Direct excitation of the forbidden clock transition in neutral 174Yb atoms confined to an optical lattice

We report direct single-laser excitation of the strictly forbidden (6s2)1S0 <--> (6s6p)3P0 clock transition in 174Yb atoms confined to a 1D optical lattice. A small (approximately 1.2 mT) static magnetic field was used to induce a nonzero electric dipole transition probability between the clock states at 578.42 nm. Narrow resonance linewidths of 20 Hz (FWHM) with high contrast were observed, demonstrating a resonance quality factor of 2.6 x 10(13). The previously unknown ac Stark shift-canceling (magic) wavelength was determined to be 759.35 +/- 0.02 nm. This method for using the metrologically superior even isotope can be easily implemented in current Yb and Sr lattice clocks and can create new clock possibilities in other alkaline-earth-like atoms such as Mg and Ca.     

Mechanism of superfluid-insulator transition in two-dimensional Bose Hubbard model

To understand the mechanism of Mott transitions in case of no magnetic influence, superfluid-insulator (Mott) transitions are studied for the S = 0 Bose Hubbard model on the square lattice, using a variational Monte Carlo approach. In trial many-body wave functions, we introduce various types of attractive correlation factors between a doubly-occupied site (doublon, D) and an empty site (holon, H), which play a central role for the transition. We propose an improved picture of D–H binding; a Mott transition occurs when the D–H pair length becomes equivalent to the minimum D–D distance, which lengths are appropriately estimated. We confirm this picture is valid for all the wave functions with attractive D–H factors we consider, and point out it can be universal for nonmagnetic Mott transitions.     

Graphene on ordered Ni-alloy surfaces formed by metal (Sn,Al) intercalation between graphene/Ni(111)

Deposition and intercalation of Al and Sn on Ni(111) supported graphene is investigated by Auger electron spectroscopy, low energy electron diffraction, and scanning tunneling microscopy. Al intercalates at ~ 200 °C while Sn intercalates at ~ 350 °C, indicating that the intercalation process is element specific. Both Al and Sn alloy with the Ni-substrate at higher annealing temperatures and form ordered alloy surfaces and surface alloys, respectively. Sn forms a (√3 × √3) R30° surface alloy by substituting surface Ni-atoms with Sn and thus the alloy maintains the same good lattice match with graphene as for Ni(111). Both Sn and Al are interacting weakly with graphene and can therefore be used to decouple graphene from the strongly interacting Ni substrate.     

Raman scattering from a two-dimensional electron gas in a weakly and periodically modulated magnetic field

An analysis of light scattering from a two-dimensional electron gas (2DEG) subject to aweak periodicmagnetic modulation of strengthB_mand to a perpendicular uniform magnetic fieldBof arbitrary strength is presented. Raman spectra are calculated for (i) electron inter-Landau-level transitions, which result in a Raman shift ∼  2ω_c, and (ii) inter-Landau-level collective electron excitations at a hybridized magnetoplasmon frequency and at anewlow-frequency, intra-Landau-level plasmon mode induced by the modulation. The dependence of the Raman cross sections on the strength ofBandB_mis assessed. The effect of a weakB_mon the Raman spectrum is similar to that of a weak electric modulation but the signals from the magnetically modulated 2DEG are much stronger. The combined effect of the electric and magnetic modulations is also considered.     

Gamow-teller strengths in proton-rich exotic nuclei deduced in the combined analysis of mirror transitions

Isospin symmetry is expected for the T(z)=+/-1-->0 isobaric analogous transitions in isobars with mass number A, where T(z) is the z component of isospin T. Assuming this symmetry, strengths of analogous Gamow-Teller (GT) transitions within A = 50 isobars were determined from a high energy-resolution study at 0 degrees in combination with the decay Q value and lifetime from the beta decay. This method can be applied to other pf-shell nuclei and can be used to study GT strengths of astrophysical interest.     

