Direct laser writing of superconducting patterns into semiconducting ceramic Y-Ba-Cu-O

Investigations on the laser-induced oxidation of YBa₂Cu₃O₆ are presented. Here, the oxygen content of the material is locally increased by laser-induced heating under cw Ar⁺ or Kr⁺ laser irradiation in 02 atmosphere. The technique permits direct-writing of superconducting patterns into the semiconducting sample surface.     

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne₂ ⁺, Ar₂ ⁺ and Kr₂ ⁺. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar₂ ⁺, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne₂ ⁺ suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne₂ ⁺ and Ar₂ ⁺ to possess a double-well potential and that of Kr₂ ⁺ to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr₂ ⁺ and Xe₂ ⁺ dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar₂ ⁺ does not.     

Rydberg excitations in rare gas clusters: structure and electronic spectra of Ar* n (3 ≤ n ≤ 25)

Likely candidates are located for the global potential energy minima of Ar* _n (3 ≤ n ≤ 25) clusters using the diatomics-in-molecules (DIM) approach. The favoured geometries are found to be different from the structures of Ar⁺ _n and correspond to the trimer Ar*₃ bound to the surface of an Ar _n?2 core via a common atom. The Ar _n?2 core is usually only slightly distorted from its own global potential minimum, although in a few cases it corresponds to a nearby local minimum. Therefore, the ‘magic’ sizes of the excimer systems are predicted to differ from those of the ions and correlate instead with the stability of Ar _n?2. The predicted electronic photoabsorption and emission spectra of Ar* _n , and photoexcitation spectra of Ar _n are discussed in terms of experimental data. Global potential energy minima for neutral Ar _n up to n = 55 with the Aziz potential are summarized also; the structure is the same as for the Lennard-Jones potential except at n = 21 where the stabilities of the two lowest Lennard-Jones minima are reversed.     

Density of states and dispersion of quasiparticles in the Emery model: Nonlocal path-integral Monte Carlo algorithm

The spectral density of states for a 108-site Cu₃₆O₇₂ cluster in the two-dimensional three-band Emery model is reconstructed with the aid of a path integral Monte Carlo algorithm. Dispersion relations are obtained for quasiparticles in the upper Hubbard band and in the correlated-states band; this corresponds to electron and hole doping of high-T _c superconductors. The form of the isoenergy surfaces is close to the experimentally observed form and confirms the existence of singularities in the density of states near the Fermi level. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 11, 860–865 (10 June 1996)     

Thermodynamical properties and defect energetics of an n-body potential adapted to Cu3Au

By using molecular statics, molecular dynamics, and Monte Carlo techniques we validate a previously developed empirical n-body potential adapted to Cu₃Au. At T=0 K, predicted cohesive energies, lattice parameters, and elastic constants in CuAu and CuAu₃ as well as the formation energy of vacancies in Cu₃Au are in good agreement with experimental data. A satisfactory behavior is also obtained at T 0 K in Cu₃Au, for atomic mean-square displacements and elastic moduli. However, this model underestimates the vacancy migration energy and the order-disorder critical temperature when the latter is evaluated by Monte Carlo including both exchanges between atoms of different species and atomic moves simulating vibrations.     

X-ray productions in symmetric heavy ion-atom collisions

X-ray measurements for 50–200 keV impact Ne⁺ → Ne, Ar⁺ → Ar and Kr⁺ → Kr show an increase of a factor of 10 in the flourescence yield for Ar-L₂₃, a similar increase may occur for Kr-M₂₃, but for Ne-K the yield does not vary much.     

Perturbation theory for mixtures of simple liquids

The perturbation approach developed by Weeks, Chandler, and Andersen (WCA) and by Verlet and Weis (VW) for pure systems is here generalized to the case of mixtures. We study binary mixtures of molecules interacting with the 12–6 Lennard-Jones potential, for which Monte Carlo simulations are available for comparison. The work is divided into two parts: The first part presents results of Monte Carlo calculations on mixtures of hard spheres of 864 and 1000 particles. The radial distribution functions generated are used to test the VW representation for the correlation functions of hard-sphere mixtures. This representation is found to work satisfactorily within the expected error limits. The second part deals with the two-step perturbation procedure for calculating the thermodynamic quantities of the Lennard-Jones system. The Lennard-Jones potential is divided into a reference potential, which is strictly repulsive, and an attractive part. The system of the reference potential is represented by a system of hard-sphere mixture with equivalent diameters determined by the WCA rule. Analytical expressions are given for evaluating these equivalent diameters. The Lennard-Jones system is then recovered to the first order by a λ expansion over the reference system. Comparison with Monte Carlo results for a mixture of Lennard-Jones molecules, obeying the Berthelot rule, shows that the total thermodynamic properties are reproduced by the perturbation theory to 1 per cent, while the agreement in excess properties is only moderately successful, similar to some other analytical theories compared here. To reproduce these excess properties, which are extremely small, a precision of 0·1 per cent in the theory is required. The present theory is estimated to be accurate to 1 per cent in view of the successive approximations made.     

