139 research outputs found
Dynamics of four-photon photoluminescence in gold nanoantennas
Two-pulse correlation is employed to investigate the temporal dynamics of
both two-photon photoluminescence (2PPL) and four-photon photoluminescence
(4PPL) in resonant and nonresonant nanoantennas excited at a wavelength of 800
nm. Our data are consistent with the same two-step model being the cause of
both 4PPL and 2PPL, implying that the first excitation step in 4PPL is a
three-photon sp->sp direct interband transition. Considering energy and parity
conservation, we also explain why 4PPL behavior is favored over three-and
five-photon photoluminescence in the power range below the damage threshold of
our antennas. Since sizeable 4PPL requires larger peak intensities of the local
field, we are able to select either 2PPL or 4PPL in the same gold nanoantennas
by choosing a suitable laser pulse duration. We thus provide a first consistent
model for the understanding of multiphoton photoluminescence generation in gold
nanoantennas, opening new perspectives for applications ranging from the
characterization of plasmonic resonances to biomedical imaging
Surfactant-like Effect and Dissolution of Ultrathin Fe Films on Ag(001)
The phase immiscibility and the excellent matching between Ag(001) and
Fe(001) unit cells (mismatch 0.8 %) make Fe/Ag growth attractive in the field
of low dimensionality magnetic systems. Intermixing could be drastically
limited at deposition temperatures as low as 140-150 K. The film structural
evolution induced by post-growth annealing presents many interesting aspects
involving activated atomic exchange processes and affecting magnetic
properties. Previous experiments, of He and low energy ion scattering on films
deposited at 150 K, indicated the formation of a segregated Ag layer upon
annealing at 550 K. Higher temperatures led to the embedding of Fe into the Ag
matrix. In those experiments, information on sub-surface layers was attained by
techniques mainly sensitive to the topmost layer. Here, systematic PED
measurements, providing chemical selectivity and structural information for a
depth of several layers, have been accompanied with a few XRD rod scans,
yielding a better sensitivity to the buried interface and to the film long
range order. The results of this paper allow a comparison with recent models
enlightening the dissolution paths of an ultra thin metal film into a different
metal, when both subsurface migration of the deposit and phase separation
between substrate and deposit are favoured. The occurrence of a surfactant-like
stage, in which a single layer of Ag covers the Fe film is demonstrated for
films of 4-6 ML heated at 500-550 K. Evidence of a stage characterized by the
formation of two Ag capping layers is also reported. As the annealing
temperature was increased beyond 700 K, the surface layers closely resembled
the structure of bare Ag(001) with the residual presence of subsurface Fe
aggregates.Comment: 4 pages, 3 figure
martensitic transition during ni growth on fe 001 evidence of a precursor phase
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 h100i 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 Nih211i crystallographic directions aligned with the Feh110i axes. In addition, the martensitic transition is detected not at 6ML, 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 h110i directions, as shown by scanning tunneling microscopy
Nanoantennas for visible and infrared radiation
Nanoantennas for visible and infrared radiation can strongly enhance the
interaction of light with nanoscale matter by their ability to efficiently link
propagating and spatially localized optical fields. This ability unlocks an
enormous potential for applications ranging from nanoscale optical microscopy
and spectroscopy over solar energy conversion, integrated optical
nanocircuitry, opto-electronics and density-ofstates engineering to
ultra-sensing as well as enhancement of optical nonlinearities. Here we review
the current understanding of optical antennas based on the background of both
well-developed radiowave antenna engineering and the emerging field of
plasmonics. In particular, we address the plasmonic behavior that emerges due
to the very high optical frequencies involved and the limitations in the choice
of antenna materials and geometrical parameters imposed by nanofabrication.
