An azo-isomerization-induced Freedericksz transition of a nematic liquid crystal

We observed an azo-isomerization-induced Freedericksz transition of the 5CB liquid crystal doped with 1% double-azo. In experiment, we found that the Freedericksz transition threshold occurred at optical intensity level of 0.3 mW/mm² for the 12.5 μm-thickness planar alignment doped 5CB, which is four orders of magnitude lower as that for the pure 5CB. Using optical phase retardation, we measured the relations between the Freedericksz transition threshold and the pump beam polarization. The maximum twist angle of liquid crystal molecules under different pump beam intensity is given. The experiment indicated that trans–cis isomerization plays an important role in the decrease of the Freedericksz transition threshold.     

Atomic force microscopy study of self-organization of chiral azobenzene derived from amino acid

A kind of chiral azobenzene amphiphile, N-[4-(4-dodecyloxyphenylazo)benzoyl]- -glutamic acid (C₁₂-Azo- -Glu; as shown in Fig. 1), was synthesized and the self-organization properties on solid substrates were investigated. C₁₂-Azo- -Glu underwent a reversible trans–cis photoisomerization in dilute solution. While the photoisomerization was suppressed on solid substrate because of the H-aggregation, indicating the formation of compact film. When C₁₂-Azo- -Glu was cast from ethanol solution onto the hydrophilic surface of mica, a stable flat-layered structure formed spontaneously in large scale. High-resolution images allowed the identification of the relative orientation of molecular rows in the ordered thin film and the crystal lattice of mica. A molecular packing model of the layered structure was proposed. There was a template effect of mica to the self-organization process. Hydrogen bonding, π–π interaction and the chiral center in the molecule played the important roles in the self-organization process. The cooperative competitive effect between them led to the highly ordered structure.     

基于咔唑-三嗪并吲哚的N-酰腙衍生物的NMR数据归属

N-酰腙类化合物在极性溶剂中存在着E/cis和E/trans两种异构体,本文利用元素分析、红外光谱、1D和2D核磁共振（NMR）波谱（包括¹H NMR、¹³C NMR、¹H-¹H COSY、¹H-¹³C HSQC、¹H-¹³C HMBC和NOESY）等技术,对基于咔唑-三嗪并吲哚的新型N-酰腙衍生物3,即2-（5H-[1,2,4]三嗪[5,6-b]吲哚-3-硫基）-N'-（9-乙基咔唑-3-亚甲基）乙酰肼的两种异构体（E/cis和E/trans）的¹H和¹³C NMR信号进行了全归属,测量了其偶合常数（J值）及两种异构体的含量,确定了其空间结构.     

Comparison of hydrodistillation and ultrasonic solvent extraction for the isolation of volatile compounds from two unifloral honeys of Robinia pseudoacacia L. and Castanea sativa L

A comparative study of ultrasound-assisted extraction (USE) with the mixture pentane:ether (1:2) and hydrodistillation (HD) with the same trapping mixture is presented for the isolation of volatile compounds from two unifloral honeys of Robinia pseudoacacia L. and Castanea sativa L. All HD isolates contained many thermally derived artefacts (especially phenylacetaldehyde with lower percentages of furfural, cis- and trans-linalool oxides and others). USE method gave the most representative profile of all honey volatiles (without artefacts). In addition, USE enabled extraction of low molecular weight semivolatile markers (especially benzoic, vanillic and phenylacetic acids) that were not extracted by HD. In this regard, low percentage of benzoic acid (0.7–7.4%), vanillic acid (0.0–1.6%) and phenylacetic acid (0.5–4.1%) was determined in Rp USE extracts, while Cs USE extracts contained phenylacetic acid (20.2–23.5%) as the major constituent with low percentage of benzoic acid (2.5–5.5%).     

Description peculiarities of viscous and elastic properties of dense molecular liquids on the basis of the model of Lennard-Jones (n−6) fluid

Results previously obtained in molecular dynamics experiment with Lennard-Jones (n−6) (L-J (n−6)) fluid were applied for the determination of viscous and elastic properties of real molecular fluids (shear viscosity coefficient and pressure). Parameters σ and of real liquids (liquid hydrocarbons) were determined by fitting p–ρ–T data of model fluids to experimental p–ρ–T data of real liquids. Using the data obtained in that way, parameters σ and viscous and elastic characteristics of real liquids were determined. The comparison of experimental and calculated viscous and elastic characteristics revealed lesser dependence of viscous properties from n in comparison with elastic properties.     

