C. Waldfried

The electronic structure of gadolinium grown on Mo(112)

The electronic structure of strained ultra-thin and thin films of Gd grown on a corrugated Mo(112) surface are described. Gadolinium overlayers order at a coverage of monolayers forming a LEED pattern. At this coverage an interface state of symmetry is formed at a binding energy of at the surface Brillouin zone centre . The effective mass of this interface state was determined to be , in both orthogonal directions along the nearly square reduced Brillouin zone. For thicker Gd films of approximately 3 to 10 ML thickness, the corresponding gadolinium state switches symmetry to the representation (Gd or 6s character) and has a much larger effective mass. The overlayer forms a rectangular surface Brillouin zone resembling the hcp surface. Gadolinium films thicker than approximately 10 ML form strained hexagonal ordered films also with substantial misfit dislocations. The strain of the thin hexagonal ordered Gd films is reflected by a reduced Brillouin zone size along by approximately 4% with respect to the less strained Gd overlayers on W(110) and Gd(0001) single crystals. The induced strain severely alters the band structure of the Gd 5d/6s bulk bands, which disperse in the opposite direction relative to the corresponding bands of the relaxed Gd(0001) structure. The surface of the strained hexagonal fcc (111) or hcp (0001) Gd films forms a localized state of symmetry (Gd or 6s character) at approximately 0.7 eV binding energy. There is little observed strain relief within the Gd films up to approximately 150 A film thickness.

Determination of the surface Debye temperature of Mo(112) using valence band photoemission

The temperature dependence of the valence-band emission from the (112) surface of molybdenum has been studied with ultraviolet photoemission spectroscopy. Valence-band features arising from the bulk and surface states exhibit different temperature dependences. The results can be explained in terms of bulk and surface vibrational disorder. A surface (bulk) Debye temperature of 313 K (422 K) is extracted from the data. These numbers will be compared to surface Debye temperatures measured with different techniques and on other materials. The conclusion is that there are major inconsistencies in the reported ratio of the surface to bulk Debye temperature, presumably as a result of the approximations used to include thermal motion in the analysis.

THE OCCUPIED AND UNOCCUPIED ELECTRONIC STRUCTURE OF ADSORBED FERROCENE

Abstract We have studied both the occupied and unoccupied molecular orbitals of adsorbed ferrocene. The occupied molecular orbitals have been identified using a combination of photoemission selection rules and resonant photoemission. Ferrocene adsorbs with a strong preferential molecular orientation on Mo(112) at 150 K, like metallocenes on many other surfaces. Though the ferrocene molecular axis is largely parallel with the Mo(112) surface there is very little perturbation of the molecular orbitals relative to the gas phase.

The nonmetal to metal transition with alkali doping of films of molecular icosahedra

Abstract Using photoemission, inverse photoemission, and electron energy loss spectroscopy, we have probed the changes in the electronic structure of molecularly adsorbed closo -1,2-dicarbadodecaborane ( closo -C 2 B 10 H 12 or orthocarborane) films as a function of Na doping. The insulator gap of this film of molecular icosahedra closes with Na doping and both new occupied and unoccupied states are introduced. The Na-doped system resembles a Mott-Hubbard insulator, even at high doping levels, and demonstrates the defining role of correlation.

The observation of a surface resonance state in the valence-band structure of the perovskite

The valence-band electronic structure of a crystalline thin film of has been investigated using angle-resolved photoemission. The observed valence-band structure has a substantial surface contribution and is strongly influenced by the surface conditions. Surface contamination and disorder attenuate the emission from a Mn-3d-derived state at a binding energy of 3.3 eV. This state was found to be localized at the surface with dispersion only in the plane of the surface and exhibited a strong intensity dependence on emission angle. On the basis of a simple LMTO-ASA bulk band calculation, it is clear that this state does not fall within a bulk band gap. We have therefore identified this Mn-3d-derived state at 3.3 eV below as a surface resonance.

Can photoemission accurately probe the bulk electronic structure of the complex oxides

The surface of the complex oxides, in particular the manganese perovskites, is dominated by a different free enthalpy at the surface. This can be demonstrated by surface segregation after annealing the samples. By comparing relative x-ray photoelectron spectroscopy intensities of the manganese perovskite La1−xCaxMnO3 (x=0.1, 0.35) for different emission angles, the profile of elemental concentration was determined. Analysis shows evidence for pronounced surface segregation of calcium. Our results suggest that there are strong differences between surface and bulk enthalpies without segregation or surface relaxation. This is consistent with the previously observed surface resonance in the electronic band structure.

The influence of the surface on the spontaneous magnetization of Gd thin films

Spin-polarized photoemission data on strained Gd(0001) films grown on Mo(112) are presented and explained in terms of an adequate mean-field approach. Experiment shows that the films exhibit a magnetic surface transition which occurs at about 10% above the bulk Curie temperature. The calculation yields an exponentially decaying surface magnetization mode whose penetration depth depends on the relative strengths of the interlayer exchange couplings . Second-order perturbation-theory refinements with respect to the long-range RKKY interactions yield a renormalization of the exchange between adjacent layers and small long-range magnetization contributions.

The origin of enhanced magnetization in strained gadolinium

Abstract From combined spin-resolved photoemission and spin-polarized inverse photoemission, the experimental spin-resolved band structure of gadolinium on Mo(112) has been constructed. The occupied spin dependent electronic structure near the Fermi level is dominated by shallow dispersion of a spin minority band with considerable surface weight. There is an occupied spin majority bulk band straddling the Fermi level whose spin minority counterpart remains largely on the unoccupied side of the Fermi level. This results in large spin majority weight in the occupied band structure relative to spin minority.

The adsorption of nickelocene Part 1: molecular bonding on Ag(100)

Abstract The molecular adsorption and desorption of nickelocene, Ni(C 5 H 5 ) 2 , on Ag(100) have been studied by both angle-resolved photoemission and thermal desorption. Photoemission results indicate that the initially adsorbed surface species closely resemble that of molecular nickelocene. The molecular axis is not along the surface normal, as determined by angle-resolved photoemission and angle-resolved thermal desorption spectroscopy. This is a different bonding orientation than that adopted by molecular ferrocene on Ag(100). The molecular nickelocene desorption energies resemble those observed for ferrocene on Ag(100) but are complicated by multiple chemisorption states and steric effects in the desorption process. Nickelocene is unusual in that the molecular orientation is seen to affect the angular dependence of molecular desoprtion.

Variations of the wave vector dependent band gaps with structural transformations of Gd thin films

Abstract Thin Gd films, of only several monolayers, form a rectangular structure on Mo(112) surface. At higher coverages on Mo(112), the Gd overlayer adopts a hexagonal packing, consistent with that expected of the hcp crystal structure of Gd(0001). The band symmetries for these Gd surfaces are found to differ substantially with structure, based both on angle resolved photoemission studies and band structure calculations performed by the LAPW method. The results suggest that increased surface polarization and exchange splitting seen for the basal face of hcp Gd are suppressed for prism and vicinal prism faces of hcp Gd. Insight into the tendency of hcp prism faces toward surface reconstruction is provided.