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Hexagonal boron nitride (hBN), sometimes referred to as white graphene, receives growing interest in the scientific community, especially when combined into van der Waals (vdW) homo- and heterostacks, in which novel and interesting phenomena may arise. hBN is also commonly used in combination with two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs). The realization of hBN-encapsulated TMDC homo- and heterostacks can indeed offer opportunities to investigate and compare TMDC excitonic properties in various stacking configurations. In this work, we investigate the optical response at the micrometric scale of mono- and homo-bilayer WS2 grown by chemical vapor deposition and encapsulated between two single layers of hBN. Imaging spectroscopic ellipsometry is exploited to extract the local dielectric functions across one single WS2 flake and detect the evolution of excitonic spectral features from monolayer to bilayer regions. Exciton energies undergo a redshift by passing from hBN-encapsulated single layer to homo-bilayer WS2, as also confirmed by photoluminescence spectra. Our results can provide a reference for the study of the dielectric properties of more complex systems where hBN is combined with other 2D vdW materials into heterostructures and are stimulating towards the investigation of the optical response of other technologically-relevant heterostacks.

The study of nonthermal electrons, generated upon photoexcitation of plasmonic nanostructures, plays a key role in a variety of contexts, from photocatalysis and energy conversion to photodetection and nonlinear optics. Their ultrafast relaxation and subsequent release of energy to a low energy distribution of thermalized hot electrons has been the subject of a myriad of papers, mostly based on femtosecond transient absorption spectroscopy (FTAS). However, the FTAS signal stems from a complex interplay of different contributions arising from both nonthermal and thermal electrons, making the disentanglement of the two a very challenging task, so far accomplished only in terms of numerical simulations. Here a combined approach is introduced, based on a post-processing of the FTAS measurements guided by a reduced semiclassical model, the so-called extended two-temperature model, which has allowed the purely nonthermal contribution to the pump-probe experimental map recorded for 2D arrays of gold nanoellipsoids to be isolated. This approach displays the intimate correlation between electron energy and probe photon energy on the ultrafast time-scale of electron thermalization. It also sheds new light on the ultrafast transient optical response of gold nanostructures, and will help the development of optimized plasmonic configurations for nonthermal electrons generation and harvesting.

We report the design and testing of a small, high vacuum chamber that allows real-time, in situ spectroscopic ellipsometry (SE) measurements; the chamber was designed to be easily inserted within the arms of a commercial ellipsometer. As a test application, we investigated the temperature-induced solid-state dewetting of thin (20 to 8 nm) Au layers on Si wafers. In situ SE measurements acquired in real time during the heating of the samples reveal features that can be related to the birth of a localized surface plasmon resonance (LSPR), and demonstrate the presence of a temperature threshold for the solid-state dewetting.

Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to investigate the role that electron correlation plays in the exchange-coupling at the ferromagnetic/antiferromagnetic interface of a Fe/CoO bilayer growth on Ag(001). The effective correlation energy U$_{\textrm{eff}}$, usually employed to assess the energy distribution of core-valence-valence Auger spectra, has been experimentally determined for each possible combination of the orbital (e$_g$ or t$_{2g}$) and the spin (majority or minority) of the two valence electrons involved in the Auger decay. Coulomb and exchange interactions have been identified and compared with the result obtained on the Fe/Ag system. The presented analysis reveals in the Fe/CoO interface an enhancement of the Coulomb interaction for the e$_g$ orbital and of the exchange interaction for the t$_{2g}$ orbital with respect to the Fe/Ag case that can be associated to the stronger electron confinement and to the exchange coupling between the two layers, respectively., Comment: 37 pages, 6 figures

Silver is an extremely appealing metal for plasmonics due to its very low optical losses in the visible and near-ultraviolet range and its relatively low reactivity. Within the emerging field of thermoplasmonics, where light-metal interactions are exploited to generate heat on the nanometric scale, knowledge of temperature-dependent complex permittivities of plasmonic materials is indispensable. We extracted the temperature-dependent complex permittivity of silver ${\ensuremath{\varepsilon}}_{Ag}$ by spectroscopic ellipsometry under high-vacuum conditions. For rising $T$, we observed an increase of the free-electron contribution to the imaginary part of the permittivity $\text{Im}[{\ensuremath{\varepsilon}}_{Ag}]$ and a temperature-dependent absorption band splitting off the interband absorption edge in the 320--360-nm range. Around 340 nm the relative increase of $\text{Im}[{\ensuremath{\varepsilon}}_{Ag}]$ at 600 K with respect to its room-temperature value is around 500%. In order to understand the implications of this behavior on silver thermoplasmonics, we computed the temperature-dependent extinction efficiency of oblate Ag ellipsoids with localized plasmon resonance within the 320--360-nm range. We predict that dramatic damping of the plasmon resonance occurs for increasing temperature, possibly leading to intriguing self-limiting effects in Ag thermoplasmonics.

