Ultrasonic light storage in a quantum well: A photon assembly line

Surface acoustic waves are used to effectively convert photogenerated excitons in a piezoelectric semiconductor quantum well into long-lived electron–hole polarizations. The strong lateral piezoelectric fields ionize the excitons and spatially separate the resulting electrons and holes. The strongly reduced wave function overlap results in very long storage times for this polarization which propagates across the sample at the speed of sound. External and deliberate screening of the piezoelectric fields triggers radiative recombination after long delay times and at a remote location of the sample.     

Strong coupling between Tamm plasmon and QW exciton

We report on the strong coupling between a Tamm plasmon mode and excitons from inorganic quantum wells. The sample is formed by an AlAs/GaAlAs Bragg reflector containing InGaAs QWs in its high refractive index layers, on top of which a thin silver film is deposited. Angle resolved reflectometry experiments at low temperature (77 K) show a clear anticrossing in the dispersion relations, evidencing the strong coupling regime. The Rabi splitting amounts to 11.5 meV. Emission from low and high energy Tamm plasmon/exciton polaritons is also demonstrated. Experimental data are in very good agreement with transfer matrix simulations.     

低温下质子辐照对Y-Ba-Cu—O和La-Sr-Cu-O性质的影响

使用外延YBa₂Cu₃O₇薄膜,测量了低温下(液氮或液氦温度)质子辐照对样品输运性质的影响。不仅观察到由低温辐照引起的样品超导转变温度T_c下降及电阻率P上升,还观察到dp/dT的上升和发生金属至非金属的相变。经室温热循环的样品显示出强烈室温退火效应。近60％由辐照引起的样品电阻升高和T_c下降可以得到恢复,并可伴随发生非金属至金属的相变,在La-Sr-Cu-O薄膜的低温辐照实验中观察到了类似的室温退 关键词：     

Emission of positive ions from KCl and NaCl crystals exposed to high electric fields

It is demonstrated by means of combined optical microscopy and field desorption mass spectrometry that alkali ions and cluster ions are emitted from microcrystals of KCl and NaCl exposed to positive high electric fields. The ion emission is very strong at temperatures where the sublimation of the sample is optically observable, indicating that an enhanced sample sublimation is a key precondition for enhanced ion emission.     

Narrow domain wall propagation and metamagnetic properties of a Tb0.5Y0.5Ni single crystal

The non-collinear magnetic structure of Tb_0.5Y_0.5Ni shows a ferromagnetic component associated with an antiferromagnetic one. At very low temperatures the domain walls are narrow and frozen by the anisotropy. Magnetization processes studied on a spherical monocrystalline sample provide the experimental evidence that, when the anisotropy is strong enough, the narrow domain wall propagation as well as the metamagnetic process depends only on exchange interactions.     

Anisotropy in the nonlinear absorption of elongated silver nanoparticles in silica, probed by femtosecond pulses

We present measurements of the anisotropy of the nonlinear absorption of a silica matrix doped with aligned spheroidal silver nanoparticles, produced by a double ion-implantation process of silver nanoparticles followed by an irradiation with Si ions. The nonlinear response was studied using the z-scan technique with fs pulses at 530 nm, which lies very close to the surface-plasmon absorption peak of the sample. The observed saturable absorption is studied for different angles of the input linear polarization of the pulses, showing a strong anisotropy, which is consistent with the fact that the nanorods are aligned.     

Mössbauer characterization of surface-coated magnetic nanoparticles for applications in transformers

In this study the synthesis of very small magnetite nanoparticles is reported. Analysis of the (311) X-ray diffraction line indicates nanoparticle with 2.2 nm average diameter. The as-synthesized nanosized magnetite sample was submitted to a chemical oxidation process for conversion to maghemite. However, the yielding of the chemical oxidation process, as indicated by the analysis of the Mössbauer data, was about 20%. This finding indicates a strong size-dependence of the oxidation process, reducing the yielding of the chemical process used as the nanoparticle size reduces down to extremely low values.     

Stability of Bose-Einstein condensates of hot magnons in yttrium iron garnet films

We investigate the stability of the recently discovered room-temperature Bose-Einstein condensate (BEC) of magnons in yttrium iron garnet films. We show that magnon-magnon interactions depend strongly on the external field orientation, and that the BEC in current experiments is actually metastable-it only survives because of finite-size effects, and because the BEC density is very low. On the other hand a strong field applied perpendicular to the sample plane leads to a repulsive magnon-magnon interaction; we predict that a high-density room-temperature magnon BEC should then form in this perpendicular field geometry.     

