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ABSTRACT: We performed photoinduced inverse spin Hall effect (ISHE) measurements on a
Pt/Ge(001) junction at room temperature. The spin-oriented electrons,
photogenerated at the direct gap of Ge using circularly polarized light,
provide a net spin current which yields an electromotive field E_ISHE in the Pt
layer. Such a signal is clearly detected at room temperature despite the strong
{\Gamma} to L scattering which electrons undergo in the Ge conduction band. The
ISHE signal dependence on the exciting photon energy is in good agreement with
the electron spin polarization expected for optical orientation at the direct
gap of Ge.
02/2013;
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F Pezzoli,
F Bottegoni,
D Trivedi, F Ciccacci,
A Giorgioni,
P Li,
S Cecchi,
E Grilli,
Y Song,
M Guzzi,
H Dery,
G Isella
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ABSTRACT: We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ∼37% and ∼85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ∼0.5 ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.
Physical Review Letters 04/2012; 108(15):156603. · 7.37 Impact Factor
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ABSTRACT: Nickel-iron interfacial intermixing, resulting from annealing ultrathin Ni films grown on Fe(001), has been investigated by low-energy electron diffraction, Auger electron spectroscopy and scanning tunnelling microscopy. The Ni/Fe interface has been found to be highly unstable at relatively low annealing temperatures, with dramatic thermal effects on the surface morphology of the Ni/Fe(001) epitaxial films.
Nanoscience and Nanotechnology Letters 01/2012; 4(11):1092-1095. · 0.53 Impact Factor
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ABSTRACT: The present study reports on the growth of thin TiO2 films onto Au(100) single crystals by Ti evaporation in a reactive O-2 atmosphere at two different substrate temperatures: room temperature (RI) and 300 degrees C. The growth of the oxide films was monitored by means of X-ray photoemission spectroscopy, while the valence and conduction band electronic structure was investigated by UV and inverse photoemission spectroscopy, respectively. The TiO2 film grows epitaxially on the Au(100) substrate at 300 degrees C exhibiting the rutile (100) surface. The evolution of the Ti 2p lineshape with the oxide coverage shows the presence of reduced oxide species (characterized by Ti3+ ions) at the Au(100) interface. A crystalline and stoichiometric TiO2 oxide is produced at high substrate temperature, while growth at RI gives a measurable concentration of defects. Post growth annealing in ultra-high vacuum of the RT grown film increases this concentration, while subsequent annealing in O-2 atmosphere restores the sample to the as-grown conditions. (c) 2012 Elsevier B.V. All rights reserved.
Thin Solid Films 01/2012; 520(11):3922-3926. · 1.89 Impact Factor
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J. Appl. Phys. 01/2012; 111(6):063916.
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ABSTRACT: We report evidence that the body-centered cubic (bcc)-face-centered cubic (fcc) transition that occurs during Ni film growth on a Fe(001) substrate is preceded by a pre-martensitic phase, as demonstrated by low-energy electron diffraction. The corresponding film superstructure is characterized by a displacement of Ni atoms along the main < 100 > crystallographic axes of iron, without any rotation of the unit cell with respect to the (001) plane, in contrast with the martensitic transition that shows four fcc Ni domains with the Ni < 211 > crystallographic directions aligned with the Fe < 110 > axes. In addition, the martensitic transition is detected not at 6 ML, as previously believed, but above 20ML if the Ni sample is rigorously kept at room temperature. The surface morphology of the bcc-fcc transition is characterized by the development of Ni mounds oriented along the < 110 > directions, as shown by scanning tunneling microscopy.
New Journal of Physics. 01/2012; 14.
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ABSTRACT: The morphology and the electronic structure of ultrathin Ni films grown on the oxygen-passivated Fe(001)-p(1 x 1)O surface are investigated by means of scanning tunneling microscopy and spectroscopy. We observe that oxygen efficiently floats on top of the Ni film and has an "anti-surfactant" effect on the Ni/Fe(001) interface, promoting the formation of two-layer-high islands since the first stages of growth, whereas the Ni growth on the oxygen-free Fe(001) surface proceeds in a layer-by-layer mode. Our experimental results show that the effectiveness of the oxygen surfactant action in the growth of metal thin films strongly depends on the choice of the substrate and overlayer combination.
Physical Review B. 01/2012; 86(7).