Monolayer tungsten disulfide ($WS_2$) has recently attracted large interest as a promising material for advanced electronic and optoelectronic devices such as photodetectors, modulators, and sensors. Since these devices can be integrated in a silicon (Si) chip via back-end-of-line (BEOL) processes, the stability of monolayer $WS_2$ in BEOL fabrication conditions should be studied. In this work, the thermal stability of monolayer single-crystal $WS_2$ at typical BEOL conditions is investigated; namely (i) heating temperature of $300$ $^\circ C$, (ii) pressures in the medium- ($10^{-3}$ mbar) and high- ($10^{-8}$ mbar) vacuum range; (iii) heating times from $30$ minutes to $20$ hours. Structural, optical and chemical analyses of $WS_2$ are performed via scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). It is found that monolayer single-crystal $WS_2$ is intrinsically stable at these temperature and pressures, even after $20$ hours of thermal treatment. The thermal stability of $WS_2$ is also preserved after exposure to low-current electron beam ($12$ pA) or low-fluence laser ($0.9$ $mJ/\mu m^2$), while higher laser fluencies cause photo-activated degradation upon thermal treatment. These results are instrumental to define fabrication and in-line monitoring procedures that allow the integration of $WS_2$ in device fabrication flows without compromising the material quality., Comment: 21 pages, 5 figures

We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse tuned around the Al L2,3 edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of femtoseconds. Following this latency time, sizable reflectivity changes were observed. Exploiting recent theoretical calculations of the EUV-excited electron dynamics [N. Medvedev, Phys. Rev. Lett. 107, 165003 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.165003], the delayed NIR-reflectivity evolution is interpreted invoking the formation of very-long-living nonthermal hot electron distributions in Al after exposure to intense EUV pulses. Our data represent the first evidence in the time domain of such an intriguing behavior.

The effects of a chemical alteration of the antiferromagnet surface in the exchange-biased ultrathin epitaxial antiferromagnetic/ferromagnetic bilayer Mn/Co/Cu(001) have been investigated. The formation of a nanometer-thick paramagnetic oxide at the surface of the Mn layer was induced through the controlled exposure to oxygen. Although the buried Mn/Co interface was not affected by the oxidation, strong changes of the exchange-bias properties of the system, which cannot be accounted for by the mere reduction of the antiferromagnetic Mn thickness, were observed. We propose that the produced surface modification affects the magnetic configuration of the near-surface region and the domain structure in the whole antiferromagnet layer which, in turn, influences the pinned uncompensated spin density responsible for the exchange bias.

Structural and electronic properties of ultrathin Mg films on Ag(1 0 0) have been investigated by means of metastable de-excitation spectroscopy (MDS), work function change (Δφ), electron diffraction and helium reflectivity (RHe) measurements. At room temperature Mg grows on Ag(1 0 0) following the Stranski–Krastanov growth mode. Successive annealing of a 10 ML thick Mg film leads to its almost complete desorption at about 390 K and to the formation of a Mg–Ag alloy. RHe and electron diffraction measurements indicate that for annealing temperatures between 480 and 570 K, the alloy orders with a c(2×2) periodicity. Further annealing at higher temperatures restores the initial Ag(1 0 0) surface.

