Chemistry. How to assemble a molecular junction

Junctions between metals and molecules play an important role in molecular electronics. Advances in this field are hampered by the lack of understanding of the electronic structure of organic-metal interfaces. In his Perspective, Kummel highlights the report by Nazin et al. (3), who have used scanning tunneling microscopy (STM) to assemble a metal-molecule-metal junction. Subsequently, they employed the STM tip to probe the atomic structure and local electronic properties of the metal-molecule interface in unprecedented detail. They find evidence for strong coupling between the molecular and metal states. Such coupling affects the conductivity of metal-molecule-metal junctions.     

Modulated chemical doping of individual carbon nanotubes

Modulation doping of a semiconducting single-walled carbon nanotube along its length leads to an intramolecular wire electronic device. The nanotube is doped n-type for half of its length and p-type for the other half. Electrostatic gating can tune the system into p-n junctions, causing it to exhibit rectifying characteristics or negative differential conductance. The system can also be tuned into n-type, exhibiting single-electron charging and negative differential conductance at low temperatures. The low-temperature behavior is manifested by a quantum dot formed by chemical inhomogeneity along the tube.     

Celestial mechanics on a microscopic scale

Classical and semiclassical methods are unrivaled in providing an intuitive and computationally tractable approach to the study of atomic, molecular, and nuclear dynamics. An important advantage of such methods is their ability to uncover in a single picture underlying structures that may be hard to extract from the profusion of data supplied by detailed quantum calculations. Modern trends in semiclassical mechanics are described, particularly the combination of group theoretical methods with techniques of nonlinear dynamics. Application is made to intramolecular energy transfer and to the electronic structure of atomic Rydberg states in external electric and magnetic fields.     

Engineered interface of magnetic oxides

Interface-selective probing of magnetism is a key issue for the design and realization of spin-electronic junction devices. Here, magnetization-induced second-harmonic generation was used to probe the local magnetic properties at the interface of the perovskite ferromagnet La(0.6)Sr(0.4)MnO3 with nonmagnetic insulating layers, as used in spin-tunnel junctions. We show that by grading the doping profile on an atomic scale at the interface, robust ferromagnetism can be realized around room temperature. The results should lead to improvements in the performance of spin-tunnel junctions.     

The optical resonances in carbon nanotubes arise from excitons

Optical transitions in carbon nanotubes are of central importance for nanotube characterization. They also provide insight into the nature of excited states in these one-dimensional systems. Recent work suggests that light absorption produces strongly correlated electron-hole states in the form of excitons. However, it has been difficult to rule out a simpler model in which resonances arise from the van Hove singularities associated with the one-dimensional band [corrected] structure of the nanotubes. Here, two-photon excitation spectroscopy bolsters the exciton picture. We found binding energies of approximately 400 millielectron volts for semiconducting single-walled nanotubes with 0.8-nanometer diameters. The results demonstrate the dominant role of many-body interactions in the excited-state properties of one-dimensional systems.     

Quantum superposition of macroscopic persistent-current states

Microwave spectroscopy experiments have been performed on two quantum levels of a macroscopic superconducting loop with three Josephson junctions. Level repulsion of the ground state and first excited state is found where two classical persistent-current states with opposite polarity are degenerate, indicating symmetric and antisymmetric quantum superpositions of macroscopic states. The two classical states have persistent currents of 0.5 microampere and correspond to the center-of-mass motion of millions of Cooper pairs.     

Quantum States and atomic structure of silicon surfaces

The electronic and geometric structures of surfaces are closely related to each other. Conventional surface science techniques can study one or the other, but not both at the same time. Recent developments in scanning tunneling microscopy have made it possible to study simultaneously the electronic and geometric structure of Si(111) and Si(001) surfaces. Surface states can be atomically resolved in space and energy; thus the electronic structure of single atoms on surfaces can be studied in detail. The various surface states observed on silicon surfaces are found to derive from different atomic-scale features in the surface geometric structure. Scanning tunneling microscopy has now bridged the gap between electronic and geometric structure, providing a unique opportunity to obtain a better understanding of many surface processes at the atomic level.     

