Melting of heterogeneous vortex matter: The vortex ‘nanoliquid’

Disorder and porosity are parameters that strongly influence the physical behavior of materials, including their mechanical, electrical, magnetic and optical properties. Vortices in superconductors can provide important insight into the effects of disorder because their size is comparable to characteristic sizes of nanofabricated structures. Here we present experimental evidence for a novel form of vortex matter that consists of inter-connected nanodroplets of vortex liquid caged in the pores of a solid vortex structure, like a liquid permeated into a nanoporous solid skeleton. Our nanoporous skeleton is formed by vortices pinned by correlated disorder created by high-energy heavy ion irradiation. By sweeping the applied magnetic field, the number of vortices in the nanodroplets is varied continuously from a few to several hundred. Upon cooling, the caged nanodroplets freeze into ordered nanocrystals through either a first-order or a continuous transition, whereas at high temperatures a uniform liquid phase is formed upon delocalization-induced melting of the solid skeleton. This new vortex nanoliquid displays unique properties and symmetries that are distinct from both solid and liquid phases.     

Development of surface magneto-optical imaging method

We have observed single vortices in a NbTiN thin film by using magneto-optical (MO) imaging method, in which the light reflected from the surface of the sample produces MO contrasts. The gap between the sample and the indicator film is estimated by the Newton ring and curve fitting to the flux density profile of single vortices.     

Novel inspection of welded joint microstructure using magneto-optical imaging technology

A novel method for measuring differences of microstructure by advanced use of the Faraday magneto-optical effect is proposed. Two groups of YAG laser welds on Q235 have been investigated in order to compare MO imaging and traditional methods. Microstructure images have been compared with MO images, and MO diagrams display different colors and gray scales for the base metal, the weld zone, and the heat affected zone. Experimental results indicate that the welded joint microstructure can be inspected by MO imaging without metallographic preparation.     

Measurement of micro weld joint position based on magneto-optical imaging

In a laser butt joint welding process,it is required that the laser beam focus should be controlled to follow the weld joint path accurately.Small focus wandering off the weld joint may result in insufficient penetration or unacceptable welds.Recognition of joint position offset,which describes the deviation between the laser beam focus and the weld joint,is important for adjusting the laser beam focus and obtaining high quality welds.A new method based on the magneto-optical(MO)imaging is applied to measure the micro weld joint whose gap is less than 0.2 mm.The weldments are excited by an external magnetic field,and an MO sensor based on principle of Faraday magneto effect is used to capture the weld joint images.A sequence of MO images which are tested under different magnetic field intensities and different weld joint widths are acquired.By analyzing the MO image characteristics and extracting the weld joint features,the influence of magnetic field intensity and weld joint width on the MO images and detection of weld joint position is observed and summarized.     

Bitter decoration and magneto-optical observations of vortex chains in high temperature superconductors

In tilted magnetic fields, vortices in anisotropic superconductors form one-dimensional arrangements, called vortex chains. We have visualized vortex chains by Bitter decoration and magneto-optical technique. The fundamental energy scale for the attractive interaction between pancake and Josephson vortices is evaluated by observing vortex chains under various conditions. We also explore how the vortex chains evolve when the large in-plane field is applied or when the anisotropy parameter of the system is changed     

Non-equilibrium melting of colloidal crystals in confinement

A novel and flexible experiment is reported for investigation of the non-equilibrium melting behaviour of model crystals made from charged colloidal spheres. In a slit geometry, polycrystalline material formed in a low salt region is driven by hydrostatic pressure up an evolving gradient in salt concentration and melts at large salt concentration. Depending on particle and initial salt concentration, driving velocity and the local salt concentration, complex morphologic evolution is observed. Crystal–melt interface positions and the melting velocity are obtained quantitatively from time-resolved Bragg and polarisation microscopic measurements. A simple theoretical model predicts the interface to first advance, then for balanced drift and melting velocities to become stationary at a salt concentration larger than the equilibrium melting concentration. It also describes the relaxation of the interface to its equilibrium position in a stationary gradient after stopping the drive in different manners. The influence of the gradient strength on the resulting interface morphology and a shear-induced morphologic transition from polycrystalline to oriented single crystalline material before melting are discussed.     