In this work we present a simple pathway to obtain large single-crystal graphene on copper (Cu) foils with high growth rates using a commercially available cold-wall chemical vapour deposition (CVD) reactor. We show that graphene nucleation density is drastically reduced and crystal growth is accelerated when: i) using ex-situ oxidised foils; ii) performing annealing in an inert atmosphere prior to growth; iii) enclosing the foils to lower the precursor impingement flux during growth. Growth rates as high as 14.7 and 17.5 micrometers per minute are obtained on flat and folded foils, respectively. Thus, single-crystal grains with lateral size of about one millimetre can be obtained in just one hour. The samples are characterised by optical microscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy as well as selected area electron diffraction (SAED) and low-energy electron diffraction (LEED), which confirm the high quality and homogeneity of the films. The development of a process for the quick production of large grain graphene in a commonly used commercial CVD reactor is a significant step towards an increased accessibility to millimetre-sized graphene crystals., Comment: Article: 7 pages, 6 figures. Supplementary Information: 5 pages, 7 figures

Codeposition of oxygen and Fe on Cu 3 Au(0 0 1) has been investigated in order to look at the modifications of the growth regime induced by the chemical additive. Oxygen deeply modifies the growth modes both at relatively low temperature (150 K) and at room temperature (RT). The bilayer growth mode active at earlier stages of growth in absence of the surfactant element is suppressed. We concentrated our investigation on RT deposition. A relatively ordered, pseudomorphic iron film has been found only at low thickness (up to 4 ML). This film is covered with an oxygen layer, whose coverage and order depends on the post-growth annealing treatment. The annealed oxygen layer has been used as a surfactant for further Fe deposition cycles. During deposition, part of the oxygen remains trapped into the film. Restoration of the chemisorbed layer is obtained after annealing at moderate temperature (700 K). As the film thickness increases as a result of successive depositions, the film is first characterized by coexistence of pseudomorphic and bcc patches. The pseudomorphic patches seem stabilized not only by presence of the oxygen, but also by Cu and Au impurities in the film. For relatively large thickness (above 25 ML) bcc domains become majority.

The magnetic properties of Ag/Fe/MgO and Cr/Fe/MgO multilayers have been studied in air by both linear (MOKE) and non-linear (MSHG) magneto-optics. Thanks to the different probing sensitivities of the two techniques, film and interface properties can be separately measured. In the Ag/Fe/MgO case, coercive field values measured with linear and non-linear optics are very similar, indicating that a sharp interface is formed and that the presence of silver on top of iron does not alter its magnetic properties. The values of coercive field measured with MOKE for the Cr/Fe/MgO multilayer differ significantly with respect to the Ag case, and furthermore different coercive fields have been measured on this system with MOKE and MSHG. We ascribe the observed effects to spin frustration caused the layered antiferromagnetic Cr spin structure along with the presence of steps at the Cr/Fe interface.

In this study, we deposited Fe films on MoS2 flakes, and investigated the microscopic magnetic behavior on individual flakes. The MoS2 flakes were fabricated on SiO2/Si(1 0 0) substrates using chemical vapor deposition. Fe coverage was deposited on the MoS2 flakes by e-beam evaporation with a thin Pd capping for protection. Investigations by atomic force microscope and Raman spectroscopy confirmed that the MoS2 flakes had a mean lateral size of 10–20 μ m and mostly single layer thick. After depositing 3.6 and 7.0 nm Fe on MoS2/SiO2, clear hysteresis loops were observable with the in-plane magnetic field. From the investigation using a magneto-optical Kerr microscope, we measured the hysteresis curves within individual MoS2 flakes. Although the Fe coverage was much thicker than the MoS2 atomic step height (~0.66 nm) and the direct connection and strong ferromagnetic coupling between Fe/MoS2 and Fe/SiO2 were expected, a magnetic decoupling between the magnetic domains of Fe/MoS2 and Fe/SiO2 was surprisingly observed. For 3.6 nm Fe/MoS2, the magnetic coercivity (H c ) was 28 ± 5 Oe, while in contrast, the H c of 3.6 nm Fe/SiO2 ranged 58 ± 5 Oe. With a thicker Fe coverage of 7.0 nm, the H c of Fe/MoS2 and Fe/SiO2 converged and the magnetic decoupling became too weak to observe. The distinct interface magnetic anisotropy of Fe on different substrates was held responsible for the observed magnetic decoupling across the MoS2 atomic step between Fe/MoS2 and Fe/SiO2 domains. These observations will be valuable in combining a magnetic coverage with a single layer MoS2 for future spintronic applications.

