T-odd correlation in the K_{13_\gamma } decay

The dependence of the K ⁺ → π ⁰ l ⁺ ν _lγ decay rate on the T-odd kinematical variable ξ = q · [p _l × p _π]/m _K ³ is investigated in the tree and one-loop approximations of the Standard Model (SM). It is shown that the partial width of the decay on the tree level is an even function of the variable ξ, whereas the odd component is generated by the electromagnetic final-state interaction and determined by the imaginary parts of one-loop diagrams. The ξ-odd components of the partial widths of the K ⁺ → π ⁰e⁺ ν _e γ and K ⁺ → μ ⁰μ⁺ ν _μ γ decays calculated in the one-loop approximation are smaller by four orders of magnitude than the even components evaluated from the tree-level SM diagrams.     

Diffusion theory of the positive column with two-stage ionization and collisional deexcitation

The diffusion theory of the low pressure positive column is extended to account for collisional deexcitation as well as for two-stage ionization by electron impact. This extension results in the equationn″(ξ)+n′(ξ)/ξ+n(ξ)+vn ²(ξ)[1+μn(ξ)]^-1=0, which is solved numerically for the value ofξ ₁ which satisfies the boundary conditionn(ξ ₁)=0 for different values of the parametersν andμ.     

Study of K^ - \to \pi ^0 e^ - \overline \nu _e \gamma decay with ISTRA+ setup

Results of study of the $K^ - \to \pi ^0 e^ - \overline \nu \gamma $ decay at the ISTRA+ setup are presented. We observed 4476 events of this decay. The branching ratio is found to be $R = \frac{{Br(K^ - \to \pi ^0 e^ - \overline \nu _e \gamma )}}{{Br(K^ - \to \pi ^0 e^ - \overline \nu _e )}}$ = (1.81±0.03(stat.)±0.07(syst.)) × 10^?2 for E*_γ > 10 MeV and θ*_eγ > 10°. For comparison with the previous experiment the branching ratio with cuts E*_γ > 10 MeV, 0.6 < cos θ*_eγ < 0.9 is calculated: R = $\frac{{Br(K^ - \to \pi ^0 e^ - \overline \nu _e \gamma )}}{{Br(K^ - \to \pi ^0 e^ - \overline \nu _e )}}$ = (0.47±0.02(stat.) ± 0.03(syst.)) × 10^?2. For the cuts E*_γ > 30 MeV and θ*_eγ > 20°, used in most theoretical papers, Br = (3.06 ± 0.09(stat.) ± 0.14(syst.)) × 10^?4. For the asymmetry we get A _ξ = ?0.015 ± 0.021. At present it is the best estimate of this asymmetry.     

Embedding of insulating nano-inclusions in Y-123 superconductor as pinning centers: The influence of size

The optimization of flux line pinning in superconductors is one of the most efficient ways to improve the transport properties of these materials. The generation of effective artificial pinning centers in a controlled way and with optimal dimensions can contribute to the enhancement of pinning capability and to the improvement of the critical current densities J_c. In this work, we examined the effectiveness of an insulating inclusion in a type II superconductor as a pinning center with a size close to the penetration depth λ instead of the coherence length ξ. To this effect, insulating nano-pinning centres (100–150 nm) have been successfully embedded into superconducting YBa₂Cu₃O_7?d (Y-123) matrix by slightly doping with nano-particle alumina dispersions. Two alumina nano-particle dispersions with mean size diameters of about 130 nm and 150 nm which are considerably larger than the coherence length ξ of Y-123 were used. A systematic study of the relationship of J_c(H,T) with different amounts of D₁ and D₂-nanoalumina additions was performed to determine the optimum nano-particle doping contents. The results indicate that slight inclusions of D₁ or D₂-nanoalumina can effectively enhance the flux pinning capability of samples. The best flux pinning was observed in the sample with 10^?2 wt.% D₁-alumina and 3 × 10^?2 wt.% D₂-alumina. The present work suggests that the use of sufficiently large insulating inclusions in the nanometer sub-scale can stabilize the flux-line lattice. It also shows that the optimal size for an insulating inclusion, acting as a pinning center in bulk material, is more likely related to λ than to ξ.     

