掺杂原子质量对硅声子散射的影响

本文采用分子动力学方法,重点研究了掺杂原子质量对硅晶格振动行为的影响,在原子尺度上清楚地展示出目前实际无法展现的点缺陷影响声子散射的演化过程。结果表明:同位素掺杂的存在会在特定声子频率区引起共振,使透射率显著减少,掺杂原子质量较大时共振区域明显扩大,且透射率随掺杂浓度增加下降得越快,会使材料导热性变差;掺杂原子质量相同时,共振区中最小透射率对应的频率值不随浓度变化而改变。在大于共振频率时,有明显LA到TA声子模式的转变,随掺杂原子质量的增加TA模式所占比例增大。这些结果对深入认识点缺陷对晶格导热微观机制的影响有积极意义。     

激光散射法测量烟幕质量浓度的试验研究

为实现实时测量烟幕粒子数浓度、质量浓度和烟幕粒子的粒径分布,开发一种烟幕质量浓度测量装置,该装置采用激光前向散射原理和消光法测量原理相结合的方法。通过测试数据,可以计算出某一时刻的烟幕粒子数浓度和质量浓度的平均值。结果表明:烟幕质量浓度测量装置运行正常,粒径及粒径分布测量值准确。     

利用CASTEP计算载流子有效质量的可靠性分析

基于第一性原理,在密度泛函理论(DFT)框架下,采用广义梯度近似(GGA)进行校正,利用超软赝势(USPP)代替离子势,计算了几种典型的半导体材料,包括金刚石结构的C、Si、Ge,闪锌矿结构的GaAs、GaP、InP、CdTe、CdS和纤锌矿结构的GaN、ZnO半导体材料的能带结构。结合半导体材料的载流子有效质量计算公式,对能带极值附近点进行拟合,得到了有效质量的理论计算结果,并对计算结果的可靠性进行了讨论。通过与实验数据的对比,计算结果显示了较好的一致性。     

同位素稀释质谱法测定α-淀粉酶的蛋白质含量

利用同位素稀释质谱法测定了盐溶液中α-淀粉酶的蛋白质含量.在含有α-淀粉酶的盐溶液中加入适量的苯丙氨酸标记物后进行酸水解,水解后的氨基酸经过高效液相色谱(HPLC)分离,质谱检测采用选择离子模式,分别监测苯丙氨酸(m/z=166)和标记苯丙氨酸(m/z=174)的离子,根据苯丙氨酸的含量计算α-淀粉酶的蛋白质含量,因此在测定中只需要氨基酸的标准物质就可以保证蛋白质含量测定结果的溯源性.实验中优化了酸水解的时间,采用HPLC-MS和MALDI-TOF质谱证明了淀粉酶已完全水解,并对测定结果的不确定度进行了评估.该方法可以准确测定溶液中蛋白质含量,并可用于蛋白质含量标准物质的定值.     

Iron-Based Layered Compounds: The Effect of Negative Interband Coupling

The experimental data relative to new superconductor LaFeAsO_1−x F _x can be analyzed in the framework of the two-band Eliashberg theory. With reasonable assumptions on input parameters of the theory such as electron–phonons and electron-antiferromagnetic spin density fluctuations spectral functions and the Coulomb pseudopotential, it is possible to calculate mean physical quantities such as critical temperature, superconductive gaps, superconductive differential conductance, and so on. In this model also the effect of disorder and magnetic impurities on the critical temperature is examined. The key ingredient is the negative interband coupling that can remarkably increase the critical temperature. This effect stems from the presence of an electron–phonon coupling constant smaller than the sum of the Coulomb pseudopotential and electron-antiferromagnetic spin density fluctuations coupling constant in the interband channel. The resulting superconductive state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets.      

Temperature Effect on Effective Mass of the Polaron in an Asymmetric Quantum Dot

We investigate the properties of the polaron in an asymmetric quantum dot by using an improved linear combination operator method. The relations between the mean number of phonons, the effective mass and the temperature are derived. It is found that the mean number of phonons and the effective mass are increasing functions of the temperature.     

Rashba Effect on the Effective Mass of the Polaron in a Parabolic Quantum Well

We investigate the influence of the Rashba effect on the properties of the polaron in a parabolic quantum well within the improved linear combination operator method. We discuss the relations among the polaron effective mass ratio with the vibrational frequency, the electron–phonon coupling strength, the velocity of the polaron and the Rashba parameter, respectively. Due to Rashba effect, the effective mass ratio occurs splitting. The effective mass ratio is an increase function of the vibrational frequency and the coupling strength. The change tendencies of the spin-up splitting effective mass ratio and the spin-down splitting effective mass ratio with the velocity and the Rashba parameter are opposite.     

