Observation of single-electron charging in double-barrier heterostructures

Incremental single-electron charging of size-quantized states has been observed in the well in submicrometer double-barrier resonant tunneling devices. In order to distinguish between the effects of size quantization and the single-electron charging, the heterostructure material was grown asymmetrical so that one barrier is substantially less transparent than the other. In the voltage polarity such that the emitter barrier is more transparent than the collector barrier, electrons accumulate in the well; incremental electron occupation of the well is accompanied by Coulomb blockade, which leads to sharp steps of the tunneling current. In the opposite voltage polarity the emitter barrier is less transparent than the collector barrier and the tunneling current reflects resonant tunneling through size-quantized well states.     

Electrically induced optical emission from a carbon nanotube FET

Polarized infrared optical emission was observed from a carbon nanotube ambipolar field-effect transistor (FET). An effective forward-biased p-n junction, without chemical dopants, was created in the nanotube by appropriately biasing the nanotube device. Electrical measurements show that the observed optical emission originates from radiative recombination of electrons and holes that are simultaneously injected into the undoped nanotube. These observations are consistent with a nanotube FET model in which thin Schottky barriers form at the source and drain contacts. This arrangement is a novel optical recombination radiation source in which the electrons and holes are injected into a nearly field-free region. Sucha source may form the basis for ultrasmall integrated photonic devices.     

Crossed nanotube junctions

Junctions consisting of two crossed single-walled carbon nanotubes were fabricated with electrical contacts at each end of each nanotube. The individual nanotubes were identified as metallic (M) or semiconducting (S), based on their two-terminal conductances; MM, MS, and SS four-terminal devices were studied. The MM and SS junctions had high conductances, on the order of 0.1 e(2)/h (where e is the electron charge and h is Planck's constant). For an MS junction, the semiconducting nanotube was depleted at the junction by the metallic nanotube, forming a rectifying Schottky barrier. We used two- and three-terminal experiments to fully characterize this junction.     

Electronic confinement and coherence in patterned epitaxial graphene

Ultrathin epitaxial graphite was grown on single-crystal silicon carbide by vacuum graphitization. The material can be patterned using standard nanolithography methods. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. All-graphene electronically coherent devices and device architectures are envisaged.     

Coupling quantum tunneling with cavity photons

Tunneling of electrons through a potential barrier is fundamental to chemical reactions, electronic transport in semiconductors and superconductors, magnetism, and devices such as terahertz oscillators. Whereas tunneling is typically controlled by electric fields, a completely different approach is to bind electrons into bosonic quasiparticles with a photonic component. Quasiparticles made of such light-matter microcavity polaritons have recently been demonstrated to Bose-condense into superfluids, whereas spatially separated Coulomb-bound electrons and holes possess strong dipole interactions. We use tunneling polaritons to connect these two realms, producing bosonic quasiparticles with static dipole moments. Our resulting three-state system yields dark polaritons analogous to those in atomic systems or optical waveguides, thereby offering new possibilities for electromagnetically induced transparency, room-temperature condensation, and adiabatic photon-to-electron transfer.     

A dielectric omnidirectional reflector

A design criterion that permits truly omnidirectional reflectivity for all polarizations of incident light over a wide selectable range of frequencies was used in fabricating an all-dielectric omnidirectional reflector consisting of multilayer films. The reflector was simply constructed as a stack of nine alternating micrometer-thick layers of polystyrene and tellurium and demonstrates omnidirectional reflection over the wavelength range from 10 to 15 micrometers. Because the omnidirectionality criterion is general, it can be used to design omnidirectional reflectors in many frequency ranges of interest. Potential uses depend on the geometry of the system. For example, coating of an enclosure will result in an optical cavity. A hollow tube will produce a low-loss, broadband waveguide, whereas a planar film could be used as an efficient radiative heat barrier or collector in thermoelectric devices.     

Inducing and viewing the rotational motion of a single molecule

Tunneling electrons from the tip of a scanning tunneling microscope were used to induce and monitor the reversible rotation of single molecules of molecular oxygen among three equivalent orientations on the platinum(111) surface. Detailed studies of the rotation rates indicate a crossover from a single-electron process to a multielectron process below a threshold tunneling voltage. Values for the energy barrier to rotation and the vibrational relaxation rate of the molecule were obtained by comparing the experimental data with a theoretical model. The ability to induce the controlled motion of single molecules enhances our understanding of basic chemical processes on surfaces and may lead to useful single-molecule devices.     