基于导模共振效应提高石墨烯表面等离子体的局域特性

本文构建了一种包含石墨烯和亚波长光栅的复合结构, 借助衍射光栅的导模共振效应, 在石墨烯表面激发高局域性表面等离子体激元, 研究了石墨烯与光栅结构对表面等离子体激元局域特性的影响规律, 并借助基于有限元法的COMSOL软件, 分析了缓冲层厚度、光栅周期、载流子迁移率和费米能级对石墨烯的表面电场、品质因子Q和有效模式面积S_eff的影响. 结果表明, 石墨烯表面等离子体激元的局域性在特定的参数点获得显著提高: 当μ = 0.7 m²/(V·s)时, 品质因子达到最大值Q_max = 1793; 当p = 235 nm或E_F = 0.72 eV时, 表面电场达到了入射光的3000倍以上. 强烈的局域性导致强烈 的光-物质相互作用, 因而本文提出的复合结构可实现高灵敏度传感器和高效率的非线性光学设备, 极大地扩展了石墨烯在纳米光学领域中的应用.     

Paramagnetic Impurities in Graphene Oxide

We report on electron paramagnetic resonance and nuclear magnetic resonance study of graphene oxide produced by the Hummers method. We show that this compound reveals isolated Mn²⁺ ions, which originate from potassium permanganate used in the process of the sample preparation. These ions are likely anchored to the graphene oxide planes and contribute to the ¹H and ¹³C spin–lattice relaxation.     

Oxidation of graphene on Ru(0 0 0 1) studied by scanning tunneling microscopy

The oxidation of graphene layer on Ru(0 0 0 1) has been investigated by means of scanning tunneling microscopy. Graphene overlayer can be formed by decomposing ethyne on Ru(0 0 0 1) at a temperature of about 1000 K. The lattice mismatch between the graphene overlayer and the substrate causes a moiré pattern with a superstructure in a periodicity of about 30 Å. The oxidation of graphene/Ru(0 0 0 1) was performed by exposure the sample to O₂ gas at 823 K. The results showed that, at the initial stage, the oxygen intercalation between the graphene and the Ru(0 0 0 1) substrate takes place at step edges, and extends on the lower steps. The oxygen intercalation decouples the graphene layer from the Ru(0 0 0 1) substrate. More oxygen intercalation yields wrinkled bumps on the graphene surface. The oxidation of graphene, or the removal of carbon atoms can be attributed to a process of the combination of the carbon atoms with atomic oxygen to form volatile reaction products. Finally, the Ru(0 0 0 1)-(2 × 1)O phase was observed after the graphene layer is fully removed by oxidation.     

Oscillatory magnetothermopower and resonant phonon drag in a high-mobility 2D electron gas

Experimental and theoretical evidence is presented for new low-magnetic-field (B<5 kG) 1/B oscillations in the thermoelectric power of a high-mobility GaAs/AlGaAs two-dimensional (2D) electron gas. The oscillations result from inter-Landau-level resonances of acoustic phonons carrying a momentum equal to twice the Fermi wave number at B=0. Numerical calculations show that both 3D and 2D phonons can contribute to this effect.     

Photoassociative spectroscopy at long range in ultracold strontium

We report photoassociative spectroscopy of 88Sr(2) in a magneto-optical trap operating on the 1S0-->3P1 intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600-2400 MHz from the 1S0-->1P1 atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the 1P1 atomic lifetime (tau=5.22+/-0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.     

Stable forms of two-dimensional crystals and graphene

We show that the “two-dimensional” graphene is stable due to transverse short-range displacements of carbon atoms, which may be described in a framework of Ising model with competing interactions. When temperature decreases, two transitions, high temperature disorder into order and order into low-temperature glass, arise. The graphene looks like a microscopic “washboard” with the wavelength of about 2–4 Å. Due to up–down asymmetry of the lattice distortions in graphene on substrate, a mini-bandgap arises. This leads to many new phenomena: a rectification of AC current induced by microwave or infrared radiation, the existence of self-trapping and a new type of fermionic mini-exciton-polaritons.     

One-step synthesis of a suspended ultrathin graphene oxide film: Application in transmission electron microscopy

Ultrathin graphene films find their use as advantageous support for nano- and biomaterials investigations. Thin film causes a very slight deterioration to measured signals, thus providing more details of the object's structure at nanoscale. The ultimate thinness of graphene works in the best way for this purpose. However, obtaining suspended thin film of a large-area, which is convenient for applications, is often a relatively complicated and time-consuming task. Here we present a one-step 1-min technique for synthesis of an extremely thin (about 1–2 nm) continuous film suspended over cells of a conventional copper grid (50–400 μm mesh). This technique enables us to acquire a large-area film which is water-resistant, stable in organic solvents and can act as a support when studying nanoparticles or biomaterials. Moreover, the very mechanism of the film formation can be interesting from the point of view of other applications of ultrathin graphene oxide papers.     

