Using photoemission spectroscopy to probe a strongly interacting Fermi gas

Ultracold atomic gases provide model systems in which to study many-body quantum physics. Recent experiments using Fermi gases have demonstrated a phase transition to a superfluid state with strong interparticle interactions. This system provides a realization of the 'BCS-BEC crossover' connecting the physics of Bardeen-Cooper-Schrieffer (BCS) superconductivity with that of Bose-Einstein condensates (BECs). Although many aspects of this system have been investigated, it has not yet been possible to measure the single-particle excitation spectrum (a fundamental property directly predicted by many-body theories). Here we use photoemission spectroscopy to directly probe the elementary excitations and energy dispersion in a strongly interacting Fermi gas of (40)K atoms. In the experiments, a radio-frequency photon ejects an atom from the strongly interacting system by means of a spin-flip transition to a weakly interacting state. We measure the occupied density of single-particle states at the cusp of the BCS-BEC crossover and on the BEC side of the crossover, and compare these results to that for a nearly ideal Fermi gas. We show that, near the critical temperature, the single-particle spectral function is dramatically altered in a way that is consistent with a large pairing gap. Our results probe the many-body physics in a way that could be compared to data for the high-transition-temperature superconductors. As in photoemission spectroscopy for electronic materials, our measurement technique for ultracold atomic gases directly probes low-energy excitations and thus can reveal excitation gaps and/or pseudogaps. Furthermore, this technique can provide an analogue of angle-resolved photoemission spectroscopy for probing anisotropic systems, such as atoms in optical lattice potentials.     

Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution and intramolecular thermalization of excited states occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution, to show that electron transfer from an aromatic adsorbate to a TiO(2) semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes.     

Kondo resonance in a single-molecule transistor

When an individual molecule, nanocrystal, nanotube or lithographically defined quantum dot is attached to metallic electrodes via tunnel barriers, electron transport is dominated by single-electron charging and energy-level quantization. As the coupling to the electrodes increases, higher-order tunnelling and correlated electron motion give rise to new phenomena, including the Kondo resonance. To date, all of the studies of Kondo phenomena in quantum dots have been performed on systems where precise control over the spin degrees of freedom is difficult. Molecules incorporating transition-metal atoms provide powerful new systems in this regard, because the spin and orbital degrees of freedom can be controlled through well-defined chemistry. Here we report the observation of the Kondo effect in single-molecule transistors, where an individual divanadium molecule serves as a spin impurity. We find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV.     

杂多阴离子[P2Mo5O23]6-的稳定结构和电子性质DFT比较研究

运用量子化学密度泛函理论(DFT)的BP86,BP91,BLYP方法系统优化[P2Mo5O23]6-杂多阴离子几何结构,并对该杂多阴离子电子性质进行分析比较.计算结果表明:三种方法几何优化结果与实验数据均具有一致性,其中BP86方法计算结果与实验值最接近;最高占有轨道主要分布在与P原子相连的端氧和P-O-Mo键桥氧的p轨道上,最低空轨道主要集中在Mo原子的d轨道和部分O原子的p轨道上;所有Mo原子和P原子的正电荷都小于正常的氧化态,所有O原子的负电荷小于-2.     

2,5-二(α-硫甲基吡啶)1,3,4噻二唑的电化学性质及离子识别性能研究

用循环伏安法和计时电量法等电化学方法研究了2,5-二(α-硫甲基吡啶)1,3,4噻二唑(DPTD)在玻碳电极(GCE)上的电化学行为.结果表明:在0.1 mol.L-1四丁基高氯酸铵(TBAP)的N,N-二甲基甲酰胺底液中于1.08 V(vs.SCE)处有一个不可逆的氧化峰,氧化峰电流与扫描速度的平方根(v1/2)呈良好的线性关系,表明DPTD在GCE上的伏安行为是一受扩散控制的不可逆电化学过程.利用电化学方法求出该电极反应过程是单电子转移,传递系数α为0.78,扩散系数为1.20×10-5cm2.s-1.利用循环伏安法还研究了该化合物对过渡金属离子的识别作用,发现该化合物在一定浓度下对Cu2+和Pb2+有较好的选择识别性.     

A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface

Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides--LaAlO3 and SrTiO3--in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V(-1) s(-1). At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.     