Pressure–temperature studies on conductivity of TlX(X=Cl,Br,I) compounds:: I: phase diagram. II: ionic mobility

We have measured the conductivity σ of TlX(X=Cl, Br, I) compounds up to 5.3 GPa and between 300–823 K. The σ–T dependence for all compounds can be divided into three distinct regions: (i) low temperature (LT), <400 K, with unusual negative σ–T dependence, (ii) intermediate temperature (IT), 400<650 K, with positive σ–T dependence and (iii) high temperature (HT), T>650 K, with positive σ–T dependence. The σ–T isobars were used to construct the T–P solid phase diagram for each compound. The LT region data indicate a new meta-stable phase in the 1.0–3.5 GPa range. The LT→IT transition is characterized by an inverse σ–T dependence followed by normal Arrhenius behavior up to and including the HT region. The extrapolation to 1 atm of the P-dependent boundary between IT and HT regions above 3 GPa for each compound in the P–T plot yields a value close to its respective normal (1 atm) T_melt suggesting a solid order–disorder transition type paralleling -AgI behavior. The abrupt drop in conductivity in the LT region for P between 2.5–4.1 GPa of all compounds is at variance with the Arrhenius behavior observed for unperturbed ion migration implying the appearance of a second factor overriding the Arrhenius temperature dependence. Normal Arrhenius σ–T dependence prevails in both IT and HT regions with Q_c values of 85–100 kJ mol⁻¹ and 50–75 kJ mol⁻¹, respectively. The higher conductivities at 0.4 GPa for TlBr and TlI relative to their 1 atm data and the increasing σ with P are in strong contrast to the normal σ-P behavior of TlCl. The dependence of activation volume ΔV^‡ on T for TlCl, i.e. ΔV^‡>0, shows abnormally high values with a maximum at 500 K for P<3.0 GPa but reasonable ΔV^‡ values appear above 3.0 GPa. The ΔV^‡–T dependence for both TlBr and TlI with ΔV^‡<0 is incompatible with an ion transport mechanism suggesting an electronic conduction process and implying an ionic–metallic transition at higher pressures. These contrasting conductivity features are discussed and interpreted in terms of electronegativity differences and bonding character rather than structure.     

Determination of localized states in porous silicon

We determined the density of state distribution near the Fermi level in porous silicon from the analysis of the current–voltage (J–V) and the current–thickness (J–T) characteristics in the space-charge-limited-current (SCLC) regime. The distribution exhibits a minimum density at the Fermi level, which is similar to the U-shape-trap-distribution observed in crystalline Si–SiO₂ interface or in amorphous Si. Theoretical analysis well explains both the J–V and the J–L characteristics, which implies that the current flow is entirely controlled by localized states situated at the quasi-Fermi level.     

Ti/Al p-GaN Schottky barrier height determined by C–V measurements

Data are presented on the rigorous method of capacitance–voltage (C–V) measurements to the barrier height of Ti/Al p-GaN Schottky junction. For a sample with Hall concentration of 5.5 × 10¹⁶/cm³ the upper limit of the modulation frequency leading the full response of the activated carriers is defined as 1.5 kHz from the capacitance versus modulation frequency (C–f) plot. The activation energy of the Mg acceptors determined from the temperature-dependent C–f plot is 0.12 eV. The barrier height estimated with this activation energy and the intercept voltage of the 1/C²–V plot drawn with the 1.5 kHz C–V data is 1.43 eV at 300 K and 1.41 eV at 500 K. This is the most reliable barrier height ever reported. A reliable room temperature C–V doping profile is demonstrated using the 1.5 KHz modulation, which is sensitive enough to resolve the presence of a 15 nm thin highly doped (8 × 10¹⁸/cm³) layer formed near the surface.     

Li-doping effects on the electrical properties of ZnO films prepared by the chemical-bath deposition method

Zn_1−xLi_xO thin films, with x varying from 0.0 to 0.5, successfully have been deposited on glass substrates using the chemical bath deposition (CBD) technique. J–E characteristics, DC conductivity and dielectric measurements have been carried out. These measurements were done as a function of temperature, Li concentration and applied electric field intensity. The J–E characteristics are explained in terms of the Pool–Frenkel and Schottky effects. The J–E relation and DC conductivity are strongly dependent on both the Li concentration and applied electric field intensity. Dielectric hysteresis was observed between heating and cooling runs which revealed that the dielectric constant often increases slowly in the low-temperature region, then increases faster above the phase transition.     