Nanostructuring of Fe films by oblique incidence deposition on a FeSi2 template onto Si(1 1 1): Growth, morphology, structure and faceting

The growth of thin Fe films deposited at oblique incidence on an iron silicide template onto Si(1 1 1) single crystal has been investigated as a function of Fe thickness (0 < t_Fe ? 180 monolayers (MLs)) and incidence angle (0 ? θ ? 80°). The growth mode is determined in situ by means of scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Stripes oriented perpendicularly to the incident atomic flux are formed for θ ? 30°. Self-correlation functions are used to extract characteristic lengths from STM images. The correlation lengths in the direction of the incident flux (ξ_x) and perpendicular to the atomic flux (ξ_y) grow with different powers versus time (ξ_x∝t^σ and ξ_y∝t^ρ, with σ = 0.34 ± 0.03 and ρ = 0.67 ± 0.03) following the exact solution of the (1 + 1) dimensional Kardar-Parisi-Zhang (KPZ) equation. The root mean square roughness follows also a scaling law for t_Fe < 120 ML leading to a growth exponent β = 0.73 ± 0.02. Shadowing and steering effects are discussed on the basis of our STM data.     

Improved calculation of the structure functions of the nucleon in the O(4, 2)-infinite multiplet model

A new investigation of the inelastic electon scattering from proton in the O(4, 2) models is presented. The resultant explicit structure functions in the limit satisfy scaling, F₁(ξ) ≠ 0 (σ_T ≠ 0), the Drell-Yan relation F₂(ξ) ～ (1?ξ)³ and, approximately, the Callan-Gross relation F₂(ξ) ≈ 2ξF₁(ξ).     

Core polarization effects and the random phase approximation solution

A metricg _ik=η _ik+Hξ _iξ_k+2Jξ _(iPk) is investigated. WhenJ=0 this reduces to the well-known Kerr metric. Conditions on the vectorp _i are obtained under which a geodetic, shear-free null congruenceξ _i in the Minkowskian space-time (with metricη _ik) will continue to remain geodetic and shear-free in the Riemannian space-time ofg _ik. A general solution of Einstein’s equationR _ik=σξ _iξ_k is obtained whenp _iξⁱ=0 andξ _i is twist-free.     

Multi-grid Monte Carlo via XY embedding. General theory and two-dimensional O(N)-symmetric non-linear σ-models

We introduce a variant of the multi-grid Monte Carlo (MGMC) method, based on the embedding of an XY model into the target model, and we study its mathematical properties for a variety of non-linear σ-models. We then apply the method to the two-dimensional O(N)-symmetric nonlinear σ-models (also called N-vector models) with N = 3,4,8 and study its dynamic critical behavior. Using lattices up to 256 × 256, we find dynamic critical exponentsZ_int, M² 0.70 ± 0.08, 0.60 ± 0.07, 0.52 ± 0.10 for N = 3, 4, 8, respectively (subjective 68% confidence intervals). Thus, for these asymptotically free models, critical slowing down is greatly reduced compared to local algorithms, but not completely eliminated; and the dynamic critical exponent does apparently vary with N. We also analyze the static data for N = 8 using a finite-size scaling extrapolation method. The correlation length ξ agrees with the four-loop asymptotic-freedom prediction to within 1 % over the interval 12 ⪅ ξ ⪅ 650.     

Lifetime measurements in25Mg

We present analytic solutions of the Eilenberger equations for the low-frequency Green's functions corresponding to those quasiparticles whose trajectories pass near the center of an isolated vortex. Using these results we find that for type II superconductors in the temperature rangeT _c ?T?T _c ²/ε_F the order parameter and the supercurrent near the vortex center increase over a lengthξ ₁～ξ _BCS T/T _c (ξ _BCS=BCS coherence length) and that the density of states at the Fermi surface isN ₀ 2π ³ ξ _BCS ² /3 In (ξ _BCS/ξ ₁). The results can be reproduced with the Bogoliubov equations for the elementary excitations. It is shown that this peculiar behavior is connected with the low-lying bound states in the vortex core. ForT? _c ²/ε _F one hasξ ₁～k _F ^?1.     

de Haas–van Alphen effect in anisotropic superconductor well below Hc2

We study a semi-classical approach to the determination of the oscillatory magnetization of an anisotropic superconductor in a magnetic field. The field is such that the spacing d between flux tubes is large compared to the coherence length ξ₀ and is small compared to the penetration depth. The low temperature de Haas van Alphen effect is enhanced by the Dingle temperature being reduced by a factor of d/ξ₀ for the case of line of zeros of the gap parameter. For point zeros a smaller effect is obtained.     