Laser induced photoemission electron and the generation of high Rydberg states of atoms and molecules

Laser induced photoemission electron was produced by directing 532 nm and/or 355 nm onto stainless steel plate on a time of flight mass spectrometer. Multiple charged ions and high Rydberg states of atoms or molecules were successfully generated by impacts of the photoemission electrons. The high Rydberg states ( n ～40-100) thus produced were separated from ions, produced by direct electron impact ionization, by a 3 V DC electric field and then ionized by a delayed pulsed HV electric field in a ZEKE-PFI manner. Relationship between generation/property of high Rydberg states of atoms/molecules and experimental conditions could be investigated. Relationship between the electron accelerating voltage and high Rydberg states of Ar was described.     

金属Rb掺杂Alq_3的电子结构及其退火与氧化行为

利用同步辐射光电子能谱实验技术研究了活泼金属铷原子掺杂8-羟基喹啉铝(Rb-Alq3)薄膜的电子结构以及低温退火与氧化行为,结果表明铷和Alq3分子发生显著反应,电子从金属原子转移到Alq3的LUMO轨道,形成活泼金属/有机接触中典型的带隙态,结合能位于1.2 eV.在低温退火后,Rb-Alq3的价带往高结合能端整体位移0.1 eV,真空能级保持不变,带隙态仍然可辨;深度氧化后,带隙态消失,最高占据态(HOS)基本回到退火氧化处理前的位置,但真空能级抬高了近1 eV.Rb-Alq3氧化后的最小电离势增大了约1 eV,而纯净Alq3薄膜只增大0.05 eV左右.     

电子显微术研究含铈铁铬合金的氧化行为

利用ＳＡＭ、ＸＰＳ和ＳＥＭ技术,研究含铈的铁－１８铬合金的氧化膜的形貌特征,测定每间隔数个纳米的不同深度的氧化层的组分,分析它的结构状态,动态分析结果,获得了铬向外迁移、改变氧化膜成分和结构、以及铈向界面扩散和偏聚的试验数据。根据结果进一步探讨了添加微量元素铈对合金元素铬在氧化过程中扩散的影响。     

过渡金属原子在半导体表面上化学吸附理论

本文研究了过渡金属原子d轨道同衬底半导体SP杂化轨道之间存在两种不同耦合系数的化学吸附问题，采用自洽的格林函数方法，计算了吸附原子d电子在强、弱耦合态的占据数，化学吸附能、电荷转移及磁性解磁矩。     

Observation of orbital waves as elementary excitations in a solid

A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise. For example, phonons are the collective excitations corresponding to lattice vibrations in a crystal, and magnons correspond to spin waves in a magnetically ordered compound. Modulations in the relative shape of the electronic clouds in an orbitally ordered state could in principle give rise to orbital waves, or 'orbitons', but this type of elementary excitation has yet to be observed experimentally. Systems in which the electrons are strongly correlated-such as high-temperature superconductors and manganites exhibiting colossal magnetoresistivity-are promising candidates for supporting orbital waves, because they contain transition-metal ions in which the orbital degree of freedom is important. Orbitally ordered states have been found in several transition-metal compounds, and orbitons have been predicted theoretically for LaMnO3. Here we report experimental evidence for orbitons in LaMnO3, using Raman scattering measurements. We perform a model calculation of orbiton resonances which provides a good fit to the experimental data.     

First-principles investigation of the impurity-kink interaction in bcc iron

Using the first-principles self-consistent discrete variational method based on density functional theory, we investigated the effect of light impurities C and N on the electronic structure of kink on the [100]（010） edge dislocation （ED） in bcc iron. Our energetic calculations show that the light impurities have a strong segregation tendency to enter the kink. The results of the charge distribution and the local density of states indicate that the strong bonds between the impurity atoms and the neighboring Fe atoms are formed due to the hybridizations of impurity atoms 2p states and Fe 3d4s4p states. The introduction of light impurities can stabilize the kink system, and impedes the sideward motion of the kink in the [100]（010） ED. This is, the light impurities induce a strong pinning effect on the [100]（010） ED and may result in the solid solute hardening.     

Spin transition of iron in magnesiowüstite in the Earth's lower mantle

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by approximately 35 percent and bulk sound velocity increases by approximately 15 percent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition.     

