Synthesis of Ba0.5Sr0.5(Ti0.80Sn0.20)O3 prepared by the soft chemical method

Single-phase Ba_0.5Sr_0.5(Ti_0.80Sn_0.20)O₃ (BST:Sn) powders with perovskite structure were prepared by the soft chemical method. Infrared data indicates that the BST:Sn powder is carbonate free while Raman analysis has shown that the transversal (TO) and longitudinal (LO) optical modes tend to disappear with tin addition. The electron diffraction pattern of the BST:Sn powder showed an interplanar distance of 3.94 Å characteristic of the pseudo-cubic structure.     

Localized suppression of longitudinal-optical-phonon-exciton coupling in bent ZnO nanowires

Using confocal micro-Raman and photoluminescence spectroscopy, we studied bending effects on optical properties of individual ZnO nanowires. Raman spectroscopy shows that local tensile strain can be introduced by bending the nanowire. The strain is expected to reduce the band gap on the bent part and modify the local phonon-exciton interaction. The corresponding micro-photoluminescence spectra indicate local suppression of the longitudinal-optical (LO) phonon-exciton interaction, which is determined by the intensity ratio of the second-order LO-phonon replica of the free exciton (FX-2LO) to the first-order process (FX-1LO). Our results may provide insight into the modulation of local electrical and optical properties by deforming the nanostructures.     

Influence of the LO Phonon Effect on the Transition Rate and the Spontaneous Emission Rate in Donor-Center Quantum Dot with Spherical Gaussian Confinement Potential

Based on Lee-Low-Pines transformation, the longitudinal optical (LO) phonon effect in a donor-center quantum dot with a spherical Gaussian confinement potential is studied. The energy expressions of the ground state and the first excited state are derived by using a Pekar-type variational method, and then, a superposition state of the two-level system is constructed. On the basis of Fermi Golden Rule, two kinds of the decoherence of superposition states caused by LO phonon effects are discussed, which are the spontaneous emission of LO phonon and the quantum transition from the ground state to the excited state by absorbing a LO phonon, respectively. The numerical results show that for the former, the superposition state can be suppressed by increasing the electron–phonon coupling constant, the dielectric constant ratio, or the dispersion coefficient. For the latter, it can be used to suppress the decoherence of the superposition state by increasing the dielectric constant ratio or decreasing the electron–phonon coupling constant, or using the low-temperature environment. This work enriches and improves the theoretical scheme to suppress the decoherence of a semiconductor quantum dot qubit caused by LO phonon-related effects.

     

Fiber optic Raman sensor to monitor the concentration ratio of nitrogen and oxygen in a cryogenic mixture

A spontaneous Raman scattering optical fiber sensor was developed for a specific need of the National Aeronautics and Space Administration (NASA) for long-term detection and monitoring of the purity of liquid oxygen (LO(2)) in the oxidizer feed line during ground testing of rocket engines. The Raman peak intensity ratios for liquid nitrogen (LN(2)) and LO(2) with varied weight ratios (LN(2)/LO(2)) were analyzed for their applicability to impurity sensing. The study of the sensor performance with different excitation light sources has helped to design a miniaturized, cost-effective system for this application. The optimal system response time of this miniaturized sensor for LN(2)/LO(2) measurement was found to be in the range of a few seconds. It will need to be further reduced to the millisecond range for real-time, quantitative monitoring of the quality of cryogenic fluids in a harsh environment.     

