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In the framework of the non relativistic quark model, an exhaustive study of radiative transitions in mesons is performed. The emphasis is put on several points. Some traditional approximations (long wave length limit, non relativistic phase space, dipole approximation for E1 transitions, gaussian wave functions) are analyzed in detail and their effects commented. A complete treatment using three different types of realistic quark-antiquark potential is made. The overall agreement with experimental data is quite good, but some improvements are suggested., 42 pages, 2 figures

The magnetoresistance (MR) of icosahedral Al-Pd-Re of nominal composition Al(70.5)Pd(21)Res(8.5), and With resistance ratios R [=rho(4 K)/rho(295 K)] from 2 to 120, has been measured in the range 0.1-40 K in magnetic fields up to 12 T. Three regions of the MR can be distinguished. For R up to 13 quantum interference effects (QIE) describe the observations well. MR in excess of 100% was observed at low temperatures in this region. For R increasing above 13, the approach of a possible metal-insulator transition (MIT) can be followed in the results. A new negative MR contribution emerges at the lowest temperature, 0.2 K, and increases in magnitude with R. From analyses within QIE the Coulomb interaction parameter above R approximate to 10 and the inelastic-scattering time decrease with R, and the results indicate that this may be the case also for the spin-orbit-scattering rate. For R greater than or equal to 50, weak localization has broken down. Samples prepared by two different methods were studied in this region of R. Although the temperature dependence of the electrical resistivity and the MR at temperatures above 4 K are similar for similar R values in both sets of samples, significant differences were observed;below 1 K. We discuss this MR in the light of current theories on both sides of an MIT and conclude that none of these theories can fully reproduce the observed features for samples in this range.

We present tunneling spectroscopy measurements at 4 K on three quasicrystalline phases i-AlPdMn, i-AlCuFe and i-AlPdRe. In the spectroscopic curves, dI/dU, we observe a structure centered about zero bias voltage and with a square root voltage dependence from 300 mV down to at least 5 mV. This structure can be interpreted as the preopening of a correlation gap in the electronic density of states (DOS), which is consistent with the proximity to a metal–insulator transition in the three phases.

We present results about electronic properties of quasi-crystalline AlPdRe samples, assumed to be on each side of the metal–insulator transition. The structural quality and the chemical composition are investigated by X-ray diffraction, scanning electron microscopy and microprobe analysis. Conductivity is reported. The density of states (DOS) at the Fermi level is probed by specific heat and diamagnetic susceptibility measurements, and the energy dependence around the Fermi level by scanning tunneling spectroscopy. The DOS at the Fermi level and the preopening of a correlation gap confirm the proximity to the metal–insulator transition, as indicated by the conductivity. The transition towards an insulating state could be related to local compositional fluctuations.

The transport properties of icosahedral Al 70.5 Pd 21 Re 8.5 have been measured. Samples were prepared either by arc melting or by melt-spinning with subsequent annealing in both cases. Measurements of the resistivity were performed between 40 mK and 300 K and of the magnetoresistance up to 12 T from 0.2 K to 4 K. The resistivity was found to become constant at different temperatures between 0.2 and 2 K, but both the temperature dependence of the resistivity and the magnetoresistance differ at low temperatures for the two kinds of samples. The differences between the samples are briefly discussed. Our results indicate that the conductivity in AlPdRe remains finite at low temperatures in the present samples, and a metal–insulator transition cannot thus be confirmed from these measurements.

The magnetoresistance, Δρ(B,T)/ρ(T), has been measured between 200 mK and 20 K, in magnetic fields up to 12 T for icosahedral AlPdRe quasicrystals of nominal composition Al70.5Pd21.5Re8. Samples of varying resistivities and residual resistance ratios, R [=ρ(4 K)/ρ(295 K)], were studied. An analysis of Δρ(B,T)/ρ(T) in terms of quantum interference effects (QIE) was made, taking into account the possibility of QIE contributions to Δρ(T) up to room temperature. The results show that the regime of diffusive metallic conduction in i-AlPdRe extends up to R in the range of 20, corresponding approximately to ρ(4 K) up to 105 μΩ cm, beyond which a description of Δρ(B,T)/ρ(T) in terms of QIE breaks down.