Current-carrying states in superconductor/insulator and superconductor/semiconductor superlattices in the mesoscopic regime

We discuss some of the basic theoretical aspects of current-carrying states in superconducting superlattices with tunnel barriers in the mesoscopic regime, when p₀ ^− 1 ⪡ a ⪡ ξ₀(a is the superconducting layer thickness, p₀is the Fermi momentum, ξ₀is the BCS coherence length and ℏ =  1). We establish the necessary conditions for the observation of the classical Josephson effect (with sinusoidal current–phase dependence) and derive self-consistent analytical expressions for the critical Josephson current. These expressions are proportional to the small factor a / ξ₀and have unusual temperature dependence as compared with the single-junction case. For certain parameter values, the superconducting gap exhibits an exponential decrease due to pair-breaking effect of the supercurrent. The supercurrent can completely destroy the superconductivity of the system above a certain characteristic temperature T ^* . In this paper, we also study the effect of intrabarrier exchange interactions. We show that this effect is strongly enhanced compared with the single-junction case and can manifest itself in an exponential decrease of the critical temperature.     

Paramagnetic resonance study of the structural phase transition in sodium azide

The electron paramagnetic resonance (EPR) of Mn²⁺ impurities in sodium azide (NaN₃) single crystals has been measured between 178 K and 300 K in order to study the transition between the monoclinic low temperature α phase and the trigonal high temperature β phase of NaN₃. The analysis of the EPR spectra yields the parameters of the zero-field splitting (zfs) in the spin Hamilton operatorD(S _ξ ² ?1/3S(S+1))+E(S _ξ ² ?S _η ² ) associated with any of four different pair states of a Mn²⁺ ion and a cation vacancy. The obtained zfs, which is temperature dependent in α-NaN₃ and constant in β-NaN₃, is consistent with a continuous phase transition atT _c =292 K. Using a point charge model for the crystal potential the zfs parameters are related to the lattice structure in the vicinity of the Mn²⁺ ion. The zfs is not consistent with the temperature dependent atomic displacements in α-NaN₃ obtained previously from an X-ray diffraction study. The critical exponents of the zfs parameters, derived in the temperature range 1 K<T _c ?T<114 K, differ from the critical exponents of atomic displacements in α-NaN₃.     

Angular asymmetries in the reactions \vec pp \to d\pi ^ + \eta and \vec pn \to d\pi ^0 \eta and a 0-f 0 mixing

The reactions pp → dπ ⁺ η and pn → dπ ⁰ η are of special interest for investigating the a ₀(980) (J ^P=0⁺) resonance in the process NN → da ₀ → dπη. We study some aspects of those reactions within a general formalism and also in a concrete phenomenological model. In particular, it is shown that the presence of nonresonant (i.e., without excitation of the a ₀ resonance) contributions to these reactions yields nonvanishing values for specific polarization observables, i.e., to effects like those generated by a ₀ ⁰ -f ₀ mixing. An experimental determination of these observables for the reaction $\vec pp \to d\pi ^ + \eta $ would provide concrete information on the magnitude of those nonresonant contributions to πη production. We also discuss the possibility of extracting information about a ₀ ⁰ -f ₀ mixing from the reaction $\vec pn \to d\pi ^0 \eta $ with a polarized proton beam.     

