2,497 research outputs found
Noninvasive Embedding of Single Co Atoms in Ge(111)2x1 Surfaces
We report on a combined scanning tunneling microscopy (STM) and density
functional theory (DFT) based investigation of Co atoms on Ge(111)2x1 surfaces.
When deposited on cold surfaces, individual Co atoms have a limited diffusivity
on the atomically flat areas and apparently reside on top of the upper
pi-bonded chain rows exclusively. Voltage-dependent STM imaging reveals a
highly anisotropic electronic perturbation of the Ge surface surrounding these
Co atoms and pronounced one-dimensional confinement along the pi-bonded chains.
DFT calculations reveal that the individual Co atoms are in fact embedded in
the Ge surface, where they occupy a quasi-stationary position within the big
7-member Ge ring in between the 3rd and 4th atomic Ge layer. The energy needed
for the Co atoms to overcome the potential barrier for penetration in the Ge
surface is provided by the kinetic energy resulting from the deposition
process. DFT calculations further demonstrate that the embedded Co atoms form
four covalent Co-Ge bonds, resulting in a Co4+ valence state and a 3d5
electronic configuration. Calculated STM images are in perfect agreement with
the experimental atomic resolution STM images for the broad range of applied
tunneling voltages.Comment: 19 pages, 15 figures, 3 table
Coulomb singularity effects in tunnelling spectroscopy of individual impurities
Non-equilibrium Coulomb effects in resonant tunnelling processes through deep
impurity states are analyzed. It is shown that Coulomb vertex corrections to
the tunnelling transfer amplitude lead to a power-law singularity in current-
voltage characteristicsComment: 7 pages, 2 figure
On hard exclusive reactions in the time-like region
The proton form factor, two-photon annihilations into as well as
exclusive charmonium decays are critically examined. It will be argued that the
standard perturbative QCD analysis of these reactions fails, i.e. the need for
additional contributions can convincingly be demonstrated. Possible dynamical
mechanisms such as colour-octet admixtures to the charmonium states or diquarks
inside baryons, will be discussed and compared to the data.Comment: Invited talk presented at the Fourth Biennal Conference on Low-Energy
Antiproton Physics, LEAP96, Dinkelsb\"{u}hl (August 1996); 8 pages, LaTeX, 4
PS-figures; uses 'espcrc2.sty' and 'psfig.sty
Nanostructured materials for optoelectronic applications
Study and optimization of new nanoscale materials useful for optoelectronic application have been considered. The search for the effective nanostructured materials has been revealed in two directions: to optimize the mechanical hardness of the inorganic systems and to increase the photorefractive parameters of the organics with nanoobjects. It has been testified that the surface mechanical properties of the inorganic materials via nanotubes treatment process can be drastically improved. For example, the surface mechanical hardness of the UV and IR range soft materials can be increased up to 3-10 times under the conditions of oriented nanotubes placement. It has been obtained that the nonlinear optical characteristics (nonlinear refraction n2 and cubic nonlinearity x(3)) of the organics thin films sensitized with fullerenes or nanotubes can be increased up to 3-4 orders of magnitude in comparison with the same parameters for bulk materials traditionally used for nonlinear optics.Selected papers presented at the Eleventh Annual Conference of the Materials Research Society of Serbia, YUCOMAT 200
Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics
Infrared reflectivity and time-domain terahertz transmission spectra of
EuTiO3 ceramics revealed a polar optic phonon at 6 - 300K, whose softening is
fully responsible for the recently observed quantum paraelectric behaviour.
Even if our EuTiO3 ceramics show lower permittivity than the single crystal due
to a reduced density and/or small amount of secondary pyrochlore Eu2Ti2O7
phase, we confirmed the magnetic field dependence of the permittivity, also
slightly smaller than in single crystal. Attempt to reveal the soft phonon
dependence at 1.8K on the magnetic field up to 13T remained below the accuracy
of our infrared reflectivity experiment
Infrared and THz studies of polar phonons and improper magnetodielectric effect in multiferroic BFO3 ceramics
BFO3 ceramics were investigated by means of infrared reflectivity and time
domain THz transmission spectroscopy at temperatures 20 - 950 K, and the
magnetodielectric effect was studied at 10 - 300 K, with the magnetic field up
to 9 T. Below 175 K, the sum of polar phonon contributions into the
permittivity corresponds to the value of measured permittivity below 1 MHz. At
higher temperatures, a giant low-frequency permittivity was observed, obviously
due to the enhanced conductivity and possible Maxwell-Wagner contribution.
Above 200 K the observed magnetodielectric effect is caused essentially through
the combination of magnetoresistance and the Maxwell-Wagner effect, as recently
predicted by Catalan (Appl. Phys. Lett. 88, 102902 (2006)). Since the
magnetodielectric effect does not occur due to a coupling of polarization and
magnetization as expected in magnetoferroelectrics, we call it improper
magnetodielectric effect. Below 175 K the magnetodielectric effect is by
several orders of magnitude lower due to the decreased conductivity. Several
phonons exhibit gradual softening with increasing temperature, which explains
the previously observed high-frequency permittivity increase on heating. The
observed non-complete phonon softening seems to be the consequence of the
first-order nature of the ferroelectric transition.Comment: subm. to PRB. revised version according to referees' report
Flux Exclusion Superconducting Quantum Metamaterial: Towards Quantum-level Switching
Nonlinear and switchable metamaterials achieved by artificial structuring on the subwavelength scale have become a central topic in photonics research. Switching with only a few quanta of excitation per metamolecule, metamaterial's elementary building block, is the ultimate goal, achieving which will open new opportunities for energy efficient signal handling and quantum information processing. Recently, arrays of Josephson junction devices have been proposed as a possible solution. However, they require extremely high levels of nanofabrication. Here we introduce a new quantum superconducting metamaterial which exploits the magnetic flux quantization for switching. It does not contain Josephson junctions, making it simple to fabricate and scale into large arrays. The metamaterial was manufactured from a high-temperature superconductor and characterized in the low intensity regime, providing the first observation of the quantum phenomenon of flux exclusion affecting the far-field electromagnetic properties of the metamaterial
- …