Search Results (86 total)

We report on a clear tendency toward organized growth, at room temperature, of Ni nanoislands deposited on a CoO(001) nanopatterned surface. The 9.2  nm square surface patterning consists in a periodic displacement field induced at the surface by an underlying interfacial dislocations network that is buried at the interface between the 5  nm thick CoO(001) thin film and the Ag(001) substrate. Grazing incidence small and wide angle x-ray scattering (GISAXS and GIXD) performed in situ, during growth, reveal that the nucleation of Ni particles is driven by the underlaying dislocation distribution. The tendency for organization is confirmed by scanning tunneling microscopy (STM), which also reveals a quite narrow size distribution of the Ni nanoparticles around 5  nm width and 0.6  nm height.

Interfacial and mesoscopic size effects in molecular beam epitaxy-grown CaF₂∕BaF₂ heterolayers have been quantitatively analyzed on the basis of Gouy-Chapman and Mott-Schottky space-charge profiles. The linear dependence of the overall parallel conductivity on the inverse interfacial spacing found for large interfacial spacings (ℓ>50  nm) can be obtained from both models being in good agreement with the experimental data. An upward bending occurring for small interfacial spacings (ℓ<30  nm) can only be reproduced by the Mott-Schottky model based on the assumption of frozen-in impurity profiles.

The structural, electronic, and mechanical properties have been calculated by using first-principles pseudopotential density functional method for three possible configurations of wurtzite BC₂N, which are deduced from four-atom wurtzite boron nitride unit cell. Our results show that the BC₂N-w3 with the maximum C-C and B-N bonds has the lowest total energy among all the reported sp³-bonded BC₂N structures. Energetically, the wurtzite structure is more stable than the zinc-blende structure for the sp³-bonded BC₂N, which is different from sp³-bonded carbon and boron nitride. The present BC₂N-w3 has the highest density, the largest bulk and shear moduli, the largest band gap, and the largest Vickers hardness among all the investigated sp³-bonded BC₂N structures. The phase stability of BC₂N-w3 indicates that it should be experimentally synthesized more easily than the zinc-blende-structured BC₂N.

Body-centered BC₂N deduced from the unit cell of the recently predicted body-centered carbon [F. J. Ribeiro , Phys. Rev. B 74, 172101 (2006)] are studied with first-principles pseudopotential density functional method. The structural, electronic, and mechanical properties are investigated for 11 possible atomic configurations of body-centered BC₂N. Our results show that the sp³-bonded body-centered BC₂N phases have lower density than the previously investigated sp³-bonded zinc-blende BC₂N, wurtzite BC₂N, and chalcopyrite BC₂N. The struc-A and struc-B composed of the maximum numbers of C-C and B-N bonds have the lowest total energy among the investigated body-centered BC₂N structures. Their calculated bulk moduli are 305 and 309  GPa, respectively. The theoretical Vickers hardness of the body-centered BC₂N is over 60  GPa, indicating that it is a potential superhard material with the hardness comparable to cubic boron nitride.

We predict a most likely structure of superhard BC₂N, named z-BC₂N, which is constructed from the sixteen-atom supercell of diamond by using first-principles calculations. z-BC₂N is a more stable metastable phase with respect to other structures and has the tetragonal symmetry belonging to the space group P-42M. z-BC₂N obeys the bond-counting rule that the chemical bonds in the unit cell consist of B-C, B-N, C-C, and C-N bonds, and C-C and B-N bonds are as many as possible. The calculated x-ray diffraction spectrum of z-BC₂N agrees with the reported experimental result more well than the previously-predicted structures. Our results indicate that z-BC₂N is perhaps the most likely structure of BC₂N. Theoretical Vickers hardness and bulk modulus of z-BC₂N are calculated to be 75.9 and 402.7  GPa exceeding those of cubic BN.

Using an ensemble Monte Carlo method, we have theoretically investigated the transport properties of a GaAs∕(Al,Ga)As double quantum well bilayer two-dimensional electron gas under an in-plane magnetic field. Intrasubband and intersubband electron-phonon interactions through polar optical and acoustic phonons are included in the simulation. The effects of the magnetic-field-induced distortions of energy dispersion on the scattering rate are fully taken into account. Our results show that the emergence of magnetic-field-induced negative effective mass does not affect appreciably the electron drift velocity (v_d) as a function of electric field (E), and no dv_d∕dE<0 region appears on the v_d-E curve. The oscillation of v_d with magnetic fields is found, which is qualitatively in accordance with experimental results.

