完全由电子关联驱动的Mott型金属-绝缘体相变研究

Mott金属-绝缘体相变(MIT)是凝聚态物理中的一个非常基本的概念.长期以来,Mott型MIT的概念被广泛应用于凝聚态物理的许多领域,特别是用于描述强关联系统的电子结构特征.然而到目前为止,完全由电子关联驱动的MIT并没有被观察到.因此,是否存在着完全由于电子之间的强关联效应导致的Mott型MIT一直是科学家们感兴趣的重要问题.近日,中国科学院物理研究所方忠研究员组、郭建东研究员组和美国Florida International大学的Jiandi Zhang教授研究组及美国Tennessee大学及橡树岭国家实验室的E.W.Plummer教授研究组、Rong Ying Jin教授研究组合作,通过实验与理论相结合的研究,在Ca1.9Sr0.1RuO4表面首次实现了纯电子驱动的Mott型MIT,发生电子结构相变时并没有相应的结构畸变出现.该研究成果对于人们认识电子-电子关联效应引起的Mott转变具有非常重要的意义.     

Spatial resolution of imaging contaminations on the GaAs surface by scanning tunneling microscope-cathodoluminescence spectroscopy

We obtained the luminescence image of the GaAs (1 1 0) surface by scanning tunneling microscope-cathodoluminescence (STM-CL) spectroscopy, where low-energy (∼100 eV) electrons field emitted from the STM tip were used as a bright excitation source. The STM-CL image with high photon signal (1.25 × 10⁴ cps) showed the dark image corresponding to the surface contamination in the STM image working as the nonradiative recombination centers of carriers. This dark image demonstrated the spatial resolution of about 100 nm in STM-CL spectroscopy of the GaAs (1 1 0) surface, which was determined by the field-emitted electron beam diameter.     

Some unusual electronic patterns on graphite surface

We report on the observation of some unusual electronic patterns on a graphite surface using scanning tunneling spectroscopy (STM). We attribute these patterns to different types of strain near the surface. One such pattern seen on a particular layer comprises of two-dimensional spatially varying super-lattice and one-dimensional fringes. This pattern is present in a finite region of a layer on the surface confined between two carbon fibers. We attribute this spatially varying super-lattice structure to the shear strain generated in the top layer due to the restraining fibers. We have also developed a model with the Moirè rotation hypothesis that gives us a better insight into such largescale spatially varying patterns. We have been able to model the above-observed pattern. We also report another pattern near a defect, which we attribute to the change in density of states due to the physical buckling of the top graphite layer. Part of this buckled layer is found to be buried under another layer and this region shows a reversed contrast and thus supporting our idea of buckling. We also performed tunneling spectroscopy measurements on various regions of these patterns which show significant variations in the density of states.      

Direct Observation of Alkyl Chain Interdigitation in Conjugated Polyquarterthiophene Self‐Organized on Graphite Surfaces

A solution processible polymer—poly(3,3‴‐didodecylquaterthiophene) (PQT‐12) is investigated at the liquid/solid interface using the scanning tunneling microscopy (STM). Two‐dimensional ordered films made up of self‐assembled domains, with dimensions of 100 nm × 50 nm adsorbed on highly oriented pyrolytic graphite (HOPG) were formed. These domains consist of parallel lamellar polymer chains, with the alkyl chains forming interdigitated structures, along with U‐shaped and closed ring segments of the polymer chains. A polymer chain packing model is proposed herein, which attempts to propose a correlation between the packing of long chains and charge mobilities. These STM results could help in understanding the relationship between the extended conjugation and molecular organization of the PQT‐12 chains.

     

Quantum Tunneling Radiation of Kerr-NUT Black Hole

Based on particles in a dynamical geometry, extending the Parikh‘s method of quantum tunneling radiation,we deeply investigate the quantum tunneling radiation of Kerr-NUT black hole. When self-gravitating action, energyconservation, and angular momentum conservation are taken into account, the emission rate of the particle on the event horizon is related to the change of Bekenstein-Hawking entropy and the emission spectrum is not precisely thermal, but is consistent with an underlying unitary theory.     

