Interaction induced non-reciprocal three-level quantum transport

Besides its fundamental importance, non-reciprocity has also found many potential applications in quantum technology. Recently, many quantum systems have been proposed to realize non-reciprocity, but stable non-reciprocal process is still experimentally difficult in general, due to the needed cyclical interactions in artificial systems or operational difficulties in solid state materials. Here, we propose a new kind of interaction induced non-reciprocal operation, based on the conventional stimulated-Raman-adiabatic-passage(STIRAP) setup, which removes the experimental difficulty of requiring cyclical interaction, and thus it is directly implementable in various quantum systems. Furthermore, we also illustrate our proposal on a chain of three coupled superconducting transmons, which can lead to a non-reciprocal circulator with high fidelity without a ring coupling configuration as in the previous schemes or implementations. Therefore, our protocol provides a promising way to explore fundamental non-reciprocal quantum physics as well as realize non-reciprocal quantum device.     

pH-Dependent Hydration Change in a Gd-Based MRI Contrast Agent with a Phosphonated Ligand

The heptadentate ligand L was shown to form an extremely stable Gd complex at neutral pH with a pGd value of 18.4 at pH 7.4. The X-ray crystal structures of the complexes formed with Gd and Tb displayed two very different coordination behaviors being, respectively, octa- and nonacoordinated. The relaxometric properties of the Gd complex were studied by field-dependent relaxivity measurements at various temperatures and by ¹⁷O NMR spectroscopy. The pH-dependence of the longitudinal relaxivity profile indicated large changes around neutral pH leading to a very large value of 10.1 mm ⁻¹⋅s⁻¹ (60 MHz, 298 K) at pH 4.7. The changes were attributed to an increase of the hydration number from one water molecule in basic conditions to two at acidic pH. A similar trend was observed for the luminescence of the Eu complex, confirming the change in hydration state. DOSY experiments were performed on the Lu analogue, pointing to the absence of dimers in solution in the considered pH range. A breathing mode of the complex was postulated, which was further supported by ¹H and ³¹P NMR spectroscopy of the Yb complex at varying pH and was finally modeled by DFT calculations.     

二维应变作用下超导薄膜LiFeAs的磁性和电子性质

基于密度泛函理论的第一性原理计算,研究了二维应变作用下LiFeAs超导薄膜的磁性结构、电子能带和态密度变化,分析了应变对其超导电性的作用.结果显示,对体系施加1%—6%的二维平面张、压应变均不改变其基态条形反铁磁性结构,费米面附近的电子态密度主要来自于Fe-3d轨道电子以及少量的As-4p电子.研究发现,与无应变情形相比,当施加压应变时,体系中Fe离子的反平行的电子自旋局域磁矩减小,薄膜反铁磁性受到抑制,费米面上电子态密度增加,超导电性来自于以反铁磁超交换耦合作用为媒介的空穴型费米面和电子型费米面间嵌套的Cooper电子对.而在张应变作用时,局域反铁磁性增强,费米面上电子态密度减小,金属性减弱,特别是张应变时费米面上空穴型能带消失, Cooper电子对出现概率显著降低,将抑制超导相变.     

CFETR CSMC装配误差下的电磁状态研究

中心螺旋管模型线圈是为了积累中心螺线管线圈设计和制造经验而预研的一套混合超导磁体线圈,该线圈由五大模块组成,高场区采用Nb_3Sn线圈,低场区采用NbTi线圈,可以满足紧凑型聚变装置对低纵横比的要求.然而模型线圈各模块结构复杂、尺寸庞大在装配过程中不可避免的存在装配误差,线圈励磁以后产生的非对称磁场和电磁力将分别影响等离子体的旋转和线圈整体的稳定性.为了详细分析装配误差对线圈磁场和电磁力的影响,选取模型线圈在装配过程中可能出现的典型的装配误差,基于电流丝等效模型计算了NbTi线圈偏置以后Nb_3Sn内外线圈上的磁场和电磁力.计算结果表明,由于装配误差,线圈磁场沿环向呈不均匀分布,轴向电磁力合力不为零;轴向和径向装配误差对线圈磁场和电磁力的影响存在明显的差异;电磁力随着装配误差呈线性增长,当线圈装配误差较大时将引发数倍于线圈自重的电磁力;该研究结果可为偏置状态下模型线圈的耦合场分析和线圈模块的装配方案设计优化提供参考.     

超导Tl2Ba2CaCu2O8+x 薄膜的太赫兹复电导率的提取

采用低温太赫兹时域光谱系统, 测试了高温超导 Tl2Ba2CaCu2O8+x 薄膜的太赫兹透射谱, 并提取了它在不同温度下的太赫兹电导率. 研究过程中发现提取后的参数存在电导率随频率波动大、与理论值偏差较大等问题.通过对基片厚度和太赫兹波入射角度的误差对高温超导薄膜电导率的影响进行了分析, 结果表明导致数据波动大是由于基片厚度的偏差引起的. 针对厚度差的影响, 一种矫正方法被提出, 通过对厚度的修正, 提高了数据提取的质量.     