Finally, we give a brief account of the current status of the field and the
major established and emerging lines of investigation in this vivid area of
research.Comment: Review article with 76 pages, 21 figure
Magnetic anisotropy at the buried CoO/Fe interface
Interfaces between antiferromagnetic CoO and ferromagnetic Fe are typically characterized by the development of Fe oxides. Recently, it was shown that the use of a proper ultra-thin Co buffer layer prevents the formation of Fe oxides [Brambilla et al., Appl. Surf. Sci. 362, 374 (2016)]. In the present work, we investigate the magnetic properties of such an interface, and we find evidence for an in-plane uniaxial magnetic anisotropy, which is characterized by a multijump reversal behavior in the magnetization hysteresis loops. X-ray photoemission spectroscopy and element-sensitive hysteresis loops reveal that the occurrence of such an anisotropy is a phenomenon developing at the very interface
Ce-L3-XAS study of the temperature dependence of the 4f occupancy in the Kondo system Ce2Rh3Al9
We have used temperature dependent x-ray absorption at the Ce-L3 edge to
investigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic
changes of the spectral lineshape with decreasing temperature are analyzed and
found to be related to a change in the occupation number, n_f, as the
system undergoes a transition into a Kondo state. The temperature dependence of
indicates a characteristic temperature of 150K, which is clearly related
with the high temperature anomaly observed in the magnetic susceptibility of
the same system. The further anomaly observed in the resistivity of this system
at low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo
origin.Comment: 7 pages, three figures, submitted to PR
High-resolution Ce 3d-edge resonant photoemission study of CeNi_2
Resonant photoemission (RPES) at the Ce 3d -> 4f threshold has been performed
for alpha-like compound CeNi_2 with extremely high energy resolution (full
width at half maximum < 0.2 eV) to obtain bulk-sensitive 4f spectral weight.
The on-resonance spectrum shows a sharp resolution-limited peak near the Fermi
energy which can be assigned to the tail of the Kondo resonance. However, the
spin-orbit side band around 0.3 eV binding energy corresponding to the f_{7/2}
peak is washed out, in contrast to the RPES spectrum at the Ce 3d -> 4f RPES
threshold. This is interpreted as due to the different surface sensitivity, and
the bulk-sensitive Ce 3d -> 4f RPES spectra are found to be consistent with
other electron spectroscopy and low energy properties for alpha-like
Ce-transition metal compounds, thus resolves controversy on the interpretation
of Ce compound photoemission. The 4f spectral weight over the whole valence
band can also be fitted fairly well with the Gunnarsson-Schoenhammer
calculation of the single impurity Anderson model, although the detailed
features show some dependence on the hybridization band shape and (possibly) Ce
5d emissions.Comment: 4 pages, 3 figur
Exchange-induced frustration in Fe/NiO multilayers
Using spin-polarized low-energy electron microscopy to study magnetization in
epitaxial layered systems, we found that the area vs perimeter relationship of
magnetic domains in the top Fe layers of Fe/NiO/Fe(100) structures follows a
power-law distribution, with very small magnetic domain cutoff radius (about 40
nm) and domain wall thickness. This unusual magnetic microstructure can be
understood as resulting from the competition between antiferromagnetic and
ferromagnetic exchange interactions at the Fe/NiO interfaces, rather than from
mechanisms involving the anisotropy and dipolar forces that govern length
scales in conventional magnetic domain structures. Statistical analysis of our
measurements validates a micromagnetic model that accounts for this interfacial
exchange coupling.Comment: 15 pages, 2 figure
Open string wavefunctions in flux compactifications
We consider compactifications of type I supergravity on manifolds with SU(3)
structure, in the presence of RR fluxes and magnetized D9-branes, and analyze
the generalized Dirac and Laplace-Beltrami operators associated to the D9-brane
worldvolume fields. These compactifications are T-dual to standard type IIB
toroidal orientifolds with NSNS and RR 3-form fluxes and D3/D7 branes. By using
techniques of representation theory and harmonic analysis, the spectrum of open
string wavefunctions can be computed for Lie groups and their quotients, as we
illustrate with explicit twisted tori examples. We find a correspondence
between irreducible unitary representations of the Kaloper-Myers algebra and
families of Kaluza-Klein excitations. We perform the computation of 2- and
3-point couplings for matter fields in the above flux compactifications, and
compare our results with those of 4d effective supergravity.Comment: 89 pages, 4 figures. v3: more typos corrected, version published in
JHE
Localized Character of 4f Electrons in CeRh(x=2,3) and CeNi(x=2,5)
We have measured Ce 4f spectral weights of extremely -like
Ce-transition metal intermetallic compounds CeRh (x=2,3) and CeNi
(x=2,5) by using the {\it bulk-sensitive} resonant photoemission technique at
the Ce ()-edge. Unprecedentedly high energy resolution and
longer escape depth of photoemitted electron at this photon energy enabled us
to distinguish the sharp Kondo resonance tails at the Fermi level, which can be
well described by the Gunnarsson-Sch\"onhammer(GS) calculation based on the
Anderson Impurity Hamiltonian. On the other hand, the itinerant 4f band
description shows big discrepancies, which implies that Ce 4f electrons retain
localized characters even in extremely -like compounds.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
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