Fluctuations in granular media

Dense slowly evolving or static granular materials exhibit strong force fluctuations even though the spatial disorder of the grains is relatively weak. Typically, forces are carried preferentially along a network of "force chains." These consist of linearly aligned grains with larger-than-average force. A growing body of work has explored the nature of these fluctuations. We first briefly review recent work concerning stress fluctuations. We then focus on a series of experiments in both two- and three-dimension [(2D) and (3D)] to characterize force fluctuations in slowly sheared systems. Both sets of experiments show strong temporal fluctuations in the local stress/force; the length scales of these fluctuations extend up to 10(2) grains. In 2D, we use photoelastic disks that permit visualization of the internal force structure. From this we can make comparisons to recent models and calculations that predict the distributions of forces. Typically, these models indicate that the distributions should fall off exponentially at large force. We find in the experiments that the force distributions change systematically as we change the mean packing fraction, gamma. For gamma's typical of dense packings of nondeformable grains, we see distributions that are consistent with an exponential decrease at large forces. For both lower and higher gamma, the observed force distributions appear to differ from this prediction, with a more Gaussian distribution at larger gamma and perhaps a power law at lower gamma. For high gamma, the distributions differ from this prediction because the grains begin to deform, allowing more grains to carry the applied force, and causing the distributions to have a local maximum at nonzero force. It is less clear why the distributions differ from the models at lower gamma. An exploration in gamma has led to the discovery of an interesting continuous or "critical" transition (the strengthening/softening transition) in which the mean stress is the order parameter, and the mean packing fraction, gamma, must be adjusted to a value gamma(c) to reach the "critical point." We also follow the motion of individual disks and obtain detailed statistical information on the kinematics, including velocities and particle rotations or spin. Distributions for the azimuthal velocity, V(theta), and spin, S, of the particles are nearly rate invariant, which is consistent with conventional wisdom. Near gamma(c), the grain motion becomes intermittent causing the mean velocity of grains to slow down. Also, the length of stress chains grows as gamma-->gamma(c). The 3D experiments show statistical rate invariance for the stress in the sense that when the power spectra and spectral frequencies of the stress time series are appropriately scaled by the shear rate, Omega, all spectra collapse onto a single curve for given particle and sample sizes. The frequency dependence of the spectra can be characterized by two different power laws, P proportional, variant omega(-alpha), in the high and low frequency regimes: alpha approximately 2 at high omega; alpha<2 at low omega. The force distributions computed from the 3D stress time series are at least qualitatively consistent with exponential fall-off at large stresses. (c) 1999 American Institute of Physics.     

Oblate high spin isomer

Energies of high spin states are calculated with the use of an oblate deformed oscillator potential with the purpose of finding possible yrast traps or isomers. Pairing forces are included and different methods of solving the pairing Hamiltonian (including the exact solution) are compared. The stability of the oblate regime against gamma vibrations is investigated with use of the random phase approximation. Pairing forces are included also here.     

光在介质中的折射

研究了光的磁场在光与介质相互作用中的行为,建立了电子云导体模型,即介质中一个电子的电子云可以看作是一个微小导体,光变化的磁场与电子云导体间的电磁感应作用导致光与电子云导体之间的能量交换。光在介质中速度减慢起因于这种能量交换。应用电子云导体模型和能量守恒原理,导出了介质折射率表达式,并利用这个表达式计算了几种介质的折射率,计算结果与实测值符合得很好。解释了折射率的各向异性,以及静电场或静磁场对介质折射率的影响。     

Intrinsic Magnetic Flux of the Electron's Orbital and Spin Motion

In analogy with the fact that there are magnetic moments associated respectively with the electron's orbital and spin motion in an atom we present several analyses on a proposal to introduce a concept of intrinsic magnetic flux associated with the electron's orbital and spin motion. It would be interesting to test or to demonstrate Faraday's and Lenz's laws of electromagnetic induction arising directly from the flux change due to transition of states in an atom and to examine applications of this concept of intrinsic flux. PACS: 03.65.-w, 03.65.Ca, 03.65.Ta.     

光与介质作用下的一种分子电流机制

光与介质分子相互作用是一个活跃的研究领域,新的现象、技术和理论不断出现,原有理论对解释某些新的现象遇到困难.本文应用法拉第电磁感应原理研究了分子中电子云与光波之间的相互作用,提出了在光波作用下介质中存在的一种分子电流机制,这种电流的密度随磁场的时间变化率增加而增大,它不同于介质分子的极化电流和磁化电流.这些工作有助于深入认识光波与介质之间的相互作用.     

Glassy spin dynamics in non-fermi-liquid UCu5-xPdx, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu5-xPdx, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function G(t) indicates strongly inhomogeneous spin fluctuations and exhibits the scaling G(t,H) = G(t/H(gamma)) expected from glassy dynamics. At 0.05 K gamma(x = 1.0) = 0.35+/-0.1, but gamma(x = 1.5) = 0.7+/-0.1. This is in contrast to inelastic neutron scattering results, which yield gamma = 0.33 for both concentrations. There is no sign of static magnetism approximately greater than 10(-3)(B)/U ion in either material above 0.05 K. Our results strongly suggest that both alloys are quantum spin glasses.     

Induced Electrical Conductivity of Gamma Irradiated S-Hydroxyquinolinc Metal Complexes

The effect of gamma radiation on the electrical conductivity σ of 8-hydroxyquinoline metal complexes was studied. The induced electrical conductivity for the investigated compounds was found to decrease on increasing radiation dose and approaches nearly that of the ligand at doses range from 3 to 5 MGy (300 to 500 Mrd). On the other hand, the increase in the activation energy ΔE by irradiation was found to depend on the crystal field stabilization energy (C.F.S.E.) in high spin tetrahedral field and the ionic potential of the metal.     