The quadratic contact process is formulated as an adsorption-desorption model on a two-dimensional square lattice. It involves random adsorption at empty sites and correlated desorption requiring diagonally adjacent pairs of empty neighbors. We assess the model behavior utilizing kinetic Monte Carlo simulations. One finds generic two-phase coexistence between a low-coverage active steady state and a completely covered or “poisoned” absorbing steady state; i.e., both states are stable over a finite range of adsorption rates or “pressures.” This behavior is in marked contrast to that for equilibrium phase separation. For spatially homogeneous systems, we provide a comprehensive characterization of the kinetics of relaxation to the steady states. We analyze rapid poisoning for higher pressures above an effective spinodal point terminating a metastable active state, nucleation-mediated poisoning in the metastable region, the dynamics of poisoned droplets within the two-phase coexistence region, and behavior reminiscent of bootstrap percolation dynamics for lower pressures. For spatially inhomogeneous systems, we analyze the propagation of planar interfaces between active and absorbing states, fully characterizing an orientation dependence which underlies the generic two-phase coexistence.

We study the direct CP violation in B̅ ⁰→ρ⁰(ω)ρ⁰(ω)→π⁺π^-π⁺π^- [with unpolarized ρ⁰(ω)] via the ρ-ω mixing mechanism which causes a large strong phase difference and consequently a large CP violating asymmetry when the masses of the π⁺π^- pairs are in the vicinity of the ω resonance. Since there are two ρ(ω) mesons in the intermediate state ρ-ω mixing contributes twice to the first order of isospin violation, leading to an even larger CP violating asymmetry (it could be 30%–50% larger) than in the case where only one ρ(ω) meson is involved. The CP violating asymmetry depends on the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements and the hadronic matrix elements. The factorization approach is applied in the calculation of the hadronic matrix elements with the nonfactorizable effects being included effectively in an effective parameter, N_c. We give the constraint on the range of N_c from the latest experimental data for the branching ratios for B̅ ⁰→ρ⁰ρ⁰ and B̅ ⁰→ρ⁺ρ^-. We find that the CP violating asymmetry could be very large (even more than 90% for some values of N_c). It is shown that the sensitivity of the CP violating asymmetry to N_c is large compared with its smaller sensitivity to the CKM matrix elements. We also discuss the possibility to remove the mod (π) ambiguity in the determination of the CP violating phase angle α through the measurement of the CP violating asymmetry in the decay B̅ ⁰→ρ⁰(ω)ρ⁰(ω)→π⁺π^-π⁺π^-.

A Comment on the Letter by Antonín Šimůnek and Jiří Vackář [Phys. Rev. Lett. 96, 085501 (2006)]. The authors of the Letter offer a Reply.

We have theoretically evaluated a recently synthesized OsN₂ that was shown to have a large bulk modulus, and we propose a structure that is isostructural with marcasite with the space group Pnnm. The calculated lattice constants, bulk modulus, and x-ray diffraction spectrum of OsN₂ are in good agreement with the experimental results. Calculations of bulk modulus and hardness indicate that the marcasite-type OsN₂ is an ultraincompressible material instead of a superhard one. The possibility of the marcasite PtN₂ has also been investigated, showing that the marcasite PtN₂ could exist as a metastable phase at pressures up to 50 GPa, being elastically stable with a slightly higher energy (32 meV∕atom at 0.1 MPa) than that in the pyrite structure.

The quadratic contact process is implemented on a square lattice as a model with random adsorption and correlated desorption requiring empty pairs of diagonal neighbors. The model exhibits a discontinuous phase transition between an active state and an absorbing state, but equistability between these states depends on the orientation of the separating interface. Correspondingly, for a generalized class of models, we find phase coexistence over a finite region of their two-dimensional parameter space. This is in stark contrast to behavior in equilibrium systems.