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

Large amplitude motions (LAMs) form a fundamental phenomenon that demands the development of specific theoretical and Hamiltonian models. In recent years, along with the strong progress in instrumental techniques on high-resolution microwave spectroscopy and computational capacity in quantum chemistry, studies on LAMs have become very diverse. Larger and more complex molecular systems have been taken under investigation, ranging from series of heteroaromatic molecules from five- and six-membered rings to polycyclic-aromatic-hydrocarbon derivatives. Such systems are ideally suited to create families of molecules in which the positions and the number of LAMs can be varied, while the heteroatoms often provide a sufficient dipole moment to the systems to warrant the observation of their rotational spectra. This review will summarize three types of LAMs: internal rotation, inversion tunneling, and ring puckering, which are frequently observed in aromatic five-membered rings such as furan, thiophene, pyrrole, thiazole, and oxazole derivatives, in aromatic six-membered rings such as benzene, pyridine, and pyrimidine derivatives, and larger combined rings such as naphthalene, indole, and indan derivatives. For each molecular class, we will present the representatives and summarize the recent insights on the molecular structure and internal dynamics and how they help to advance the field of quantum mechanics.     

STM and AFM of bio/organic molecules and structures

Applications of scanning tunneling and atomic force microscopes in bio/organic researches are reviewed with a special emphasis on the types of researches that are expected to contribute to the creation of a new field of “single molecule biochemistry” in the near future. The reviewed articles within the scope as stated above actually include a fairly broad spectrum of researches. It is, therefore, a hope of the author that this review will be useful to those who are considering biological applications of the probe microscopy techniques but are not quite familiar with the types of experiments that have been done in the past. In the section on scanning tunneling microscopy, identification of chemically distinct functional groups by the difference in their tunneling properties will be discussed as a main focus because it is fundamental for biochemists to identify molecules by their shapes and properties. In the section on atomic force microscopy, recent progresses in the imaging techniques of proteins and DNAs are closely reviewed, and rapidly advancing technologies of single molecule measurements and manipulation of nanometer sized structures are given extensive coverage because the author considers that such new applications are extremely promising to open an entirely new field in biological sciences.     

高磁电阻磁性隧道结的几种微制备方法研究

利用金属掩模法优化了制备磁性隧道结的实验和工艺条件，金属掩模的狭缝宽度为100 μm. 采用4 nm厚的Co₇₅Fe₂₅为铁磁电极和10或08 nm厚的铝氧化物 为势垒膜， 直接制备出了室温隧穿磁电阻（TMR）为30%—48％的磁性隧道结，其结构为Ta(5 nm)/Cu(25 nm)/Ni₇₉Fe₂₁(5 nm)/Ir₂₂Mn₇₈(10 nm)/ Co₇₅Fe₂₅ (4 nm)/Al(08 nm)-O/Co₇₅Fe₂₅(4 nm)/Ni₇₉Fe ₂₁(20 nm)/Ta(5 nm).同时，利用刻槽打孔法和去胶掀离法两种光刻技术并结合Ar离子束刻蚀及化学反应刻 蚀，制备出面积在4 μm×8 μm—20 μm×40 μm、具有室温高TMR和低电阻的高质量磁性 隧道结.300 ℃ 退火前后其室温TMR可分别达到22% 和50%.研究结果表明，采用光刻中的刻 槽打孔或去胶掀离工艺方法制备的小尺寸磁性隧道结，可用于研制磁动态随机存储器和磁读 出头及其他传感器件的磁敏单元. 关键词： 磁性隧道结 隧穿磁电阻 金属掩模法 光刻法     

纳米多晶La0.7Sr0.3MnO3－δ的输运性质和磁电阻效应研究

利用溶胶-凝胶法制备了纳米多晶La_0.7Sr_0.3MnO_3-δ(LSM)块体样品.详细研究了在不同烧结温度下的LSM样品电阻率随测量温度的变化关系和磁电阻效应.随着测量温度从室温降低,电阻率ρ都在250K附近存在最大值,低于该温度后,样品表现为金属导电特性,随后在50K左右存在一极小值.即随着温度从50K左右降低到4.2K,ρ反而逐渐升高,表现为绝缘体性的导电特性.研究表明,在低温下(<50K),ρ随温度降低而升高的现象与隧穿效应的理论模型(lnρ∝T^1/2)符合得很好,表明这种现象是由于传导电子在通过邻近LSM晶粒间表面/界面层时的隧道效应所致.而在50—250K的温度范围内,其电阻率与T²成正比,表现为LSM本征的金属导电特性.因此这种低温下电阻率的极小值现象来源于隧穿效应和LSM晶粒本征的金属导电特性的相互竞争.本文还详细研究了相应的隧道磁电阻效应. 关键词： 0.7Sr_0.3MnO_3-δ')" href="#">多晶La_0.7Sr_0.3MnO_3-δ 隧道效应 隧道磁电阻效应