Excess properties of Lennard-Jones binary mixtures from computer simulation and theory

Monte Carlo simulation and theory are used to calculate the excess thermodynamic properties of binary mixtures of spherical Lennard-Jones molecules. We study the excess functions of three binary mixtures characterized by the following size and dispersive energy ratios: (1) (σ₂₂/σ₁₁)³ = 2 and ?₂₂/?₁₁ = 2; (2) (σ₂₂/σ₁₁)³ = 1 and ?₂₂/?₁₁ = 1/2 and (3) (σ₂₂/σ₁₁)³ = 1/2 and ?₂₂/?₁₁ = 2. In all cases, the unlike size parameter, σ₁₂, is kept constant and equal to the value given by the Lorentz combining rule (σ₁₂ = (σ₁₁ + σ₂₂)/2). However, different unlike dispersive energy parameter values are considered through the following combining rules: (a) ?₁₂ = (?₁₁?₂₂)^1/2 (Berthelot rule); (b) ?₁₂ = ?₁₁ (association); and (c) ?₁₂ = ?₂₂ (solvation). The pressure and temperature dependence of the excess volume and excess enthalpy is studied using the NpT Monte Carlo simulation technique for all the systems considered. Additionally, the simplest conformal solution theory is used to check the adequacy of this approach in predicting the excess properties in a wide range of thermodynamic conditions and variety of binary mixtures. In particular, we have applied the van der Waals one-fluid theory to describe Lennard-Jones binary mixtures through the use of the Johnson et al. [1993, Molec. Phys., 78, 591] Helmholtz free energy. Agreement between simulation results and theoretical predictions is excellent in all cases and thermodynamic conditions considered. This work confirms the applicability of the van der Waals one-fluid theory in predicting excess thermodynamic properties of mixtures of spherical molecules. Furthermore, since binary mixtures of spherical Lennard-Jones molecules constitute the reference fluid to be used in perturbation theories for complex fluids, such as the statistical association fluid theory (SAFT), this work shows clearly the applicability of the conformal solution theory within the framework of SAFT for predicting excess functions.     

Regular approximated scalar relativistic correlated ab initio schemes: applications to rare gas dimers

The ab initio scalar ZORA approach, which was previously tested within the context of numerical and basis set SCF calculations, is generalized to include electron correlation. The technical details of the method are investigated in calculations on the systems Ne₂, Ar₂, Kr₂ and Xe₂. For the weakly bonded rare gas dimers we calculated the bond lengths and well depths using the non-relativistic ZORA and scaled ZORA MP2 method. The relativistic effect on the potential energy minimum, obtained with the most accurate method (scaled ZORA) is shown to be very close to that of the Dirac-Fock MP2 method.     

Alternative Oblique-Incidence Reflectometry for Measuring Tissue Optical Properties

To deduce the optical properties, the absorption coefficient SmU_aand reduced scattering coefficient μ’_s, of turbid medium, Lin et al. (Appl. Opt. 34 (1995) 2362) proposed an oblique incidence reflectometry in which the diffusion approximation was assumed. In this paper we propose an alternative method which does not assume the diffusion approximation but uses a Monte Carlo light propagation model. Two features are extracted from the diffuse reflectance distribution detected on the medium surface, and optical properties are then estimated by looking up the predetermined table generated by Monte Carlo simulations. The validity of the proposed method has been confirmed by computer simulations.     

Production of Kr2+ in collisions of rare gas ions with Kr

Total and differential cross sections for the production of fast Kr²⁺ ions in collisions of He⁺, Ne⁺ and Ar⁺ with Kr were measured at primary energies below 500 eV. In the system Ar⁺+Kr most of these reactions occur in close collisions and are accompanied by a large momentum transfer. For Ne⁺+ Kr collisions the angular distribution in the centre of mass system is approximately isotropic over a wide angular range. The cross section values for the Kr²⁺ production amount to 1% of the total charge transfer cross section in the investigated energy range.     

Possible scheme of synthesis-assembling of fullerenes

A new scheme of fullerene formation is proposed on the basis of the similarity between the experimentally detected carbon structures. According to experimental data, the microclusters of C₂ and C₁₀ are synthesized first and then either an intermediate nucleus cluster or an obtainable lower fullerene is assembled from them. A high-symmetry fullerene can be assembled with a high probability from a nucleus cluster with a “good” symmetry. The atomic and electronic structures of molecules such as C₃₆, C₆₀, C₇₀, and C₇₆ are analyzed. For C₃₆, the NMR spectra are calculated and compared with the experimental data.     