We present the study on the infrared and nonresonant Raman spectra of β-BC₂N within the framework of ab initio pseudopotential density functional perturbation theory in a four-atom orthorhombic unit cell. The Raman tensors are calculated from the second-order response of the electronic density matrix with respect to a uniform electric field. Comparison between experiments and calculations for cubic BN is presented to test our method in reproducing all the measured features quantitatively. The LO/TO splitting is well imposed by adding the nonanalytical part deduced from the Born effective charge and macroscopic dielectric constant. Finally, different Raman experiment configurations for β-BC₂N are discussed.

We calculated the matter effect, including both the Earth and supernova, on the detection of neutrinos from type II supernovae at the proposed Daya Bay reactor neutrino experiment. It is found that apart from the dependence on the flip probability P_H inside the supernova and the mass hierarchy of neutrinos, the amount of the Earth matter effect depends on the direction of the incoming supernova neutrinos, and reaches the biggest value when the incident angle of neutrinos is around 93°. In the reaction channel ν̅ _e+p→e⁺+n the Earth matter effect can be as big as about 12%. For other detection processes the amount of the Earth matter effect is a few per cent.

Our first principles calculations predict a tetragonal BC₃ (t-BC₃) phase originating from the cubic diamond structure. The t-BC₃ structure is formed by alternately stacking sequence of metallic CBC and insulating CCC blocks, exhibiting a sandwichlike metal and/or insulator layered structure with the anisotropic conductivity on the basal planes formed by the metallic CBC blocks. Its stability has been confirmed by our calculations of the total energy, elastic constants, and phonon frequencies. A low critical pressure of ∼4 GPa for a synthesis of the t-BC₃ from the graphitelike BC₃ (g-BC₃) is expected.

Based on the fact that ionicity exists in chemical bonds between identical atoms in α-boron, investigation has been performed on the influences of X atoms on the ionicities of B-B bonds in B₁₃C₂-like structured B_yX (X=C, N, O, P, and As) compounds. For the four types of B-B bonds in the B₁₂ icosahedra of the B_yX compounds, their overlap population P, population ionicity f_h, and Phillips ionicity f_i have been calculated. Comparing to the overlap population P_c value for the structure of the regular B₁₂ icosahedron, the difference between P and P_c in the distorted B₁₂ icosahedron and its origin have been discussed. Our results have shown that the charge transfer occurs between B_p and B_e atoms in the distorted B₁₂ icosahedra of B_yX crystals, different from the α-boron. Electronegativity of the X atom in B_yX crystals plays an important role in the ionicities of B-B bonds in the icosahedra. Among the five investigated B_yX crystals, the largest ionicity of 0.449 has been found to exist in the B_p-B_e bond in the B₆O crystal, which is comparable to that found in AlN. By taking into account the effect of the ionicities of B-B bonds in the B₁₂ icosahedra on hardness, the calculated hardness is in agreement with the experimental values for B₆O and B₁₃C₂. In addition, the calculations of hardness indicate that B₆N is a potential superhard material, but B₆P and B₆As are not.

Based on the higher order Cauchy-Born rule, a continuum-based constitutive model, which taking the interatomic potentials and atomic microstructure of carbon nanotubes into consideration, is presented for the analysis of the mechanical properties of carbon nanotubes. With the use of Tersoff-Brenner potential, the influences of the potential parameters, tube diameters, chirality and the inner displacement on the energies, the axial Young’s moduli, the circular elastic moduli of graphite and single-walled carbon nanotubes as well as the bending stiffness of SWNT are investigated. The results obtained are in good agreement with that of molecular dynamics.

Heterodiamond BC₂N, as a kind of superhard material expectable, is studied using the ab initio pseudopotential density functional method. All the calculations are performed after geometric optimization starting from an eight-atom zinc-blende structure unit cell. For all the structures possible, we calculate in detail the structural parameters, charge transfers, bond populations, band structures, density of states, and optical properties (dielectric function, refractive index, absorption coefficient, reflectivity, electron energy loss spectrum, and photoconductivity). In addition, the optical anisotropy of some structures is also discussed. Our calculated results show that all the structures are metastable and some of them tend to form graphitelike structures and exhibit semimetallic behavior leading to interesting optical properties.