太阳真空集热管真空度测量模型与特性参数分析

依据经典热力学理论，提出了太阳真空集热管真空度检测的负压腔原理，并建立了理论数学模型。同时运用现代气体动理论，分析了负压腔测量系统模型的动态特性以及分子平均速率、热导率以及空间布置等特性参数对真空度测量的影响。     

高压4H-SiC 肖特基二极管的模拟及研制

结终端技术能提高4H-SiC肖特基势垒二极管器件的耐压性能.利用仿真软件ISE-TCAD10.0对具有结终端扩展JTE保护的4H-SiC SBD器件进行了仿真研究，并依据仿真优化好的参数试制了器件.实验测试结果表明，模拟优化结果与实验测试器件的结果一致性较好，实测此器件的反向电压值达2 000 V，接近理想击穿耐压88%，漏电流数值为0.1 mA/cm.     

The bonding electron density in aluminum

Aluminum is considered to approach an "ideal" metal or free electron gas. The valence electrons move freely, as if unaffected by the presence of the metal ions. Therefore, the electron redistribution due to chemical bonding is subtle and has proven extremely difficult to determine. Experimental measurements and ab initio calculations have yielded substantially different results. We applied quantitative convergent-beam electron diffraction to aluminum to provide an experimental determination of the bonding electron distribution. Calculation of the electron distribution based on density functional theory is shown to be in close agreement. Our results yield an accurate quantitative correlation between the anisotropic elastic properties of aluminum and the bonding electron and electrostatic potential distributions.     

Piezoelectric nanogenerators based on zinc oxide nanowire arrays

We have converted nanoscale mechanical energy into electrical energy by means of piezoelectric zinc oxide nanowire (NW) arrays. The aligned NWs are deflected with a conductive atomic force microscope tip in contact mode. The coupling of piezoelectric and semiconducting properties in zinc oxide creates a strain field and charge separation across the NW as a result of its bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. The efficiency of the NW-based piezoelectric power generator is estimated to be 17 to 30%. This approach has the potential of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices.     

A paramagnetic bonding mechanism for diatomics in strong magnetic fields

Elementary chemistry distinguishes two kinds of strong bonds between atoms in molecules: the covalent bond, where bonding arises from valence electron pairs shared between neighboring atoms, and the ionic bond, where transfer of electrons from one atom to another leads to Coulombic attraction between the resulting ions. We present a third, distinct bonding mechanism: perpendicular paramagnetic bonding, generated by the stabilization of antibonding orbitals in their perpendicular orientation relative to an external magnetic field. In strong fields such as those present in the atmospheres of white dwarfs (on the order of 10(5) teslas) and other stellar objects, our calculations suggest that this mechanism underlies the strong bonding of H(2) in the (3)Σ(u)(+)(1σ(g)1σ(u)*) triplet state and of He(2) in the (1)Σ(g)(+)(1σ(g)(2)1σ(u)(*2)) singlet state, as well as their preferred perpendicular orientation in the external field.     

Synthesis of imido analogs of the uranyl ion

Here we describe the synthesis of two imido analogs of the uranyl ion, UO(2+)2, in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(NtBu)2I2(THF)2 (1) and U(NPh)2I2(THF)3 (2) (where tBu is tert-butyl and THF is tetrahydrofuran). Both compounds have been fully characterized by standard analytical techniques, including x-ray crystallography, and the chemical bonding between the metal center and the nitrogen ligands was quantified by using hybrid density functional theory calculations. As expected for a uranyl analog, these complexes exhibit linear N-U-N linkages and very short U-N bonds. In addition, the theoretical calculations show strong involvement of the 5f and 6d electrons in the U-N bonding.     

太阳能新风系统在北方农村地区的应用研究

我国太阳能资源丰富,尤其是我国北方地区,日照时间长、日照强度强,相对其他地区的太阳能利用有很大的优势,可以利用太阳能生产生活热水、采暖、干燥等。目前太阳能热水系统在我国应用十分广泛,尤其是农村地区,几乎每家都装有真空管式热水集热器,但是太阳能空气集热器作为采暖方式应用较少。我国北方地区冬季以晴朗天气居多,但是温度较低,一般需要进行建筑的供热,太阳能集热器采暖基于自身的优势将成为未来采暖的趋势。该文介绍了我国太阳资源的分布情况,描述了北方采暖的现状,提出将太阳能集热器应用在北方采暖系统中,尤其是在农村地区应用具有实际意义,并通过试验验证了其推广的可行性。     

