X. R. Qin

Scanning tunneling microscopy of single-layer MoS2 in water and butanol

Abstract Single layers of MoS 2 have been studied with a scanning tunneling microscope operating in air. Films were prepared by depositing either an aqueous or butanol [CH 3 (CH 2 ) 3 OH] suspension of single-layer MoS 2 on graphite substrates. The images obtained from both types of films showed that the unit cell of the single layers corresponds to an approximate 2x1 superlattice of the hexagonal 2H-MoS 2 structure. The image of dry restacked MoS 2 from an aqueous suspension transforms back to the hexagonal MoS 2 pattern.

Structure of tetracene films on hydrogen-passivated Si(001) studied via STM, AFM, and NEXAFS

Scanning tunneling microscopy STM, atomic force microscopy AFM, and near-edge x-ray absorption fine structure NEXAFS have been used to study the structure of tetracene films on hydrogen-passivated Si001. STM imaging of the films with nominal thickness of three monolayers 3M L exhibits the characteristic “herringbone” molecular packing known from the bulk crystalline tetracene, showing standing molecules on the ab plane. The dimensions and orientation of the herringbone lattice indicate a commensurate structural relationship between the lattice and the crystalline substrate. The corresponding AFM images illustrate that at and above the third layer of the films, the islands are anisotropic, in contrast with the submonolayer fractals, with two preferred growth directions appearing orthogonal to each other. The polarization dependent NEXAFS measurements indicate that the average molecular tilting angle with respect to the surface first increases with the film thickness up to 3 ML, then stabilizes at a value close to the bulk tetracene case afterwards. The combined results indicate a distinct growth morphological change that occurs around a few monolayers of thickness.

Formation of glass fiber tips for scanning near-field optical microscopy by sealed- and open-tube etching

A method, sealed-tube etching, for fabricating glass fiber tips used in scanning near-field optical microscopy is reported. Two glass tips are formed simultaneously inside the acrylate jacket of an optical fiber that is sealed at the bottom end with plastic prior to etching in hydrofluoric acid. The qualities of the tips generated from this and conventional tube etching methods are compared in terms of their taper length, surface smoothness, as well as time evolution of the tip shapes. Etching mechanisms are proposed for this method, which provides more insight into the conventional tube etching method than previously considered.

Thickness-dependent mobility in tetracene thin-film field-effect-transistors

The authors report the thickness-dependent mobility of tetracene field-effect transistors with silicon oxide as the gate dielectric. The lowest field-effect mobility starts at ∼3 monolayers (ML). The mobility increases with film thickness linearly and saturates at ∼0.38 cm2V−1 s−1 at the coverage of 8 ML. Surface morphology of tetracene films and possible mechanisms that could account for the dependence of the mobility on thickness are discussed.

Initial beam emission spectroscopy diagnostic system on HL-2A tokamak

A beam emission spectroscopy system is being developed and deployed on the HL-2A tokamak to measure local low wavenumber (k ⊥ ρ i < 1) density fluctuations by measuring the Doppler-shifted emission from a 50 kV deuterium heating neutral beam. High spatial resolution (Δr ≤ 1 cm, Δz ≤ 1.5 cm) measurements are achieved with customized in-vacuum optics. High frequency, high-gain preamplifiers sample the light intensity at a Nyquist frequency of 1 MHz and achieve a high S/N ratio via high optical throughput, low-noise preamplifiers, and high quantum efficiency photodiodes. A first set of 16 detector channels [configured in an 8 (radial) × 2 (poloidal) array] has been installed and tested at HL-2A, covering the radial range r/a = 0.8-1.1. The frequency and wavenumber spectra have been measured under different plasma conditions. Initial measurements have demonstrated the capability of measuring edge plasma density fluctuation spectra and the poloidal flow velocity fields with a high S/N ratio.

Flux dependence of the morphology of a tetracene film on hydrogen-passivated Si(100)

