonant tunneling is obstructed by the correlation (Coulomb) energy which causes voltage, temperature and time dependencies. The obstruction increases with distance of the localized state from the metal and this increased obstruction causes the transition from resonant to intermediate-state tunneling. This corresponds to a space-wise metal-insulator transition. In oxides, like Nb2Os, the correlation energy is small and the hybridization is strong and thus resonant tunneling through localized states at the Fermi energy can account for various tunnel anomalies observed in the normal or superconducting state.