Based on a molecular mechanics model, analytical solutions are obtained for the critical buckling strain of multiwalled carbon nanotubes (MWNT’s) under axial compression and bending. We show that only part of the outer layers buckles first while the remaining inner part remains stable in a very thick MWNT, which is quite different from the initial buckling mode of a relatively thin MWNT in which all individual tubes buckle simultaneously. Such a difference in the initial buckling modes results in quite different size effects on the critical buckling strain of thin and thick MWNT’s. For instance, inserting more inner individual tubes may increase the critical buckling strain of a thin MWNT, but cannot increase the critical buckling strain of a thick tube. The effects of tube size on the initial buckling wavelength are also examined, and it is shown that the initial buckling wavelength is weakly dependent on the thickness of the MWNT.

Nanoscale organic electroluminescence was induced by positioning a sharp tungsten tip on the surface of a free-base porphyrin (H₂TBPP) monolayer on the top of PtTBP porphyrin (PtTBPP) multilayers on a Cu(100) substrate in an ultrahigh vacuum scanning tunneling microscope (STM) system. The well-defined molecular fluorescence spectra are perfectly matched with the conventional photoluminescence spectrum from bulk H₂TBPP molecules. The nanoscale PtTBPP multilayers do not fluoresce; rather, they act as spacers to enhance the decoupling of the electronic state of the H₂TBPP monolayer from the Cu surface. The electronic property of molecules and the energy-level alignment of molecules with respect to the Fermi levels of electrodes are probably quite critical for observing STM-induced molecular fluorescence from molecular layers with a similar thickness. The molecule in proximity to the tip apex of a scanning tunneling microscope is locally excited by the hot electron injection mechanism, followed by radiative decay via Franck-Condon transitions.

Bound states of twin-pulse solitons were experimentally observed in a passively mode-locked fiber ring laser. Similar to those of single-pulse solitons, the bound states of twin-pulse solitons are marginally stable and occur at some fixed, quantized soliton separations. Our experimental investigations revealed that the formation of such bound states might be resulted from the dispersive wave mediated long-range soliton interaction in the laser.

A Comment on the Letter by Pascal Hersen, Stéphane Douady, and Bruno Andreotti, Phys. Rev. Lett. 89, 264301 (2002).

The effect of random distribution of Mn atoms on the electronic and magnetic properties of Ga_1-xMn_xAs with x=0.0625 is studied with Vienna ab initio simulation package. We use a 2×2×2 supercell, in which 64 atoms are included, and two Mn atoms substitute Ga positions. Five different Mn-Mn configurations are considered. Local structural relaxations show that the bond lengths of Mn-As are smaller than those of the nearby Ga-As. The total-energy calculation indicates that because of the inhomogeneous spatial distribution of holes, the ferromagnetic interaction between two Mn ions strongly depends on the direction along the two Mn ions. The inhomogeneous distribution of Mn atoms makes the holes stay in slightly stronger localized states. The value of magnetic moments and their spatial distributions are slightly affected by random distribution of Mn atoms.

Intrinsic molecular fluorescence from porphyrin molecules on Au(100) has been realized by using a nanoscale multimonolayer decoupling approach with nanoprobe excitation in the tunneling regime. The molecular origin of luminescence is established by the observed well-defined vibrationally resolved fluorescence spectra. The molecules fluoresce at low “turn-on” voltages for both bias polarities, suggesting an excitation mechanism via hot electron injection from either tip or substrate. The excited molecules decay radiatively through Franck-Condon π^*-π transitions.

The role of molecules in tunneling current induced photon emission from luminescent molecules on metal surfaces is still an open issue despite the recent observation of molecular fluorescence. We found that at low tunneling currents (<1 nA), the molecular fluorescence from the surface of a perinone derivative (PD) molecular monolayer on Au(100) is completely quenched through non-radiative energy dissipation to the Au substrate. The PD molecular monolayer acts as a spacer to modify the tip-induced plasmon modes, enhance the intensity of the plasmon-mediated emission, and cause a blueshift of the emission spectra. However, at relatively high tunneling currents (1 nA<I_t<∼5 nA), broadened emission spectra were observed, which suggests that molecular fluorescence may be feasible upon further decoupling of the PD molecular electronic states from the metal substrate.