430 research outputs found
Pseudospin for Raman D Band in Armchair Graphene Nanoribbons
By analytically constructing the matrix elements of an electron-phonon
interaction for the band in the Raman spectra of armchair graphene
nanoribbons, we show that pseudospin and momentum conservation result in (i) a
band consisting of two components, (ii) a band Raman intensity that is
enhanced only when the polarizations of the incident and scattered light are
parallel to the armchair edge, and (iii) the band softening/hardening
behavior caused by the Kohn anomaly effect is correlated with that of the
band. Several experiments are mentioned that are relevant to these results. It
is also suggested that pseudospin is independent of the boundary condition for
the phonon mode, while momentum conservation depends on it.Comment: 25 pages, 5 figure
A numerical investigation of a piezoelectric surface acoustic wave interaction with a one-dimensional channel
We investigate the propagation of a piezoelectric surface acoustic wave (SAW)
across a GaAs/AlGaAs heterostructure surface, on which there is
fixed a metallic split-gate. Our method is based on a finite element
formulation of the underlying equations of motion, and is performed in
three-dimensions fully incorporating the geometry and material composition of
the substrate and gates. We demonstrate attenuation of the SAW amplitude as a
result of the presence of both mechanical and electrical gates on the surface.
We show that the incorporation of a simple model for the screening by the
two-dimensional electron gas (2DEG), results in a total electric potential
modulation that suggests a mechanism for the capture and release of electrons
by the SAW. Our simulations suggest the absence of any significant turbulence
in the SAW motion which could hamper the operation of SAW based quantum devices
of a more complex geometry.Comment: 8 pages, 8 figure
Mesoscopic spin confinement during acoustically induced transport
Long coherence lifetimes of electron spins transported using moving potential
dots are shown to result from the mesoscopic confinement of the spin vector.
The confinement dimensions required for spin control are governed by the
characteristic spin-orbit length of the electron spins, which must be larger
than the dimensions of the dot potential. We show that the coherence lifetime
of the electron spins is independent of the local carrier densities within each
potential dot and that the precession frequency, which is determined by the
Dresselhaus contribution to the spin-orbit coupling, can be modified by varying
the sample dimensions resulting in predictable changes in the spin-orbit length
and, consequently, in the spin coherence lifetime.Comment: 10 pages, 2 figure
The onset of X-ray emission in young stellar objects: a Chandra observation of the Serpens star-forming region
AIMS: To study the properties of X-ray emissions from young stellar objects
(YSOs), through their evolution from Class I to Class III and determine whether
Class 0 protostars emit in X-rays. METHODS: A deep Chandra X-ray observation of
the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally
suited for this type of investigation, being populated by a dense and extremely
young cluster whose members are found in all different evolutionary stages,
including six well studied Class 0 sources. RESULTS: None of the six Class 0
protostars is detected in our observations, excluding the presence of sources
with X-ray luminosities > 0.4 10^30 erg/s (for column densities of the order of
4 10^{23} cm^-2, or A_V ~ 200). A total of 85 X-ray sources are detected and
the light curves and spectra of 35 YSOs are derived. There is a clear trend of
decreasing absorbing column densities as one moves from Class I to Class III
sources, and, possibly, evidence of decreasing plasma temperatures, too. We
observe a strong, long-duration, flare from a Class II low-mass star, for which
we derive a flaring loop length of the order of 20 stellar radii. We interpret
the flaring event as originating from a magnetic flux tube connecting the star
to its circumstellar disk. The presence of such a disk is supported by the
detection, in the spectrum of this star, of 6.4 keV Fe fluorescent emission.Comment: Accepted for publication in A&
Spin diffusion and injection in semiconductor structures: Electric field effects
In semiconductor spintronic devices, the semiconductor is usually lightly
doped and nondegenerate, and moderate electric fields can dominate the carrier
motion. We recently derived a drift-diffusion equation for spin polarization in
the semiconductors by consistently taking into account electric-field effects
and nondegenerate electron statistics and identified a high-field diffusive
regime which has no analogue in metals. Here spin injection from a ferromagnet
(FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying
this spin drift-diffusion equation to several typical injection structures such
as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field
regime spin injection from a ferromagnet into a semiconductor is enhanced by
several orders of magnitude. For injection structures with interfacial
barriers, the electric field further enhances spin injection considerably. In
FM/NS/FM structures high electric fields destroy the symmetry between the two
magnets at low fields, where both magnets are equally important for spin
injection, and spin injection becomes locally determined by the magnet from
which carriers flow into the semiconductor. The field-induced spin injection
enhancement should also be insensitive to the presence of a highly doped
nonmagnetic semiconductor (NS) at the FM interface, thus FM/NS/NS
structures should also manifest efficient spin injection at high fields.
Furthermore, high fields substantially reduce the magnetoresistance observable
in a recent experiment on spin injection from magnetic semiconductors
Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves
Individual self-assembled Quantum Dots and Quantum Posts are studied under
the influence of a surface acoustic wave. In optical experiments we observe an
acoustically induced switching of the occupancy of the nanostructures along
with an overall increase of the emission intensity. For Quantum Posts,
switching occurs continuously from predominantely charged excitons (dissimilar
number of electrons and holes) to neutral excitons (same number of electrons
and holes) and is independent on whether the surface acoustic wave amplitude is
increased or decreased. For quantum dots, switching is non-monotonic and shows
a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of
positively charged and neutral excitons is observed at high surface acoustic
wave amplitudes. These findings are explained by carrier trapping and
localization in the thin and disordered two-dimensional wetting layer on top of
which Quantum Dots nucleate. This limitation can be overcome for Quantum Posts
where acoustically induced charge transport is highly efficient in a wide
lateral Matrix-Quantum Well.Comment: 11 pages, 5 figure
PoboljĆĄan postupak sinteze nekih novih 1,3-diaril-2-propen-1-ona koristeÄi PEG-400 kao reciklirajuÄe otapalo i njihovo antimikrobno vrednovanje
A simple and convenient route is described for the synthesis of novel hetero 1,3-diaryl-2-propen-1-ones (chalcones) by using recyclable poly PEG-400 as an alternative reaction solvent. The reaction is clean with excellent yield, shorter reaction time and reduces the use of volatile organic compounds (VOCs). All the synthesized compounds were evaluated for their antimicrobial activities against several pathogenic representatives.Opisana je jednostavna i pogodna metoda sinteze novih hetero 1,3-diaril-2-propen-1-ona (kalkona) koristeÄi poli(etilenglikol) (PEG-400) kao alternativno otapalo. Reakcija je jednoznaÄna, a uporaba hlapljivih organskih otapala je smanjena. IskoriĆĄtenja na produktima su visoka, a reakcijska vremena kraÄa. Svi sintetizirani spojevi testirani su na antimikrobno djelovanje na nekoliko patogenih mikroorganizama
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