基于强影响度的近红外奇异样本识别算法研究

校正样本选择以及奇异样本剔除对于近红外光谱定量和定性建模非常重要。现有的识别奇异样本的方法一般都基于数据重心估计,需要一个经验的判断阈值,在很大程度上限制了其识别准确性和实用性。针对现有方法奇异样本识别准确率低的问题,改进了一种现有度量尺度-杠杆值,构造出一种新的基于强影响度的奇异样本识别算法。这种度量尺度在一定程度上减少了对数据重心的依赖,使正常样本更加聚集,拉开了奇异样本与正常样本的距离;同时,为了避免人工根据经验设定阈值的不合理性,引入统计学领域中跳跃度的概念,提出了一种自动阈值设定方法判别奇异样本。为了验证该算法的有效性,利用马氏距离、杠杆值-光谱残差法与该算法分别对200个代表性校正集样本中的异常样品进行剔除,然后通过偏最小二乘法(PLS)对剩余的校正集样本(以烟碱为指标)定量建模,并对60个代表性测试集样本进行预测,以交互验证均方根误差(RMSECV)、相关系数(r)和预测均方根误差(RMSEP)为评价指标比较各算法的优劣。实验对比结果表明,基于强影响度的奇异样本识别算法较现有方法明显提高了奇异样本识别的准确率,具有较低的RMSECV(0.104),RMSEP(0.112)以及较高的R(0.983),提高了模型的稳定性和预测能力。     

1.54 μm room temperature emission from Er-doped Si nanocrystals deposited by ECR-PECVD

In this work, silicon nanocrystals (Si-nc) embedded in a silicon-rich silicon oxide (SRSO) matrix doped with Er³⁺ ions for different erbium and silicon concentrations have been deposited by electron-cyclotron resonance plasma-enhanced chemical-vapor-deposition (ECR-PECVD) technique. Their optical properties have been investigated by photoluminescence (PL) and reflectance spectroscopy.Room temperature emission bands centered at ∼1.54 and at 0.75 μm have been obtained for all samples. The most intense emission band at ∼1.54 μm was obtained for samples with concentrations of 0.45% and 39% for erbium and silicon, respectively. Moreover, it has been found that the broad emission band centered at ∼0.75 μm for all samples shows a very strong interference pattern related to the a specific sample structure and a high sample quality.     

Robust estimation of the cerebral blood flow in arterial spin labelling

The introduction of arterial spin labelling (ASL) techniques in magnetic resonance imaging (MRI) has made feasible a non-invasive measurement of the cerebral blood flow (CBF). However, to date, the low signal-to-noise ratio of ASL gives us no option but to repeat the acquisition to accumulate enough data in order to get a reliable signal. The perfusion signal is then usually extracted by averaging across the repetitions. But the sample mean is very sensitive to outliers. A single incorrect observation can therefore be the source of strong detrimental effects on the perfusion-weighted image estimated with the sample mean.     

Interacting electrons in disordered potentials: The inverse compressibility

The inverse compressibility, i.e., the change in the chemical potential as the number of particles in the sample is changed, is studied for a small quantum dot. It is found that the inverse compressibility behaves differently for different values of disorder and electron-electron interactions. For weak interactions or strong disorder one may understand this behavior in the framework of a random matrix theory.I am grateful to A. Auerbach, M. Feild, B. I. Shklovskii, and U. Sivan for very useful discussions and to the US-Israel Binational Science Foundation for financial support.     

Crystallization behavior of iron-based amorphous nanoparticles prepared sonochemically

In general, a rapid quenching is required to obtain an amorphous metal. It is known that an intensive ultrasonication generates a very high temperature within cavitation bubbles in a very short moment, which enables a rapid quenching process in a liquid phase synthesis. In this study, the sonochemically-derived “amorphous iron” from Fe(CO)₅ was carefully examined by XRD, TEM, TG-DTA. The product was found to be an amorphous containing a certain amount (∼15%) of volatile component that can be removed by heating in a nitrogen flow. After annealed in the inert atmosphere at 600 °C, cooled down to room temperature, and then exposed in air (oxygen), the sample showed a strong exotherm accompanied by a weight gain. This is due to oxidation of fine metallic iron. Experimental operations of such a reactive material were examined.     

Variable-range hopping conduction in doped germanium at very low temperatures and high magnetic fields

The conductivity of doped Ge below the metal-insulator transition is measured at temperatures between 4 K and 40 mK and in magnetic fields up to 7 Tesla. In zero field the resistivity exponent diverges asT ^–1/2. In weak fields the magnetoresistance increases asB ² and becomes exponentially large in strong fields and at low temperatures. The results can be described quantitatively in terms of variable-range hopping between localized states having a Coulomb gap in the density of states at the Fermi level. The magnetoresistance is calculated for arbitrary fields by means of a quasi-classical method. A fit to the data gives the radius of the localized states and the density of states. The sample is found to be very close to the metal-insulator transition. A small increase of the binding energy is observed in strong fields.     