We present indications for a metal-insulator transition in the icosahedral AlPdRe phase of high structural quality. We follow the conductivity through the transition and its dependence on temperature and magnetic field in a series of i-AlPdRe samples. Features typical for systems on both sides of the metal-insulator transition are found. Moreover, the variation of conductivity with temperature is very unusual with σ(T) varying like Tα (1 ≤ α ≤ 1.5) in a wide temperature range from 7 K to 700 K.

In this paper we investigate when the transition from weak to strong localization takes place in icosahedral AlPdRe. This was made by determining an upper limit of the residual resistance ratio, RRR = ϱ 4.2 K /ϱ 295 K , where theories of quantum interference effects (QIE) can account for the magnetoresistance in these materials. It was found that QIE can describe the magnetoresistance in samples with RRR up to about 10. For samples with higher RRR several observations strongly suggest that other transport mechanisms have to be considered.

We present tunneling spectroscopy measurements for icosahedral i-AlPdRe, i-AlCuFe and approximant α-AlMnSi phases. We show that for both oxidized and bare surfaces the density of states (DOS) close to the Fermi level exhibits specific features for quasicrystals and approximants as compared with the crystalline non approximant ω-AlCuFe phase. The Fermi energy lies in the middle of a narrow pseudo-gap of about 50 meV width. For higher energies, the DOS varies as the square root of energy, which is attributed to electron-electron interaction. The overall DOS differs from ab initio calculations and the possible roles of electron scattering and non homogeneous electronic structure close to the surface are discussed.

Tunneling spectroscopy studies on oxidized and bare surfaces of icosahedral {ital i}-AlPdRe, {ital i}-AlCuFe, and approximant {alpha}-AlMnSi phases reveal specific features of the density of states (DOS) close to the Fermi level as compared to the crystalline nonapproximant {omega}-AlCuFe phase. The Fermi energy lies in the middle of a narrow pseudogap of about 50meV width. For higher energies, the DOS exhibits a square root energy dependence attributed to electron-electron interaction effects. The DOS differs from the linear muffin tin orbital calculated DOS and the possible roles of electron scattering and inhomogeneous electronic structure close to the surface are discussed. {copyright} {ital 1996 The American Physical Society.}

The crystalline approximant phases α-AlMnSi and R-AlCuFe present for alloys of metallic elements anomalous electronic transport properties, with low conductivity values and strong temperature dependence of both the conductivity and the Hall coefficient. In both the α- and R-phases, the high field magnetoconductivity can be analysed by quantum interference effects. The transport behaviours are very similar to those of the high structural quality icosahedral phases. Moreover, the R-AlCuFe phase properties are identical to those of the AlCuFe icosahedral phases of the same composition. Surprisingly, the α-AlMnSi phase, of a much simpler structure than R-AlCuFe, presents the same anomalous behaviours. Approximants are very different from the icosahedral AlMnSi counterparts which behave like amorphous metals. The results indicate that the relevant length scale for understanding the transport properties is about the size of these crystalline unit cells, i.e. one or two nanometers.

A charged particle is moving inside a planar billiard embedded in a uniform constant magnetic field which is directed perpendicular to the plane. The stability of classical trajectories is studied by the area-preserving tangent map which is derived for any billiard shape having a smooth convex boundary. As an example, two generic billiards are considered here, namely the ellipse and the stadium, classical trajectories of which are well known to be regular or chaotic, respectively, for an uncharged particle. Lyapunov exponents and Poincare sections are studied as a function of the field strength and of the billiard deformation. Regular motion is restored by increasing the magnetic field and/or the deformation.