Determination of Trace Level Iodine in Biological and Botanical Reference Materials by Isotope Dilution Mass Spectrometry

A method has been developed for the determination of trace level iodine in biological and botanical materials. The method consists of spiking a sample with ¹²⁹I, equilibration of the spike with the natural iodine, wet ashing under carefully controlled conditions, and separation of the iodine by co-precipitation with silver chloride. Measurement of the ¹²⁹I/¹²⁷I ratio is accomplished by negative thermal ionization mass spectrometry using LaB₆ for ionization enhancement. The application of the method to the certification of trace iodine in two Standard Reference Materials is described.     

Effective Mass Close to a Quantum Critical Point

The effective mass in heavy fermion materials is calculated, taking into consideration both the antiferromagnetic and ferromagnetic fluctuations. The temperature and the proximity of a quantum critical point effects are considered. The effective mass diverges at the quantum critical point and in the zero temperature limit.     

On Effective Mass of Charged Clusters in Liquid Media

General difficulties related to experimental determination of charged cluster masses M _i in liquid media are stressed (normally M _i strongly exceed typical masses m ₀ of ambient solvent molecules, M _i ?m ₀). This well-known feature deteriorates the product ω _i τ vital in different resonance phenomena by shifting it to the domain ω _i τ≤1 which is hardly suitable for the experiment. Here ω _i is one of the typical frequencies used to measure the mass M _i in the liquid (e.g., the cyclotron resonance frequency) and τ is the relevant relaxation time. Discussed in the paper are the details of the scenario allowing to extract experimental data on M _i under the conditions ω _i τ≤1 where most usual techniques are very difficult to apply.     

Physical Mechanisms for Effective Mass Enhancement in 3He

We use structural information from simulations and from variational ground state calculations for calculating the effective mass of ³He at zero temperature. It is found that the relatively large effective mass is due to a combination of several physical effects: Density fluctuations cause an effective mass enhancement due to predominantly hydrodynamic backflow. This effect is, around the Fermi momentum, a smooth function of the single particle wave number; its magnitude is consistent with the effective mass of ⁴He impurities in ³He. Spin-fluctuations, on the other hand, cause a pronounced peak of the effective mass around the Fermi wave number. We also find, consistent with earlier work, an instability of the single particle spectrum at about 2.5 k _F, this is due to the coupling to density fluctuations in the maxon region.     

The Effective Two-Particle Interaction of Cold Atoms as Derived from Bragg Scattering

We study the dynamic structure function of ultracold alkali-metal gases for large scattering lengths and momenta where corrections to the mean field approximation become important. We compare our result with the Bragg-scattering measurements in ⁸⁵Rb by Papp et al. (Phys. Rev. Lett. 101:135301, 2008) and show that these experiments set very strict limits to the shape of the effective two-particle interaction ruling out the contact and hard spheres potentials. Using the Feshbach resonance approximation we derive the effective interaction, which turns out to be very similar to the soft spheres potential in momentum space. At large scattering lengths the interaction becomes universal and could be directly measured by Bragg scattering. We also discuss the experimental conditions needed for the appearance of the maxon-roton structure in the excitation spectrum and finally show that when the scattering length becomes larger than 2000 Bohr radii the uniform gas phase undergoes a phase transition into the density wave state.     

Isotope Effect on Properties of CMR Manganites

A unique feature of incredibly large magnetoresistance and also the properties, in which electronlattice interactions play a key role, has attracted a lot of attention to manganese oxides. Oxygen isotope effect has proved to be spectacular in this system and it gives insight into many characteristics of interactions and charge transfer in this system. This review concentrates on transformation of properties induced by oxygen ¹⁶O–¹⁸O isotope substitution and mechanisms leading to this extraordinary effect.     

