排序方式: 共有44条查询结果,搜索用时 187 毫秒
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固相过渡金属原子簇化合物Nb_3X_4(X=S,Se,Te)在低温下出现超导行为,且从S到Te,Nb_3X_4的超导转变温度Tc呈下降趋势。本文采用EHT近似下的紧束缚能带方法,计算了Nb_3X_4的能带结构。讨论其电子结构与超导电性的关系,并从化学键的观点出发对其Tc的递变加以解释。此外,本文还给出了Nb金属晶体的能带结构。 相似文献
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Heteronuclear intermolecular single-quantum coherences in liquid nuclear magnetic resonance 下载免费PDF全文
This paper analyses the heteronuclear Cosy Revamped by Asymmetric Z-gradient Echo Detection pulse sequence. General theoretical expressions of the pulse sequence with arbitrary flip angles were derived by using dipolar field treatment and signals originating from heteronuclear intermolecular single-quantum coherences (iSQCs) in highly-polarized two spin-1/2 systems were mainly discussed in order to find the optimal flip angles. The results show that signals from heteronuclear iSQCs decay slower than those from intermolecular double-quantum coherences or intermolecular zero-quantum coherences. Magical angle experiments validate that the signals are from heteronuclear iSQCs and insensitive to the imperfection of radio-frequency flip angles. All experimental observations are in excellent agreement with theoretical predictions. The quantum-mechanical treatment leads to similar predictions to the dipolar field treatment. 相似文献
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二核钼簇合物Mo~2S~4(i-mnt)~2(Et~4N)~2和(Et~4N)~3K[Mo~2S~4(i-mnt)~2]~2[i-mnt=S~2C(CN)^2^-]的阴离子在晶体中构成独特的链状结构,本文采用扩展的Huckel近似下的紧束缚能带方法,计算了它们的能带结构。结果表明,相邻簇阴离子间存在弱的相互作用,它是形成链状结构的基础。链状结构与晶体的半导体性质相关联。态密度和晶体轨道重叠布居反映了晶体中电荷分布状况及化学成键特点,与晶体结构分析、NMR谱学表征结果相符合。 相似文献
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单扫描时空编码磁共振成像是一种新型超快速磁共振成像技术,它对磁场不均匀和化学位移伪影有较强的抵抗性,但是其固有的空间分辨率较低,因此通常需要进行超分辨率重建,以在不增加采样点数的情况下提高时空编码磁共振图像的空间分辨率.然而,现有的重建方法存在迭代求解时间长、重建结果有混叠伪影残留等问题.为此,本文提出了一种基于深度神经网络的单扫描时空编码磁共振成像超分辨率重建方法.该方法采用模拟样本训练深度神经网络,再利用训练好的网络模型对实际采样信号进行重建.数值模拟、水模和活体鼠脑的实验结果表明,该方法能快速重建出无残留混叠伪影、纹理信息清楚的超分辨率时空编码磁共振图像.适当增加训练样本数量以及在训练样本中加入适当的随机噪声水平,有助于改善重建效果. 相似文献
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Fast high-resolution nuclear magnetic resonance spectroscopy through indirect zero-quantum coherence detection in inhomogeneous fields 下载免费PDF全文
In many cases, high-resolution nuclear magnetic resonance (NMR) spectra are virtually impossible to obtain by con- ventional nuclear magnetic resonance methods because of inhomogeneity of magnetic field and inherent heterogeneity of sample. Although conventional intramolecular zero-quantum coherence (ZQC) can be used to obtain high-resolution spectrum in inhomogeneous field, the acquisition takes rather long time. In this paper, a spatially encoded intramolecular ZQC technique is proposed to fast acquire high-resolution NMR spectrum in inhomogeneous field. For the first time, the gradient-driven decoding technique is employed to selectively acquire intramolecular ZQC signals. Theoretical analyses and experimental observations demonstrate that high-resolution NMR spectral information can be retrieved within several scans even when the field inhomogeneity is severe enough to erase most spectral information. This work provides a new way to enhance the acquisition efficiency of high-resolution intramolecular ZQC spectroscopy in inhomogeneous fields. 相似文献
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在合成和表征了一系列新的异核异价三核过渡金属羰酸配合物〖Fe2^ⅢMⅡO(OOCCH2H5)6L3〗(M=Co,Ni,Mn,L=C5H5N,H2O)的基础上,利用多种NMR技术并结合UV研究了这些配合物在不同溶剂介质和温度下的谱学特征和动力学性质。利用谱峰积分比例、线宽、相同骨架分子的配体取代和纵向驰豫时间对^1HNMR谱进行了归属。实验结果表明:这类配合物的金属离子间宫心桥存在一定的反铁磁作用, 相似文献