空气介质阻挡放电等离子体对云南松单板表面的处理工艺

采用空气介质阻挡放电等离子体处理云南松单板表面,对处理后的单板进行胶合性能及表面性能分析,探讨放电等离子体处理单板表面的最佳工艺。结果表明：在1000—4000W范围,随处理功率的增加,单板的胶合强度增大,水接触角减小,表面能增大;在处理功率5000W时,各指标变化趋势相反。当处理次数为1—7次时,随处理次数的增加,单板的胶合强度增大,水接触角减小,表面能增大;在处理9、11次时,各指标变化趋势相反。当放电等离子体处理功率为4000W,处理次数为7次时,木材表面改性效果最佳,其胶合强度、水接触角及表面能分别为2．1MPa、43°、56．7MJ／m2。     

The Kondo effect in an artificial quantum dot molecule

Double quantum dots provide an ideal model system for studying interactions between localized impurity spins. We report on the transport properties of a series-coupled double quantum dot as electrons are added one by one onto the dots. When the many-body molecular states are formed, we observe a splitting of the Kondo resonance peak in the differential conductance. This splitting reflects the energy difference between the bonding and antibonding states formed by the coherent superposition of the Kondo states of each dot. The occurrence of the Kondo resonance and its magnetic field dependence agree with a simple interpretation of the spin status of a double quantum dot.     

Graphene barristor, a triode device with a gate-controlled Schottky barrier

Despite several years of research into graphene electronics, sufficient on/off current ratio I(on)/I(off) in graphene transistors with conventional device structures has been impossible to obtain. We report on a three-terminal active device, a graphene variable-barrier "barristor" (GB), in which the key is an atomically sharp interface between graphene and hydrogenated silicon. Large modulation on the device current (on/off ratio of 10(5)) is achieved by adjusting the gate voltage to control the graphene-silicon Schottky barrier. The absence of Fermi-level pinning at the interface allows the barrier's height to be tuned to 0.2 electron volt by adjusting graphene's work function, which results in large shifts of diode threshold voltages. Fabricating GBs on respective 150-mm wafers and combining complementary p- and n-type GBs, we demonstrate inverter and half-adder logic circuits.     

Vacuum-induced transparency

Photons are excellent information carriers but normally pass through each other without consequence. Engineered interactions between photons would enable applications as varied as quantum information processing and simulation of condensed matter systems. Using an ensemble of cold atoms strongly coupled to an optical cavity, we found that the transmission of light through a medium may be controlled with few photons and even by the electromagnetic vacuum field. The vacuum induces a group delay of 25 nanoseconds on the input optical pulse, corresponding to a light velocity of 1600 meters per second, and a transparency of 40% that increases to 80% when the cavity is filled with 10 photons. This strongly nonlinear effect provides prospects for advanced quantum devices such as photon number-state filters.     

Reactions of solvated electrons initiated by sodium atom ionization at the vacuum-liquid interface

Solvated electrons are powerful reagents in the liquid phase that break chemical bonds and thereby create additional reactive species, including hydrogen atoms. We explored the distinct chemistry that ensues when electrons are liberated near the liquid surface rather than within the bulk. Specifically, we detected the products resulting from exposure of liquid glycerol to a beam of sodium atoms. The Na atoms ionized in the surface region, generating electrons that reacted with deuterated glycerol, C(3)D(5)(OD)(3), to produce D atoms, D(2), D(2)O, and glycerol fragments. Surprisingly, 43 ± 4% of the D atoms traversed the interfacial region and desorbed into vacuum before attacking C-D bonds to produce D(2).     

Electronic, magnetic, and geometric structure of metallo-carbohedrenes

The energetics and the electronic, magnetic, and geometric structure of the metallocarbohedrene Ti(8)C(12) have been calculated self-consistently in the density functional formulation. The structure of Ti(8)C(12) is a distorted dodecahedron with a binding energy of 6.1 electron volts per atom. The unusual stability is derived from covalent-like bonding between carbon atoms and between titanium and carbon atoms with no appreciable interaction between titanium atoms. The density of states at the Fermi energy is high and is derived from a strong hybridization between titanium 3d and carbon sp electrons. Titanium sites carry a small magnetic moment of 0.35 Bohr magneton per atom and the cluster is only weakly magnetic.