Mechanism of current leakage through metal/n-GaN interfaces

Detailed current–voltage–temperature (I–V–T) measurements were performed on the Schottky diodes fabricated on MOVPE-grown n-GaN layers. A large deviation from the thermionic emission (TE) transport was observed in the reverse I–V curves with a large excess leakage. From the calculation based on the thermionic-field emission (TFE) model, it was found that the tunneling plays an important role in the carrier transport across the GaN Schottky barrier even for doping densities as low as 1×10¹⁷ cm⁻³. A novel barrier-modified TFE model based on presence of near-surface fixed charges or surface states is proposed to explain the observed large reverse leakage currents.     

Effects of oxygen flow rate on the properties of HfO2 layers grown by metalorganic molecular beam epitaxy

The investigations on the properties of HfO₂ dielectric layers grown by metalorganic molecular beam epitaxy were performed. Hafnium-tetra-tert-butoxide, Hf(C₄H₉O)₄ was used as a Hf precursor and pure oxygen was introduced to form an oxide layer. The grown film was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and capacitance–voltage (C–V) and current–voltage (I–V) analyses. As an experimental variable, the O₂ flow rate was changed from 2 to 8 sccm while the other experimental conditions were fixed. The XPS spectra of Hf 4f and O 1s shifted to the higher binding energy due to the charge transfer effect and the density of trapped charges in the interfacial layer was increased as the oxygen flow rate increased. The observed microstructure indicated the HfO₂ layer was polycrystalline, and the monoclinic phases are the dominant crystal structure. From the C–V analyses, k = 14–16 and EOT = 44–52 were obtained, and the current densities of (3.2–3.3) × 10⁻³ A/cm² were measured at −1.5 V gate voltage from the I–V analyses.     

Ga primary ion ToF-SIMS fragment pattern of inorganic compounds and metals

Regularity of Ga⁺ primary ion ToF-SIMS fragment pattern of inorganic compounds is discussed. For an inorganic compound as formulated M–A, where the valence of cation M is +n and that of anion A is −p, the chemical composition of appeared ToF-SIMS fragment are M_xA_y, which satisfy the rule nx≥py+1 for positive ion fragments and nx≤py+1 for negative ones. For example, for oxide fragment of chemical composition, M_xA_y (valence of M is +n), the fragment obeys the rule nx≥2y+1 for positive ions and nx≤2y+1 for negative ones, respectively. The regularity of ToF-SIMS fragment patterns of sulfides, nitrates, sulfates etc. is discussed.     

Characterization of metal/GaN Schottky interfaces based on I–V–T characteristics

Interface properties of metal/n- and p-GaN Schottky diodes are studied by I–V–T and C–V–T measurements, and simulation of their characteristics. On the basis of the previously proposed “surface patch” model, the gross behavior of I–V–T characteristics, which includes Richardson plots together with temperature dependence of the effective Schottky barrier heights (SBHs) and n-values, can be well reproduced. Furthermore, the dependence of the true SBH on the metal work function was also deduced from high-temperature I–V curves, giving S-values of 0.28 and 0.20 for n- and p-GaN samples, respectively, and the interface Fermi level tends to be pinned at a characteristic energy of about two-third of the bandgap.     

DNA-based organic-on-inorganic semiconductor Schottky structures

A sandwich device has been fabricated from DNA molecular film by solution processing located between Al and p-type silicon inorganic semiconductor. We have performed the electrical characteristics of the device such as current–voltage (I–V), capacitance–voltage (C–V) and capacitance–frequency (C–f) at room temperature and in dark. The DNA-based structure has showed the rectifying behavior. From its optical absorbance spectrum, it has been seen that DNA has been a semiconductor-like material with wide optical band energy gap of 4.12 eV and resistivity of 1.6 × 10¹⁰ Ω cm representing a p-type conductivity.     