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M Cantoni,
R Bertacco,
A Brambilla,
M Finazzi,
L Duo, F Ciccacci,
A Verdini,
L Floreano,
A Morgante,
M Passoni,
C S Casari,
Li A Bassi
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ABSTRACT: The Fe growth on ZnSe(001) takes place via the initial formation of superparamagnetic nano-islands that subsequently coalesce, giving rise to a continuous film for a nominal thickness of 8 Fe monolayers. For a very low Fe coverage (2 Fe monolayers), we show that the surface barrier potential (i.e. the barrier potential seen by electrons incident on the surface), measured by absorbed current spectroscopy, attains very large values (6.9 eV at room temperature) and dramatically changes as a function of temperature, with an increase of∼1.5 eV from room temperature down to 130 K, largely exceeding similar changes observed in both thin films and nanoparticles. This phenomenon disappears as the thickness increases and is fully reversible with temperature. Nonequilibrium phenomena due to the experimental conditions are present, but are not able to explain the observed data. Inverse photoemission, core level photoemission, x-ray photoemission diffraction, and scanning tunneling microscopy are employed in order to find temperature-dependent properties of the Fe islands: while only minor changes as a function of temperature are present in the electronic band structure, the Fe crystal structure, and the morphology of the islands, a noticeable temperature dependence of the Se segregation through the Fe islands is found.
PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS. 01/2012; 85:155456-1-155456-8.
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ABSTRACT: We present a confocal microscopy setup that is able to record magneto-optical hysteresis cycles separating the in-plane and out-of-plane magnetization components. This apparatus is based on a modified commercial microscope, where the light beam has been deviated from the cylindrical symmetry axis of the objective lenses by inserting a translating plate in the optical path. The instrument allows for the magneto-optical imaging with a lateral resolution of 600 nm at λ = 635 nm light wavelength.
The Review of scientific instruments 02/2011; 82(2):023709. · 1.52 Impact Factor
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ABSTRACT: The oxygen-induced effects on the morphology of out-of-equilibrium iron surfaces have been studied by means of scanning tunneling microscopy and spectroscopy applied to either the Fe(001) or the Fe(001)-p(1 x 1)O surface. With respect to the pristine Fe(100) surface, step coalescence is observed upon oxygen exposure when the Fe(001)-p(1 x 1)O surface is formed, with a final morphology characterized by wide terraces separated by steps bunches. We have also investigated the surfactant properties of the chemisorbed oxygen by comparing Fe/Fe(001) and Fe/Fe(001)-p(1 x 1)O homoepitaxy and analyzing the influence of the oxygen overlayer on the growth kinetics. The intralayer mass transport is found to be unaffected by the presence of oxygen, suggesting oxygen-enhanced interlayer mass transport as a mechanism able to explain the surfactant effect. We attribute both phenomena to the presence of a strained FeO overlayer.
Physical Review B. 01/2011; 83(23).
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A Picone,
G Fratesi,
A Brambilla,
P Sessi,
F Donati,
S Achilli,
L Maini,
M I Trioni,
C S Casari,
M Passoni,
Li A Bassi,
M Finazzi,
L Duò, F Ciccacci
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ABSTRACT: We have investigated the formation of scanning tunneling microscopy (STM) atomically resolved images of the Fe(001)-p(1x1)O surface. The latter is characterized by a high in-plane symmetry for both oxygen and iron atoms, thus representing a very appealing template for understanding how to distinguish between oxygen and metal atoms in STM images of an oxidized metal surface. We report on the occurrence of a corrugation reversal as a function of the tip-sample distance, and we use the conclusions of such an investigation for interpreting the experimental images of an oxygen vacancy created on the surface. Our experimental work, supported by ab initio density-functional theory calculations, allows us to assess the fundamental role of the local potential barrier in determining the STM image formation.
PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS. 01/2010; 81:115450-1-115450-6.
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F Donati,
P Sessi,
S Achilli,
Li A Bassi,
M Passoni,
C S Casari,
C E Bottani,
A Brambilla,
A Picone,
M Finazzi,
L Duo,
M I Trioni, F Ciccacci
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ABSTRACT: In this work the density of states close to the Fermi level EF of the Fe(001)-p(1x1)O surface is investigated, by means of scanning tunneling spectroscopy (STS). STS spectra are dominated by two features, located at about 0.5 eV below EF and 0.9 eV above EF. The comparison with ab initio density-functional theory simulations of the surface electronic structure shows a very good agreement and allows assigning the observed features to minority states of the sample surface.
PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS. 01/2009; 79:195430-1-195430-6.
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ABSTRACT: The surface band bending in ZnSe(001), as a function of the temperature, is investigated both in the valence band (by photoemission) and in the conduction band (by inverse photoemission and absorbed current spectroscopies). Two different mechanisms are invoked for interpreting the experimental data: the band bending due to surface states, and the surface voltage induced by the incident beam. While the latter is well known in photoemission (surface photovoltage), we demonstrate the existence of a similar effect in inverse photoemission and absorbed current spectroscopies, induced by the incident electrons instead of photons. These results point to the importance of considering the surface voltage effect even in electron-in techniques for a correct evaluation of the band bending. (c) 2009 Elsevier B.V. All rights reserved.