The surface magnetism of an iron film grown on a MgO(1 0 0) substrate has been studied by means of spin polarized metastable de-excitation spectroscopy (SPMDS) as a function of the exposure to oxygen. The O 2p states are magnetically coupled to the iron film and exhibit a splitting in energy Δ E O 2p between the majority and the minority components. The behavior of Δ E O 2p with oxygen exposure demonstrates that the magnetic coupling of the O 2p states is strongly dependent on the local environment of the adsorbed oxygen atoms and provides a clear indication of the magnetic shielding induced by oxygen diffusion in sub-surface sites.

The surface magnetic properties of ultrathin Cr films grown on an O/Fe/Ag(100) substrate have been studied by means of spin polarized metastable de-excitation spectroscopy. The presence of oxygen on top of the Fe film inhibits the segregation of Ag and strongly reduces Fe/Cr intermixing. The magnetization of the outermost Cr layer is zero immediately after its deposition, whereas, upon annealing up to 500 K, a net magnetization is observed which reverse its direction as a function of the film thickness already from a 3 ML thick film. The magnetization reversal is in agreement with the layered antiferromagnetic structure of Cr clearly observed for thicker films.

The correlation between crystallographic order and surface magnetism has been investigated by studying the chemisorption of oxygen on iron films grown on Ag(100) and MgO(100). The exchange splitting of O 2p states depends strongly on surface order as it decreases from Δ E O 2p ≃0.5 eV for the O(1×1) phase to Δ E O 2p ≃0.2 eV for the O/Fe/MgO disordered phase. Even smaller values of Δ E O 2p are obtained for the highly disordered O/Fe/Ag system, demonstrating that the coupling of oxygen states with the magnetic film underneath is strongly affected by crystallographic order.

We present a systematic investigation of the optical properties of cluster-assembled TiO2 films (20–150 nm nominal thickness) deposited onto Si wafers and processed with two types of ex-situ oxidation treatments. Spectroscopic ellipsometry (0.7–5 eV energy range), supported by x-ray photoelectron spectroscopy and atomic force microscopy measurements, allowed us to obtain information on the depth-dependent morphology of the films in a non-destructive mode. The characteristic grainy and porous structure of the films was modelled by means of a flexible effective-medium approach, allowing us to obtain a reliable estimate of the amount and distribution of voids into the films and their accessibility to foreign liquids. The absorption-edge broadening and significant optical absorptions within the gap were attributed to the intrinsically nanogranular morphology and to the interrelated defective structure/stoichiometry of the particles, also in relation to the occurrence of poorly accessible interstices or pores in the inner parts of the film.

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.

""Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to. probe electronic structure in antiferromagnetic thin films. In particular, exploiting the AR-APECS capability to. discriminate Auger electron emission events characterized by a different spin of the ion in its final state, a sharp. multiplet structure in the Ni MVV Auger line shape of NiO\\\/Ag(001) thin films is measured below the critical. Néel temperature. The assignment of multiplet terms follows from a close comparison of the experimental. AR-APECS line shapes with the predictions based on semiempirical calculations on a cluster model and an. open-band extension of the Cini-Sawatzky approach. In analogy to CoO, also in NiO, above the Néel temperature. a more featureless Auger spectrum appears and AR-APECS does not disentangle anymore high-spin and low-spin. contributions to the total Auger intensity. Such a behavior, which seems to be a general result for metal oxide. antiferromagnetic systems, is discussed.""

A. Akin Unal,1 Aimo Winkelmann,1 Christian Tusche,1 Francesco Bisio,2 Martin Ellguth,1 Cheng-Tien Chiang,1 Jurgen Henk,3 and Jurgen Kirschner1 1Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany 2CNR-SPIN, Corso Perrone 24, I-16152 Genova, Italy 3Institut fur Physik-Theoretische Physik, Martin-Luther-Universitat Halle-Wittenberg, 06099 Halle, Germany (Received 6 July 2012; revised manuscript received 31 August 2012; published 27 September 2012)

We observe an enhanced spin polarization and reduction of surface sensitivity due to the intermediate unoccupied electronic states in spin-resolved two-photon photoemission (2PPE) from ultrathin ferromagnetic cobalt films. By comparison to one-photon photoemission (1PPE) measurements, we disentangle the 2PPE signals coming from the initial and intermediate electronic states and determine the influence of the unoccupied quantum-well states in the nonlinear two-photon transitions. Our results point out the importance of spin-dependent pathways in nonlinear photoemission and provide a possible mechanism of spin-filtering in optical transitions.