We analyze, within a non-relativistic quark model, the low energy part of the baryonic spectrum in the octet and decuplet flavour representations. The relevance of a strong Coulomb potential is emphasized in order to explain its general features. The addition of a three-body force allows to solve the ‘Roper puzzle’, giving a consistent explanation to its relative position in the spectrum.

Matrix elements of the spin-isospin operators have been computed between Faddeev wavefunctions of the A = 3 nuclei, incorporating the ϱ- and π-mesons exchange contributions. The corresponding form factors at the 3 He-t vertices are evaluated and used as input in a free-parameter calculation of the cross sections for the reaction p( 3 He, t)Δ ++ . The results agree with experimental data, giving more support to the sensitivity of ( 3 He, t) reactions to the nuclear pionic mode.

We attempt to explain the experimental sparsity of diquonia candidates given the theoretical abundance of such states. We do this by investigating the lowest-order contributions of such states as intermediates in {ital p{bar p}} scattering into exclusive baryon-antibaryon final states. We find that the contributions depend on the partial widths for the meson-meson decays of the diquonia, and that resonant effects can be easily made to disappear. We conclude that if the meson-meson widths of diquonia are larger than about 50 MeV, most of these states will be extremely difficult to observe in {ital p{bar p}} scattering, for instance. We note that diquonia may offer a convenient means of describing some aspects of the dynamics of baryon-antibaryon scattering.

The wave equation in a one–dimensional cavity is equivalent to the motion of massless particles in a two-dimensional space-time billiard. This allows us to consider, in a simple way, the case of a cavity with one or two oscillating walls. It is shown that a set of continuous families of frequencies of the oscillating walls leads to an exponential growth of the energy of a wave. For such cases, the wave energy is localized in narrow space regions moving at the wave velocity. As a consequence the electromagnetic vacuum fluctuations inside the cavity increase exponentially and the possibility to observe experimentally photon creation from the vacuum could be reconsidered.

[New Search] [General Search Help] [ Back To Hit List | Next Document | Issue Table of Contents | Bottom of Page] AIP Conference Proceedings -- June 7, 2002 -- Volume 619, Issue 1, pp. 839-844 Full Text: [ PDF (1439 kB) GZipped PS ] Order Rightslink Permissions for Reuse About Rightslink view MyArticles What is this? Radiative transitions in mesons within a non relativistic quark model R. Bonnaz, B. Silvestre-Brac, and C. Gignoux Institut des Sciences Nucléaires, 53 Avenue des martyrs, F38026 Grenoble-Cedex, France An exhaustive study of radiative transitions in mesons is performed in a non relativistic quark model. Three different types of mesons wave functions are tested. The effect of some usual approximations is commented. Overall agreement with experimental data is obtained.

The Faddeev-Yakubovsky equations have been solved in configuration space for the four nucleon system. The n+t elastic cross section has been calculated using several realistic interactions and a phenomenological three-nucleon force. Special attention is devoted to the description of the observed resonant structure at $T_{lab}\approx 4$ MeV. In the present state of the calculations, the realistic NN interactions are unable to describe the energy region considered, i.e. n+t S- and P- waves. Whereas the inclusion of a three-nucleon force improves the zero energy region, the disagreement in the resonant peak remains. This failure contrasts with the results of a pure phenomenological model. The origin of this disagreement as well as the possible issues are discussed., Comment: 14 pages, 5 figures .eps latex.tar.gz file

The results for the n+t reaction obtained by solving the Faddeev-Yakubovsky and AGS equations are presented. We show how the resonant behaviour, which is the main feature of the n+t elastic cross section, is missed by the realistic NN potentials when limited to the j = 1 waves.

