Pion–Photon Transition Form Factor

An analysis of all available data (CELLO, CLEO, B A B AR) in the range [1÷ 40] GeV² for the pion–photon transition form factor in terms of light-cone sum rules with next-to-leading-order accuracy is discussed, including twist-four contributions and next-to-next-to-leading order and twist-six corrections—the latter two via uncertainties. The antithetic trend between the B A B AR data for the γ*γπ ⁰ and those for the γ*γ η(η′) transition is pointed out, emphasizing the underlying antagonistic mechanisms: endpoint enhancement for the first and endpoint-suppression for the second—each associated with pseudoscalar meson distribution amplitudes with distinct endpoint characteristics.     

Heat Conduction and Entropy Production in Anharmonic Crystals with Self-Consistent Stochastic Reservoirs

We investigate a class of anharmonic crystals in d dimensions, d≥1, coupled to both external and internal heat baths of the Ornstein-Uhlenbeck type. The external heat baths, applied at the boundaries in the 1-direction, are at specified, unequal, temperatures T _l and T _r . The temperatures of the internal baths are determined in a self-consistent way by the requirement that there be no net energy exchange with the system in the non-equilibrium stationary state (NESS). We prove the existence of such a stationary self-consistent profile of temperatures for a finite system and show that it minimizes the entropy production to leading order in (T _l ?T _r ). In the NESS the heat conductivity κ is defined as the heat flux per unit area divided by the length of the system and (T _l ?T _r ). In the limit when the temperatures of the external reservoirs go to the same temperature T, κ(T) is given by the Green-Kubo formula, evaluated in an equilibrium system coupled to reservoirs all having the temperature T. This κ(T) remains bounded as the size of the system goes to infinity. We also show that the corresponding infinite system Green-Kubo formula yields a finite result. Stronger results are obtained under the assumption that the self-consistent profile remains bounded.     

Optical measurements in single crystal ErCrO3

The optical⁴ I _15/2→⁴ S _3/2 transition of Er³⁺ in single crystal ErCrO₃ has been studied with polarized light as a function of temperature between 1.7 and 140 °K and of applied magnetic fields in the temperature region between 1.7 and 10 °K. The temperature dependence of the ground state splitting has been deduced from these measurements. ErCrO₃ undergoes a spin reorientation with a reorientation temperature depending on applied external field. The reorientation temperature in zero field was determined to beT _r =9.4±0.5 °K. The experiments in external magnetic field along thez-axis have given the magnetic moment of the Er³⁺ ions to be μ _z =5.55 (±0.45) μ _b and the reorientation fieldH _z ^r as a function of temperature. Specific heat and magnetization data have been calculated from the optical data of the Er³⁺ ground state and compared with recent specific heat and magnetization studies in ErCrO₃ by other authors.     

Atomic beam magnetic resonance investigations in the3 F 2 Ground State of177Hf,179Hf and180Hf

The 5d ²6s ^{2 3} F ₂ ground state of¹⁷⁷Hf,¹⁷⁹Hf and¹⁸⁰Hf has been studied using the atomic beam magnetic resonance method. The atomic beam was produced by an universal evaporation technique described in a previous paper. The results are¹⁸⁰Hfg _j (³ F ₂)=0.695812 (10)¹⁷⁷Hf Δv(³ F ₂;F=11/2?F=9/2)=991.7917 (10) MHz Δv(³ F ₂;F=9/2?F=7/2)=477.0081 (10) MHz Δv(³ F ₂;F=7/2?F=5/2)=162.8890 (10) MHz¹⁷⁹HfΔv(³ F ₂;F=13/2?F=11/2)=82.1320 (10) MHz Δv(³ F ₂;F=11/2?F=9/2)=392.8498 (10) MHz. The magnetic dipole and electric quadrupole moments of the¹⁷⁷Hf and¹⁷⁹Hf nuclear ground states as calculated from these hyperfine structure measurements are the following: μ(177)=0.75(8)μ _k , Q(177)=4.34 (65) barns μ(179)=?0.61 (6)μ _k , Q(179)=4.90 (75) barns.     

An analysis of the binary and ternary fission of uranium induced by 12.2 GeV protons

In this work the binary and ternary fission of uranium induced by 12.2 GeV protons has been investigated with a polycarbonate detector registering the tracks of particles with mass numbersA>16. The basic characteristics of binary and ternary fission are discussed and the corresponding cross-sections are given. The values of the cross-sections for binary and ternary fission are calculated to be σ _b =(915 ± 120) mb and σ _t -(12.8 ± 2.5) mb, respectively.     

Temperature dependence of upper critical fields for clean superconductors of the second kind. II

A numerical mistake made in a previous paper is corrected. For specular reflexion of electrons by the surface the result now agrees with a result obtained earlier byEbneth andTewordt. In order to facilitate a comparison with experimental data the calculation of the ratioH _c3(T)H_c2(T) is extended up to terms linear (T _c ?T)/T _c for arbitrary reflexion kernels.     