Induced decay of positronium and grasers

The differential cross section and the total cross section for the stimulated decay of positronium by an incident photon of frequencyΩ is calculated as a function of the dimensionless variableξ=hΩ/mc ². Forξ?1 the total cross section is found to decrease asξ ^?2. We also look at the particular case of positronium in a black-body radiation field. Expressions for the number of induced annihilations per second as functions of the dimensionless ratiomc ²/kT and the number of positronium atoms are obtained. It is found that this rate is proportional to (kT/mc ²)² forkT?mc ² and to (kT/mc ²)ln(kT/mc ²) forkT?mc ². The possibility of utilizing induced two-photon decay of positronium as a γ-ray laser at the wavelength λ _C /2 is examined, where λ _C is the Compton wavelength.     

The process γγ → X by parton model and prediction of equivalent-photon method for ee → ee + X

A parton model for the virtual photon process γ → X is examined. It is assumed that a scaling property applies and the total cross section is σ_γγ→X = s^?1φ(ξ), where the function ? is dependent on the scaling variable ξ defined by ξ = s/(q²₁q²₂). The cross section for the process ee → + X is calculated using the equivalent-photon technique.     

Observation of a tensor glueball in the reactions p\bar p \to \pi \pi ,\eta \eta ,\eta \eta '

Partial wave analysis of the reactions $p\bar p \to \pi \pi ,\eta \eta ,\eta \eta '$ in the region of invariant masses 1900–2400 MeV indicates the existence of four relatively narrow tensor-isoscalar resonances ? ₂(1920), ? ₂(2020), ? ₂(2240), ? ₂(2300), as well as the broad state ? ₂(2000). The determined decay couplings of the broad resonance ? ₂(2000) → π⁰π⁰, ηη, ηη′ satisfy the relations appropriate to those of a tensor glueball, while the couplings of other tensor states do not, thus verifying the glueball nature of ? ₂(2000).     

An experimental study of the K+ → πoμ+ν decay dalitz plot

A sample of K⁺_μ3 events, detected in the CERN 1.1 m³ heavy-liquid bubble chamber, has been used to investigate the q² dependence of the form factors, giving ξ(0) = ?1.1 ± 1.0 and ξ(6.6m²_π) = ?0.34 ± 0.20, for λ₊ = 0.027. A three parameter fit gives a value for λ₊ = 0.025 ± 0.017 in good agreement with the preceding K_e3 analysis.     

Doublon–holon-binding mechanism of Mott transition in 2D Hubbard model

The mechanism of nonmagnetic Mott transitions in the Hubbard model on the square lattice is studied, using a variational Monte Carlo method. A simple doublon (D)–holon (H) binding mechanism a previous study proposed [J. Phys. Soc. Jpn. 75 (2006) 114706] has to be modified, because even a wave function with completely bound D–H pairs brings about a Mott transition at a finite correlation strength. By introducing two characteristic lengths, D–H pair binding length, ξ_DH, and minimum inter-doublon distance, ξ_DD, we can properly describe the physics of Mott transitions, and determine the critical point by ξ_DD ～ ξ_DH. This concept seems universal, because it is valid not only for newly introduced wave functions with long-range D–H and D–D (H–H) correlation factors discussed here, but for a wide range of wave functions with D–H binding factors.     

A comparative study of thermionic emission from vertically grown carbon nanotubes and tungsten cathodes

Thermionic emission from vertically grown carbon nanotubes (CNTs) by water-assisted chemical vapor deposition (WA-CVD) is investigated. I-V characteristics of WA-CNT samples exhibit strong Schottky effect leading to field proportionality factor β ∼ 10⁴ cm⁻¹in contrast to β ∼ 200 cm⁻¹ for the bare tungsten substrate. Non-contact atomic force microscopy imaging of CNT samples show propensity of nanoasperities over a scale of micron size over which the tungsten surface is seen to be atomically smooth. The values of root mean-square roughness for CNTs and W were found to be 24.2 nm and 0.44 nm respectively. The Richardson-Dushman plots yield work function values of Φ_CNT ? 4.5 and Φ_W ? 4.3 eV. Current versus time data shows that CNT cathodes are fifteen times noisier than tungsten cathode presumably due to increased importance of individual atomic events on the sharp CNT tips of bristle like structures. Power spectral density of current exhibited 1/f^ξ behavior with ξ ? 1.5, and 2 for W and CNTs. The former suggests surface diffusion whereas the latter indicates adsorption/desorption of atomic/molecular species as a dominant mechanism of noise generation.