Bistability in atomic-scale antiferromagnets

Control of magnetism on the atomic scale is becoming essential as data storage devices are miniaturized. We show that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Néel states, that are stable for hours at low temperature. For the smallest structures, we observed transitions between Néel states due to quantum tunneling of magnetization. We sensed the magnetic states of the designed structures using spin-polarized tunneling and switched between them electrically with nanosecond speed. Tailoring the properties of neighboring antiferromagnetic nanostructures enables a low-temperature demonstration of dense nonvolatile storage of information.     

Femtosecond dynamics of electron localization at interfaces

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.     

Theory and modeling of stereoselective organic reactions

Theoretical investigations of the transition structures of additions and cycloadditions reveal details about the geometries of bond-forming processes that are not directly accessible by experiment. The conformational analysis of transition states has been developed from theoretical generalizations about the preferred angle of attack by reagents on multiple bonds and predictions of conformations with respect to partially formed bonds. Qualitative rules for the prediction of the stereochemistries of organic reactions have been devised, and semi-empirical computational models have also been developed to predict the stereoselectivities of reactions of large organic molecules, such as nucleophilic additions to carbonyls, electrophilic hydroborations and cycloadditions, and intramolecular radical additions and cycloadditions.     

Quantum confinement and host/guest chemistry: probing a new dimension

Nanoparticulate metals and semiconductors that have atomic arrangements at the interface of molecular clusters and "infinite" solid-state arrays of atoms have distinctive properties determined by the extent of confinement of highly delocalized valence electrons. At this interface, the total number of atoms and the geometrical disposition of each atom can be used to significantly modify the electronic and photonic response of the medium. In addition to teh novel inherent physical properties of the quantum-confined moieties, their "packaging" into nanocomposite bulk materials can be used to define the confinement surface states and environment, intercluster interactions, the quantum-confinement geometry, and the effective charge-carrier density of the bulk. Current approaches for generating nanostructures of conducting materials are briefly reviewed, especially the use of three-dimensional crystalline superlattices as hosts for quantum-confined semiconductor atom arrays (such as quantum wires and dots) with controlled inter-quantum-structure tunneling.     

Splice junctions: association with variation in protein structure

A comparison between eukaryotic gene sequences and protein sequences of homologous enzymes from bacterial and mammalian organisms shows that intron-exon junctions frequently coincide with variable surface loops of the protein structures. The altered surface structures can account for functional differences among the members of a family. Sliding of the intron-exon junctions may constitute one mechanism for generating length polymorphisms and divergent sequences found in protein families. Since intron-exon junctions map to protein surfaces, the alterations mediated by sliding of these junctions can be effected without disrupting the stability of the protein core.     

Junctions between cancer cells in culture: ultrastructure and permeability

Cell junctions between Novikoff hepatoma cells (N1S1-67) growing as small clumps or chains in suspension culture have been studied with ultrastructural, electrophysiological, and dye-injection techniques. Cells within clumps are commonly electrically coupled and can exchange dyes with a molecular weight of 332 to 500. Gap junctions and intermediate junctions are present, whereas true tight junctions and desmosomes are absent or very rare. This system should provide a useful model for studying the properties of "communicating" junctions.     

Interface structure and atomic bonding characteristics in silicon nitride ceramics

Direct atomic resolution images have been obtained that illustrate how a range of rare-earth atoms bond to the interface between the intergranular phase and the matrix grains in an advanced silicon nitride ceramic. It has been found that each rare-earth atom bonds to the interface at a different location, depending on atom size, electronic configuration, and the presence of oxygen at the interface. This is the key factor to understanding the origin of the mechanical properties in these ceramics and will enable precise tailoring in the future to critically improve the materials' performance in wide-ranging applications.     