Theoretical study of the Dick effect in a continuously operatedRamsey resonator

It is well established that passive frequency standards operated in pulsed mode may suffer a degradation of their frequency stability due to the frequency (FM) noise of the Local Oscillator (LO). In continuously operated frequency standards, it has been shown that a similar degradation of the frequency stability may arise, depending on the used modulation-demodulation scheme. In this paper, we report a theoretical analysis on the possible degradations of the frequency stability of a continuous fountain due to the LO FM noise. A simple model is developed to evaluate whether or not aliasing persists. This model is based on a continuous frequency control loop of a frequency standard using a Ramsey resonator. From this model, we derive a general formula, valid for all usual modulation-demodulation schemes, for the LO frequency fluctuations due to aliasing in closed loop operation. We demonstrate that in an ideal situation and for all usual modulation waveforms, no aliasing occurs if the half-period of modulation equals the transit time of atoms in the Ramsey resonator. We also deduce that in the same conditions, square-wave phase modulation provides the strongest cancellation of the LO instabilities in closed loop operation. Finally, we show that the “Dick formula” for the specific case of the pulsed fountain can be recovered from the model by a sampling operation     

The electrical and interface properties of metal-ferroelectric (lanthanum substituted bismuth titanate: BLT)-insulator-semiconductor (MFIS) structures with various insulators

We have investigated metal-ferroelectric-insulator semiconductor (MFIS) structures with lanthanum substituted bismuth titanate (BLT) as a ferroelectric layer and lanthanum oxide (LO) or zirconium silicate (ZSO) as an insulating buffer layer between BLT and Si substrate. The morphology of BLT films deposited on LO or ZSO oxide was not changed due to the good thermal stability of LO and ZSO films. But an interface reaction between BLT and buffer layer started at high annealing temperature (750 °C), which was confirmed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The maximum memory window was 3.59 V at a sweep voltage of 7 V with the LO film annealed at 650 °C and a thickness of 5 nm. With BLT/LO annealed at 750 °C, the window was decreased due to the reaction between the BLT film and LO. The memory window was about 1 V lower with a ZSO film because ZSO film has a lower dielectric constant than LO film. The MFIS structure annealed at 750 °C had a lower leakage current density because the electrical properties of the buffer layer (La oxide or Zr silicate) were improved by the thermal process.     

A simple analysis of the Dick effect in terms of phase noise spectral densities

In cold-atom frequency standards based on the Ramsey double interaction method, the phase noise of the interrogating signal appears as a random "end-to-end phase difference", thereby introducing frequency noise in the loop. This phenomenon is analyzed in this paper in the Fourier frequency domain, using phase noise power spectral densities S(phi)(f). In continuously operated standards, the excess noise thus introduced is servoed out in the long term to become eventually smaller than the atomic shot noise, whereas in standards with pulsed operation the phase noise around even harmonics of the pulse rate is down-converted by aliasing to base band. This latter mechanism is referred to in the literature as Dick effect. In this paper, a model of the frequency control servo system is proposed, in which the input signal is the (known) local oscillator (LO) phase noise S(phi)(f) and the output signal is the (unknown) phase noise S(phi)(f) of the standard in closed loop operation. The level of excess white frequency noise added by aliasing on the stabilized LO through the Dick effect can be related analytically to the characteristics of the free LO phase noise. From this, the stability limitation (with slope tau(-1/2)) typical of the Dick effect can then be obtained by the usual conversion formulas based on the power law model.     

Technical limitations to homodyne detection at audio frequencies

Homodyne detection relies on the beat between a relatively strong local oscillator (LO) field at the carrier frequency and a signal beam with sidebands centered around the carrier frequency. This type of signal detection, or signal readout, is widely used in quantum optics applications and is expected to be used in advanced interferometric gravitational wave detectors. We investigate experimentally the limitations to making such measurements in a laboratory environment at audio frequencies. We find that beam jitter noise, electronic noise of the photodetectors, and the LO intensity noise can limit the homodyne detection in this frequency band, and we discuss potential solutions.     

Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion

We have investigated normal and resonant Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion. Experimental results show that the normal Raman spectra consist of the conventional modes associated with wurtzite ZnO and impurity-related additional modes. Under resonant conditions, only longitudinal optical (LO) phonon scattering and its overtones are observed. The number of LO phonon lines and their relative intensity depend on the doping element and level. For the nanorods doped with Cu and Ni, we have observed LO phonon overtones up to eleventh order. This situation does not happen for the Mn-doped nanorods, which show only five LO phonon modes. By co-doping Mn and Co into the ZnO host lattice, however, the LO phonon overtones up to eleventh order are observed again. The nature of this phenomenon is explained by means of the study of XRD, TEM and photoluminescence.     

