Unexpectedly Large Electron Correlation Measured in Auger Spectra of Ferromagnetic Iron Thin Films: Orbital-Selected Coulomb and Exchange Contributions.

A set of electron-correlation energies as large as 10 eV have been measured for a magnetic 2 ML Fe film deposited on Ag(001). By exploiting the spin selectivity in angle-resolved Auger-photoelectron coincidence spectroscopy and the Cini-Sawatzky theory, the core-valence-valence Auger spectrum of a spin-polarized system have been resolved: correlation energies have been determined for each individual combination of the two holes created in the four subbands involved in the decay: majority and minority spin, as well as e_{g} and t_{2g}. The energy difference between final states with parallel and antiparallel spin of the two emitted electrons is ascribed to the spin-flip energy for the final ion state, thus disentangling the contributions of Coulomb and exchange interactions.

Rippling of graphitic surfaces: a comparison between few-layer graphene and HOPG.

The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with dilute aqueous solutions of bio-organic molecules (l-methionine and dimethyl sulfoxide, DMSO). Before interaction with molecular solutions, we observe nicely ordered, three-fold oriented rippled domains, with a 4.7 ± 0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction with the molecular solution, the ripple periodicity "relaxes" to 6.2 ± 0.2 nm (large periodicity, LP), while the peak-to-valley height increases to 0.2-0.3 nm. When additional energy is transferred to the system through sonication in solution, graphene planes are peeled off, as shown by quantitative analysis of Raman spectroscopy and X-ray photoelectron spectroscopy which indicate a neat reduction of thickness. Upon exfoliation rippled domains are no longer observed. In comparative experiments on cleaved HOPG, we could not observe ripples on pristine samples in ambient air, while LP ripples develop upon interaction with the molecular solutions. Recent literature on similar systems is not univocal regarding the interpretation of rippling. The ensemble of our comparative observations on FLG and HOPG can be hardly rationalized solely on the basis of the surface assembly of molecules, either organic molecules coming from the solution or adventitious species. We propose to consider rippling as the manifestation of the free-energy minimization of quasi-2D layers, eventually affected by factors such as interplanar stacking, and interactions with molecules and/or with the AFM tip.

Oscillations of the orbital magnetic moment due to d-band quantum well states.

The effect of electron confinement on the magnetocrystalline anisotropy of ultrathin bcc Fe films is explored by combining photoemission spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect measurements. Pronounced thickness-dependent variations in the magnetocrystalline anisotropy are ascribed to periodic changes in the density of states at the Fermi level, induced by quantization of d(xz), d(yz) out-of-plane orbitals. Our results reveal a direct correlation between quantum well states, the orbital magnetic moment, and the magnetocrystalline anisotropy.

Reentrant surface anisotropy in the antiferromagnetic/ferromagnetic bilayer Mn/Co/Cu(001).

We investigated the magnetic anisotropy energy of monatomic surface-step atoms in antiferromagnetic/ferromagnetic (AF/FM) epitaxial Mn/Co bilayers grown on vicinal Cu(001) surfaces. The step-induced anisotropy of the Co/Cu(001) films was quenched upon submonolayer Mn deposition, but a reentrant uniaxial surface anisotropy was observed for Mn thickness (tMn) between 1 and 2 monolayers, which disappears for Mn thickness above 2 monolayers. In the Mn/Co/Cu(001) system, Mn films undergo a tMn-dependent transition from FM to AF in the 1-2 Mn monolayer thickness range, which entails the coexistence of FM and AF Mn phases in the film. The observation of a sizeable uniaxial anisotropy exclusively in the Mn-thickness range of coexistence of the FM and AF phases points out the crucial role of the boundaries between FM and AF regions within the Mn film. A symmetry-breaking mechanism of a magnetic type, rather than a purely geometric one, is therefore proposed as the origin of the reentrant anisotropy.

Tuning the magneto-optical response of iron oxide nanocrystals in Au- and Ag-based plasmonic media.

We investigated the magneto-optical response of chemically synthesized iron oxide magnetic nanocrystals, optically coupled with ordered planar arrays of plasmonic nanoparticles. We compare the signals from two classes of systems, featuring either Au or Ag as the plasmonic counterpart. The localized surface plasmon resonance of the Ag and Au nanoparticles arrays were superimposed or detuned, respectively, with respect to the dominant magneto-optical transitions of the magnetic material. Under resonance, a significant enhancement of the magneto-optical signal was observed. In both cases, we could separate the purely plasmonic and the magnetic contributions in the magneto-optical spectrum of the optically coupled composite based on their different magnetic-field dependence.

Band structure effects in above threshold photoemission.

We have performed an angle-resolved two-photon and three-photon photoemission study of the Ag(111) surface employing ultrashort laser pulses as the excitation source. We show the presence of multi-photon resonances between electronic states at selected points of the Brillouin zone which appear in the high-order photoemission spectral region. We observe clear signatures of electronic band structure effects of the Ag crystal in above-threshold photoemission (ATP) processes, identifying two types of transitions, which either proceed via non-resonant multi-photon excitation from an occupied initial state, or involve a real intermediate state located above the vacuum level of the solid directly influencing the ATP process. For this latter class of phenomena, we suggest that electron populations created by incoherent processes give a contribution to the multi-photon transition, possibly allowing us to trace the transmission of photoexcited electrons through the crystal.

