66 research outputs found
Helicity sensitive terahertz radiation detection by dual-grating-gate high electron mobility transistors
We report on the observation of a radiation helicity sensitive photocurrent
excited by terahertz (THz) radiation in dual-grating-gate (DGG)
InAlAs/InGaAs/InAlAs/InP high electron mobility transistors (HEMT). For a
circular polarization the current measured between source and drain contacts
changes its sign with the inversion of the radiation helicity. For elliptically
polarized radiation the total current is described by superposition of the
Stokes parameters with different weights. Moreover, by variation of gate
voltages applied to individual gratings the photocurrent can be defined either
by the Stokes parameter defining the radiation helicity or those for linear
polarization. We show that artificial non-centrosymmetric microperiodic
structures with a two-dimensional electron system excited by THz radiation
exhibit a dc photocurrent caused by the combined action of a spatially periodic
in-plane potential and spatially modulated light. The results provide a proof
of principle for the application of DGG HEMT for all-electric detection of the
radiation's polarization state.Comment: 7 pages, 4 figure
Collective Charge Excitation in a Dimer Mott Insulating System
Charge dynamics in a dimer Mott insulating system, where a non-polar
dimer-Mott (DM) phase and a polar charge-ordered (CO) phase compete with each
other, are studied. In particular, collective charge excitations are analyzed
in the three different models where the internal-degree of freedom in a dimer
is taken into account. Collective charge excitation exists both in the
non-polar DM phase and the polar CO phase, and softens in the phase boundary.
This mode is observable by the optical conductivity spectra where the light
polarization is parallel to the electric polarization in the polar CO phase.
Connections between the present theory and the recent experimental results in
kappa-(BEDT-TTF)2Cu2(CN)3 are discussed.Comment: 5 pages, 4 figure
Bandwidth-controlled Mott transition in I. Optical studies of localized charge excitations
Infrared reflection measurements of the half-filled two-dimensional organic
conductors -(BEDT-TTF)Cu[N(CN)]BrCl were
performed as a function of temperature ( K) and
Br-substitution (, 40%, 73%, 85%, and 90%) in order to study the
metal-insulator transition. We can distinguish absorption processes due to
itinerant and localized charge carriers. The broad mid-infrared absorption has
two contributions: transitions between the two Hubbard bands and intradimer
excitations from the charges localized on the (BEDT-TTF) dimer. Since the
latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both
electronic and vibrational features provides a tool to disentangle these
contributions and to follow their temperature and electronic-correlations
dependence. Calculations based on the cluster model support our interpretation.Comment: 12 pages, 12 figure
Bandwidth-controlled Mott transition in kappa-(BEDT-TTF)2Cu[N(CN)2]Br{x}Cl{1-x}: Optical studies of correlated carriers
In the two-dimensional organic charge-transfer salts
kappa-(BEDT-TTF)2Cu[N(CN)2]Br{x}Cl{1-x} a systematic variation of the Br
content from x = 0 to 0.9 allows us to tune the Mott transition by increasing
the bandwidth. At temperatures below 50 K, an energy gap develops in the
Cl-rich samples and grows to approximately 1000 cm-1 for T -> 0. With
increasing Br concentration spectral weight shifts into the gap region and
eventually fills it up completely. As the samples with x = 0.73, 0.85 and 0.9
become metallic at low temperatures, a Drude-like response develops due to the
coherent quasiparticles. Here, the quasiparticle scattering rate shows a
omega^2 dependence and the effective mass of the carriers is enhanced in
agreement with the predictions for a Fermi liquid. These typical signatures of
strong electron-electron interactions are more pronounced for compositions
close to the critical value x_c \approx 0.7 where the metal-to-insulator
transition occurs.Comment: 11 pages, 12 figure
Photoexcitation-Energy-Dependent Transition Pathways from a Dimer Mott Insulator to a Metal
We theoretically study pump-photon-energy-dependent pathways of a
photoinduced dimer-Mott-insulator-to-metal transition, on the basis of
numerical solutions to the time-dependent Schr\"odinger equation for the exact
many-body wave function of a two-dimensional three-quarter-filled extended
Peierls-Hubbard model. When molecular degrees of freedom inside a dimer are
utilized, photoexcitation can weaken the effective interaction or increase the
density of photocarriers. In the organic dimer Mott insulator, -(BEDT-TTF)Cu[N(CN)]Br, the intradimer and the interdimer
charge-transfer excitations have broad bands that overlap with each other. Even
in this disadvantageous situation, the photoinduced conductivity change depends
largely on the pump photon energy, confirming the two pathways recently
observed experimentally. The characteristic of each pathway is clarified by
calculating the modulation of the effective interaction and the number of
carriers involved in low-energy optical excitations. The
pump-photon-energy-dependent pathways are confirmed to be realized from the
finding that, although the effective interaction is always and slowly weakened,
the introduction of carriers is sensitive to the pump-photon energy and
proceeds much faster.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Effects of hypodontia on craniofacial structures and mandibular growth pattern
Introduction
This study was performed to examine craniofacial structures in persons with hypodontia and to reveal any differences, that may occur, when agenetic teeth are only found in the maxilla, the mandible or in both jaws. The groups consistent of 50 children (33 girls, 17 boys) aged between 9 and 13.5 years were analyzed and assigned to three subgroups. Group 1= upper jaw hypodontia. Group 2= lower jaw hypodontia. Group 3= hypodontia in both jaws.
Material and methods
Eleven angular and three index measurements from lateral encephalographs and two linear measurements from dental blaster casts were calculated. All data was statistically analyzed, parameters with p<5% were investigated for each subgroup respectively.
Results
In comparison with standards the study group showed bimaxillary retrognathism and a reduction of the lower anterior facial height. Moreover both overbite and overjet significantly increased. Other values laid within the normal ranges. Evaluating results of the subgroups, differences in the means of SNA, SNB and overjet between the groups were observed. Analysis of the mandibular growth pattern revealed, that neither vertical nor horizontal patterns are dominant in hypodontia patients.
Conclusions
In certain dentofacial parameters differences between persons with hypodontia and such with full dentition exist. According to our findings agenetic teeth may have a negative influence on the saggital development of a jaw and the lower face and may be responsible for increased overbites. This should receive attention in orthodontic treatment of hypodontia patients
A força de atrito em braquetes plásticos e de aço inoxidável com a utilização de quatro diferentes tipos de amarração
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