Die amorphisierung von kristallen durch ionenbeschuss und mechanismen von sekundärprozessen

Results of an experimental study of the ion-electron-emission coefficient of monocrystalline and amorphous germanium as a function of the angle of incidence for 10–30 keV Ne⁺, Ar⁺, Kr⁺ ions are presented. Using the concept of ion channeling in crystals the energy dependence of anisotropy of the ion-electron-emission coefficient is explained.     

Calculation of ion energy distributions of argon excimer ions generated in helicon plasma

A program is developed to calculate the ion energy distributions （IEDs） of Ar2^＋ making use of a simplified kinetic model with a combination of Monte Carlo method. Several coefficients are used to realize good match between the calculated and measured results. Some important assumptions are confirmed： argon excimer ions have short lifetime, hence they are formed in a short range before the collecting electrode. The excimer ions that encounter collisions will be discarded because they turn to other ion species after they collide with argon atoms. From the calculated results some plasma parameters such as the cross section or neutral density in discharge could be evaluated.     

Many-body effects in the formation of multiply charged ions in a strong laser field

Some of the many-body effects in the formation of multiply charged ions in a laser field have been taken into account: inelastic tunneling, collective tunneling, and magnetic moment projection relaxation of the atomic core. Strong fields with an intensity exceeding 10¹⁷ W cm⁻² are considered when the magnetic component of the laser field acts on the free motion of a photoelectron; therefore, the formation of multiply charged ions through rescattering becomes unlikely. Numerical calculations have been performed for Ar⁹⁺ … Ar¹³⁺, Kr¹⁹⁺ … Kr²³⁺, Rb¹⁰⁺, and Rb¹¹⁺ ions. A significant contribution of collective tunneling, which was not observed in weaker fields investigated previously, has been revealed. Allowance for collective tunneling is shown to reduce the intensity leading to saturation by more than 10%. In this case, the yield of multiply charged Rb ions changes by an order of magnitude, while the yield of multiply charged Ar and Kr ions changes by more than a factor of 2. Comparison with experimental data on the formation of argon ions under the action of a linearly polarized laser pulse is made.     

Molecular physics and chemistry applications of quantum Monte Carlo

We discuss recent work with the diffusion quantum Monte Carlo (QMC) method in its application to molecular systems. The formal correspondence of the imaginary-time Schrödinger equation to a diffusion equation allows one to calculate quantum mechanical expectation values as Monte Carlo averages over an ensemble of random walks. We report work on atomic and molecular total energies, as well as properties including electron affinities, binding energies, reaction barriers, and moments of the electronic charge distribution. A brief discussion is given on how standard QMC must be modified for calculating properties. Calculated energies and properties are presented for a number of molecular systems, including He, F, F^?, H₂, N, and N₂. Recent progress in extending the basic QMC approach to the calculation of “analytic” (as opposed to finite-difference) derivatives of the energy is presented, together with an H₂ potential-energy curve obtained using analytic derivatives.     

Messungen des γi-Koeffizienten,der Zündspannung und des normalen Katodenfalls der Glimmentladung an gasbedeckten Mo- und Fe-Oberflächen in Edelgasen

Measurements of the γ_i-coefficient, the ignition voltage and the normal cathode fall of the glow discharge on gas-covered cathode surfaces for the combinations Mo—Ne⁺, Ar⁺, Kr⁺, Xe⁺ and Fe—Ne⁺ are described. H₂, N₂ and O₂ are used for covering the cathodes with a monomolecular adsorption layer. Measurements are carried out with a dynamic method according to VARNEY in the range 30 ? X/p₀ ? 400 V cm^?1 Torr^?1.     

Role of translation-rotation coupling on phase diagrams of N2-solids and related systems

Molecular solids composed of N₂-molecules are studied by Monte Carlo simulations in the constant-stress ensemble utilizing Lennard-Jones and electrostatic interactions. A phase transition was found from a high-temperature orientationally-disordered cubic phase to a low-temperature phase with Pa3 structure. The transition temperature and the jump in volume are in qualitative agreement with experimental findings. An increase in the elastic constants C₁₁ and C₄₄ and a decrease in C₁₂ at the fcc-Pa3 transition are predicted. An additional study was done by neglecting the electrostatic interaction in order to study the role of the intermolecular potential. In this case a transition to a low-temperature phase with trigonal structure was obtained. If, however, translationrotation coupling is omitted, the Lennard-Jones model exhibits the Pa3 phase too. In this study, phase transitions to hexagonal phases are suppressed by the choice of periodic boundary conditions. Many similarities are found with theoretical predicition of the translation-rotation coupling induced phase instabilities in molecular C₆₀- and C₇₀-solids.     

Virial-bias Monte Carlo methods

A new sampling technique is proposed for the isothermal-isobaric (T, P, N) ensemble Monte Carlo computer simulation. The new method selects the volume perturbations by using the partial derivative of the volume with respect to the internal energy. Test calculations on the Lennard-Jones fluid show significant improvements over the conventionally used method.