Scanning probe microscopy investigations of the electrical properties of chemical vapor deposited graphene grown on a 6H-SiC substrate

Sublimated graphene grown on SiC is an attractive material for scientific investigations. Nevertheless the self limiting process on the Si face and its sensitivity to the surface quality of the SiC substrates may be unfavourable for later microelectronic processes. On the other hand, chemical vapor deposited (CVD) graphene does not posses such disadvantages, so further experimental investigation is needed. In this paper CVD grown graphene on 6H-SiC (0 0 0 1) substrate was investigated using scanning probe microscopy (SPM). Electrical properties of graphene were characterized with the use of: scanning tunnelling microscopy, conductive atomic force microscopy (C-AFM) with locally performed C-AFM current–voltage measurements and Kelvin probe force microscopy (KPFM). Based on the contact potential difference data from the KPFM measurements, the work function of graphene was estimated. We observed conductance variations not only on structural edges, existing surface corrugations or accidental bilayers, but also on a flat graphene surface.     

Spectroscopy of the 1S0-3P0 clock transition of 87Sr in an optical lattice

We report on the spectroscopy of the 5s(2) 1S0(F=9/2)-->5s5p 3P0(F=9/2) clock transition of 87Sr atoms (natural linewidth of 1 mHz) trapped in a one-dimensional optical lattice. Recoilless transitions with a linewidth of 0.7 kHz as well as the vibrational structure of the lattice potential were observed. By investigating the wavelength dependence of the carrier linewidth, we determined the magic wavelength, where the light shift in the clock transition vanishes, to be 813.5+/-0.9 nm.     

Graphene domain boundaries on Pt(111) as nucleation sites for Pt nanocluster formation

The growth of Pt nanoclusters on a graphene layer on Pt(111) was studied with ultra high vacuum scanning tunneling microscopy. Different periodicities in the moiré patterns of the graphene layer are observed corresponding to different orientations with respect to the Pt(111) lattice. Various graphene orientations are possible because of a relatively weak graphene–Pt interaction. Following Pt deposition onto the graphene-covered surface, small Pt nanoclusters are observed to preferentially form along the moiré domain boundaries. The weak interaction of graphene with Pt(111) leads to a weak corrugation in the superlattice compared to other transition metals, such as Ru, but it is found even this weak corrugation is sufficient to serve as a template for the formation of mono-dispersed one-dimensional Pt nanocluster chains. These Pt nanoclusters are relatively stable and only undergo agglomeration at annealing temperatures above 600 K.     

Bag Boundaries for Quasispinor Confinement Within Nanolanes on a Graphene Sheet

The problem of bag boundary conditions within a field-theoretic approach is revisited to study confinement of massless Dirac quasispinors in monolayer graphene. While no-flux bag boundaries have previously been used to model lattice termination sites in graphene nanoribbons, a generalized setting is considered in which the confining boundaries are envisaged as arbitrary straight lines drawn across a graphene sheet and the quasispinor currents are allowed to partially permeate (leak) through such boundaries. Specifically focus is on rectangular nanolanes defined as areas confined between a pair of parallel lines at arbitrary separation on an unbounded lattice. It is shown that such nanolanes exhibit a considerable range of bandgap tunability depending on their widths and armchair, zigzag, or intermediate orientation. The case of nanoribbons can be derived as a special limit from the nanolane model. In this case, certain inconsistencies are clarified in previous implementations of no-flux bag boundaries and show that the continuum approach reproduces the tight-binding bandgaps accurately (within just a few percent in relative deviation) even as the nanoribbon width is decreased to just a couple of lattice spacings. This accentuates the proper use of boundary conditions when field-theoretic approaches are applied to graphene systems.     