Observation of hotspot in BSCCO thin film structure by fluorescent thermal imaging

Hotspot formation is observed in a structured thin superconducting film of Bi₂Sr₂CaCu₂O_x (BSCCO) using the fluorescent thermal imaging technique. The BSCCO film is deposited on SrTiO₃ (STO) and has a superconducting transition at 80 K. A film of rare-earth doped polymer film deposited directly on the superconductor is used as thermal sensor down to 4 K.     

磁通格子的有序-无序相变和反向熔化

考虑了无序钉扎、热涨落和磁通互作用, 用Monte Carlo分子动力学模拟方法研究二维磁通格子在无序钉扎强度和温度空间的相图, 以及由布拉格玻璃相到非晶磁通玻璃相和到磁通液体相的有序-无序相变.为了决定磁通格子的序，计算了静态结构因子和磁通格子位形的有限尺寸指数.计算结果表明，Bragg玻璃相在低温的无序磁通玻璃相和高温的磁通液体相之间 , 表现出磁通格子的反向熔化行为.分析后认为，这一反向熔化行为起因于磁通之间互作用的温度效应. 关键词： Ⅱ类超导体 磁通格子 相图 结构因子 反向熔化     

基于横向磁光效应磁化等离子体光子晶体的光子带隙特性

通过外加与电磁波传播方向垂直的磁场来调控等离子体光子晶体的光子带隙结构。采用时域有限差分方法数值分析了由本征层为等离子体层和其他电介质材料层交替堆叠而成的磁化等离子体光子晶体的光子带隙结构和频谱特性。数值结果表明,光子晶体中等离子体的介电常数随着外磁场大小的变化而改变,从而使磁化等离子体光子晶体的带隙特性在一定的频率范围相应地得到调节。     

等轴晶在过热溶液落管中的下落熔化特性研究

研究了NH4Cl等轴晶在过热NH4Cl-70 wt％H2O溶液落管内熔化中的形貌演化过程,并分析了溶液过热度及等轴晶初始尺寸对其下落速度和熔化速率的影响规律和机理.结果表明:等轴晶在过热溶液中下落时,若不发生旋转,其形貌由准轴对称形貌演变成非轴对称准三角形貌;若发生旋转,则有助于保持其准轴对称形貌.通过分析不同条件下等轴晶下落时的阻力系数,认为提高溶液过热度可获得更为光顺的等轴晶外形,增大其下落速度.等轴晶初始尺寸越大,其形貌复杂性提升及下落速度增大,会导致熔化速率加快.在等轴晶下落熔化的过程中,逐渐减小的下落速度减弱了界面前沿对流传质条件,使得单个等轴晶的熔化速率基本保持恒定.     

Digital imaging studies of submicron colloidal spheres confined into a single layer between two smooth glass plates: Two dimensional melting

Digital imaging is used to study the particle positions and trajectories of a layer of highly charged 0.3 μm diameter polystyrene spheres in water suspension, rigidly confined into two-dimensions between two smooth glass plates. As the in-plane sphere density is varied, we find a two-stage melting transition in which we observe separate divergences in the orientational and translational order at different densities, separated by roughly 4%. The topological defect statistics are complex, but qualitatively consistent with the predictions of Halperin and Nelson.     

High-resolution cardiac imaging using an interleaved 3D double slab technique

A three-dimensional (3D) gradient-echo sequence with interleaved double-slab excitation was developed and optimized for the requirements in pediatric cardiac imaging. For this purpose high contrast between blood and myocardium signal should be obtained without the use of contrast agents. An acceptable measuring time for a large region examined with high spatial resolution should be achieved as well, especially with regard to the small structures of the heart and vessels of infants. The presented approach works with gradient moment nulling and a short echo time of 5.5 ms resulting in generally high signal intensity and only minor signal losses due to turbulent flow. The sequence allows simultaneous ECG-gated recording of two separately excited slabs with small thickness (10 mm) and with a distance of several centimeters between them. Thus, common effects of presaturation in 3D imaging can be avoided, although a relatively short measuring time is achievable. In order to get a 3D data set with good signal homogeneity of blood and of the other structures across a large volume of interest several double-slab measurements with suitable positions must be performed. The latter aspect is especially important for postprocessing techniques as multiple planar reconstruction and maximum intensity projection. Examples of applications of the new technique and appropriately postprocessed images are presented allowing demonstration even of subtle cardiac malformations.     