We have recently argued that manganites do not possess stripes of charge order, implying that the electron-lattice coupling is weak [Loudon et al., Phys. Rev. Lett. 94, 097202 (2005)]. Here we independently argue the same conclusion based on transmission electron microscopy measurements of a nanopatterned epitaxial film of La{sub 0.5}Ca{sub 0.5}MnO{sub 3}. In strain relaxed regions, the superlattice period is modified by 2% to 3% with respect to the parent lattice, suggesting that the two are not strongly tied.Minimalist theories of complex systems are broadly of two kinds: mean field and axiomatic. So far, all theories of complex properties absent from simple systems and intrinsic to glasses are axiomatic. Stretched Exponential Relaxation (SER) is the prototypical complex temporal property of glasses, discovered by Kohlrausch 150 years ago, and now observed almost universally in microscopically homogeneous, complex nonequilibrium materials, including luminescent electronic Coulomb glasses. A critical comparison of alternative axiomatic theories with both numerical simulations and experiments strongly favors channeled dynamical trap models over static percolative or energy landscape models. The topics discussed cover those reported since the authors review article in 1996, with an emphasis on parallels between channel bifurcation in electronic and molecular relaxation.The local atomic structure of the Ag induced Si(111)-({radical}(3)x{radical}(3)) surface has been investigated using photoelectron diffraction (PED) at 10 and 300 K. Two surface components, whose intensities varied by changing the photon energy as a consequence of diffraction effects, were observed in the Si 2p core-level spectra at both temperatures. The good agreement between the experimental PED patterns of the Si 2p surface components and the simulated PED patterns indicates that the atomic structure of this surface follows the inequivalent triangle model. Further, since the PED patterns obtained at 10 and 300 K resemble each other closely, we conclude that the local atomic structure of the Ag/Si(111)-({radical}(3)x{radical}(3)) surface is the same at the two temperatures, and thus that the origin of the transition reported in the literature is an order-disorder transition.Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La{sub 2}CuO{sub 4} single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined to drive a continuous reorientation of the copper spins from the orthorhombic b axis to the c axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for nonstoichiometric La{sub 2}CuO{sub 4} is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work.Positron annihilation spectroscopy was applied to study relaxed P-doped n-type and undoped Si{sub 1-x}Ge{sub x} layers with x up to 0.30. The as-grown SiGe layers were found to be defect free and annihilation parameters in a random SiGe alloy could be represented as superpositions of annihilations in bulk Si and Ge. A 2 MeV proton irradiation with a 1.6x10{sup 15} cm{sup -2} fluence was used to produce saturated positron trapping in monovacancy related defects in the n-type layers. The defects were identified as V-P pairs, the E center. The distribution of Si and Ge atoms surrounding the E center was the same as in the host lattice. The process leading to the formation of V-P pairs therefore does not seem to have a significant preference for either Si or Ge atoms. In undoped Si{sub 1-x}Ge{sub x} we find that a similar irradiation produces a low concentration of divacancies or larger vacancy defects and found no evidence of monovacancies surrounded by several Ge atoms.Structural properties of the spin chain and ladder compound Sr{sub 14}Cu{sub 24}O{sub 41} have been studied using high energy x-ray diffraction. Strong incommensurate modulation reflections are observed due to the lattice mismatch of the chain and ladder structure, respectively. While modulation reflections of low orders display only a weak temperature dependence, higher orders dramatically increase in intensity when cooling the sample to 10 K. All observed modulation reflections are indexed within a super space group symmetry and no structural phase transition could be identified between 10 K and room temperature. We argue that these modulation reflections are not caused by a fivefold periodicity of the chain lattice, as claimed by Fukuda et al., Phys. Rev. B 66, 012104 (2002), but that holes localize in the potential given by the lattice modulation, which in turn gives rise to a further deformation of the lattice.We report neutron diffraction experiments on the light-induced metastable state SI in single crystals of Na{sub 2}[Fe(CN){sub 5}NO]{center_dot}2D{sub 2}O. It is shown that the metastable state SI corresponds to a linkage isomer of the NO group, the so-called isonitrosyl configuration where the NO ligand is oxygen bound to the central iron atom.The impact of group-III vacancy diffusion, generated during dielectric cap induced intermixing, on the energy state transition and the inhomogeneity reduction in the InGaAs/GaAs quantum-dot structure is investigated. We use a three-dimensional quantum-dot diffusion model and photoluminescence data to determine the thermal and the interdiffusion properties of the quantum dot. The band gap energy variation related to the dot uniformity is found to be dominantly affected by the height fluctuation. A group-III vacancies migration