98 research outputs found
Spin disorder in maghemite nanoparticles investigated using polarized neutrons and nuclear resonant scattering
The manuscript reports the investigation of spin disorder in maghemite nanoparticles of different shape by a combination of polarized small-angle neutron scattering (SANSPOL) and nuclear forward scattering (NFS) techniques. Both methods are sensitive to magnetization on the nanoscale. SANSPOL allows for investigation of the particle morphology and spatial magnetization distribution and NFS extends this nanoscale information to the atomic scale, namely the orientation of the hyperfine field experienced by the iron nuclei. The studied nanospheres and nanocubes with diameters of 7.4 nm and 10.6 nm, respectively, exhibit a significant spin disorder. This effect leads to a reduction of the magnetization to 44% and 58% of the theoretical maghemite bulk value, observed consistently by both techniques
Ab initio and nuclear inelastic scattering studies of FeSi/GaAs heterostructures
The structure and dynamical properties of the FeSi/GaAs(001) interface
are investigated by density functional theory and nuclear inelastic scattering
measurements. The stability of four different atomic configurations of the
FeSi/GaAs multilayers is analyzed by calculating the formation energies and
phonon dispersion curves. The differences in charge density, magnetization, and
electronic density of states between the configurations are examined. Our
calculations unveil that magnetic moments of the Fe atoms tend to align in a
plane parallel to the interface, along the [110] direction of the FeSi
crystallographic unit cell. In some configurations, the spin polarization of
interface layers is larger than that of bulk FeSi. The effect of the
interface on element-specific and layer-resolved phonon density of states is
discussed. The Fe-partial phonon density of states measured for the FeSi
layer thickness of three monolayers is compared with theoretical results
obtained for each interface atomic configuration. The best agreement is found
for one of the configurations with a mixed Fe-Si interface layer, which
reproduces the anomalous enhancement of the phonon density of states below 10
meVComment: 14 pages, 9 figures, 4 table
Dynamics of Metal Centers Monitored by Nuclear Inelastic Scattering
Nuclear inelastic scattering of synchrotron radiation has been used now since
10 years as a tool for vibrational spectroscopy. This method has turned out
especially useful in case of large molecules that contain a M\"ossbauer active
metal center. Recent applications to iron-sulfur proteins, to iron(II) spin
crossover complexes and to tin-DNA complexes are discussed. Special emphasis is
given to the combination of nuclear inelastic scattering and density functional
calculations
A Simple and Effective Method for Construction of Escherichia coli Strains Proficient for Genome Engineering
Multiplex genome engineering is a standalone recombineering tool for large-scale programming and accelerated evolution of cells. However, this advanced genome engineering technique has been limited to use in selected bacterial strains. We developed a simple and effective strain-independent method for effective genome engineering in Escherichia coli. The method involves introducing a suicide plasmid carrying the l Red recombination system into the mutS gene. The suicide plasmid can be excised from the chromosome via selection in the absence of antibiotics, thus allowing transient inactivation of the mismatch repair system during genome engineering. In addition, we developed another suicide plasmid that enables integration of large DNA fragments into the lacZ genomic locus. These features enable this system to be applied in the exploitation of the benefits of genome engineering in synthetic biology, as well as the metabolic engineering of different strains of E. coli.open7
Spin-Polarized Transprot through Double Quantum Dots
We investigate spin-polarized transport phenomena through double quantum dots
coupled to ferromagnetic leads in series. By means of the slave-boson
mean-field approximation, we calculate the conductance in the Kondo regime for
two different configurations of the leads: spin-polarization of two
ferromagnetic leads is parallel or anti-parallel. It is found that transport
shows some remarkable properties depending on the tunneling strength between
two dots. These properties are explained in terms of the Kondo resonances in
the local density of states.Comment: 8 pages, 11 figure
Spintronic transport and Kondo effect in quantum dots
We investigate the spin-dependent transport properties of quantum-dot based
structures where Kondo correlations dominate the electronic dynamics. The
coupling to ferromagnetic leads with parallel magnetizations is known to give
rise to nontrivial effects in the local density of states of a single quantum
dot. We show that this influence strongly depends on whether charge
fluctuations are present or absent in the dot. This result is confirmed with
numerical renormalization group calculations and perturbation theory in the
on-site interaction. In the Fermi-liquid fixed point, we determine the
correlations of the electric current at zero temperature (shot noise) and
demonstrate that the Fano factor is suppressed below the Poissonian limit for
the symmetric point of the Anderson Hamiltonian even for nonzero lead
magnetizations. We discuss possible avenues of future research in this field:
coupling to the low energy excitations of the ferromagnets (magnons), extension
to double quantum dot systems with interdot antiferromagnetic interaction and
effect of spin-polarized currents on higher symmetry Kondo states such as
SU(4).Comment: 11 pages, 5 figures. Proceedings of the 3rd Intl. Conf. on Physics
and Applications of Spin-Related Phenomena in Semiconductors, Santa Barbara,
200
Lattice dynamics of endotaxial silicide nanowires
Self-organized silicide nanowires are considered as main building blocks of
future nanoelectronics and have been intensively investigated. In
nanostructures, the lattice vibrational waves (phonons) deviate drastically
from those in bulk crystals, which gives rise to anomalies in thermodynamic,
elastic, electronic, and magnetic properties. Hence, a thorough understanding
of the physical properties of these materials requires a comprehensive
investigation of the lattice dynamics as a function of the nanowire size. We
performed a systematic lattice dynamics study of endotaxial FeSi nanowires,
forming the metastable, surface-stabilized -phase, which are in-plane
embedded into the Si(110) surface. The average widths of the nanowires ranged
from 24 to 3 nm, their lengths ranged from several m to about 100 nm. The
Fe-partial phonon density of states, obtained by nuclear inelastic scattering,
exhibits a broadening of the spectral features with decreasing nanowire width.
The experimental data obtained along and across the nanowires unveiled a
pronounced vibrational anisotropy that originates from the specific orientation
of the tetragonal -FeSi unit cell on the Si(110) surface. The
results from first-principles calculations are fully consistent with the
experimental data and allow for a comprehensive understanding of the lattice
dynamics of endotaxial silicide nanowires.Comment: 9 pages, 7 figures, 3 table
Large-Scale Atomistic Simulations of Environmental Effects on the Formation and Properties of Molecular Junctions
Using an updated simulation tool, we examine molecular junctions comprised of
benzene-1,4-dithiolate bonded between gold nanotips, focusing on the importance
of environmental factors and inter-electrode distance on the formation and
structure of bridged molecules. We investigate the complex relationship between
monolayer density and tip separation, finding that the formation of
multi-molecule junctions is favored at low monolayer density, while
single-molecule junctions are favored at high density. We demonstrate that tip
geometry and monolayer interactions, two factors that are often neglected in
simulation, affect the bonding geometry and tilt angle of bridged molecules. We
further show that the structures of bridged molecules at 298 and 77 K are
similar.Comment: To appear in ACS Nano, 30 pages, 5 figure
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