15 Search Results

We report that bottom-up graphene nanoribbons (GNRs) have recently been shown to host nontrivial topological phases. Here, we report the fabrication and characterization of deterministic GNR quantum dots whose orbital character is defined by zero-mode states arising from nontrivial topological interfaces. Topological control was achieved through the synthesis and on-surface assembly of three distinct molecular precursors designed to exhibit structurally derived topological electronic states. Using a combination of low-temperature scanning tunneling microscopy and spectroscopy, we have characterized two GNR topological quantum dot arrangements synthesized under ultrahigh vacuum conditions. Our results are supported by density-functional theory and tight-binding calculations, revealing thatmore » the magnitude and sign of orbital hopping between topological zero-mode states can be tuned based on the bonding geometry of the interconnecting region. These results demonstrate the utility of topological zero modes as components for designer quantum dots and advanced electronic devices.« less

The incorporation of nanoscale pores into a sheet of graphene allows it to switch from an impermeable semimetal to a semiconducting nanosieve. Nanoporous graphenes are desirable for applications ranging from high-performance semiconductor device channels to atomically thin molecular sieve membranes, and their performance is highly dependent on the periodicity and reproducibility of pores at the atomic level. Achieving precise nanopore topologies in graphene using top-down lithographic approaches has proven to be challenging due to poor structural control at the atomic level. Alternatively, atomically precise nanometer-sized pores can be fabricated via lateral fusion of bottom-up synthesized graphene nanoribbons. This technique, however,more » typically requires an additional high temperature cross-coupling step following the nanoribbon formation that inherently yields poor lateral conjugation, resulting in 2D materials that are weakly connected both mechanically and electronically. Here, we demonstrate a novel bottom-up approach for forming fully conjugated nanoporous graphene through a single, mild annealing step following the initial polymer formation. We find emergent interface-localized electronic states within the bulk band gap of the graphene nanoribbon that hybridize to yield a dispersive two-dimensional low-energy band of states. We show that this low-energy band can be rationalized in terms of edge states of the constituent single-strand nanoribbons. The localization of these 2D states around pores makes this material particularly attractive for applications requiring electronically sensitive molecular sieves.« less

The design and fabrication of robust metallic states in graphene nanoribbons (GNRs) are challenging because lateral quantum confinement and many-electron interactions induce electronic band gaps when graphene is patterned at nanometer length scales. Recent developments in bottom-up synthesis have enabled the design and characterization of atomically precise GNRs, but strategies for realizing GNR metallicity have been elusive. Here we demonstrate a general technique for inducing metallicity in GNRs by inserting a symmetric superlattice of zero-energy modes into otherwise semiconducting GNRs. We verify the resulting metallicity using scanning tunneling spectroscopy as well as first-principles density-functional theory and tight-binding calculations. Our resultsmore » reveal that the metallic bandwidth in GNRs can be tuned over a wide range by controlling the overlap of zero-mode wave functions through intentional sublattice symmetry breaking.« less

Topological insulators are an emerging class of materials that host highly robust in-gap surface or interface states while maintaining an insulating bulk1,2. Most advances in this field have focused on topological insulators and related topological crystalline insulators3 in two dimensions4,5,6 and three dimensions7,8,9,10, but more recent theoretical work has predicted the existence of one-dimensional symmetry-protected topological phases in graphene nanoribbons (GNRs)11. The topological phase of these laterally confined, semiconducting strips of graphene is determined by their width, edge shape and terminating crystallographic unit cell and is characterized by a Z2 invariant12 (that is, an index of either 0 or 1,more » indicating two topological classes—similar to quasi-one-dimensional solitonic systems13,14,15,16). Interfaces between topologically distinct GNRs characterized by different values of Z2 are predicted to support half-filled, in-gap localized electronic states that could, in principle, be used as a tool for material engineering11. Here we present the rational design and experimental realization of a topologically engineered GNR superlattice that hosts a one-dimensional array of such states, thus generating otherwise inaccessible electronic structures. This strategy also enables new end states to be engineered directly into the termini of the one-dimensional GNR superlattice. Atomically precise topological GNR superlattices were synthesized from molecular precursors on a gold surface, Au(111), under ultrahigh-vacuum conditions and characterized by low-temperature scanning tunnelling microscopy and spectroscopy. Our experimental results and first-principles calculations reveal that the frontier band structure (the bands bracketing filled and empty states) of these GNR superlattices is defined purely by the coupling between adjacent topological interface states. This manifestation of non-trivial one-dimensional topological phases presents a route to band engineering in one-dimensional materials based on precise control of their electronic topology, and is a promising platform for studies of one-dimensional quantum spin physics.« less

Current applications of paratellurite TeO{sub 2} in optics and nuclear physics require improving the yield and optical quality of synthesized single crystals. Modelling of the growth process is a useful tool for such purpose, however, involving the knowledge of several thermodynamic parameters of both solid and liquid phase of the compound. This work describes the determination of two main unknown ones, namely dynamic viscosity {eta} and thermal conductivity {lambda}{sub l} of the liquid phase. Measurements of {eta} are performed by means of a specifically designed apparatus; over a 40 K temperature interval above the melting point we find a meanmore » value of 0.028 Pa.s. This data is used in commercial software to depict the evolution of the solid-liquid interface in our Bridgman growth set-up. Comparison of the distribution of defects observed in grown ingots with the results of modelling allows estimating {lambda}{sub l} as lying in the range 3.50-3.75 W m{sup -1} K{sup -1}.« less