Interaction of alkanethiols with nanoporous cluster-assembled Au films.

This article presents a study of the interaction of octadecanethiol molecules (C(18)) with nanoporous cluster-assembled gold films under a liquid environment based on a combined spectroscopic ellipsometry and X-ray photoelectron spectroscopy investigation. By comparing the optical response, following the deposition of C(18), of cluster-assembled films with varying degrees of porosity with that of flat surfaces and by resolving the corresponding features of the molecule-Au bond, we have been able to define the conditions that either favor molecular in-depth diffusion into the pores or promote the formation of a molecular self-assembled monolayer (SAM) restricted to the film surface. In the presence of abundant open pores, C(18) molecules strongly diffuse within the film interior and bind to the pore walls, whereas in the presence of porous films with less abundant open pores we have observed that the molecules tend to remain confined to the surface region, adopting a SAM-like configuration.

Growth dynamics of L-cysteine SAMs on single-crystal gold surfaces: a metastable deexcitation spectroscopy study.

We report on a metastable deexcitation spectroscopy investigation of the growth of L-cysteine layers deposited under UHV conditions on well-defined Au(110)- (1 × 2) and Au(111) surfaces. The interaction of He(*) with molecular orbitals gave rise to well-defined UPS-like Penning spectra which provided information on the SAM assembly dynamics and adsorption configurations. Penning spectra have been interpreted through comparison with molecular orbital DFT calculations of the free molecule and have been compared with XPS results of previous works. Regarding adsorption of first-layer molecules at room temperature (RT), two different growth regimes were observed. On Au(110), the absence of spectral features related to orbitals associated with SH groups indicated the formation of a compact SAM of thiolate molecules. On Au(111), the data demonstrated the simultaneous presence, since the early stages of growth, of strongly and weakly bound molecules, the latter showing intact SH groups. The different growth mode was tentatively assigned to the added rows of the reconstructed Au(110) surface which behave as extended defects effectively promoting the formation of the S-Au bond. The growth of the second molecular layer was instead observed to proceed similarly for both substrates. Second-layer molecules preferably adopt an adsorption configuration in which the SH group protrudes into the vacuum side.

Soft-tissue infection with Absidia corymbifera and kidney complications in an AIDS patient.

We describe a case of primary cutaneous Absidia corymbifera infection in an AIDS patient with renal complications. The Sensititre YeastOne panel was adopted to determine antifungal susceptibility and liposomial amphotericin B was used which initially produced a significant clinical response.

Ultrafast optical spin injection into image-potential states of Cu(001).

We report the observation of a net spin polarization in the n=1 image-potential state at the Cu(001) surface. The spin polarization is achieved by spin-selective multiphoton excitation of electrons from the spin-orbit split Cu d bands to the image-potential state using circularly polarized ultrafast light pulses. We show that by tuning the exciting photon energy, we can adjust the resonant coupling of the image-potential state to d bands of different double-group symmetry. This allows us to tune the spin polarization injected into the image-potential state.

Mechanisms of high-order perturbative photoemission from Cu(001).

We observed high-order 2- to 4-photon photoemission and above threshold photoemission (ATP) processes with 3.07 eV light from the Cu(001) surface. The intensity of 3-photon photoemission via excitation through the n = 1 image potential state significantly exceeded that of the 2-photon process. The ATP occurs either via single photon transitions from the image potential resonances above the vacuum level or by multiphoton transitions from image potential states below the vacuum level. The experimental ratio of the m- to (m + 1)-photon process yields is sensitive to the electronic band structure of the solid.

Isolating the step contribution to the uniaxial magnetic anisotropy in nanostructured Fe/Ag(001) films.

We have investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in Fe/Ag(001) films on which nanoscale surface ripples were fabricated by the ion sculpting technique. For rippled Fe films deposited on flat Ag(001), the steps created along the ripple sidewalls are shown to be the only source of uniaxial anisotropy. Ion sculpting of ultrathin magnetic films allows one to selectively study the step-induced anisotropy and to investigate the correlation between local atomic environment and magnetic properties.

Surface magnetism during oxygen-aided Fe homoepitaxy.

Magnetization-induced optical surface second harmonic generation (SHG) in the longitudinal geometry was used to study magnetism on the p(1 x 1)O/Fe(001) surface during a layer-by-layer Fe growth with surfactant oxygen. Considerable one-monolayer oscillations were observed. Minima of the magnetization-induced contributions to the second order dielectric susceptibility were detected at half-filled monolayers. These contributions were accessed either by measuring a purely magnetic SHG yield, or indirectly from the magnetization dependence of the overall SHG signal, both providing consistent results. The origin of the oscillatory surface magnetism is consistent with the expected oscillating oxygen induced relaxation due to the surface Fe islands.

Temperature driven reversible breakdown of pseudomorphism in ultrathin Fe/Cu3Au films.

We observe that ultrathin Fe/Cu(3)Au(001) films in the 6-13 A thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range T(c) approximately 345-380 K. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition.