108 research outputs found
Strong coupling between single-electron tunneling and nano-mechanical motion
Nanoscale resonators that oscillate at high frequencies are useful in many
measurement applications. We studied a high-quality mechanical resonator made
from a suspended carbon nanotube driven into motion by applying a periodic
radio frequency potential using a nearby antenna. Single-electron charge
fluctuations created periodic modulations of the mechanical resonance
frequency. A quality factor exceeding 10^5 allows the detection of a shift in
resonance frequency caused by the addition of a single-electron charge on the
nanotube. Additional evidence for the strong coupling of mechanical motion and
electron tunneling is provided by an energy transfer to the electrons causing
mechanical damping and unusual nonlinear behavior. We also discovered that a
direct current through the nanotube spontaneously drives the mechanical
resonator, exerting a force that is coherent with the high-frequency resonant
mechanical motion.Comment: Main text 12 pages, 4 Figures, Supplement 13 pages, 6 Figure
Magnetism of small V clusters embedded in a Cu fcc matrix: an ab initio study
We present extensive first principles density functional theory (DFT)
calculations dedicated to analyze the magnetic and electronic properties of
small V clusters (n=1,2,3,4,5,6) embedded in a Cu fcc matrix. We consider
different cluster structures such as: i) a single V impurity, ii) several
V dimers having different interatomic distance and varying local atomic
environment, iii) V and iv) V clusters for which we assume compact
as well as 2- and 1-dimensional atomic configurations and finally, in the case
of the v) V and vi) V structures we consider a square pyramid and a
square bipyramid together with linear arrays, respectively. In all cases, the V
atoms are embedded as substitutional impurities in the Cu network. In general,
and as in the free standing case, we have found that the V clusters tend to
form compact atomic arrays within the cooper matrix. Our calculated non
spin-polarized density of states at the V sites shows a complex peaked
structure around the Fermi level that strongly changes as a function of both
the interatomic distance and local atomic environment, a result that
anticipates a non trivial magnetic behavior. In fact, our DFT calculations
reveal, in each one of our clusters systems, the existence of different
magnetic solutions (ferromagnetic, ferrimagnetic, and antiferromagnetic) with
very small energy differences among them, a result that could lead to the
existence of complex finite-temperature magnetic properties. Finally, we
compare our results with recent experimental measurements.Comment: 7 pages and 4 figure
Spontaneous Formation of Core@shell Co@Cr Nanoparticles by Gas Phase Synthesis
This work presents the gas phase synthesis of CoCr nanoparticles using a magnetron-based gas aggregation source. The effect of the particle size and Co/Cr ratio on the properties of the nanoparticles is investigated. In particular, we report the synthesis of nanoparticles from two alloy targets, Co90Cr10 and Co80Cr20. In the first case, we observe a size threshold for the spontaneous formation of a segregated core@shell structure, related to the surface to volume ratio. When this ratio is above one, a shell cannot be properly formed, whereas when this ratio decreases below unity the proportion of Cr atoms is high enough to allow the formation of a shell. In the latter case, the segregation of the Cr atoms towards the surface gives rise to the formation of a shell surrounding the Co core. When the proportion of Cr is increased in the target (Co80Cr20), a thicker shell is spontaneously formed for a similar nanoparticle size. The magnetic response was evaluated, and the influence of the structure and composition of the nanoparticles is discussed. An enhancement of the global magnetic anisotropy caused by exchange bias and dipolar interactions, which enables the thermal stability of the studied small particles up to relatively large temperatures, is reported
The CAT-ACT Beamline at ANKA : A new high energy X-ray spectroscopy facility for CATalysis and ACTinide research
A new hard X-ray beamline for CATalysis and ACTinide research has been built at the synchrotron radiation facility ANKA. The beamline design is dedicated to X-ray spectroscopy, including ‘flux hungry’ photon-in/photon-out and correlative techniques with a special infrastructure for radionuclide and catalysis research. The CAT-ACT beamline will help serve the growing need for high flux/hard X-ray spectroscopy in these communities. The design, the first spectra and the current status of this project are reported
The CAT-ACT Beamline at ANKA: A new high energy X-ray spectroscopy facility for CATalysis and ACTinide research
A new hard X-ray beamline for CATalysis and ACTinide research has been built at the synchrotron radiation facility ANKA. The beamline design is dedicated to X-ray spectroscopy, including ‘flux hungry’ photon-in/photon-out and correlative techniques with a special infrastructure for radionuclide and catalysis research. The CAT-ACT beamline will help serve the growing need for high flux/hard X-ray spectroscopy in these communities. The design, the first spectra and the current status of this project are reported
Low-energy p-d Scattering: High Precision Data, Comparisons with Theory, and Phase-Shift Analyses
Angular distributions of sigma(theta), A_y, iT_11, T_20, T_21, and T_22 have
been measured for d-p scattering at E_c.m.=667 keV. This set of high-precision
data is compared to variational calculations with the nucleon-nucleon potential
alone and also to calculations including a three-nucleon (3N) potential.