Journal of Electron Spectroscopy and Related Phenomena 01/2009; 173(2-3):84-87. · 1.96 Impact Factor
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ABSTRACT: We have investigated the micromagnetic structure of magnetic domains in Fe/CoO/Fe trilayer systems and the magnetization coupling between the iron layers. We observe very small magnetic domains separated by nanometer-sized domain walls in the top Fe layer for a narrow CoO thickness range. Such domains have lateral dimensions as low as 30 nm and present topologies which are very similar to those observed in the top layer of Fe/NiO/Fe trilayers. Both magnetic domain structure and Fe interlayer coupling dramatically change with the CoO thickness. The role of magnetocrystalline anisotropy and magnetic frustrations on the observed phenomenology is discussed.
Physical Review B. 01/2009; 79(17).
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ABSTRACT: We report an x-ray photoemission spectroscopy study of the Au/La0.67Sr0.33MnO3 interface formation, aiming to investigate interface abruptness and possible chemical interdiffusion. Our results indicate that the gold deposition does not affect the chemical properties of manganite film and no interdiffusion or segregation takes place.
Journal of Applied Physics 02/2008; 103(4):044903-044903-4. · 2.17 Impact Factor
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ABSTRACT: Hybrid interfaces in semiconductor heterostructures consisting of C(60)/GaAs bilayers have been investigated by means of ultrafast pump-probe spectroscopy. From previous results, the energy levels alignment at the interface shows no injection barrier for electrons from GaAs to C(60). Measurements have been carried out in order to monitor photoinduced electron transfer across the interface. Upon selective excitation of the GaAs layer we observe photobleaching (PB) of C(60) due to a decrease of C(60) neutral molecules. We monitor the PB dynamics concluding that electron transfer proceeds in about 4 ps and then relaxes back to ground state in timescales beyond 100 ps. (C) 2008 Elsevier B.V. All rights reserved.
Chemical Physics Letters 01/2008; 466(1-3):65-67. · 2.34 Impact Factor
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ABSTRACT: By means of X-ray photoemission spectroscopy and low energy electron diffraction, we show that it is possible to grow good quality thin epitaxial CoO films on Fe(001) substrates, through deposition in oxygen atmosphere. In particular, the composition and the structure of CoO(001)/Fe(001) bilayer systems and Fe(001)/CoO(001)/Fe(001) trilayer systems have been investigated by monitoring the evolution of the chemical interactions at the interfaces as a function of CoO thickness and growth temperature. We observe the presence of Fe oxides at the CoO/Fe interface and of a thin layer of metallic cobalt at the upper Fe/CoO interface of trilayer systems. (C) 2008 Elsevier B.V. All rights reserved.
Thin Solid Films 01/2008; 516(21):7519-7524. · 1.89 Impact Factor
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A. Li Bassi,
C. S. Casari,
D. Cattaneo,
F. Donati,
S. Foglio,
M. Passoni,
C. E. Bottani,
P Biagioni,
A. Brambilla,
M Finazzi, F. Ciccacci,
L. Duo
[show abstract]
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ABSTRACT: A simple, reliable method for preparation of bulk Cr tips for Scanning Tunneling Microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and Spin Polarized-STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7x7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations.
10/2007;
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R. Bertacco,
S. Brivio,
M. Cantoni,
A. Cattoni,
D. Petti,
M. Finazzi, F. Ciccacci,
A. A. Sidorenko,
M. Ghidini,
G. Allodi,
R. De Renzi
[show abstract]
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ABSTRACT: The authors report about La <sub>0.67</sub> Sr <sub>0.33</sub> Mn O <sub>3</sub> single crystal manganite thin films in interaction with a gold capping layer. With respect to uncoated manganite layers of the same thickness, Au-capped 4 nm thick manganite films reveal a dramatic reduction (≃185 K ) of the Curie temperature T<sub>C</sub> and a lower saturation low temperature magnetization M<sub>0</sub> . A sizable T<sub>C</sub> reduction (≃60 K ) is observed even when an inert Sr Ti O <sub>3</sub> layer is inserted between the gold film and the 4 nm thick manganite layer, suggesting that this effect might have an electrostatic origin.
Applied Physics Letters 10/2007; · 3.84 Impact Factor
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ABSTRACT: We present a detailed investigation of the electronic properties of C-60 grown on GaAs(1 0 0) substrates, as a function of the fullerene coverage, from the very early stages of interface formation up to the development of a bulk-like fullerene film. We monitor the chemical interactions and the energy levels alignment by means of X-rays, ultraviolet and inverse photoemission spectroscopies. The two latter techniques allow to investigate the electronic structure close to the Fermi level. Energy levels alignment at the interfaces of C-60 with p-doped and GaAs(1 0 0) are obtained and discussed. (C) 2007 Elsevier B.V. All rights reserved.
Surface Science. 01/2007; 601(18):4078-4081.