Protein fragment imaging using ink jet printing digestion technique

We have developed a capable tool for visualizing protein distribution at the sub-cell level with TOF-SIMS. This new method of visualizing a protein includes: (1) printing a digestive enzyme with ink jet (IJ) printing technology onto a tissue section, (2) measuring the two-dimensional distributions of decomposed peptide ions derived from digested protein on the tissue section by TOF-SIMS with a bismuth cluster ion (Bi₃⁺) source and using the aqueous ion enhancement method, and (3) analyzing the two-dimensional distributions of proteins using the results of proteome analysis. We have not yet tried this method onto a real tissue section. To show the possibility of this method, TOF-SIMS results of visualizing dot-patterned digested fragment peptide ions using IJ printing digestion technique onto a protein film are presented.     

Emergence and stability of vortex clusters in Bose-Einstein condensates: A bifurcation approach near the linear limit

We study the existence and stability properties of clusters of alternating charge vortices in repulsive Bose-Einstein condensates. It is illustrated that such states emerge from cascades of symmetry-breaking bifurcations that can be analytically tracked near the linear limit of the system via weakly nonlinear few-mode expansions. We present the resulting states that emerge near the first few eigenvalues of the linear limit, and illustrate how the nature of the bifurcations can be used to understand their stability. Rectilinear, polygonal and diagonal vortex clusters are only some of the obtained states while mixed states, consisting of dark solitons and vortex clusters, are identified as well. We also explore the evolution of unstable states and their transient dynamics exploring configurations of nearby bifurcation branches.     

Magneto-optical imaging of vortex arrangements in Pb finite superconducting networks

We have fabricated finite-sized Pb superconducting networks with 10 × 10 square (each 6 × 6 μm²) holes by using the electron beam lithography and vortex arrangements are visualized by using magneto-optical imaging. We find that the vortex penetration at low temperature is controlled by defects in the network. We also find nearly regular arrangements of vortices with defects close to 1/2 and1/3 of the matching field.     

Pipe flow measurement by using a side-scattering holographic particle imaging technique

A side scattering holographic particle imaging technique has been developed and demonstrated to be a capable tool for obtaining good quality images of a particle field in a high Reynolds number pipe flow. Instantaneous three-dimensional velocity components can be measured. The streamwise velocity error is estimated to be about 5%. Limitations of this optical set-up are discussed. The methodology and results presented will be of use for designing a pulsed laser holographic technique for turbulent velocity measurement or particle diagnostics in a pipe flow.     

Measurement of diffusion coefficients using a quick echo split NMR imaging technique

A new method for the ultrafast generation of diffusion-weighted images is reported. The technique combines a quick echo split NMR imaging sequence with the principle of Stejskal and Tanner. It allows to determine the diffusion constant with nearly the same accuracy as the conventional spin-echo technique, requiring only a fraction of the time. The determined values for water doped with 1 g Cu(NO₃)₂ per liter of H₂O and pure acetone were D_water = (1.95 ± 0.02) × 10⁻⁹ m²/s and D_acetone = (4.05 ± 0.02) × 10⁻⁹ m²/s at 18.5°C. They are in good agreement both with literature and our own reference measurements using a diffusion-weighted spin-echo sequence. In addition, the temperature dependence of D_water was measured in the range of 18.5–45.9°C and a good correspondence with reported data was found.     

Diagnosis of colon cancer using frequency domain fluorescence imaging technique

Early detection and treatment of colon cancer has been associated with better disease prognosis. Conventional and reported optical techniques have limitations in detecting early stages of colon cancer growth. In this paper, a homodyne signal processing assisted frequency domain (FD) fluorescence imaging methodology is proposed for the early diagnosis of colon cancer. Simulated phantom tissues representing the biopsy samples at different stages of colon cancer growth are prepared and used for the imaging study. Selective imaging of healthy and diseased sites simulated in the samples was achieved even for fluorescence emissions having close lifetimes and wavelength values. Possible extension of the methodology for in vivo investigations is also discussed.     

用吸收法对铯原子磁光阱中冷原子数目的测量

介绍了吸收法测量冷原子数的原理以及对铯原子气室磁光阱中俘获的冷原子数目的测量过程及结果. 与通常的荧光收集法相比，在原理上与静止二能级原子同共振单模光场作用的模型更加接近，同时大大减小了测量中的误差累积，提高了测量精度. 测得的冷原子数为(8±0.3)×10⁶，同时还利用测得的阱中俘获的稳态冷原子数和磁光阱中冷原子的寿命间接获得了磁光阱的俘获率. 关键词： 激光冷却与俘获 磁光阱 冷原子数目 俘获率