The Yb-content Li{sub 6}Ln(BO{sub 3}){sub 3} (Ln: Gd, Y) solid solution has been investigated. Crystal growth has been successful for several compositions. A 22% molar content of ytterbium ions was determined by chemical analysis (ICP). Physical properties relevant to laser operation like mechanical hardness, thermal expansion and thermal conductivity were measured on single crystals. Optical measurements, including refractive index and low temperature spectroscopy, were also performed. Finally, the effect of the Y/Gd ratio is discussed. -- Graphical abstract: Several solid solutions of a rare earth borate were studied. The figure illustrates one of these single crystals obtained by Czochralski andmore » shows thermal behaviour and absorption spectra at low temperature. Display Omitted Research highlights: {yields} We have grown by Czochralski method five Li{sub 6}Ln(BO{sub 3}){sub 3} (Ln=Y, Gd,Yb) single crystals. {yields} Chemical, physical and spectroscopic characteristics are reported. {yields} Data relevant to laser operation are listed.« less

A procedure for calculating the relevant concentrations of sulfur dioxide in an MgO-SO{sub 2}-H{sub 2}O system and the construction of an equilibrium-operational diagram showing the relation between c{sub tot} (the amount of total SO{sub 2} per unit mass of solution), c{sub com} (the total concentration of Mg{sup 2+} in the solution expressed with SO{sub 3}{sup 2{minus}} and HSO{sub 3}{sup {minus}} using the corresponding stoichiometric factors), Y (the excess of SO{sub 2} over amount necessary to form Mg(HSO{sub 3}){sub 2} only, per unit mass of solution), pH, p{sub SO{sub 2}} (the partial pressure of SO{sub 2}), and the solubility of MgSO{submore » 3} at different temperatures in the range from 298 to 323 K is described. The procedure uses the Rudzinski method for determining the pH of a complex aqueous solution and Pitzer`s ion interaction model to calculate the activity coefficients in the mixture at various temperatures. The calculated concentrations match closely to those obtained by experiment. The design diagram at 323 K is used for graphical representation of the operating lines for absorption of SO{sub 2} taking place within an industrial SO{sub 2} absorption unit. Stage efficiency and SO{sub 2} removal of commercial Venturi units was computed. The number of overall gas-phase mass transfer units for each stage was found to be in the range from 1 to 3.« less

Described herein is a procedure for calculating the relevant concentrations of sulfur dioxide in a MgO-SO{sub 2}-H{sub 2}O system and the construction of an equilibrium diagram showing the relationships between total SO{sub 2}, combined SO{sub 2}, true free SO{sub 2}, and pH. The procedure used the Rudzinski method for determining the pH of complex aqueous solutions and the modified Debye-Hueckel expression for calculating activity coefficients. The calculated concentrations matched closely those obtained from Hagefeldt`s diagram and measured concentrations from an actual industrial magnesium based pulping process. The equilibrium diagram was used for the graphical representation of the mass balances ofmore » the industrial absorption process.« less

Thermal storage in water has become very common in the last decades in many countries. Short term water heat storage systems play an important role in combined heat and power plants, in the process industry, in solar thermal systems and for domestic hot water production. The water volume must be thermally stratified to avoid degradation of heat quality. The temperature distribution in the storage is, however, the integrated result of a complicated process where many different phenomena act together. Temperature measurements only will not be sufficient to explain and evaluate the thermal behavior of a storage system; increased knowledge aboutmore » the velocity field is needed. A video based Particle Tracking Velocimetry technique has been developed and used to document the non-stationary velocity field in a water heat storage system. An ordinary video camera is used to record the in-plane movements of particles in a light sheet in seeded water. The technique offers a powerful method for studies of fluid flow and velocity field. The combination of particle traces and vector plots have proven to be an efficient tool in gaining knowledge about crucial phenomena. Velocity fields are presented, showing mixing near the inlet, natural convection at the wall and exchange of water between the boundary layer and the core, and their impact on the stratification in the storage.« less

Juberg-Marsidi syndrome (McKusick 309590) is a rare X-linked recessive condition characterized by severe mental retardation, growth failure, sensorineural deafness, and microgenitalism. Here the authors report on the genetic mapping of the Juberg-Marsidi gene to the proximal long arm of the X chromosome (Xq12-q21) by linkage to probe pRX214H1 at the DXS441 locus (Z = 3.24 at [theta] = .00). Multipoint linkage analysis placed the Juberg-Marsidi gene within the interval defined by the DXS159 and the DXYS1X loci in the Xq12-q21 region. These data provide evidence for the genetic distinction between Juberg-Marsidi syndrome and several other X-linked mental retardation syndromes thatmore » have hypogonadism and hypogenitalism and that have been localized previously. Finally, the mapping of the Juberg-Marsidi gene is of potential interest for reliable genetic counseling of at-risk women. 25 refs., 2 figs., 3 tabs.« less