Resonant Raman scattering of a-SiNx:H

Micro-Raman measurements were carried out to investigate the microstructure of amorphous silicon–nitrogen alloy (a-SiN_x:H) samples with different N contents prepared by plasma enhanced chemical vapor deposition (PECVD). Resonant Raman effect was discovered by using 647.1- and 514.5-nm excitation wavelengths. The frequency of TO mode downshifts with increasing photon energy without varying its width, while LO mode expands to a great extent. The frequency-dependent shift of TO band is explained by heterogeneous structure model and quantum confinement model, and the width expansion of LO mode may be related to the overlapping of LA and LO bands.     

Characteristics of In_(0.52)Al_(0.48)As Grown on InP(100) Substrates by Molecular Beam Epitaxy: Growth Optimisation and Effects of Si Doping

Growth of In0.52Al0.48As epilayers on InP (100) substrates by molecular beam epitaxy at a wide range of substrate tempreatures (470~550℃) and at different Si doping levels has been carried out. Low temperature photoluminescence (PL) and double-axis X-ray diffraction (XRD) analyses shaw a strong dependence of the PL and XRD linewidths, XRD intensity ratio (Lepi/Isub), and lattice-mismatch on the substrate temperature. The X-ray diffraction peaks of samples grown at law temperatures show a composition of smaller peaks, indicating the presence of disorder due to alloy clustering. Raman scattering measurements of the same samples show an additional higher energy mode at 273 cm-1 in addition to the InAs-like and AlAs-like longitudinal-optic (LO) phonon modes. Samples doped with Si show an inverted S-shaped dependence of the PL peak energy variation with the temperature which weakens at high doping levels due to a possible reduction in the donor binding energy. Supported be observations of a reduction in both the AlAs-like and InAs-like LO phonon frequencies and a broadening of the LO phonon line shape as the doping level is increased, the PL intensity also shows in increasing degrees at higher doping levels, a temperature dependence which is characteristic of disordered and amorphous materials.     

液氧过冷用热交换器的热力设计

研究了液氧过冷器的换热性能,并对过冷器的结构型式和所需要热交换面积进行了设计计算.过冷器采用液氮作冷却剂,液氧释放的热量由液氮来吸收,从而将液氧过冷到一定温度.主要对列管型换热器进行了设计计算,并用FLUENT流体类软件对设计结果进行模拟,根据模拟结果,考虑采用强化换热措施.     

A derivation of the long-term degradation of a pulsed atomic frequency standard from a control-loop model

The phase of a frequency standard that uses periodic interrogation and control of a local oscillator (LO) is degraded by a long-term random-walk component induced by downconversion of LO noise into the loop passband. The Dick formula for the noise level of this degradation is derived from an explicit solution of an LO control-loop model.     

Anomalous Dispersion of LO Phonons inLa.1.85Sr0.15CuO4

The dispersion of the highest energy LO phonon branch in La. _1.85 Sr _0.15 CuO ₄ in the (100) direction has been reinvestigated by high resolution inelastic neutron scattering. In contrast to what has been recently reported by McQueeney et al. (Phys. Rev. Lett. 82. , 628 (1999)), we find that the dispersion is continuous throughout the Brillouin zone. These results are in conflict with the idea of charge stripe order existing on a time scale which is long compared to that of the LO frequencies. On the other hand, the strong softening and boradening of LO phonons observed on doping might be taken as a precursor phenomenon to a stripe order phase with a periodicity of about 4 lattice constants. also at     

Learning organisation and lean production: an empirical research on their relationship

This study aims at identifying the pairwise relationship between specific sets of Lean Production (LP) practices and Learning Organisation (LO) dimensions. More specifically, the association between 10 LP operational constructs and seven LO dimensions, based upon previous literature, is empirically examined. A survey-based study was carried out with 135 companies undergoing a LP implementation. The study sample was categorised according to two control variables: type of organisation and company’s LP implementation experience. Collected data was analysed through multivariate techniques. Results indicate that LP implementation and LO development are highly correlated. Additionally, findings also show that the extent of relationships varies according to the context, allowing the identification of synergistic associations and conflicting ones. The identification of these relationships provides means to better comprehend how the implementation of specific LP practices corroborates to enhancing LO capabilities within the organisation and vice-versa.     