Optical detected ESR in the4 S 3/2 excited state of Er:YCl3

The electron spin resonance in the⁴ S _3/2 excited state of Er³⁺ in yttrium trichloride was studied by optical detection techniques. From angular dependence of the resonance field the principle value of theg-tensor in direction of the twofold crystal axis was deduced to beg∥=3.350±0.004 and the perpendicular valueg⊥ in the crystallographica-b-plane was extrapolated to beg⊥=2.857±0.004. The lifetime of the excited state is found to be temperature independent with τ _r =(1.62±0.02)·10^?3 sec and the spin lattice relaxation timeT ₁ was determined in the temperature region 1.5 to 2.1 °K by observing the recovery of the fluorescent light signal after a microwave saturation pulse was switched off.T ₁ is found to follow a direct process with \(T_1^{ - 1} = k \cdot cth\left( {\frac{{\rlap{--} h\omega }}{{2kT}}} \right)\) .     

Die elektrischen Eigenschaften des Kondo-Supraleiters (La,Ce)Al2

In the ternary alloy (La, Ce)Al₂ the Kondo anomalies due to the Cerium impurities are studied in the normal and superconducting state. A vanishing of superconductivity in certain (La, Ce)Al₂ alloys below a second transition temperature has been observed for the narrow range of 0.6–0.7 at% Ce-additions. The transition back into the normal state has been observed by means of dc-conductivity and low frequency mutual inductance measurements down to 30 mK. Curves of the upper critical field versus temperature show a maximum at finite temperatures even for lower Ce-concentrations. The temperature dependence of the pair-breaking parameter has been deduced empirically from the upper critical field curves, assuming additive pair breaking theory. In contradiction to the theory no decrease of the pair-breaking parameter forT → 0 can be observed. Therefore the reentrance of superconductivity at ultra low temperatures cannot be expected.     

Das kritische Magnetfeld Hc∥ von dünnen supraleitenden Filmen

Thin superconducting films in a surface parallel magnetic field are considered, assuming that the order parameter is constant in space. Without any further restrictions on the film thickness,a, an equation for the critical field is derived. In the clean limit, this equation is valid only in the vicinity of T_c, but in the dirty limit it is valid for all temperatures. Our results, for specular reflection, are identical with those obtained byThompson andBaratoff. But for not too thin films we find essential differences between the results for specular and for diffus reflection. For very thin films (a?v/2πT _c), considered in detail byde Gennes andTinkham and byShapoval, our results are in agreement with those obtained byShapoval. In the opposit limit (a?v/2π T _c), the most essential corrections to the Ginzburg-Landau result obtained byLüders are rederived.     

Dynamics of impurity spin above the Kondo temperature

The dynamics of impurity spin contained by nonmagnetic host metal is investigated theoretically. The pseudofermion representation proposed byAbrikosov is applied to impurity spin. The calculations are carried out keeping only the leading logarithmic terms in any order of the perturbation theory. This approximation is adequate only above the Kondo temperature. Abrikosov's method is slightly modified to treat the spin dynamics. The real and the imaginary part of the pseudofermion self-energy is calculated. The imaginary part of the self-energy satisfies a simple relation which holds between the electron and pseudofermion self-energies. The decrease in the effective gyromagnetic factor is determined, which shows how the spin compensated state begins to form at low temperature. The first terms of the power series of the static susceptibility calculated from the pseudofermion Green function are in agreement with the results of the previous perturbative calculations given by e.g.Yosida andOkiji. The spectral function of the pseudofermion propagator is discussed in details. It has a long tail at large positive energies and satisfies the sum rule \(\int\limits_{{}^\_\infty }^\infty {d\omega \rho (\omega ) = 1} \) . The dynamic susceptibility and other physical quantities will be presented in the second part of this paper.     

Deep inelastic Compton scattering and large-p T pion photoproduction in QCD hard collision model

Inclusive photon(pion) production at large-p _T in γp collisions is studied in the framework of QCD perturbation theory, taking into account the effects due to parton transverse momenta. The contributions from various components of photons are equally important at relatively smallp _T (e.g.,p _T?4 Ge V/c at \(\sqrt s = 17GeV\) ). At largep _T, however, the contributions from the subprocesses with direct photon coupling are most important; quark Compton scattering is the dominant subprocess in inelastic Compton scattering.     

Green's function theory of the Kondo effect in dilute magnetic alloys

The exact solution of thet-matrix integral equation derived from the self-consistent Nagaoka equations in the theory of dilute magnetic alloys is established. It is shown that the unique solution for thet-matrix involving all Kondo type anomalies can be found under quite general assumptions. Using the exact solution we have calculated thermodynamic properties of dilute magnetic alloys. It is found that the excessive specific heat of the system due to the anomalous scattering of conduction electrons from the magnetic impurities is of the order ofBoltzmann's constant per local moment at low temperatures. In the limit of vanishing temperature the specific heat goes to zero asymptotically as (lnT)^?4. Finally the entropy difference of the interacting system as compared to the free system is calculated.     

