Self-Sustained Microwave Absorption Induced by Extremely High Radiation Intensities in Surface Electrons on Liquid Helium

A new nonlinear-optical absorption effect is observed in electrons bound to the liquid helium surface. We study absorption of mm-wave radiation due to resonant excitation of electron bound states. Below 1 K, almost all electrons occupy the ground state. Therefore, the system should be transparent for resonant radiation connecting any two excited states. On the contrary, we observe strong absorption peaks associated with transitions between the first excited and the higher excited states. We show that this anomaly results from the bistability of the electron system induced by extremely high radiation intensities and the long electron relaxation time.     

Size-dependent Tunneling and Optical Spectroscopy of InAs Nanocrystal Quantum Dots

Scanning tunneling spectroscopy is used to investigate singleInAs nanocrystals, 2 to 8 nm in diameter. The tunnelingcurrent-voltage characteristics exhibit a pronounced sizedependence of both single electron charging energy andquantum-confined electronic states. These are well describedusing the Orthodox Model for single electron tunnelingthrough a quantum dot with a discrete level structure. Quantumconfined states having atomic-like S and P symmetries aredirectly identified in the tunneling spectra as two andsixfold single electron charging multiplets, respectively. Thetunneling spectra acquired on un-passivated nanocrystals showclear evidence of sub-gap surface states. Surprisingly,excellent agreement is found between the strongly allowedoptical transitions and spacing of levels detected in thetunneling experiment.     

The effect of phase segregation during structural relaxation of Fe10Ni65Si10B15 spin glass in the amorphous state

The structural relaxation process of Fe₁₀Ni₆₅B₁₅Si₁₀ amorphous spin glass is described. The ribbon samples were prepared by melt spinning and heat treated using two different annealing processes: AP1 and AP2. The techniques employed to investigate structural changes in the three alloy states as-quenched (AQ), AP1 and AP2 were as follows: transmission electron microscopy, scanning electron microscopy with X-ray analysis, differential scanning calorimetry. Differences were detected in the amorphous structures for two states of relaxation in the alloy. The formation of clusters is proposed for one alloy state and the influence of clustering on the early stages of crystallization is discussed.     

The effect of adsorbed oxygen on the conductivity of perylenetetracarboxylic acid <Emphasis Type="Italic">N,N</Emphasis>-dimethyldiimide films

The effect of adsorbed oxygen on the conductivity of vacuum-deposited perylenetetracarboxylic acid N, N-dimethyldiimide films with variable surface topography was studied using the method of cyclic thermodesorption. The results are interpreted in terms of a two-level hopping conductivity model. Microscopic parameters of the electron hopping transport (the radius of electron localization in the intrinsic and impurity states and the concentration of localization centers) are evaluated. The regions of oxygen concentrations in which the electron transport proceeds by hopping via intrinsic states, or impurity localized electron states, or a combined system involving the states of both types are determined.     

A Chain Model of a Zigzag Contact of Lateral Graphene-Like Heterostructures

A simple structural model is proposed for the zigzag interface formed by contacting two-dimensional graphene-like compounds AB and CD (both free and formed on a metal). For the graphene–hexagonal boron nitride system, analytical expressions for the electron spectrum, density of states, and atom occupation numbers at the interface are obtained. The results of calculating the densities of states and occupation numbers within two alternative approximations are in good agreement.     

Electron properties of open semiconductor quantum dots

The energy spectrum and lifetimes of electron states in an open semiconductor quantum dot (QD) have been studied using the scattering S-matrix method. It is established that the lifetime of electron states in the QD is highly sensitive to changes in the QD radius and the thickness of an external coating layer. As the coating layer thickness increases from one to five monolayers, the electron lifetime grows by a factor of 20–60.     

Wavelength dependent laser-induced etching of Cr-O doped GaAs: Morphology studies by SEM and AFM

The laser induced etching of semi-insulating GaAs 〈100〉 is carried out to create porous structure under super- and sub-bandgap photon illumination (h v). The etching mechanism is different for these separate illuminations where defect states play the key role in making distinction between these two processes. Separate models are proposed for both the cases to explain the etching efficiency. It is observed that under sub-bandgap photon illumination the etching process starts vigorously through the mediation of intermediate defect states. The defect states initiate the pits formation and subsequently pore propagation occurs due to asymmetric electric field in the pore. Formation of GaAs nanostructures is observed using scanning electron (SEM) and atomic force microscopy (AFM).     

