Chiral Diarylethene Dopant Induces Large Photostimulated Pitch Change in Nematic Liquid Crystals
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dimensional impurity channel (2DIC) on the bottom side of the quantum well. The peaks observed in the IQHE regime of the 2DEG reflect the resistivity of the 2DIC (ρ 2xxDIC ). In that regime, the coupling between the two channels leads to a nonequilibrium charge distribution in the 2DEG with very long RC characteristic times, which causes the peak phenomena to last for a long time. The influence of the 2DEG nonequilibrium charge distribution on the 2DIC resistivity pattern is observed as the hysteretic peaks. The density of 2DIC, estimated at about 5.7 × 1010 cm-2, can drop below the conducting limit when a negative bias is applied to the back-side gate, causing the hysteresis to disappear. WIRAWAN PURWANTO
Lattice-Dynamics Properties May Explain Low Dielectric Constant of Thin Films Soft-mode phonons are known to play a key role in the physics of ferroelectrics. The softening of one particular phonon usually results in a dramatic increase in the dielectric constant and an appearance of the ferroelectric phase. The relative variation of the soft-mode frequency with the dielectric constant is fully described by the Lyddane–Sachs–Teller (LST) formula. Researchers from The Pennsylvania State University and Max Planck Institute for Solid State Research have reported in the March 23 issue of Nature the relationship between the lower dielectric constant of thin films and the reduced softening of the lowest optical-phonon mode. This research may explain the mechanism of low dielectric constant in thin films. After applying the dead-layer model— which takes into account interface effects and results in a decrease of the average dielectric constant—to derive a temperature-dependent dielectric constant for thin SrTiO3 (STO) films, the researchers found that the model was not able to accurately describe the measured dielectric constants. According to the researchers, latticedynamics studies are necessary for a better explanation of the lower dielectric constant of thin films. The researchers used pulsed laser deposition to prepare the samples: They heated the substrate to 720°C in an oxygen pressure of 100 mTorr, then cooled the as-deposited film to room temperature in 400 Torr of oxygen. They then deposited STO films with thicknesses of 0.5, 1, and 2 µm on a 0.35-µm conducting oxide SrRuO3 buffer layer. Although the researchers observed optical-phonon peaks in STO films by means of conventional Raman scattering,
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the soft mode (the lowest optical mode) was not detected in this way. For this reason, they carried out far-infrared ellipsometry measurements at the National Synchrotron Light Source. Taking advantage of the high brightness of the synchrotron and Fourier transform infrared (FTIR) spectroscopy, the complex dielectric functions had been directly measured in the frequency range of 30–700 cm -1 and at a temperature range of 5–300 K. From the plots of the real and imaginary parts of the measured complex dielectric constants versus wave numbers, the soft-mode optical TO1 (transverse) phonon has been clearly observed,
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