Evaluation of the Isotope Ratio Performance of an Axial Time-of-Flight ICP Mass Spectrometer

The isotope ratio performance of an axial time-of-flight ICP mass spectrometer (Renaissance TOF-ICPMS, LECO Corp.) was evaluated. The isotope ratio precision, expressed as the relative standard deviation (RSD) for 10 successive measurements, was evaluated using multielement standard solutions with analyte concentrations of 50-500 μg/L. The influence of the acquisition time per replicate measurement was studied by varying it between 0.5 and 300 s. For an acquisition time of 30 s per replicate and an elemental concentration of 500 μg/L, typical isotope ratio precisions of ≤0.05% RSD were obtained. The fact that this isotope ratio precision can be obtained for many ratios simultaneously is an especially attractive feature of TOF-ICPMS. In contrast to what was expected, increasing the acquisition time per replicate to values of >30 s resulted in a slightly deteriorated isotope ratio precision. At short acquisition times (<10 s), isotope ratio precisions similar to, or better than, the best values ever reported for quadrupole-based instruments were obtained. The latter observation remained valid when working with transient signals of corresponding duration. Mass discrimination was observed to be analogous to that observed with other types of ICPMS instrumentation (～1% per mass unit at midmass). The accuracy attainable was evaluated by comparing Pb isotopic results for a "natural" Pb standard solution obtained by TOF-ICPMS with those obtained by thermal ionization mass spectrometry.     

Precise Measurement of Isotope Ratios with a Double-Focusing Magnetic Sector ICP Mass Spectrometer

The potential of a commercially available double-focusing magnetic sector ICP mass spectrometer (Element, Finnigan MAT, Bremen, Germany) for precise isotope ratio measurement at the low-resolution setting (R = 300) was evaluated. Optimization of scanning conditions led to a relative standard deviation for a set of 10 consecutive 2 min measurements of ～0.1% ((206)Pb(+)/(207)Pb(+)) at signal intensities of ～200?000 counts/s (peak height). This compares favorably with the best values ever reported for quadrupole ICPMS and barely exceeds the theoretical value (counting statistics). Increasing the signal intensity to values ≥500?000 counts/s (peak height) resulted in a further reduction of the RSDs obtained (for both (25)Mg(+)/(26)Mg(+) and (206)Pb(+)/(207)Pb(+)) to typically 0.04%. These figures are remarkably better than those reported for commercially available quadrupole ICPMS systems. This improvement significantly reduces the difference between isotope ratio precision of ICPMS on one hand and those of thermal ionization mass spectrometry and plasma source multiple collector mass spectrometry on the other.     

同位素内标-UPLC-MS/MS法检测易拉罐啤酒中双酚A含量

目的为了提高金属易拉罐啤酒中双酚A (BPA)的检测能力,建立一种基于同位素内标的超高效液相色谱-串联质谱分析方法。方法样品经超声处理后除去CO₂,随后添加内标物BPA-¹³C12,用超高效液相色谱-串联质谱法进行检测分析,采用内标法定量。结果经同位素内标标准曲线校正,方法的定量限为1.00μg/L,线性方程为y=1.73x+0.206,线性相关系数为0.9992,相对标准偏差为4.68%6.63%,加标回收率为91.0%～109%。使用该方法对厦门市市售6种易拉罐啤酒样品进行了BPA污染物迁移量检测,结果显示6种啤酒中均检出BPA,平均质量浓度为1.48μg/L。结论此方法具有前处理简便快捷、检测限高、灵敏度高、准确度好等特点,能够满足金属易拉罐啤酒中低含量BPA的检测要求。     

Effective Mass of an Electron Bubble in Superfluid Helium-4

We present the results of computer simulations of the motion of an electron bubble through superfluid helium-4 when acted upon by an electric field. The simulations are based on an extended version of the Gross–Pitaevskii equation. The temperature is assumed to be sufficiently low for the drag exerted on the bubble by thermal excitations to be negligible, and the calculations are made for velocities below the critical velocitie for nucleation of vortices and roton production. We calculate the effective mass \(m*\) of the bubble and obtain results in excellent agreement with the measurements of Poitrenaud and Williams, and Ellis, McClintock, and Bowley.     

Validation of New Instrumentation for Isotope Dilution Mass Spectrometric Determination of Organic Serum Analytes

A major activity in the 20 year collaboration between the Analytical Chemistry Division at NIST and the College of American Pathologists (CAP) has been the development of highly accurate and precise “definitive” methods for important clinical analytes in human serum. Definitive methods for organic analytes use isotope dilution/gas chromatography/mass spectrometry and require a mass spectrometer capable of making highly precise measurements of the ratio between the ion intensities of a characteristic ion from the analyte of interest and its stable-isotope-labeled analog. Recently, the mass spectrometer used for 20 years for definitive method development and measurements was replaced with a modern instrument capable of automated operation, with accompanying gains in convenience and sample throughput. Switching to the new instrument required modifications of measurement protocols, acceptance criteria, and ratio calculations with background corrections to go along with automated instrument operation. Results demonstrated that the two instruments gave comparable results for measurements of both urea and cholesterol in samples from various serum-based Standard Reference Materials [SRMs] and College of American Pathologists materials.