Characterisation of one-dimensional CdS nanorods synthesised by solvothermal method

One-dimensional (1D) cadmium sulfide nanorods were successfully synthesised using cadmium nitrate and sulfur powder as starting materials and polyvinyl alcohol (PVA, MW = 1 25,000) as a capping agent in ethylenediamine as a solvent by solvothermal method at 200°C for 24 and 72 h. X-ray diffraction patterns (XRD) indicated a single phase of hexagonal wurtzite CdS structure, of which the results are in accordance with those of selected area electron diffraction (SAED). The morphologies of CdS were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which showed 1D nanorods. The length and diameter of CdS nanorods were increased when PVA was added and the reaction times were prolonged. High resolution transmission electron microscopy (HRTEM) showed that growth direction of wurtzite CdS nanorods is along [001] direction or c-axis. Raman spectra presented the 1LO and 2LO at 299.36 and 600.72 cm^?1, respectively. The 2LO/1LO intensity ratios were increased when the length of CdS nanorods became longer.     

A 60 GHz retrodirective array system with efficient power management for wireless multimedia sensor server applications

A 60 GHz retrodirective array system with efficient power management for wireless multimedia sensor server applications is proposed. The system consists of three sections: a retrodirective array, a local oscillator (LO) and a power management system. The proposed system is able to efficiently control battery power by turning off when it is not in use. In particular, the power management circuit controls the power of the LO by a rectenna for efficient battery operation. Although there is no interrogating signal, the system is in a sleeping mode where extremely low power consumption occurs. When the system is interrogated by an RF signal, the system is awaken and starts consuming full power. When the received power density is larger than 0.013 mW/cm ^{, the system turns on and starts working as a retrodirective array. Furthermore, the proposed system can avoid the use of an expensive LO by employing a fourth subharmonic mixer. Experimental data are used to verify the proposed idea.}     

Size effects on Raman spectra of small CdSe nanoparticles in polymer films

The results of a resonant Raman scattering (RRS) study of polymer-stabilized colloidal CdSe nanoparticles (NPs) are reported. The size-selective nature of the RRS is demonstrated by analysing the NP ensembles with different average size [Formula: see text] and size distribution Δd using a set of excitation wavelengths. The effect of size selection on the estimation of [Formula: see text] and Δd values from the RRS spectra is discussed, as well as some peculiarities of RRS on surface optical phonons. From the experimentally observed small variation of the I(2LO)/I(LO) ratio for 2-5?nm NPs a minor effect of [Formula: see text] on the electron-phonon coupling strength in this [Formula: see text] range is supposed.     

Quantum confinement effects in Gd-doped CdS nanoparticles prepared by chemical precipitation technique

CdS and Gd-doped CdS nanoparticles have been synthesized by chemical precipitation technique. The X-ray diffraction patterns show that the CdS and Gd-doped CdS nanoparticles exhibit hexagonal structure. The high resolution transmission electron microscope image shows that CdS and Gd-doped CdS nanoparticles have particle size lying in the range of 3.5 to 4.0 nm. Raman spectra show that 1LO, 2LO and 3LO peaks of the Gd-doped CdS nanoparticles are slightly shifted to lower wavenumber side when compared to that of CdS. Optical absorption spectra of Gd-doped CdS nanoparticles shows that absorption edge is slightly shifted towards longer wavelength side (red shift) when compared to that of CdS and this shift is due to the quantum confinement effect present in the samples.