Exact boundary conditions at finite distance for the solutions of the time-dependent Schrodinger equation are derived. A numerical scheme based on Crank-Nicholson method is proposed to illustrate its applicability in several examples., Comment: Latex.tar.gz file, 20 pages, 9 figures

The Faddeev-Yakubowski equations have been solved in configuration space for the four nucleons system. Results for bound and scattering states in the isospin and S-wave approximation for different (T,S) channels are presented. The n-t elastic cross section has been also calculated with realistic interactions. A special interest was devoted to the description of the observed resonant structure at $T_{lab}\approx 3$ MeV which is well reproduced only by the simple MT I-III potential., latex.tar.gz file, 5 pages, 2 figures. Contribution to the XVth International Conference on Few-Body Problems in Physics - Groningen, 22-26 July 1997

A new family of long living antiprotonic states in He is presented. The origin of these states is the long range \(\bar{pHe}\) polarization potential which generates two-body \(\bar{p}\)—He and three-body \(\bar{p}\) — He — He charged clusters

A new family of stationary coherent states for the two-dimensional harmonic oscillator is presented. These states are coherent in the sense that they minimize an uncertainty relation for observables related to the orientation and the eccentricity of an ellipse. The wavefunction of these states is particularly simple and well localized on the corresponding classical elliptical trajectory. As the number of quanta increases, the localization on the classical invariant structure is more pronounced. These coherent states give a useful tool to compare classical and quantum mechanics and form a convenient basis to study weak perturbations.

The Faddeev-Noyes equations are solved in their natural Cartesian Jacobi coordinates for scattering below break-up threshold and for bound states. This approach is particularly well adapted to deal with strongly varying interactions. The method is proved to be successful in the three-nucleon system. First results concerning the4He trimer in configuration space are presented and further generalizations are suggested.

Faddeev equations for a low energy elastic scattering of a particle on a two body-bound state are solved in cartesian Jacobi coordinates. The method is applied successfully to calculate n-d scattering length with realistic interactions.

We present a calculation of the e − − (e − e +) scattering length in the framework of the bipolar expansion applied to the modified Faddeev equations (FE). Also, a new method of direct solving the three-dimensional FE for the three-body Coulomb bound state problem is described.

The flavor SU(3) multiplets of T-diquonia (q2q2) are classified, their masses estimated, and their decay widths into baryon-antibaryon pairs are evaluated. Using the same formalism, but with broken SU(2), the contributions of such states to some exclusive pp scattering cross sections are estimated, in an attempt to understand the experimental sparsity of diquonia candidates. It is found that if the meson-meson decay widths of these states are of the order of 50 MeV, they are difficult to detect experimentally. It is noted that, in general, except for the elastic scattering channel, the assumption of scattering via diquonia intermediates gives a reasonable description of physical reality.

The behavior of the low-transfer vertex for baryon--baryon-meson interaction and threshold effects plays an important role for the spin-orbit interaction in baryons. If treated correctly they are capable of explaining, at least schematically, the order and importance of spin-orbit splittings in {ital L}=1 baryons as well as partial and total widths.

We classify all the SU(3) multiplets of $T$-diquonia consistent with the Pauli principle, and estimate their masses using a potential model. Within the framework of the $^{3}P_{0}$ model, we calculate the total and partial decay widths of these states into baryon-antibaryon pairs. We find that both the total widths and the partial widths range from a few MeV to several hundred MeV. We briefly discuss implications for experimental detection of diquonia.

The long range ordered quasicrystals present anomalous electronic properties for metallic alloys. We present here for the first time a correlation between the temperature dependences of resistivity (p), Hall effect (RH) and thermoelectric power (S), in high structural, quality AlPdMn and AlCuFe quasicrystalline phases for various compositions. Strong composition effects are emphasized, for the high p values and the strong temperature dependences of the transport properties, as well as for sign reversals of both RH(T) and S(T). This is an indication for band structure effects in these phases. A simple two band model well accounts for these behaviours.

The magnetoresistance of seven icosahedral AlPdRe samples with residual resistivity ratios (RRR) between 2 and 45 has been measured in the temperature range 0.23 K to 4.2 K in magnetic fields up to 12 T. The results indicate that weak localization breaks down when RRR exceeds values in the range 5 to 10.

Deuteron-α scattering with n, p break-up is calculated in a three-body model with local interactions. The exclusion principle is taken into account in an approximate way.