energy H{sub m} for InGaAs quantum dots of 1.7 eV was deduced. This result is similar to the value obtained from the bulk and GaAs/AlGaAs quantum-well materials confirming the role of SiO{sub 2} capping enhanced group-III vacancy induced interdiffusion in the InGaAs quantum dots.We report vibrating wire viscometer experiments in the concentrated and dilute phase of saturated {sup 3}He-{sup 4}He mixtures showing that the slip length may become orders of magnitude larger than the mean free path due to specular scattering of the {sup 3}He quasiparticles with a {sup 4}He coating adsorbed at the surface of the wire. Since the liquid does not almost stick to the surface, the boundary conditions for fluid flow are unusual and not accounted for by the current theory for slip [H. Hoejgaard Jensen et al., J. Low Temp. Phys. 41, 473 (1980)]. The experimental results are in excellent agreement with a recent theory for slip [R. Bowley and J. Owers-Bradley, J. Low Temp. Phys. 136, 15 (2004)] which accounts for the effect of the cylindrical geometry and for velocity slip in directions normal as well as tangential to the surface of the wire. We find that our viscosity measurements in the dilute phase resulting from the data analysis based on the recent slip theory are in better agreement with the Fermi liquid theory than previous experimental results.Magnetization measurements prove that the magnetic properties of large-angle ({theta}>30 deg. ) bismuth bicrystals with a crystallite interface (CI) of twisting types essentially differ from well-known results on single-crystalline specimens. Two superconducting phases with T{sub c}{approx}8.4 K and {approx}4.3 K were observed at the CI of bicrystals while ordinary rhombohedral Bi is not a superconductor. We conclude that these phases have to do with the central part and the adjacent layers of the CI of bicrystals.The local structure around Ag ions in silver borate glasses g-Ag{sub 2}O{center_dot}nB{sub 2}O{sub 3} (n=2,4) was studied by x-ray absorption spectroscopy at the Ag K edge for temperatures from 77 to 450 K. Extended x-ray absorption fine structure (EXAFS) analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribution functions (RDFs) around Ag ions were reconstructed using a method based on the direct inversion of the EXAFS expression. The RDFs consist of about eight atoms (oxygens and borons), exhibit a relatively weak temperature dependence, and indicate the presence of strong static disorder. Two main components can be identified in RDFs, located at about 2.3-2.4 A and 2.5-3.4 A, respectively. The chemical types of atoms contributing to the RDF were determined via a simulation of configurationally averaged x-ray absorption near-edge structure (XANES) and EXAFS signals. The immediate neighborhood of Ag contains mostly oxygens while borons dominate at larger distances. The combination of EXAFS and XANES techniques allowed us to determine a more complete structural model than would be possible by relying solely on either EXAFS or XANES alone.Hall effects of the La{sub 0.7}Ce{sub 0.3}MnO{sub 3+{delta}} film, which is believed an electron-doped manganite, have been experimentally studied, and a positive normal Hall coefficient is observed below the Curie temperature when the oxygen content of the film varies in a wide range. These observations may be attributed to the presence of excessive oxygen and composition distribution in the film, which may occur companying tetravalent ion doping. Removing excessive oxygen drives the system into the electron-doping state, however, the resistivity increases monotonically with oxygen loss, and the metal-to-semiconductor transition typical for a hole-doped manganite disappears. These results suggest the determinative role of hole doping for the resistive and magnetic behaviors in La{sub 0.7}Ce{sub 0.3}MnO{sub 3+{delta}}.We studied the influence of the disorder introduced in polycrystalline MgB{sub 2} samples by neutron irradiation. To circumvent self-shielding effects due to the strong interaction between thermal neutrons and {sup 10}B we employed isotopically enriched {sup 11}B which contains 40 times less {sup 10}B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low {sup 10}B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1-2x10{sup 18} cm{sup -2}. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behavior has been carried out to identify what scattering and pinning mechanisms come into play. Finally, the correlation between some characteristic lengths and the transition widths is analyzed.The structure of Na{sub 0.5}CoO{sub 2}, the low-temperature insulator that separates the antiferromagnetic and normal metals in the Na{sub x}CoO{sub 2} phase diagram, is studied by high-resolution powder neutron diffraction at temperatures between 10 and 300 K. Profile analysis confirms that it has an orthorhombic symmetry structure, space group Pnmm, consisting of layers of edge-sharing CoO{sub 6} octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The oxygen content is found to be stoichiometric within 1%, indicating that the Na concentration accurately determines the electronic doping. The Na ordering creates two distinct Co sites, in parallel chains running along one crystallographic direction. The differences in their Co-O bond distances and the derived bond valence sums, reflections of the degree of charge ordering in this phase, are very small.The temperature