Photodetachment of Metastable Helium Molecules from the Surface of Helium Nanodroplets

Metastable triplet helium molecules, created in helium nanodroplets by electron bombardment, become trapped in surface states. Photo induced electronic transitions of a molecule result in its detachment from the cluster. It is subsequently detected when it emits a secondary electron upon reaching a nearby surface. We have found that there are two different characteristic energies with which the molecules are emitted, one thermal and one much larger. We have also found that the spectral lines are asymmetric, but the peaks are only slightly blue shifted from their values in the free molecule.     

Sketching the Theory of Copper Oxide High Temperature Superconductivity

The role of the antibonding state in the electron correlations in copper oxide HTSC is analyzed. Then the t-J Hamiltonian is used to establish the formation of the charge stripes in underdoped oxides. It is proposed that these stripes make up the boundaries between the two degenerate antiferromagnetic (AFM) states, and that they are a key factor in switching between these states. We also provide a theoretical expression to the charge driven AFM magnons that have been observed by Neutron scattering experiments. Finally, the double correlation theory is applied to the stripe phase of holes to result in the superconductive gap and in the “pseudogap.”     

Two carriers in vertically coupled quantum dots: magnetic field effect

We study a two-charge-carrier (two holes or two electrons) quantum dot molecule in a magnetic field. In comparison with the electron states in the double quantum dot, the switching between the hole states is achieved by changing both the inter-dot distance and magnetic field. We use harmonic potentials to model the confining of two charge carriers and calculate the energy difference delta E between the two lowest energy states with the Hund-Mulliken technique, including the Coulomb interaction. Introducing the Zeeman effect, we note a ground-state crossing, which can be observed as a pronounced jump in the magnetization at a perpendicular magnetic field of a few Tesla. The ground states of the molecule provide a possible realization for a quantum gate.     

The growth and structure of copper films on (0001) ruthenium surfaces

The structural and electronic properties of clean evaporated copper films on (0001) Ru surfaces were studied by low energy electron diffraction, Auger electron spectroscopy and thermal desorption and work function measurements. Two-dimensional epitaxial growth and three-dimensional epitaxial growth are clearly discernible from “extra” spots in the low energy electron diffraction patterns and from Auger spectroscopy as well as from thermal desorption measurements which show the formation of two states β₁ and β₂. The β₂ state is identified as arising from two-dimensional copper growth and is almost saturated after the deposition of about 5 × 10¹⁴ Cu atoms cm^-2; the β₁ state is caused by the three-dimensional growth of copper. The binding energies of the two states were evaluated from the maximum desorption temperatures; we found that the binding energy of β₂ exceeds that of β₁ (which is identical with the sublimation energy of copper) by about 3–4 kcal mol^-1. The work function increases up to 800 meV; an intermediate maximum indicates a small charge transfer from ruthenium to copper.     

Off-Axis Electron Holography of Single Ferromagnetic Nanowires

Off-axis electron holography has been applied to study the remanent magnetization state of single ferromagnetic Co₉₃Cu₇ nanowires a few micrometer in length and a typical radius of 40nm. Because the objective lens of the electron microscope has to be switched off, the spatial resolution of the reconstructed phase images is presently limited to approximately 70nm. The magnetization reversal of an individual nanowire has been followed by observing a series of remanent states, obtained ex situ by applying different external magnetic field sweeps parallel to the nanowire axis. The relation between misoriented crystal grains and nonuniform magnetization states has been studied by the combination of electron holography and conventional transmission electron microscopy.     

Quantum Coherence of Charge States in the Single Electron Box

A metallic electrode connected to electron reservoirs by tunnel junctions has a series of charge states corresponding to the number of excess electrons in the electrode. In contrast to the charge state of an atomic or molecular ion, the charge states of such an island are mesoscopic states involving all the conduction electrons of the island. Island charge states bear some resemblance to the photon number states of the cavity in cavity QED, the phase conjugate to the number of electrons being analogous to the phase of the field in the cavity. For a normal island, charge states decay irreversibly into charge states of lower energies. However, the gound state of a superconducting island connected to superconducting reservoirs can be a coherent superposition of charge states differing by two electrons (i.e., a Cooper pair). We describe an experiment in which this Josephson effect involving only one Cooper pair is measured.     

