Weak Interaction Effects Research Articles

Superconductivity in Zn-doped tetragonalLaBaCaCu3O7−δsystems

Resistivity and ac susceptibility of Zn-doped samples of the LaBaCaCu{sub 3}O{sub 7{minus}{delta}} (LBCCO) system have been measured for Zn content of 0.5{percent}, 1.0{percent}, 1.5{percent}, 2.0{percent}, 2.5{percent}, and 3.0{percent} at.wt. X-ray diffraction has been used to find the lattice parameters of the samples. The samples remain tetragonal for all the considered concentrations of Zn. Idometry and thermogravimetric analyses have been done to estimate the oxygen content of different samples. Analyzing the resistivity and ac susceptibility data, the following conclusions are drawn about the role of Zn in the LBCCO samples. (1) The conduction mechanism of electrons appears to follow a crossover from the purely metallic regime to the localization regime due to either weak localization or electron-electron interaction effects after about 1.5{percent} Zn. (2) The superconducting transition as revealed by the resistivity vs temperature curves or susceptibility vs temperature curves becomes sharper with Zn increasing content of Zn up to 1.0{percent}. After 1.5{percent} the resistive and ac susceptibility transitions become broader with increasing Zn. (3) T{sub c} depression up to Zn content of 1.0{percent} seems due to direct suppression of the effective pairing interaction, while at and above 2.5{percent} Zn T{sub c} depression is expected to be due to disorder effectsmore » such as reduction of density of states at the Fermi energy. {copyright} {ital 1997} {ital The American Physical Society}« less

Theoretical review of B-physics

Weak decays and mixing of B-hadrons play a special role in our understanding of the physics of the Standard Model and beyond. The measured amplitudes, however, result from a complicated interplay of weak and strong interaction effects. Understanding strong interaction dynamics, which becomes simpler for heavy quarks, is thus a fundamental part of the game. In this review, several theoretical aspects of B dynamics which are relevant for current and future experimental measurements are discussed.

Laser traps for radioactive isotopes

The techniques of laser cooling and trapping now make it possible to observe large samples of stable atoms in a small volume at low temperature. This capability was recently extended to radioactive isotopes. This opens up new opportunities for the investigation of fundamental symmetries through measurements using radioactive atoms. In this paper we will discuss several fundamental measurements in atomic systems and how the ability to trap radioactive atoms will play an important role in improving the precision of such measurements. Measurements of the effects of the weak interaction are of particular note since they are becoming quite precise. In particular, we will describe in detail the system developed at Stony Brook to trap radioactive alkali atoms and measure weak interaction effects in francium isotopes.

Effect of an ac electric field on weak electron localization in Bi films

We have studied the effects of weak localization and electron-electron interaction in conductivity of Bi films at temperatures between 1.1 and 4.2 K in high-frequency electric field (≅9.4 GHz). The experiments are carried out under conditions of no appreciable overheating of electrons due to microwave radiation. The data obtained permit to examine the influence of temperature on quantum corrections Δσ(ω) in high-frequency electric field more comprehensively and reliably than in earlier works. The experimental dependences Δσ(ω,T) are found to be in good agreement with the known theoretical models.

Weak localisation and interaction of electrons in MBE δ<Sb>-layers inSi

Temperature (1.6–20K) and magnetic field (up to 2.5T) dependences of quantum corrections to the conductivity and Hall coefficient ofSi epitaxial films containing a δ -layer of different sheet densityN Sb are studied. The quantum corrections are due to the effects of electron weak localization (WL) and electron-electron interaction (EEI). Analysis of the quantum corrections made it possible to determine the temperature dependence of electron phase relaxation time τϕ, the spin-orbit interaction time τ80 and the screening factorF. It is found that the dependence τϕ(T) is determined by the electron-electron scattering processes, τϕ∝T −p withp≈1, and an increase in the parameterF with decreasing electron concentrationn in the δ-layer is related to the specific features of the screening processes in two-dimensional electron systems.

Avoided critical behavior in a uniformly frustrated system

We study the effects of weak long-ranged antiferromagnetic interactions of strength Q on a spin model with predominant short-ranged ferromagnetic interactions. In three dimensions, this model exhibits an avoided critical point in the sense that the critical temperature T c ( Q = 0) is strictly greater than lim Q→0 T c ( Q). The behavior of this system at temperatures less than T c ( Q = 0) is controlled by the proximity to the avoided critical point. We also quantize the model in a novel way to study the interplay between charge-density wave and superconducting order.

