C. Krembel

STRONG CORRELATION SATELLITES IN CORE LEVEL PHOTOEMISSION FROM MN IN THE MONOLAYER RANGE ON AG(001)

Abstract We observe a complex shape of the Mn2p core level photoemission spectra for Mn in structures of reduced dimensionality such as an ideal monolayer on Ag(001). The Mn2p 3/2 and Mn2p 1/2 spectral intensity is found to split into doublets with separation about 4 and 5 eV, respectively. The data indicate an interpretation in terms of a poorly screened and a well screened final state of the Mn with a core hole. In the monolayer range these two final states have comparable intensities and dominant contributions from atomic 2p 5 3d 5 and 2p 5 3d 6 configurations, respectively. Their intensity ratio is found to be extremely sensitive to the Mn atomic environment and directly reflects the degree of the Mn3d localization, electron correlation and reduction in charge fluctuations. Additional fine structure is clearly visible in high-resolution data for both final state 3d electrons counts and can be assigned to direct and exchange Coulomb interactions of the unfilled core and 3d shells. Slow 3d-charge fluctuations also result in a remarkable three-peak structure of the Mn3s line.

Metastable film growth of Cr on Cu(OO1)

The growth and structure of Cr films (0–30 nominal monolayers (ML)) on Cu(001) held at room temperature have been investigated by photoemission, photoelectron diffraction and low energy electron diffraction techniques. The films exhibit ordered body-centred cubic (bcc) domains typically ∼ 10 lattice parameters in size with Cr(110) ∥Cu(001) and Cr[1111] ∥ Cu[110] and equivalent epitaxial orientations. LEED and photoelectron diffraction reveal a substantial deviation from relaxed bcc structure in ultra-thin films ( < 3 ML) with an in-plane distortion and expansion and an interlayer contraction of the Cr(110) layers. The initial mode of growth is characterized by the formation of multilayer islands and 90% substrate coverage occurs by ∼ 4 ML.

Initial growth and structure of Mn on Ag(100) : formation of a superficial alloy

Abstract We have investigated the crystallographic structure of ultra-thin Mn films deposited at room temperature on a clean Ag(100) single crystal, using angle-resolved ultra-violet photoemission, X-ray photoelectron diffraction (XPD) and low-energy electron diffraction. The evolution of the Ag 4d valence band shows deviations from simple layer by layer growth. These results, combined with the XPD data, indicate that by ∼1–1.5 monolayers of Mn a two layers thick Mn-rich epitaxial MnAg alloy is formed that continues the face centered cubic lattice of the Ag substrate.

Epitaxy of thin Cr layers on Cu(001)

Thin Cr films (6-30 monolayers) have been evaporated in ultra-high vacuum onto a Cu(001) single crystal kept at room temperature and have been studied by low energy electron diffraction (LEED), X-ray photoelectron diffraction (XPD), and angle resolved ultra-violet photoemission (ARUPS). The LEED diagrams obtained for these layers reveal the formation of epitaxial body-centered cubic (bcc) Cr domains with Cr(110)∥Cu(001) and Cr[111]∥[Cu[110]. The Cu(001) substrate symmetry results in four different orientations of these domains. This conclusion is confirmed by the XPD data. Indeed the modulations of the Cr2p 3/2 core level intensity as a function of emission angle are in agreement with single scattering simulations performed for a superposition of equivalent bcc Cr(110) domains on a Cu(001) substrate

Formation of c(2×2) Mn–Ag superficial bilayer alloys on Ag(001): role of thermally activated surface atomic exchange and ordering

Ultra-thin Mn films have been evaporated in ultra-high vacuum on a Ag(001) substrate kept at temperatures between 210 and 340 K. A c(2×2) LEED (low-energy electron diffraction) diagram is obtained due to the formation of a two-layer-thick superficial Mn–Ag alloy. The intensity and width of the half integer spots are investigated as a function of coverage, deposition temperature, and time elapsed after Mn deposition. The brightest and sharpest diagram is obtained for ∼1.5 monolayers deposited at room temperature, which corresponds to a surface plane made of c(2×2) domains formed by Mn and Ag atoms in a checkerboard arrangement, while the subsurface plane is saturated in Mn. Depending on the preparation conditions, various c(2×2) surface alloys may be obtained that mainly differ in the MnxAg1−x composition of the second atomic plane. A comparison of the time evolution of the LEED spot intensity with ion scattering spectroscopy data reveals that two different thermally activated phenomena determine the formation and nature of the c(2×2) superstructure: on the one hand, part of the deposited Mn atoms must exchange with Ag atoms from the substrate first plane, and on the other hand, the Mn and Ag atoms of the film surface plane diffuse laterally so that nucleation and growth of the c(2×2) domains take place.