Interface formation between two organic films based on phthalocyanine and perylene derivatives

The process of interface formation between two organic films composed of donor (copper phthalocyanine, CuPc) and acceptor (perylene-3,4,9,10-tetracarboxylic dianhydride, PTCDA) molecules has been studied in situ using the total current spectroscopy technique. It is established that the donor-acceptor interaction between CuPc and PTCDA molecules do not distort the energy structure of the density of electron states. The main π*, σ*₁, and σ*₂ bands of antibonding (unoccupied) electron states are identified, which are determined both by C-C bonds in the aromatic rings and by additional C-N and C-O bonds. The width of the interface potential barrier is evaluated and its relation to the limiting polarizability of molecules is demonstrated. The interface potential barrier is formed in the course of negative charge transfer between donor (CuPc) and acceptor (PTCDA) molecules.     

Interference in tunneling ionization involving an electron bound by two short-range potentials

The phenomenon of tunneling ionization involving electron bound by two delta-potentials under the action of a constant electric field has been studied. Distributions of the electron-current density for two different initial states are found. Dependence of the emission current on the orientation of potentials relative to the field direction and the distance between their centers is established. Conditions of manifestation of the interference effects are determined.

     

Semi-empirical simulations of the electron centers in MgO crystal

Semi-empirical quantum chemical simulations of 125-atom clusters have been undertaken to obtain the self-consistent atomic and electronic structure of the two basic electron defects in MgO crystals — F⁺ and F centers (one and two electrons trapped by an O vacancy). The calculated absorption and luminescence energies agree well with the experimental data, the excited states of both defects are found to be essentially delocalized over nearest-neighbour cations. The mechanism of the F⁺ → F photoconversion is discussed.     

SrTiO3材料中氧空位的密度泛函理论

计算了SrTiO3-δ（δ=0，δ=0．125）体系电子结构，分析了氧空位对SrTiO3晶体的价键结构、能带、态密度、分波态密度、差分电荷密度的影响。所有计算都是基于密度泛函理论（DFT）框架下的第一性原理平面波超软赝势方法。计算结果表明：当氧空位浓度δ=0．125时，空位在母体化合物SrTiO3中引入了大量的传导电子，费米能级进入导带，体系显示金属型导电性。由于空位掺杂，导带底附近的态密度发生了畸变，刚性能带模型不再适合描述SrTiO2.875体系。同时，在导带底附近距离费米能级0．3eV处引入了空位能级，这和实验测得的SrTiO3材料内中性氧空位的电离能约为0．4eV相符。此外，Mulliken布局分析、分波态密度和差分电荷密度分析表明，该空位能级主要由与其最近邻的两个Ti原子的3d电子态贡献，并且由该空位引入的导电电子大部分都局域在空位最近邻的两个Ti原子周围。最后，计算了三种典型平衡条件下SrTiO3晶体内中性氧空位的形成能。     

Ion nitriding of a sintered iron molybdenum alloy

The behaviour of a sintered alloy Fe Mo 1.5 C 0.01 in two density states when subjected to an ion nitriding process is analysed. The thermochemical treatment was carried out with three different combinations of parameters (time, nitriding atmosphere). The analyses of the material after treatment included dimensional measurements, macro- and micro hardness determinations, diffractometric analysis and microstructural examination with an optical microscope and a scanning electron microscope equipped with an EDX system. All treatments led to satisfactory results for the material in the two density states: the volume mass is not a discriminative parameter in this case.     

Ground and Excited State Electronic Interactions in Push–Pull-Chromophore–[60]Fullerene Conjugates

A variety of push–pull-chromophore–[60]fullerene conjugates connected with different spacers was successfully synthesized by applying “click chemistry” to the corresponding acetylene-appended fullerene precursors. Direct connection of the fullerene cages to push–pull motifs gives rise to ground state electronic interactions, which were characterized by electrochemical studies. On the other hand, when the two moieties were linked through pyrrolidine rings, no interactions occurred between the C₆₀ units and the push–pull motifs in the ground states. Instead, an electron transfer proceeded upon light exitation, giving the charge-separated states, which was corroborated by time-resolved transient absorption measurements.     

A discharge with runaway electrons: Efficient process for the formation of Rydberg states in oxygen molecules

A new method for exciting high-lying states in oxygen molecules is proposed, which is based on the interaction of O₂ molecules with a low-energy (～10²–10⁴ eV) electron beam formed in a discharge with runaway electrons. An algorithm for calculating the electron energy distribution function (EEDF) using the Monte Carlo simulation technique is developed. The EEDF calculation for a discharge in pure oxygen is performed for a plasma pumped with monoenergetic electron beam in the absence of an external electric field. Efficiencies of the formation of Rydberg and Herzberg states in oxygen molecules under conditions of a quasi-stationary dc discharge and a discharge with runaway electrons are compared.