94 research outputs found
Study of the generator/motor operation of induction machines in a high frequency link space power system
Static power conversion systems have traditionally utilized dc current or voltage source links for converting power from one ac or dc form to another since it readily achieves the temporary energy storage required to decouple the input from the output. Such links, however, result in bulky dc capacitors and/or inductors and lead to relatively high losses in the converters due to stresses on the semiconductor switches. The feasibility of utilizing a high frequency sinusoidal voltage link to accomplish the energy storage and decoupling function is examined. In particular, a type of resonant six pulse bridge interface converter is proposed which utilizes zero voltage switching principles to minimize switching losses and uses an easy to implement technique for pulse density modulation to control the amplitude, frequency, and the waveshape of the synthesized low frequency voltage or current. Adaptation of the proposed topology for power conversion to single-phase ac and dc voltage or current outputs is shown to be straight forward. The feasibility of the proposed power circuit and control technique for both active and passive loads are verified by means of simulation and experiment
Ultrafast photoinduced enhancement of nonlinear optical response in 15-atom gold clusters on indium tin oxide conducting film
We show that the third order optical nonlinearity of 15-atom gold clusters is
significantly enhanced when in contact with indium tin oxide (ITO) conducting
film. Open and close aperture z-scan experiments together with non-degenerate
pump-probe differential transmission experiments were done using 80 fs laser
pulses centered at 395 nm and 790 nm on gold clusters encased inside
cyclodextrin cavities. We show that two photon absorption coefficient is
enhanced by an order of magnitude as compared to that when the clusters are on
pristine glass plate. The enhancement for the nonlinear optical refraction
coefficient is ~3 times. The photo-induced excited state absorption using
pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also
show an enhancement by an order of magnitude. These results attributed to the
excited state energy transfer in the coupled gold cluster-ITO system are
different from the enhancement seen so far in charge donor-acceptor complexes
and nanoparticle-conjugate polymer composites.Comment: To appear in Optics Express (2013);
http://dx.doi.org/10.1364/OE.21.00848
Phonon Anomalies, Orbital-Ordering and Electronic Raman Scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: Temperature-dependent Raman Study
We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a
wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal
to orthorhombic structural transition as well as magnetic transition at Tsm ~
160K. The mode frequencies of two first-order Raman modes B1g and Eg, both
involving displacement of Fe atoms, show sharp increase below Tsm.
Concomitantly, the linewidths of all the first-order Raman modes show anomalous
broadening below Tsm, attributed to strong spin-phonon coupling. The high
frequency modes observed between 400-1200 cm-1 are attributed to the electronic
Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The
splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which
increases as temperature decreases below Tsm. A broad Raman band observed at ~
3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d
antiferromagnet.Comment: Accepted for Publication in JPC
Anomalous Raman scattering from phonons and electrons of superconducting FeSe
We report interesting anomalies in the temperature dependent Raman spectra of
FeSe measured from 3K to 300K in the spectral range from 60 to 1800
cm and determine their origin using complementary first-principles
density functional calculations. A phonon mode near 100 cm exhibits a
sharp increase by 5% in frequency below a temperature T ( 100
K) attributed to strong spin-phonon coupling and onset of short-range
antiferromagnetic order. In addition, two high frequency modes are observed at
1350 cm and 1600 cm, attributed to electronic Raman scattering
from ()to / -orbitals of Fe.Comment: 19 pages, 4 figures, 1 tabl
Superconducting Fluctuations and Anomalous Phonon Renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2
Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of
5 K to 300 K, covering the superconducting transition temperature Tc ~ 28.3 K,
reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2
% and linewidth increases by ~ 175 % at To ~ 60 K. Below To, anomalous
softening of the mode frequency and a large decrease by ~ 10 cm-1 in the
linewidth is observed. These precursor effects at T0 (~ 2Tc) are attributed to
significant superconducting fluctuations, possibly enhanced due to reduced
dimensionality arising from weaked coupling between the well separated (~ 15
{\AA}) Fe-As layers in the unit cell. A large blue-shift of the mode frequency
between 300 K to 60 K (~7%) indicates strong spin-phonon coupling in this
superconductor.Comment: Iron based Superconducto
Raman evidence for Orbiton-Mediated Multiphonon Scattering in Multiferroic TbMnO
Temperature-dependent Raman spectra of TbMnO from 5 K to 300 K in the
spectral range of 200 to 1525 cm show five first-order Raman allowed
modes and two high frequency modes. The intensity ratio of the high frequency
Raman band to the corresponding first order Raman mode is nearly constant and
high ( 0.6) at all temperatures, suggesting a orbiton-phonon mixed nature
of the high frequency mode. One of the first order phonon modes shows anomalous
softening below T ( 46 K), suggesting a strong spin-phonon coupling.Comment: 14 pages, 3 figures, 1 tabl
Coupled Phonons, Magnetic Excitations and Ferroelectricity in AlFeO3: Raman and First-principles Studies
We determine the nature of coupled phonons and magnetic excitations in AlFeO3
using inelastic light scattering from 5 K to 315 K covering a spectral range
from 100-2200 cm-1 and complementary first-principles density functional
theory-based calculations. A strong spin-phonon coupling and magnetic ordering
induced phonon renormalization are evident in (a) anomalous temperature
dependence of many modes with frequencies below 850 cm-1, particularly near the
magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band
positions of high frequency Raman bands between 1100-1800 cm-1, in particular a
broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon
Raman scattering. We also observe weak anomalies in the mode frequencies at ~
100 K, due to a magnetically driven ferroelectric phase transition.
Understanding of these experimental observations has been possible on the basis
of first-principles calculations of phonons spectrum and their coupling with
spins
Temperature-dependent Raman study of CeFeAsO0.9F0.1 Superconductor: Crystal field excitations, phonons and their coupling
We report temperature-dependent Raman spectra of CeFeAsO0.9F0.1 from 4 K to
300 K in spectral range of 60 to 1800 cm-1 and interpret them using estimates
of phonon frequencies obtained from first-principles density functional
calculations. We find evidence for a strong coupling between the phonons and
crystal field excitations; in particular Ce3+ crystal field excitation at 432
cm-1 couples strongly with Eg oxygen vibration at 389 cm-1 . Below the
superconducting transition temperature, the phonon mode near 280 cm-1 shows
softening, signaling its coupling with the superconducting gap. The ratio of
the superconducting gap to Tc thus estimated to be ~ 10 suggests CeFeAsO0.9F0.1
as a strong coupling superconductor. In addition, two high frequency modes
observed at 1342 cm-1 and 1600 cm-
Review of Graphene Technology and Its Applications for Electronic Devices
Graphene has amazing abilities due to its unique band structure characteristics defining its enhanced electrical capabilities for a material with the highest characteristic mobility known to exist at room temperature. The high mobility of graphene occurs due to electron delocalization and weak electron–phonon interaction, making graphene an ideal material for electrical applications requiring high mobility and fast response times. In this review, we cover graphene’s integration into infrared (IR) devices, electro-optic (EO) devices, and field effect transistors (FETs) for radio frequency (RF) applications. The benefits of utilizing graphene for each case are discussed, along with examples showing the current state-of-the-art solutions for these applications
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