Metallohelices: Effects of Weak Interactions on Helical Morphology

Metallohelices: Effects of Weak Interactions on Helical Morphology

Electrical conductivity and structural behaviour of Al65Cu20Ru15 quasicrystalline alloy

The low temperature conductivity σ (T) and the magnetoresistance (up to 8 T) of a phason disorder free pure Al65Cu20Ru15 quasicrystalline alloy, prepared by melt casting, has been investigated in the temperature range from 1.3 K to 200 K. In the temperature range 1.3 to 20 K the conductivity exhibits a √T-variation, whereas for the range 20 to 200 K a linear T-dependence has been observed. The analysis of magnetoresistance data reveals the presence of weak localization and electron-electron interaction effects. The positive nature of the magnetoresistance indicates the presence of spin orbit scattering. The σ∞ √T behaviour prevailing over such a wide temperature range has been observed for the first time for Al65Cu20Ru15. A quantitative analysis of the √T-variation in terms of electron-electron interaction yields a very low density of states at the Fermi level.

Weak localization and interaction effects in amorphous Ta/Ge multilayers

We have measured the in-plane resistivity of amorphous Ta/Ge multilayers with individual layer thicknesses of between 0.4 and 34 nm from 1.5 to 300 K. Conduction occurs principally through the Ta layers, allowing us to study the effects of reduced dimensions in a structurally disordered metal. At high temperatures the negative temperature coefficient of resistance can be described in terms of the destruction of quantum interference by phonons. At lower temperatures a turnover to a positive temperature coefficient of resistance is seen, suggesting that spin-orbit scattering becomes important. At still lower temperatures the resistance is dominated by electron-electron interaction effects and we have observed a transition from two-dimensional to three-dimensional behaviour as the Ta layer thickness is increased.

Self-esteem, stress, and depression among graduate students.

In a study of 29 graduate students, self-ratings of stress correlated with low scores on self-esteem but were not related to an objective indicator of actual stress. Both self-rated stress and low self-esteem scores were related to scores on depression, with a weak interaction effect.

The Effects of Weak Interaction and Reynolds Number on Boundary Layer Scalars With Tandem Turbulent Spots

Introduction The dynamics of the intermittent portion of a boundary layer transition are determined by the formation, con­vection, and merging of individual turbulent spots (Schubauer and Klebanoff, 1955). As these localized regions of turbulence convect downstream, they grow by destabilizing the surround­ing laminar boundary layer (Gad-el-Hak et al., 1981), causing it to become turbulent at the edges of each spot. The spots grow and merge to form a fully turbulent boundary layer. Many studies have documented the space-time development of single, isolated turbulent spots (Schubauer and Klebanoff, 1955; Wygnanskietal., 1976; Antoniaet al., 1981; Wygnanski et al., 1982). It is expected, however, that turbulent spots in a natural transition zone will be influenced by spot interaction and merging. The characteristics of this interaction may dictate the dynamics of the transition region. Elder (1960) found that a side-by-side pair of turbulent spots merged without detectable change to either's growth. Wygnanski (1981) studied turbulent spots generated periodically along a wind tunnel centerline with different spacings and found that sufficiently close spacing produced velocity profiles similar to a fully-turbulent boundary layer. Gutmark and Black welder (1987) showed that the tur­bulence level and the convection velocity of a closely following trailing spot of a tandem pair was attenuated by interaction. The growth of the trailing spot was reduced by the highly stable calmed region behind the leading spot. Zilberman et al. (1977) and Wygnanski (1981) demonstrated that key fea­tures of spots are quite persistent, showing that portions of the spot structure furthest from the wall retain their identity for a considerable length of time even after the spot has merged with a fully turbulent boundary layer. Lauchle (1981) applied knowledge of the scalar descriptors of intermittence in the transition to predict the radiated sound in terms of the time and length scales of the unsteady displacement thickness. Krane and Pauley (1992a) quantified the temporal and spatial scales of the unsteady displacement thickness for a single turbulent spot and used them to determine the level and character of sound produced. It was shown that the large positive and negative peaks in displacement thickness during passage of the spot can result in significant noise pro­duction. The behavior of these peaks can be described in terms of scalars such as their convection speeds and their rise and fall times. Wygnanski (1981) showed that a typical transition exhibits characteristics of weak rather than strong in­teraction. To reliably extend the predictions based on the iso­lated spot to a natural transition, therefore, it is necessary to know if the spot descriptors are modified significantly by weak spot interaction. Further, it was desired to assess the univer­sality of the findings by investigating the influence of Reynolds number on the spot descriptors.