Growth of thin chemically bonded diamondlike films by ion beam deposition

Carbon films with a diamondlike structure that are chemically bonded to surfaces have been deposited by means of low-energy C(+) ion beams. When mass-selected C(+) beams at energies in the range from 20 to 200 electron volts impinge on atomically clean surfaces, the first carbon monolayer grows as a carbide structure that is chemically bonded to the surface. As deposition continues, the structure evolves over the next several atomic layers into a diamondlike structure. These pure carbon films are strongly adhered to the surface through the carbide bonds, which also provide for an intimate interface. There are significant applications for such films, particularly as insulators and doped semiconductors.     

Packing C60 in boron nitride nanotubes

We have created insulated C60 nanowire by packing C60 molecules into the interior of insulating boron nitride nanotubes (BNNTs). For small-diameter BNNTs, the wire consists of a linear chain of C60 molecules.With increasing BNNT inner diameter, unusual C60 stacking configurations are obtained (including helical, hollow core, and incommensurate) that are unknown for bulk or thin-film forms of C60.C60 in BNNTs thus presents a model system for studying the properties of dimensionally constrained "silo" crystal structures. For the linear-chain case, we have fused the C60 molecules to form a single-walled carbon nanotube inside the insulating BNNT.     

Structural transitions upon ligand binding in a cooperative dimeric hemoglobin

Comparison of the 2.4 angstrom resolution crystal structures of dimeric clam hemoglobin in the deoxygenated and carbon-monoxide liganded states shows how radically different the structural basis for cooperative oxygen binding is from that operative in mammalian hemoglobins. Heme groups are in direct communication across a novel subunit interface formed by the E and F helices. The conformational changes at this interface that accompany ligand binding are more dramatic at a tertiary level but more subtle at a quaternary level than those in mammalian hemoglobins. These findings suggest a cooperative mechanism that links ligation at one subunit with potentiation of affinity at the second subunit.     

Energy gaps in "metallic" single-walled carbon nanotubes

Metallic single-walled carbon nanotubes have been proposed to be good one-dimensional conductors. However, the finite curvature of the graphene sheet that forms the nanotubes and the broken symmetry due to the local environment may modify their electronic properties. We used low-temperature atomically resolved scanning tunneling microscopy to investigate zigzag and armchair nanotubes, both thought to be metallic. "Metallic" zigzag nanotubes were found to have energy gaps with magnitudes that depend inversely on the square of the tube radius, whereas isolated armchair tubes do not have energy gaps. Additionally, armchair nanotubes packed in bundles have pseudogaps, which exhibit an inverse dependence on tube radius. These observed energy gaps suggest that most "metallic" single-walled nanotubes are not true metals, and they have implications for our understanding of the electronic properties and potential applications of carbon nanotubes.     

Quantum state engineering and precision metrology using state-insensitive light traps

Precision metrology and quantum measurement often demand that matter be prepared in well-defined quantum states for both internal and external degrees of freedom. Laser-cooled neutral atoms localized in a deeply confining optical potential satisfy this requirement. With an appropriate choice of wavelength and polarization for the optical trap, two electronic states of an atom can experience the same trapping potential, permitting coherent control of electronic transitions independent of the atomic center-of-mass motion. Here, we review a number of recent experiments that use this approach to investigate precision quantum metrology for optical atomic clocks and coherent control of optical interactions of single atoms and photons within the context of cavity quantum electrodynamics. We also provide a brief survey of promising prospects for future work.     

Josephson junction arrays with Bose-Einstein condensates

We report on the direct observation of an oscillating atomic current in a one-dimensional array of Josephson junctions realized with an atomic Bose-Einstein condensate. The array is created by a laser standing wave, with the condensates trapped in the valleys of the periodic potential and weakly coupled by the interwell barriers. The coherence of multiple tunneling between adjacent wells is continuously probed by atomic interference. The square of the small-amplitude oscillation frequency is proportional to the microscopic tunneling rate of each condensate through the barriers and provides a direct measurement of the Josephson critical current as a function of the intermediate barrier heights. Our superfluid array may allow investigation of phenomena so far inaccessible to superconducting Josephson junctions and lays a bridge between the condensate dynamics and the physics of discrete nonlinear media.