过渡金属敏化稀土化合物近红外发光性能研究进展

稀土元素由于具有特殊的电子构型而使得稀土原子或离子的化合物(配合物)有很多独特的物理和化学性质,尤其是稀土化合物的发光性能,但由于跃迁选择定则,稀土化合物的发光性能比较弱,需要用过渡金属离子对稀土化合物进行敏化,以增强其发光性能。在这个过程中,过渡金属离子的最低激发态不直接参与发光的辐射跃迁,却可以改善稀土化合物的发光性能。围绕过渡金属敏化稀土化合物发光的研究方法、研究成果以及其发展前景进行了综述。     

The first-principles study on the doping effect of Re in Ni3A1

Using first-principles density function for molecules method (DMol) and discrete variational method (DVM) based on the density functional theory, we studied the doping effect of Re in Ni3Al. The structure relaxation and the alloying energy show that Re has a strong A1 site preference and leads to the local deformation, which is in agreement with the experimental results and other theoretical results. In addition, the charge density difference and the bond order show that Re can strongly enhance the interatomic interaction between the nearest neighbor atoms. From the density of states and the Pauli spectrum, we find that resonance states and localized states are induced by doping Re, and the doped Re atom forms the hybridized bond with the nearest neighbor atoms.     

苯丙氨酸电荷转移百分数的理论计算

采用密度泛函理论中的CAM B3LYP方法, 在6-31G(d)基组水平上优化不同条件下左旋体苯丙氨酸(S-Phe)分子的几何构型. 结果表明： 在隐式溶剂甲醇与气相条件下, S-Phe分子体系电荷转移的定性结果基本一致, 定量结果略有差别, 其中各片段的S2激发态基本一致, 仅COOH片段的S2激发态差别为2.2%, S4激发态差别为0.4%～2.8%; 在不同环境下, 定性分析缺少一个S5激发态, 由Becke方法计算的激发态S2和S4电荷转移相差分别约为2.5%～8%.     

多缺陷碳纳米管的制备及其光催化性能研究

低温合成的碳纳米管在纯化后进行高温煅烧(真空),X射线光电子能谱(XPS)和比表面积(BET)分析显示,O/C原子比下降,比表面积增大.拉曼光谱和透射电子显微镜(TEM)进一步证实,该热处理的碳管呈现较多的缺陷.光催化测试显示此种多缺陷碳管具有明显的光催化性能.这主要是因为,大部分化学吸附的氧原子将被解吸,很容易使局部晶格重新排布,形成大量的拓扑学缺陷和空位缺陷.由此在碳管的能隙间形成缺陷态能级,以致可以被可见光激发,即表现出光催化特性.     

Atom-by-atom spectroscopy at graphene edge

The properties of many nanoscale devices are sensitive to local atomic configurations, and so elemental identification and electronic state analysis at the scale of individual atoms is becoming increasingly important. For example, graphene is regarded as a promising candidate for future devices, and the electronic properties of nanodevices constructed from this material are in large part governed by the edge structures. The atomic configurations at graphene boundaries have been investigated by transmission electron microscopy and scanning tunnelling microscopy, but the electronic properties of these edge states have not yet been determined with atomic resolution. Whereas simple elemental analysis at the level of single atoms can now be achieved by means of annular dark field imaging or electron energy-loss spectroscopy, obtaining fine-structure spectroscopic information about individual light atoms such as those of carbon has been hampered by a combination of extremely weak signals and specimen damage by the electron beam. Here we overcome these difficulties to demonstrate site-specific single-atom spectroscopy at a graphene boundary, enabling direct investigation of the electronic and bonding structures of the edge atoms-in particular, discrimination of single-, double- and triple-coordinated carbon atoms is achieved with atomic resolution. By demonstrating how rich chemical information can be obtained from single atoms through energy-loss near-edge fine-structure analysis, our results should open the way to exploring the local electronic structures of various nanodevices and individual molecules.     

铁磁金属合金中的内磁场和磁矩

本文研究了若干晶态和非晶态铁磁金属合金中Fe原子的内磁场和磁矩,认为这些合金中存在两种不同的机制。类金属原子的键合效应和过渡金属原子的电子转移效应,它们导致Fe原子内磁场和磁矩明显降低。所得结果表明,虽然铁磁金属合金中Fe原子的内磁场和磁矩具有同样的变化趋势,但在一般情况下,两者之间不一定存在某种简单的正比关系或线性关系。     

The first-principles study on the doping effect of Re in Ni3Al

Using first-principles density function for molecules method （DMol） and discrete variational method （DVM） based on the density functional theory, we studied the doping effect of Re in Ni3Al. The structure relaxation and the alloying energy show that Re has a strong Al site preference and leads to the local deformation, which is in agreement with the experimental results and other theoretical results. In addition, the charge density difference and the bond order show that Re can strongly enhance the interatomic interaction between the nearest neighbor atoms. From the density of states and the Pauli spectrum, we find that resonance states and localized states are induced by doping Re, and the doped Re atom forms the hybridized bond with the nearest neighbor atoms.