An exact method to obtain the wave function of the neutron-deuteron system at energies below the breakup threshold is presented. The scattering length and the effective range are calculated in the case of three local potentials. The quantity $k cot\ensuremath{\delta}$ as a function of energy is given for one of these interactions. As for the triton case, realistic potentials are unable to cope completely with experimental data.NUCLEAR REACTIONS $d(n,n)d$, $E=0$ MeV (cold neutrons); calculated scattering length and effective range; wave function also obtained.

The ground state energy and wave functions are found for 6 Li, treated as an inert α-particle, a proton and a neutron, interacting with each other by local, realistic forces, taking the Pauli principle into account. The Faddeev equations in space are solved, and values for theenergy and charge radius are obtained which are in agreement with experimental values.

The validity of the instantaneous approximation for the Bethe-Salpeter equation is questioned within the framework of the simple scalar-scalar model of Cutkosky. Detailed numerous results for various approximations are compared to the exact ones. WKB quantization is applied to these relativistic approximations. An unexpected question arises: is the currently used Bethe-Salpeter equation (i.e., the ladder approximation) well suited to describe two interacting relativistic particles?

The differential equations for Yakubovsky components of the four-body wave functions are derived. An asymptotic form of these components is described. A symmetrized form of the Yakubovsky differential equations for four identical particles is given and its angular analysis is performed. Bound-state calculations are presented for various potentials.

A formalism has been developed to treat the two nucleon behaviour with the incorporation of three-quark dynamics from Faddeev equations. This formalism in which a six-quark hamiltonian is decomposed in terms of nucleons, internal hamiltonians and the internucleon q-q interaction permits us to treat the nucleon internal dynamics properly. The short distance N-N behaviour has been described very well.

We use the non-relativistic constituent quark model and the bag model to study the stability of the dilambda H = (uuddss) and pentaquark states P = ( Q uuds ), ( Q ddus ) and ( Q ssud ) . While they are stable in the limit of exact SU(3)F flavour symmetry between u, d, and s quarks, the H and the P become unbound when a realistic breaking of SU(3)F is introduced.

Les equations de Faddeev-Lovelace resolues numeriquement ont permis d'eprouver la validite de l'approximation de la matrice de transition (DWTA) et de la DWBA pour les reactions d'echange, ainsi que l'approximation soudaine utilisee dans les reactions de stripping.

The Faddeev equations are presented in configuration space together with a method for solving the three-particle bound states. Triton observables are calculated for four realistic nucleon-nucleon interactions of Reid and of de Tourreil and Sprung. Two-body correlations in the triton are given as well as the corrections to the triton energy from neglected interaction states. It is concluded that the relevance of the triton for distinguishing different local realistic nucleon-nucleon interactions is questionable.

We solve Faddeev equations, written as coupled integro-differential equations, by using a relaxation method. The initial guess solution for this method is obtained by solving Faddeev equations by means of separable expansions for two-body Green functions. We apply this method to the problem of the bound states for three bosons interacting via two potentials, one of them having a repulsive part. Preliminary results on the triton problem are given.

A closed and analytical formula is given for the free Green’s function in a harmonic oscillator basis. It is very useful for solving the Lippman–Schwinger equation for the scattering of two clusters within the resonating group method formula.

The sensitivity of a three-particle bound state to the choice among a variety of typical potentials each giving the same low energy scattering parameters and bound state energies is studied. As a simple test of this model dependence of the binding energy of the LAMBDA -d bound system is obtained using the separable Yamaguchi potential for a range of values for the scattering length and effective range.

The Faddeev equations in configuration space are solved with the p 2 -dependent nucleon-nucleon Paris potential. Resulting binding energies and charge form factors for 3-nucleon state are calculated. One finds close agreement with previous calculations based on more phenomenological potentials. This brings confidence before utilizing the new features of the Paris potential in investigations involving the explicit description of mesonic degrees of freedom.