Double Jeopardy: Interaction Effects of Marital and Poverty Status on the Risk of Mortality

The purpose of this paper is to examine the hypothesis that marital and poverty status interact in their effects on mortality risks beyond their main effects. This study examines the epidemiological bases for applying an additive rather than a multiplicative specification when testing for interaction between two discrete risk factors. We specifically predict that risks associated with being nonmarried and with being poor interact to produce mortality risks that are greater than each risk acting independently. The analysis is based on men and women who were ages 25-74 during the 1971-1975 National Health and Nutrition Examination Survey I (NHANES I) and who were traced successfully in the NHANES I Epidemiologic Follow-Up Study in 1982-1984. Overall, being both poor and nonmarried places nonelderly (ages 25-64) men, but not women, at risk of mortality greater than that expected from the main effects. This study shows that for all-cause mortality, marital and poverty status interact for men but less so for women; these findings exist when interaction is assessed with either a multiplicative or an additive standard. This difference is most pronounced for poor, widowed men and (to a lesser degree) poor, divorced men. For violent/accidental deaths among men, the interaction effects are large on the basis of an additive model. Weak main and interaction effects were detected for the elderly (age 65+).

Effects of weak interaction on kaon condensation and cooling of neutron stars

Kaon condensation and its implication in the cooling mechanism in neutron stars are investigated within a framework of current algebra and PCAC. The weak interaction, n ⇌ p + K −, is shown to play a significant role in determining not only the critical density but also the equation of state of the K − condensed phase. The chemical equilibrium for the weak interaction leads to large proton-admixture. In connection with this result, the possibility of the direct URCA process, n → p + e − + v ̄ e , p + e − → n + v e , is investigated. It is shown that, within a simple treatment without the nuclear interactions such as the symmetry energy, the kinematical condition for the direct URCA process is not satisfied despite the large proton-mixing, due to the resulting small electron Fermi momentum. The physical content of the K − condensation from a viewpoint of strangeness degrees of freedom is also discussed.

The Persistence of Inoculation in Conferring Resistance to Smoking Initiation Among Adolescents

The authors previously reported the results of an investigation into the potential of inoculation to promote resistance to smoking onset in young adolescents. Inoculation videos were administered to 1,047 adolescents in a midwestern city. Results for the first year indicated that inoculation promotes resistance to smoking initiation but only among adolescents of low self-esteem. This report follows up the first-year results in an effort to assess the permanence of the initial effects. The results during the second year revealed main effects for inoculation on attitudes, suggesting modest persistence for the initial pretreatments, and a weak interaction effect in the fall, which dissipated by spring. In addition, the pattern of results suggests that if resistance approaches are to have a chance of success in smoking prevention they must target young adolescents at the point of transition from primary (elementary) to secondary (middle or junior high) school. Beyond this pivotal juncture, adolescents’attitudes opposing smoking quickly deteriorate, therefore rendering the resistance paradigm inappropriate.

Analysis of weak-interaction effects in high energy hadron-hadron collisions.

Parity-violating (pv) effects in inclusive hadron and jet productions in high energy hadron-hadron collisions are analyzed. Such effects arise from the interference between strong and weak amplitudes. This interference gives rise to a nonzero value of the pv parameters $A_L$ and $P_L$, where $A_L$ measures the difference in the inclusive cross sections of, for example, $p+p\ri {\rm jet}+X$ ($X$=anything), with one of incident proton beams in a state of $\pm$ helicity, and $P_L$ denotes the longitudinal polarization of a high-energy baryon (e.g., $\Lambda$) produced in $p+p\ri\Lambda+X$ with the initial proton beams unpolarized. In the present paper, the single helicity asymmetry $A_L$ in one-jet, two-jet and two-jet plus photon productions as well as in the Drell-Yan process $p+p\ri\ell^+\ell^-+{\rm jet}+X$ is probed, and the longitudinal polarization $P_L$ of the $\Lambda$ produced in unpolarized $pp$ collisions is studied. We conclude that the pv effects in high energy proton-proton collisions are in general only sensitive to the spin dependent valence quark distributions.