Initial growth mode of Cr on Cu(100) and Ag(100): a comparison

Abstract The initial growth mode of Cr on Cu(100) and Ag(100) surfaces is studied by photoemission methods. In particular photoemission is used as a titration method allowing to determine the clean surface area left after Cr deposit. For Cu(100) it is found that at room temperature, or above, body centered cubic (bcc) Cr grows initially in the form of bilayer or multilayer islands with (110) orientation and with 90% substrate coverage by ∼ 4 monolayers. This contrasts with the growth mode observed on Ag(100) where, at moderate substrate temperatures (430–450 K), only monolayer bcc Cr islands with (100) orientation are formed initially. This behaviour is confirmed by Cr 2p core level photoemission measurements which indicate that in the monolayer range Cr is already in bulk-like environment on Cu(100) in contrast to Ag(100) where a flat monolayer with strongly enhanced magnetism is obtained.

Crystallographic and electronic structure of ultra thin Ag/Cr/Ag(100) sandwiches

Abstract We have investigated the electronic and crystallographic structure of thin Ag/Cr/Ag(100) sandwiches using angle-resolved ultraviolet (ARUPS) and X-ray (ARXPS) photoemission. First an ordered two-dimensional (2D) Cr monolayer is formed by deposition of one layer equivalent (LE) Cr onto a clean Ag(100) single crystal kept at 440 K under UHV conditions. The relevant ARXPS curves for Cr 2 p 1 2 are structureless as expected for a single monolayer. Then Ag films (from the submonolayer range to 5 LE) are deposited at RT onto this Cr monolayer. Even after deposition of 3 LE Ag, the ARUPS spectra show the persistence of Cr3d induced features characteristic of the 2D Cr monolayer. However after Ag deposition the valence band spectra also reveal an additional sharp Cr3d peak at 1.2 eV binding energy, and the Cr 2 p 1 2 ARXPS curves are structured and exhibit strong forward scattering features characteristic of a face-centered cubic environment. It is concluded that only a part of the Cr monolayer is destroyed and dissolves in the deposited Ag which grows in the form of 3D islands.

Formation of a two-dimensional Ag(001) monolayer on epitaxially stabilized Mn(001)

Abstract The growth mode and electronic properties of ultrathin films of silver deposited at room temperature (RT) on a 9 monolayers (ML) pseudomorphic Mn film on a Ag(001) surface have been studied by means of low energy electron diffraction (LEED), X-ray photoelectron diffraction (XPD) and angle-resolved ultraviolet photoemission (ARUPS). In particular ARUPS was used to investigate the valence band structure. Initially the data show some Ag segregation on top of the Mn film grown at RT. In the 0.5–0.9 ML coverage range the Ag adlayer exhibits d-band spectral behavior characteristic of two-dimensional dispersion. Further evaporation of Ag results in the formation of Ag bilayer platelets, and > 3 ML deposition the system exhibits a three-dimensional electronic structure converging towards a bulk Ag(001)-like system. Thus the growth mode of Ag on the Mn film is found to be nearly layer by layer up to 2 ML.

Segregation of Cu on annealed ultrathin Cr films deposited on Cu(001)

Thin Cr films (∼20 A) deposited in vacuum at 300 K on a Cu(001) single crystal have been annealed at various temperatures and analyzed by angle resolved ultraviolet photoemission, X-ray photoemission, X-ray photoelectron diffraction and low energy electron diffraction. The data show that Cu segregation occurs above 425 K, resulting in an atomic monolayer wetting the whole Cr film. At 775 K, the Cr overlayer collapses and the Cu(001) substrate surface becomes partly uncovered.

X-ray photoelectron diffraction from ultra-thin Cr layers on Au(100) and Ag(100): a comparison

Abstract Ultra-thin Cr films deposited at room temperature on Au(100) and Ag(100) single crystals have been studied by X-ray photoelectron diffraction (XPD). For Cr coverages higher than 7 layer equivalents (LE), the modulations of the Cr2p level intensities versus the polar angle θ present maxima indicating for both substrates the growth of epitaxial body centered cubic (bcc) Cr. However, at low coverages (1–3 LE), the curves show Cr2p core line enhancement at angles different from those expected for a bcc layer. We show that, for Cr/Au(100), the Cr atoms are in a face centered cubic (fcc) environment and are incorporated in the Au(100) matrix. In contrast, for Cr/Ag(100), the shift of the forward scattering angular positions can be mainly related to Ag segregation on top of the bcc Cr layers and, possibly, to some distortion from a bcc environment in these ultra-thin layers.