Self‐emulsion polymerization of amphiphilic monomers—a green route to synthesis of polymeric nanoscaffolds

Self‐emulsion polymerization (SEP), a green route developed by us for the polymerization of amphiphilic monomers, does not require any emulsifier or an organic solvent except that the water‐soluble initiators such as 2,2′‐azobis[2‐(2‐imidazolin‐2‐yl)propane]dihydrochloride (VA‐044) and potassium persulfate (KPS) are only used. We report here the polymer nanoscaffolds from a number of amphiphilic monomers, which can be used for in situ encapsulation of a variety of nanoparticles. As a demonstration of the efficacy of these nanoscaffolds, the synthesis of a biocompatible hybrid nanoparticle (nanohybrid), prepared by encapsulating Fe₃O₄ magnetic nanoparticle (Fe₃O₄ MNPs) in poly(2‐hydroxyethyl methacrylate) in water, for MRI application is presented. The nanohybrid prepared following the SEP in the form of an emulsion does not involve the use of any stabilizing agent, crosslinker, polymeric emulsifier, or surfactant. This water‐soluble, spherical, and stable nanohybrid containing Fe₃O₄ MNPs of average size 10 ± 2 nm has a zeta potential value of ?41.89 mV under physiological conditions. Magnetic measurement confirmed that the nanohybrid shows typical magnetic behavior having a saturation magnetization (Ms) value of 32.3 emu/g and a transverse relaxivity (r2) value of 29.97 mM^?1 s^?1, which signifies that it can be used as a T2 contrast agent in MRI. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019     

14T MRI大口径全身自屏蔽超导磁体的一种优化设计方法

磁共振成像(MRI)的磁体设计首先是确保中心成像区的场值和均匀性, 二是尽可能减少场值耗散的距离即漏磁5Gs 线. 基于此本文提出了一种线性与非线性规划联合优化的方法. 首先将导体作为基本单元, 在预布置线圈的空间范围内构建二维连续导体网格. 通过线性规划搜索满足磁场约束条件的网格电流分布图. 再将存在电流的网格离散为一个个矩形线圈区域, 在保证场值均匀性、 杂散场5 Gs 线范围以及线圈位置间隔、 导体超导线安全裕度的前提下利用非线性规划, 具体确定各个线圈的轴向和径向位置、 线圈内导体层数和各层匝数以及通电流大小等. 采用这种联合优化方法, 不仅节省优化时间, 还可以自行设计线圈形状有利于工程实现. 文中由此方法给出了14 T MRI 磁体的一种设计方案, 依靠4 组线圈使得45 cm 中心球形成像内不均匀度降低到5 ppm, 而高场耗散的5 Gs 线通过磁体自屏蔽减小到15 m 以内. 满足了设计的要求.     

Characterisation of magnetic resonance imaging (MRI) contrast agents using NMR relaxometry

ABSTRACT

This contribution describes the use of Fast Field-cycling relaxometry (FFC-NMR) for the characterisation of Gd(III)- and Mn(II)-based contrast agents for MRI. Through a series of selected examples, we analyse the role of different structural and dynamic parameters on ¹H relaxivity and on the shape of the ¹H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles. The amplitude and shape of the profiles is affected by the number of water molecules coordinated to the metal ion, the water exchange rate, the rotational correlation time of the complex and the relaxation of the electron spin. As a result, ¹H NMRD profiles represent a powerful tool for the understanding of the properties of MRI contrast agent candidates at the molecular level.     

基于级联卷积神经网络的前列腺磁共振图像分类

针对深度学习训练成本高，以及基于磁共振图像的前列腺癌临床诊断需要大量医学常识且极为耗时的问题，本文提出了一种基于级联卷积神经网络（Convolutional Neural Network，CNN）和磁共振图像的前列腺癌（Prostate Cancer，PCa）自动分类诊断方法，该网络以Faster-RCNN作为前网络，对前列腺区域进行提取分割，用于排除前列腺附近组织器官的干扰；以基于ResNet改进的网络结构CNN40_bottleneck作为后网络，用于对前列腺区域病变进行分类.后网络由瓶颈结构串联组成，其中使用批量标准化（Batch Normalization，BN）、全局平均池化（Global Average Pooling，GAP）进行优化.实验结果证明，本文方法对前列腺癌诊断结果较好，而且缩减了训练时间和参数量，有效降低了训练成本.     

Chemical Exchange Saturation Transfer (CEST) Agents: Quantum Chemistry and MRI

Diamagnetic chemical exchange saturation transfer (CEST) contrast agents offer an alternative to Gd³⁺‐based contrast agents for MRI. They are characterized by containing protons that can rapidly exchange with water and it is advantageous to have these protons resonate in a spectral window that is far removed from water. Herein, we report the first results of DFT calculations of the ¹H nuclear magnetic shieldings in 41 CEST agents, finding that the experimental shifts can be well predicted (R²=0.882). We tested a subset of compounds with the best MRI properties for toxicity and for activity as uncouplers, then obtained mice kidney CEST MRI images for three of the most promising leads finding 16 (2,4‐dihydroxybenzoic acid) to be one of the most promising CEST MRI contrast agents to date. Overall, the results are of interest since they show that ¹H NMR shifts for CEST agents—charged species—can be well predicted, and that several leads have low toxicity and yield good in vivo MR images.