Intermolecular perturbation of a Jahn–Teller system: The triptycene⋅Nen (n=1–3) van der Waals clusters

The effect of rare gas complexation on the electronically excited S1(E′) state of triptycene (T), which is Jahn–Teller distorted, was investigated by two-color resonant two-photon ionization (2C-R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene⋅Nen, n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high-symmetry positions, the system symmetry is lowered from D3h(n=0) to C2v for n=1 and 2, but reverts to D3h for n=3. A Jahn–Teller (A1⊕E)⊗e coupling model including a uniaxial external strain component was applied successfully to calculate the S1 state levels and S1←S0 electronic spectra of all three complexes. The spectrum of T⋅Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D3h structure in which each of the three V-shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic spectra of T⋅Ne and T⋅Ne2 were fitted with a considerable uniaxial strain of −19.73 and 19.07 cm−1, respectively, confirming both the predicted C2v geometry of the complexes as well as the equal magnitude, but opposite sign of the intramolecular distortion induced by one or two Ne atoms. These distortions correspond to a small change of the angle of the V-shaped compartments by ∼0.8°. The spectra of T⋅Ne and T⋅Ne2 are much more complex compared to triptycene and T⋅Ne3 due to the splitting of the E vibronic levels in C2v symmetry, and the appearance of additional transitions to levels that are weak or symmetry forbidden in bare triptycene. The calculated Jahn–Teller potential energy surfaces and vibronic wave functions are discussed and classified in terms of their symmetry and localization/delocalization properties.

Anapole moments of atoms.

We analyze the anapole moment or toroidal dipole moment (TDM) of an atom, resulting from the weak interaction between electrons. A simple estimate can be made if the electron density is assumed to be constant. The TDM is found to grow like ${\mathit{Z}}^{4/3}$, reaching approximately (${\mathit{eG}}_{\mathit{F}}$/\ensuremath{\Elzxh}c) for the heaviest atoms. An algorithm for calculating the TDM of an atom with a slowly varying electron density is derived using a unitary transformation which removes the weak interaction from the Hamiltonian. We have not attempted to apply this method, but it appears to be a useful method for calculating anapole moments and other effects of weak interactions in complex atoms.

Nyquist phase relaxation in one-dimensional metal films.

We have measured the magnetoresistance of narrow (600\char21{}1000 \AA{}) and thin (150\char21{}200 \AA{}) gold films, one dimensional with respect to weak localization and electron-electron interaction effects. It is shown that electron-electron collisions with small energy transfer (Nyquist phase-breaking mechanism) govern the phase relaxation in such films over a wide temperature range. The Nyquist time ${\mathrm{\ensuremath{\tau}}}_{\mathit{N}}$ was estimated from the magnetoresistance data on the basis of the theoretical dependence \ensuremath{\Delta}${\mathit{R}}_{\mathrm{loc}}$(H) that is applicable to the case when Nyquist phase relaxation dominates other phase-breaking proceses. The temperature dependence ${\mathrm{\ensuremath{\tau}}}_{\mathit{N}}$(T)\ensuremath{\propto}${\mathit{T}}^{\mathrm{\ensuremath{-}}2/3}$ obtained in this way is in good agreement with the theoretical prediction for one-dimensional conductors.

Finite-element calculations for the S states of helium.

The finite-element method provides a convenient and accurate procedure for the calculation of the expectation values of quantum observables. We calculated energies, wave functions, and expectation values of [ital r][sub 1][sup [ital n]] for [ital n]=[minus]1, 1, and 2, and of [pi][delta]([bold r][sub 1]) for the singlet [ital n] [sup 1]S and triplet [ital n] [sup 3]S states ([ital n]=1,2,3,4) of helium. In contrast to the standard methods with globally defined basis functions, the accuracy of the expectation values of physical observables is comparable to the accuracy of the eigenvalues. The results are reported here and compared with those of Baker [ital et] [ital al]. 2 [Relativistic, Quantum Electrodynamic, and Weak Interaction Effects in Atoms, edited by Walter Johnson, Peter Mohr, and Joseph Sucher, AIP Conf. Proc. No. 189 (AIP, New York, 1989); Phys. Rev. A 41, 1247 (1990)], Drake [Nucl. Instrum. Methods Phys. Res. B 31, 7 (1988)], Pekeris [Phys. Rev. 115, 1216 (1959)], Accad [ital et] [ital al]. [Phys. Rev. A 4, 516 (1971)], and Haftel and Mandelzweig [Phys. Rev. A 38, 5995 (1988)].

Electron-electron scattering in metal-insulator-metal sandwiches.

We have studied weak localization and electron-electron interaction effects in samples consisting of two thin metal films separated by an insulating layer of SiO. When the SiO thickness was less than about 200 A, the electron-electron scattering rate was enhanced with respect to that found in isolated, i.e., well-separated, films. This suggests that in the sandwich structures, electrons in one metal film are able to «communicate» with, i.e., scatter from, electrons in the other film across distances of order 200 A, and that this process enhances the phase-breaking rate