Agreement with cross-section and tensor analyzing power data is excellent when
a 3N potential is used. However, a comparison between the vector analyzing
powers reveals differences of approximately 40% in the maxima of the angular
distributions which is larger than reported at higher energies for both p-d and
n-d scattering. Single-energy phase-shift analyses were performed on this data
set and a similar data set at E_c.m.=431.3 keV. The role of the different
phase-shift parameters in fitting these data is discussed.Comment: 18 pages, 6 figure
Curie temperature enhancement of electron doped SrFeMoO perovskites studied by photoemission spectroscopy
We report here on the electronic structure of electron-doped half-metallic
ferromagnetic perovskites such SrLaFeMoO (=0-0.6) as
obtained from high-resolved valence-band photoemission spectroscopy (PES). By
comparing the PES spectra with band structure calculations, a distinctive peak
at the Fermi level (E) with predominantly (Fe+Mo) t
character has been evidenced for all samples, irrespectively of the values
investigated. Moreover, we show that the electron doping due to the La
substitution provides selectively delocalized carriers to the
t metallic spin channel. Consequently, a gradual rising of
the density of states at the E has been observed as a function of the La
doping. By changing the incoming photon energy we have shown that electron
doping mainly rises the density of states of Mo parentage. These findings
provide fundamental clues for understanding the origin of ferromagnetism in
these oxides and shall be of relevance for tailoring oxides having still higher
T
Signatures of three-nucleon interactions in few-nucleon systems
Recent experimental results in three-body systems have unambiguously shown
that calculations based only on nucleon-nucleon forces fail to accurately
describe many experimental observables and one needs to include effects which
are beyond the realm of the two-body potentials. This conclusion owes its
significance to the fact that experiments and calculations can both be
performed with a high accuracy. In this review, both theoretical and
experimental achievements of the past decade will be underlined. Selected
results will be presented. The discussion on the effects of the three-nucleon
forces is, however, limited to the hadronic sector. It will be shown that
despite the major successes in describing these seemingly simple systems, there
are still clear discrepancies between data and the state-of-the-art
calculations.Comment: accepted for publication in Rep. Prog. Phy
Growth and magnetic characterization of Co nanoparticles obtained by femtosecond pulsed laser deposition
We present a detailed study on the morphology and magnetic properties of Co nanostructures deposited onto oxidized Si substrates by femtosecond pulsed laser deposition. Generally, Co disks of nanometric dimensions are obtained just above the ablation threshold, with a size distribution characterized by an increasingly larger number of disks as their size diminishes, and with a maximum disk size that depends on the laser power
density. In Au/Co/Au structures, in-plane magnetic anisotropy is observed in all cases, with no indication of superparamagnetism regardless of the amount of material or the laser power density. Magnetic force microscopy observations show coexistence of single-domain and vortex states for the magnetic domain structure of
the disks. Superconducting quantum interference device magnetometry and x-ray magnetic circular dichroism measurements point to saturation magnetization values lower than the bulk, probably due to partial oxidation of the Co resulting from incomplete coverage by the Au capping layer.Work was supported in part by the U.S. Department of Energy, Basic Energy Sciences (Grant No. DE-FG02-06ER46273), NSF FOCUS Center, the Spanish Ministerio de
Educación y Ciencia (References No. PR2005-0017 and No.MAT2005-05524-C02), Comunidad de Madrid (Reference No. S-0505/MAT/0194 NANOMAGNET), and CSIC (Reference No. 200650I130). Support from the SRS staff during the XMCD experiments is greatly acknowledged. Y.H. and L.M. also acknowledge financial support from the “Ramón y
Cajal” and “Juan de la Cierva” programs, respectively, from the Spanish Ministerio de Investigación y Ciencia and Consejo
Superior de Investigaciones Científicas (CSIC).Peer reviewe
Angle-resolved photoemission study and first principles calculation of the electronic structure of GaTe
The electronic band structure of GaTe has been calculated by numerical atomic
orbitals density-functional theory, in the local density approximation. In
addition, the valence-band dispersion along various directions of the GaTe
Brillouin zone has been determined experimentally by angle-resolved
photoelectron spectroscopy. Along these directions, the calculated valence-band
structure is in good concordance with the valence-band dispersion obtained by
these measurements. It has been established that GaTe is a direct-gap
semiconductor with the band gap located at the Z point, that is, at Brillouin
zone border in the direction perpendicular to the layers. The valence-band
maximum shows a marked \textit{p}-like behavior, with a pronounced anion
contribution. The conduction band minimum arises from states with a comparable
\textit{s}- \textit{p}-cation and \textit{p}-anion orbital contribution.
Spin-orbit interaction appears to specially alter dispersion and binding energy
of states of the topmost valence bands lying at . By spin-orbit, it is
favored hybridization of the topmost \textit{p}-valence band with deeper
and flatter \textit{p}-\textit{p} bands and the valence-band minimum at
is raised towards the Fermi level since it appears to be determined by
the shifted up \textit{p}-\textit{p} bands.Comment: 7 text pages, 6 eps figures, submitted to PR
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