We report on the dispersion relations of surface plasmon resonance (SPR) modes in spatially extended self-organized two-dimensional arrays of closely spaced gold nanoparticles (NPs). The arrays are supported on uniaxially rippled LiF substrates that trigger the organization of the metallic gold NPs in sequences of closely spaced chains. We measured the dispersion of the longitudinal and transverse SPR modes of these chains and successfully reproduced the experimental data by means of a modified effective-medium model. The calculations show that the interchain electromagnetic coupling increases by almost one order of magnitude the SP group velocity in well-defined regions of the Brillouin zone with respect to isolated chains.

We demonstrate the contribution of coherent excitation pathways in three-photon photoemission from Cu(001). We identify separate peaks in the photoelectron spectra as originating from two specific $\text{Cu}\text{ }d$ bands that are excited at different $k$ points via the same unoccupied image-potential state. By model calculations we show that our experimental observations impose lower limits on the relevant $\text{Cu}\text{ }d$-hole dephasing time on the order of 10--20 fs. Our findings suggest extensions to future studies in electronic structure mapping with tunable laboratory and free-electron laser sources, exploiting possible multiphoton resonances between occupied bulk and unoccupied surface states.

Received 29 June 2009; revised manuscript received 31 August 2009; published 30 September 2009 We have performed a study of simultaneous nonlinear optical second harmonic generation SHG and angle-resolved nonlinear photoemission at the Cu001 surface excited by fundamental photon energies near 3 eV. At these excitation energies, we identify a dominant contribution to the SHG signal that is determined by a two-photon resonance between the d bands and the n = 1 image-potential state at the Cu001 surface. The near-resonant behavior of SHG and nonlinear photoemission is studied via Cs adsorption, which allows to systematically lower the work function and thus the reference vacuum level of the image potential. Comparison of the angle-resolved photoemission signal, arising from a restricted region of electronic momentum space, with the simultaneous optical SHG signal allows to identify an additional contribution of electronic states with nonzero surface-parallel momentum to the SHG signal enhancements. A simple model accounting for the observed behavior is developed and the implications for the quantitative understanding of SHG are discussed.

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.

We have investigated the exchange bias (EB) in ultrathin epitaxial Cr/Fe/Ag(001) nanopatterned bilayers characterized by atomic-scale-tailored interfaces consisting of nanoripples aligned along the ${[100]}_{\text{Fe}}$ direction. Precisely measuring the magnetic anisotropy of the system, we quantitatively characterized the energetics of the magnetization reversal in terms of domain-wall propagation in the Fe layer and discussed the influence of the interface structure in determining the exchange bias and promoting the domain-wall processes. We found no influence of the relative orientation of the field-cooling direction with respect to the nanoripple orientation in determining the EB magnitude, but we observed that the anisotropic morphology generates characteristic spin frustrations affecting the energetics of different domain-wall processes.

We report a second harmonic generation (SHG) investigation of the compensated antiferromagnetic NiO(001) surface. We show that bulk and surface contributions to the total SHG signal can be distinguished based on their symmetry properties and spectral dependence. We observed that the coupling of antiferromagnetic order with crystalline structure via crystal distortion effects gives rise to a clearly observable SHG signal of magnetic origin.

We investigate the effect of oxygen adsorption on the in-plane magnetic anisotropy of monatomic Fe steps in nanopatterned $\mathrm{Fe}∕\mathrm{Ag}(001)$ films. We selectively resolve the effect of different adsorbate binding sites on the anisotropy, deducing that the step anisotropy was strongly quenched upon decoration of the atomic steps by oxygen, whereas it was considerably enhanced by adsorption on fourfold-coordinated terrace sites. We exploit this previously unreported correlation between adsorption sites and magnetic anisotropy to delve into the contributions to the overall step-induced anisotropy of differently coordinated atoms in the proximity of the step.