Effect of oxide thickness on the capacitance and conductance characteristics of MOS structures

In this work, the investigation of the interface states density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics in Au/SnO₂/n-Si (MOS) structures prepared at various SnO₂ layer thicknesses by spray deposition technique have been reported. It is fabricated five samples depending on deposition time. The thicknesses of SnO₂ films obtained from the measurement of the oxide capacitance in the strong accumulation region for MOS Schottky diodes are 37, 79, 274, 401, and 446 Å, for D1, D2, D3, D4, and D5 samples, respectively. The C–V and G–V measurements of Au/SnO₂/n-Si MOS structures are performed in the voltage range from −6 to +10 V and the frequency range from 500 Hz to 10 MHz at room temperature. It is observed that peaks in the forward C–V characteristics appeared because of the series resistance. It has been seen that the value of the series resistance R_s of samples D1 (47 Ω), D2 (64 Ω), D3 (98 Ω), D4 (151 Ω), and D5 (163 Ω) increases with increasing the oxide layer thickness. The interface state density D_it ranges from 2.40×10¹³ cm⁻² eV⁻¹ for D1 sample to 2.73×10¹² cm⁻² eV⁻¹ for D5 sample and increases with increasing the oxide layer thickness.     

Analysis of frequency-dependent series resistance and interface states of In/SiO2/p-Si (MIS) structures

In this work, the investigation of the interface state density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G/ω−V) characteristics in In/SiO₂/p-Si metal–insulator–semiconductor (MIS) structures with thin interfacial insulator layer have been reported. The thickness of SiO₂ film obtained from the measurement of the oxide capacitance corrected for series resistance in the strong accumulation region is 220 Å. The forward and reverse bias C–V and G/ω−V characteristics of MIS structures have been studied at the frequency range 30 kHz–1 MHz at room temperature. The frequency dispersion in capacitance and conductance can be interpreted in terms of the series resistance (R_s) and interface state density (D_it) values. Both the series resistance R_s and density of interface states D_it are strongly frequency-dependent and decrease with increasing frequency. The distribution profile of R_s–V gives a peak at low frequencies in the depletion region and disappears with increasing frequency. Experimental results show that the interfacial polarization contributes to the improvement of the dielectric properties of In/SiO₂/p-Si MIS structures. The interface state density value of In/SiO₂/p-Si MIS diode calculated at strong accumulation region is 1.11×10¹² eV⁻¹ cm⁻² at 1 MHz. It is found that the calculated value of D_it (≈10¹² eV⁻¹ cm⁻²) is not high enough to pin the Fermi level of the Si substrate disrupting the device operation.     

Anisotropic dielectric properties of PbYb1/2Ta1/2O3 single crystals

PbYb_1/2Ta_1/2O₃ single crystals were obtained for the first time. They were grown by the flux method. The PbOPbF₂B₂O₃ system was used as a solvent. Dielectric investigations were carried out in 1 0 0_c, 1 1 0_c and 1 1 1_c pseudocubic directions. These studies pointed to anisotropy of dielectric properties. Frequency-independent ε′(T) and ε″(T) maxima related to the antiferroelectric–paraelectric (AFE—PE) phase transition are observed for all directions at 562 K. The frequency-dependent ε′(T) and ε″(T) maxima near 400 K related to the ferroelectric (FE)–AFE phase transition are observed only in 1 1 1_c direction. The hysteresis loops were observed in this direction only. These results point that ferroelectric relaxor properties appear only in 1 1 1_c direction. We propose to consider the ferroelectric phase as ferrielectric one.     

Non-Fermi liquid regimes in U1−xMxPd2Al3(M=Y,Th)

The temperature-composition (T–x) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U_1−xM_xPd₂Al₃ (M=Y,Th) are described. The T–x phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y³⁺ and Th⁴⁺, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.     

The quantum mechanics of affine variables

We study the wrapping of N-type IIB Dp-branes on a compact Riemann surface Σ in genus g>1 by means of the Sen–Witten construction, as a superposition of N′-type IIB Dp′-brane/antibrane pairs, with p′>p. A background Neveu–Schwarz field B deforms the commutative C-algebra of functions on Σ to a non-commutative C-algebra. Our construction provides an explicit example of the N′→∞ limit advocated by Bouwknegt-Mathai and Witten in order to deal with twisted K-theory. We provide the necessary elements to formulate M(atrix) theory on this new C-algebra, by explicitly constructing a family of projective C-modules admitting constant-curvature connections. This allows us to define the g>1 analogue of the BPS spectrum of states in g=1, by means of Donaldson’s formulation of the Narasimhan–Seshadri theorem.