Isotopic effects in the establishment of thermal equilibrium of stationary H2- and D2-glow plasmas

In stationary glow plasmas, thermal equilibrium of the electron gas with the neutral gas is established, if, with increasing gas pressure, the diffusion lifetime of plasma electrons becomes high enough. In D₂-plasmas, thermal equilibrium sets in at higher pressures than in H₂. Equal electron temperatures are measured in H₂- and D₂-glows, if the corresponding gas pressures are in the ratiop d ₂/P h ₂=1.22. This isotopic effect of electron temperature and of thermal equilibrium is compared with calculations of the Boltzmann equation, taking into account the influence of ambipolar diffusion on the electron velocity distribution. There is a good agreement between measurement and theory.     

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La,Gd)B6, (Y,Gd)Al2 and (La,Gd)Al2

The frequency dependence of the freezing temperatureT _f(ν) is determined for the dilute spin glass systems (La, Gd)B₆ and (Y, Gd)Al₂ in the frequency range 10–1,000 Hz. While for (La, Gd)B₆,T _f(ν) is found to be weak, for (Y, Gd)Al₂ T _f(ν) is even stronger than for the previously studied system (La, Gd)Al₂. Both, measurements of the temperature dependence of the susceptibility nearT _f and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B₆ and ferromagnetic for (Y, Gd)Al₂ as well as (La, Gd)Al₂.     

Crystalline field effects in superconducting La1−xTbxAl2 under pressure

The pressure dependence of the superconducting transition temperature of LaAl₂ and alloys with Tb and Gd impurities has been measured. The results are compared with a theoretical calculation of the pressure dependence ofT _c , which is based on the variation by pressure of the crystalline field experienced by the Tb ions. Measurements onLaGdAl₂ where crystal field effects are absent have been used to study the influence of pressure on the scattering rate.     

Effective hamiltonian of Bloch electrons in a homogeneous electrical field

In order to construct a band mechanics of Bloch electrons in a homogeneous electrical field E with the interband interaction taken into account, a method of determining the exact single-band Hamiltonian $$H_q = \varepsilon _q^F (\kappa ) + Fi\frac{\partial }{{\partial \kappa }}$$ is proposed, where ε _q ^F (κ) is the renormalized (effective) electron dispersion law for R = 0 and the q-th Bloch band,F= ¦e¦·E. The function ε _q ^F (κ) is expressed in terms of the interband element coordinates as well as in terms of periodic solutions of the system of ordinary differential equations which degenerateinto a common Riccati equation in a two-band approximation. A solution of the system and the form of ε _q ^F (κ), in agreement with the Wanhier result, is found in the quasiclassical approximation.     

QCD Chiral Restoration at Finite T Under the Magnetic Field

We investigate the chiral restoration at finite temperature (T) under the strong external magnetic field ${{\bf B}=B_{0}\hat{z}}$ of the SU(2) light-flavor QCD matter. We employ the instanton-liquid QCD vacuum configuration accompanied with the linear Schwinger method for inducing the magnetic field. The Harrington–Shepard caloron solution is used to modify the instanton parameters, i.e. the average instanton size ${(\bar{\rho})}$ and inter-instanton distance ${(\bar{R})}$ , as functions of T. In addition, we include the meson-loop corrections as the large-N _c corrections because they are critical for reproducing the universal chiral restoration pattern. We present the numerical results for the constituent-quark mass as well as chiral condensate which signal the spontaneous breakdown of chiral-symmetry (SBχS), as functions of T and B. Besides we find that the changes for the F _π and m _π due to the magnetic field is relatively small, in comparison to those caused by the finite T effect.     

High-energy scattering by a superposition of two power potentials

For a superposition of two power potentials the scattering amplitude and the total cross section are evaluated making use of the high-energy formula derived byBlankenbecler andGoldberger. The scattering amplitude is expressed by a finite sum of impact-parameter pole contributions in analogy to Regge poles in the angular momentum plane. It is shown that each pole contribution can be expressed in terms of known functions. Furthermore the large momentum transfer asymptotics of an impact parameter pole is investigated in detail.     

Renormalization scheme invariant analysis of the DIS structure functionsF 2 andF L

A renormalization scheme invariant analysis of the deep inelastic scattering structure functionF ₂ andF _L is performed. Expressions for the moments are given in this approach. We find a significant improvement of the agreement withR=σ _L /σ _T -SLAC data with respect to the conventional perturvative \((\overline {MS} )\) analysis. Higher twist corrections are also required confirming previous evidence.     

Crossed diagram contribution to the electron self-energy due to impurity scattering in a magnetic field

We consider the contribution of crossed diagrams to the self-energy for a system of free electrons and fixed point impurities in the presence of a strong magnetic field. At the peaks of the Shubnikov-de Haas (S.d.H.) oscillations the dependence on the impurity concentration of the crossed diagram is the same as in the generalized Born approximation (n _i ^2/3 ). Nevertheless, for the S.d.H. regime the contribution of the crossed diagrams is shown to be unimportant.