We have experimentally determined the magnitude of the in-plane magnetocrystalline anisotropy of monatomic Fe steps on the surface of nanopatterned $\mathrm{Fe}∕\mathrm{Ag}(001)$ films. The films have been nanopatterned by the ion-sculpting technique, in which grazing-incidence ion sputtering is exploited to induce the formation of nanometer-scale surface ripples oriented along the ion-beam direction. We quantitatively assessed the ripple morphology in order to determine the density of Fe monatomic steps parallel to the ripple ridges. By correlating the morphology with the measured uniaxial magnetic anisotropy, we obtained the strength of the magnetocrystalline anisotropy of Fe monatomic steps ${\ensuremath{\kappa}}_{u}^{\mathit{mc}}=(6.5\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}\mathrm{erg}∕\mathrm{cm}$.

The possibility of modifying the magnetic properties of ultrathin films by means of ion sculpting has been verified for Co/Cu(0 0 1) and Fe/Ag(0 0 1). For both these systems, ion sputtering at grazing incidence performed in a proper, material-dependent, temperature range induces the formation of ripples at the surface of the films. Following ripple formation, the magnetic anisotropy in the plane of the films undergoes dramatic changes brought about by the out-of-equilibrium distribution of defect sites induced by the ripple structure itself.

The onset of in-plane uniaxial magnetic anisotropy in ion-sculpted ultrathin Fe/Ag(0 0 1) films has been investigated. Uniaxial anisotropy already develops at ion fluences where the self-organisation of the ripple structure has not yet started. The evolution of the strength of the uniaxial anisotropy as a function of ion fluence allows to identify the step unbalance at the ion-sculpted surface as the origin of the magnetic anisotropy.

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.

Ion sculpting of ultrathin Fe/Ag(001) films induces the self-assembled formation of nanometric surface ripples oriented along the ion beam direction and the corresponding onset of an in-plane uniaxial contribution to magnetic anisotropy. The authors show that fine tuning of the ion dose impinging on the film allows to tailor the in-plane magnetic anisotropy of such films from purely biaxial in the as-grown state to purely uniaxial. The magnitude of magnetic anisotropy in the pure uniaxial state can be tailored by varying the Fe film thickness prior to irradiation. (c) 2006 American Institute of Physics.

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 dependence of the magnetic properties of ultrathin Fe/Cu 3 Au films (thickness

Islands of 1\char21{}2 nm lateral size and double-layer height are formed when one monolayer (ML) of Fe is deposited on ${\mathrm{Cu}}_{3}\mathrm{Au}(001)$ at low temperature [M. Canepa et al., Phys. Rev. B 62, 13 121 (2000)]. We used the photoelectron diffraction technique to investigate the atomic structure and chemical composition of these nanoislands just after the deposition at 140 K and after annealing at 400 K. We show that only bilayer islands are formed at low temperature, without any surface segregation. After annealing, the Fe atoms are reaggregated to form mainly trilayer islands. Surface segregation is shown to be inhibited also after the annealing process. The implications for the film magnetic properties and the growth model are discussed.

We investigated the plasmonic response of arrays of Al nanowires fabricated in high-vacuum and embedded within a transparent protective medium. The nanostructures exhibited a strongly-birefringent plasmonic response which, depending on the mutual orientation of the incident-field polarization and the nanowire axis, allowed the plasmon resonance to span the whole spectral range from the visible to the deep-ultraviolet regime. Comparing the experimental data with theoretical calculations allowed to rationalize the optical response in terms of non-ideal nanowire morphologies arising from the bottom-up character of the nanofabrication method. The broadband plasmonic response suggests the potential application of these systems in plasmon-enhanced photovoltaics, exploiting the great advantage of the low-cost of aluminium.

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.

The growth of ultra thin iron films (up to a thickness of 5–6 ML) on O(1×1)–Fe/Ag(0 0 1) has been investigated by means of He reflectivity ( R He ) and metastable de-excitation spectroscopy. The presence of oxygen induces a quasi-ideal layer-by-layer growth at variance with the case of the homoepitaxial growth of iron on Fe(0 0 1). The surface electronic density of states suddenly changes upon the deposition of the first half of a monolayer. After the sudden change in the first stages of the growth, the surface density of states of both O 2p and Fe 3d states remains essentially unchanged, irrespectively of the thickness of the deposited film. This provides a clear indication that oxygen floats at the surface acting as a surfactant for the growth of iron on O(1×1)–Fe/Ag(0 0 1). The stationary fraction of oxygen that remains on the topmost layer as growth proceeds depends on the substrate temperature. Post-growth annealing up to 650 K restores the initial coverage of oxygen and the ordered O(1×1) phase.