dependence of the local structure of V{sub 2}O{sub 3} in the vicinity of the metal-to-insulator transition (MIT) has been investigated using hard x-ray absorption spectroscopy. It is shown that the vanadium pair distance along the hexagonal c axis changes abruptly at the MIT as expected. However, a continuous increase of the tilt of these pairs sets in already at higher temperatures and reaches its maximum value at the onset of the electronic and magnetic transition. These findings confirm recent theoretical results which claim that electron-lattice coupling is important for the MIT in V{sub 2}O{sub 3}. Our results suggest that the distortion of the symmetry of the basal plane plays a decisive role for the MIT and orbital degrees of freedom drive the MIT via changes in hybridization.We present here ab initio determinations of the nuclear-quadrupole moment Q of hyperfine-probe-nuclear states of three different In isotopes: the 5{sup +} 192 keV excited state of {sup 114}In (probe for nuclear quadrupole alignment spectroscopy), the 9/2{sup +} ground state of {sup 115}In (nuclear magnetic and nuclear quadrupole resonance probe), and the 3/2{sup +} 659 keV excited state of {sup 117}In (perturbed angular correlations probe). These nuclear-quadrupole moments were determined by comparing experimental nuclear-quadrupole frequencies to the electric field gradient tensor calculated with high accuracy at In sites in metallic indium within the density functional theory. These ab initio calculations were performed with the full-potential linearized augmented plane wave method. The results obtained for the quadrupole moments of {sup 114}In [Q({sup 114}In)=-0.14(1) b] are in clear discrepancy with those reported in the literature [Q({sup 114}In)=+0.16(6) b and +0.739(12) b]. For {sup 115}In and {sup 117}In our results are in excellent agreement with the literature and in the last case Q({sup 117}In) is determined with more precision. In the case of Q({sup 117}In), its sign cannot be determined because standard {gamma}-{gamma} perturbed angular correlations experiments are not sensitive to the sign of the nuclear-quadrupole frequency.An original epitaxial system consisting of two ferrimagnetic insulator layers (CoFe{sub 2}O{sub 4} and Fe{sub 3}O{sub 4}) separated by a nonmagnetic metallic layer (Au) has been grown. The transport properties in the current in plane geometry indicate that the conduction of the CoFe{sub 2}O{sub 4}/Au/Fe{sub 3}O{sub 4} trilayer takes place within the thin metallic layer. The giant magnetoresistance (GMR) observed (2.6% at 10 K) is associated to the switching from a parallel to an antiparallel configuration of the magnetization of the two ferrite layers and corresponds to the spin dependence of electron reflection at the interfaces with a large contribution of specular reflections. The increase of the GMR (5% at 10 K) in the symmetrical interface CoFe{sub 2}O{sub 4}/Fe{sub 3}O{sub 4}/Au/Fe{sub 3}O{sub 4} system and the effect of the interface roughness on the GMR confirm the presence of this spin-dependent specular reflection.The effect of externally applied pressure on the magnetic behavior of Cu{sub 2}Te{sub 2}O{sub 5}(Br{sub x}Cl{sub 1-x}){sub 2} with x=0, 0.73, and 1, is investigated by a combination of magnetic susceptibility, neutron diffraction, and neutron inelastic scattering measurements. The magnetic transition temperatures of the x=0 and 0.73 compositions are observed to increase linearly with increasing pressure at a rate of 0.23(2) and 0.04(1) K/kbar, respectively. However, the bromide shows contrasting behavior with a large suppression of the transition temperature under pressure, at a rate of -0.95(9) K/kbar. In neutron inelastic scattering measurements of Cu{sub 2}Te{sub 2}O{sub 5}Br{sub 2} under pressure only a small change to the ambient pressure magnetic excitations were observed. A peak in the density of states was seen to shift from {approx}5 meV in ambient pressure to {approx}6 meV under an applied pressure of 11.3 kbar, which was associated with an increase in the overall magnetic coupling strength.A KH{sub 2}PO{sub 4} (KDP) crystal, irradiated by a 1 MeV hydrogen ion beam to a dose of 10{sup 15} ions/cm{sup 2}, was studied by means of x-ray diffraction (XRD), {sup 1}H nuclear magnetic resonance (NMR), and dielectric constant measurements. The XRD pattern for the a-cut KDP crystal revealed a decrease in the lattice constant along the a axis after the proton irradiation. According to the {sup 1}H NMR spin-lattice relaxation rate measurements, the proton irradiation gave rise to reduction in the activation energy in the paraelectric phase, from 0.42 to 0.28 eV, in agreement with the temperature dependent second moment measurements indicating the proton motion is more activated after the proton irradiation. Besides, analysis of the temperature-dependent dielectric constants using a mean-field approximation revealed a change in the hydrogen bond induced by the proton irradiation.

Structure and Morphology of Tetracene Thin Films on Hydrogen-Terminated Si(001)

Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the structure of tetracene films on hydrogen-passivated Si(001). A distinct growth morphology change that occurs around a few monolayers of film thickness was characterized. This coverage-dependent film structural phase transition leads to a molecularly ordered film structure commensurate with the crystalline substrate.