Fabrication and magnetic properties of amorphous Co0.71Pt0.29 nanowire arrays

Highly ordered Co_0.71Pt_0.29 alloy nanowire arrays have been fabricated successfully by direct current electro-deposition into the pores of a porous anodic aluminum oxide (AAO) template. SEM and TEM images reveal that the nanowires of array are uniform, well isolated, and parallel to one another. The aspect ratio of nanowires is over 200. XRD and EDS pattern indicates that amorphous Co_0.71Pt_0.29 structure was formed during electro-deposition. In amorphous sample, magnetocrystal anisotropy is very small, therefore, shape anisotropy plays a dominant role which leads to strong perpendicular anisotropy. High coercivity (Hc=1.7 kOe) and squareness (Mr/Ms) around 0.7 were obtained in the samples when the field was applied parallel to the axis of the nanowires. However, when it changed to polycrystalline structure after annealing, due to the competition of magnetocrystal anisotropy and shape anisotropy, the sample did not display perpendicular anisotropy.     

Cold ignition of combustion-like waves of cryo-chemical reactions in solids

Fronts of weakly exothermal chemical reaction may propagate in solids at very low temperatures ( 4K≤T≤77K) thanks to a quite unusual mechanism, involving a feedback between the heat produced by the reaction and the disruption of the solid matrix. In this class of phenomena, the reaction may be induced by mechanical constraints, without a large elevation of temperature. On the basis of a simple phenomenological model, we investigate ignition of a propagating front by initially (i) disrupting a localized zone of the solid matrix, or by (ii) introducing a temperature jump, leading to a thermal shock with strong temperature gradients. In particular, we show that reaction can be initiated by disrupting only a very small fraction of the sample. Applications to the problem of initiation of solid explosives by friction or shocks is briefly discussed. Received 26 January 2001 and Received in final form 3 May 2001     

Magnetic anisotropy of textured CrO2 thin films investigated by X-ray magnetic circular dichroism

Highly a-axis-textured CrO₂ films have been deposited on Al₂O₃ (0001) substrates by chemical vapor deposition. CrO₂ has been found to have highly a-axis (010)-oriented columnar growth on a Cr₂O₃ (0001) initial layer. The six-fold surface symmetry of the Cr₂O₃ initial layer leads to three equivalent in-plane orientations of the a-axis-oriented CrO₂ unit cell. We report Cr L_2,3 X-ray magnetic circular dichroism data along the surface normal and at 60° off-normal sample orientation. For a 60° sample alignment, a strong increase of the projected orbital moment could be observed for unoccupied majority t_2g states using moment analysis. Therefore, the c axis is identified as the intrinsic magnetic easy axis of CrO₂. In addition, a small spin moment and a very strong magnetic dipole term T_z have been found. Received: 8 January 2002 / Accepted: 8 January 2002     

NMR strong off-resonance irradiation without sample overheating

A prototype NMR probe for long RF pulse has been constructed. Its main elements are two coils mounted in the concentric position. The first bigger coil is wound around a glass dewar tube and the second smaller coil is placed inside the dewar. These two coils are thermally isolated by the dewar. A long and strong RF pulse is applied to the bigger coil. The smaller detection coil inside the dewar contains a sample and to this coil a short RF pulse is applied. The two coils are independently tuned and electrically isolated. During the operation of the strong RF pulse the smaller coil has a high resistance to ground (very low Q factor) and does not absorb energy from the bigger coil. During the operation of the short on-resonance RF pulse the bigger coil is detuned to a higher frequency, but the resonance circuit with the small coil is in the electrical resonance. The NMR probe may be used in off-resonance experiments in which long and strong RF pulses are applied to the bigger coil and thereby the problem of the sample overheating is avoided.     

两步法制备空间取向高度一致的ZnO纳米棒阵列

采用两步法,即先用磁控溅射在Si(100)表面生长一层ZnO籽晶层、再利用液相法制备空间取向高度一致的ZnO纳米棒阵列.用扫描电子显微镜、X射线衍射、高分辨透射电子显微镜和选区电子衍射对样品形貌和结构特征进行了表征.结果表明,ZnO纳米棒具有垂直于衬底沿c轴择优生长和空间取向高度一致的特性和比较大的长径比,X射线衍射的(XRD)(0002)峰半高宽只有0.06°,选区电子衍射也显示了优异的单晶特性.光致发光谱表明ZnO纳米棒具有非常强的紫外本征发光和非常弱的杂质或缺陷发光特性. 关键词： ZnO纳米棒阵列 ZnO籽晶层 两步法 液相生长     

Measurement of the magnetic field of resin-impregnated bulk superconductor annuli

Large single-grain bulk RE–Ba–Cu–O (RE: rare earth elements) superconductors can trap large fields exceeding several teslas and thus can function as very strong quasi-permanent magnets. We have found that the resin can penetrate into a bulk superconductor, when the sample was immersed in molten resin. Hence, resin impregnation was effective in improving mechanical properties. Three bulk superconductors‘ annuli with resin impregnation, each 50-mm i.d. and 80-mm o.d. was built and energized, by a field-cool method, to generate, in a bath of liquid nitrogen, a persistent trapped field of 1.62 T.