Schonland ambiguity in the electron nuclear double resonance analysis of hyperfine interactions: principles and practice

For the analysis of the angular dependence of electron paramagnetic resonance (EPR) spectra of low-symmetry centres with S=1/2 in three independent planes, it is well-established-but often overlooked-that an ambiguity may arise in the best-fit g<--> tensor result. We investigate here whether a corresponding ambiguity also arises when determining the hyperfine coupling (HFC) A<--> tensor for nuclei with I=1/2 from angular dependent electron nuclear double resonance (ENDOR) measurements. It is shown via a perturbation treatment that for each set of M(S) ENDOR branches two best-fit A<--> tensors can be derived, but in general only one unique solution simultaneously fits both. The ambiguity thus only arises when experimental data of only one M(S) multiplet are used in analysis or in certain limiting cases. It is important to realise that the ambiguity occurs in the ENDOR frequencies and therefore the other best-fit result for an ENDOR determined A<--> tensor depends on various details of the ENDOR experiment: the M(S) state of the fitted transitions, the microwave frequency (or static magnetic field) in the ENDOR measurements and the rotation planes in which data have been collected. The results are of particular importance in the identification of radicals based on comparison of theoretical predictions of HFCs with published literature data. A procedure for obtaining the other best-fit result for an ENDOR determined A<--> tensor is outlined.     

The effect of gradually constricted channel on the I–V characteristics of graphene sheets

Ideal graphene is a gapless semiconductor consisting of a single layer of carbon atoms regularly arranged in a honeycomb lattice having infinite spatial extent in the (x,y)-plane, in which electrons behave as Dirac massless fermions. Even neglecting interactions with the anchoring substrate, a graphene sheet in real world has finite extent, leading to distinctive features in the conductivity of a given sample. In this letter we study the effect of a gradual channel constriction in graphene nanoribbons on their I–V characteristics, using non-equilibrium Green's function formalism. The constriction width and the border cutting angle are the main parameters to be varied. We found that transmission through the channel is considerably affected by these parameters, presenting sharp peaks at specific energies, which can be attributed to a resonance due to the tuning of energy eigenvalues.     

Theoretical understanding of adlayer structure,thermal stability and electronic property of graphene molecules

The geometry of hexafluorotribenzo[a,g,m]coronene with n-carbon alkyl chains [FTBC-Cn (n = 4, 6, 8, 12)] and their supramolecule self-assembly on a highly oriented pyrolytic graphite (HOPG) surface has been optimized by molecular dynamics simulations using COMPASS force field at 0 K, 298 K, 333 K and 353 K. Electronic properties and intermolecular interactions in graphene supramolecule assembly have been studied by the first principle methods based on the density functional theory (DFT). It is indicated that the thermal stability and electronic properties of graphene molecules can be tunable by attaching alkyl chains to a triangular graphene sheet, and changing the length of the alkyl chain, and self-assembling on a certain substrate. The main results are as follows. The geometry and energy gap of the FTBC-Cn single molecule and their supramolecule self-assembly on HOPG are both stable with the changes of the temperature from 0 K to 353 K and the number of carbon atoms on the alkyl chain. The simulation results of geometry, energy gap as well as STM images of graphene supramolecule assembly are in good agreement with the corresponding experimental results in room temperature. Furthermore, the electronic properties of graphene supramolecule assembly at the temperatures of 0 K, 333 K and 353 K are also predicted. When a triangular graphene molecule attached with six alkyl chains, the energy gaps are increased and stabilized at the temperature from 0 K to 353 K. After FTBC-Cn molecule self-assembly on a HOPG substrate, the energy gap is reduced but still stable.     

An improved image alignment procedure for high-resolution transmission electron microscopy

Image alignment is essential for image processing methods such as through-focus exit-wavefunction reconstruction and image averaging in high-resolution transmission electron microscopy. Relative image displacements exist in any experimentally recorded image series due to the specimen drifts and image shifts, hence image alignment for correcting the image displacements has to be done prior to any further image processing. The image displacement between two successive images is determined by the correlation function of the two relatively shifted images. Here it is shown that more accurate image alignment can be achieved by using an appropriate aperture to filter the high-frequency components of the images being aligned, especially for a crystalline specimen with little non-periodic information. For the image series of crystalline specimens with little amorphous, the radius of the filter aperture should be as small as possible, so long as it covers the innermost lattice reflections. Testing with an experimental through-focus series of Si[1 1 0] images, the accuracies of image alignment with different correlation functions are compared with respect to the error functions in through-focus exit-wavefunction reconstruction based on the maximum-likelihood method. Testing with image averaging over noisy experimental images from graphene and carbon-nanotube samples, clear and sharp crystal lattice fringes are recovered after applying optimal image alignment.