Nuclear orientation as a tool for studying the structure of very unstable nuclei

With the availability of modern isotope separator on-line systems it has become possible to make broad and systematic studies of low-energy low-spin nuclear structure. A vital ingredient in such a program is unique spin-parity assignments to all low-lying levels. A most desirable complement to spin-parity information is detailed spectroscopic information. Obtaining such information far from stability is difficult because of low activity production. Nuclear orientation provides a means for obtaining spin assignments usingsingles measurements. This is less demanding on source intensities than - angular correlation coincidence measurements. Further, nuclear orientation can provide information on magnetic moments and on multipole mixing ratios. A number of structural problems are discussed: the need for unique spin assignments in systematics schemes; the need to distinguish between E2+E0 and M1 transitions; the importance of measuring E2-M1 mixing ratios; and the value of magnetic moment information. Particular emphasis is placed on the desirability of obtaining such information in the neutron-deficient Pt, Au, Hg, Tl, Pb and Bi isotopes, based upon the experimental program at the UNISOR facility.Work supported in part by U.S.Dept. of Energy, Contract No. DE-AS05-80ER10599.     

New excited states of the nucleus129Xe

Excited states of¹²⁹Xe have been investigated through the¹²⁶Te(α,n)¹²⁹Xe reaction at the incident energy E=16MeV by means of in-beam gamma ray spectroscopy. Numerous levels below the excitation energy of 2.5MeV were found. Spin assignments were made from side feeding excitation functions. A new level at 274.2keV, which is suggested as spin 9/2 negtive parity bandhead, was observed for the first time.     

Effect of gamma softness on the stability of chiral geometry: spectroscopy of 106Ag

A study of the nucleus 106Ag has revealed the presence of two strongly coupled negative-parity rotational bands up to the 19- and 20- states, respectively, which cross each other at spin I approximately 14. The data suggest that near the crossover point the bands correspond to different shapes, which is different to the behavior expected from a pair of chiral bands. Inspection of the properties of these bands indicates a triaxial and a planar nature of rotation for the two structures. Possible causes for this may be understood in terms of a shape transformation resulting from the large degree of gamma softness of 106Ag. These data, along with the systematics of the odd-odd structures in the mass 100 region, suggest that gamma softness has marked implications for the phenomenon of nuclear chirality.     

Finite-range nucleon-nucleon interaction in the radiative capture of fast nucleons

The direct-semidirect model for nucleon radiative capture has so far been formulated assuming a zero-range nucleon-nucleon interaction. An extension of the theory to finite-range forces is proposed here. The usual δ-function in the interaction is replaced by a range-dependent gaussian function and then the radial form factor in the particle-vibration coupling (responsible for excitation of the E1 and E2 giant resonance states) is derived. Calculations are performed for the ¹⁴²(p, γ) reaction in the 5–30 MeV energy region. A detailed analysis provides indications on effects related to the interaction range as regards both the E1 and E2 integrated cross sections and the forward-backward asymmetry of the γ-ray angular distributions. Sensitivity to the interaction range shows connections with the proton final-level spin.     

Observation of parity doublets in the odd-odd nucleus220Ac

Excited states of²²⁰Ac have been investigated by means of the²⁰⁹Bi(¹⁴C,3n) reaction and using the “Château de Cristal” gamma ray facility. The observation of bands with alternating parity states reveals the reflection asymmetric nuclear shape of²²⁰Ac. The extracted B(E1)/B(E2) ratios display a clear spin dependence.     

Quantum correction to conductivity close to a ferromagnetic quantum critical point in two dimensions

We study the temperature dependence of the conductivity due to quantum interference processes for a two-dimensional disordered itinerant electron system close to a ferromagnetic quantum critical point. Near the quantum critical point, the crossover between diffusive and ballistic regimes of quantum interference effects occurs at a temperature T*=1/taugamma(E(F)tau)2, where gamma is the parameter associated with the Landau damping of the spin fluctuations, tau is the impurity scattering time, and E(F) is the Fermi energy. For a generic choice of parameters, T* is smaller than the nominal crossover scale 1/tau. In the ballistic quantum critical regime, the conductivity behaves as T1/3.     

Observation of anomalous spin-state segregation in a trapped ultracold vapor

We observe counterintuitive spin segregation in an inhomogeneous sample of ultracold, noncondensed rubidium atoms in a magnetic trap. We use spatially selective microwave spectroscopy to verify a model that accounts for the differential forces on two internal spin states. In any simple understanding of the cloud dynamics, the forces are far too small to account for the dramatic transient spin polarizations observed. The underlying mechanism remains to be elucidated.     

Polarized spin density distribution in the chromyl ethyleneglycolate anion

Electron spin resonance (E.S.R.) studies of the long-lived paramagnetic species formed in dichromate ethylene glycol solutions are presented. These results, together with detailed calculations of the electronic structure, indicate that a fairly long-range atom-to-atom spin polarization occurs, producing small negative spin densities at the distant protons.     

A semiclassical description of Fermions with spin-dependent forces

It is shown how to generalize the semiclassical treatment of dense Fermi gases in the presence of spin-dependent forces. One constructs the Wigner transform of the singlet density matrix in that representation in which the position operator and thez component of the spin operator are diagonal. This way one obtains a matrix in the spin indices which is a function of the semiclassicalp andq variables. Rules are then given how to calculate with this matrix, theμ-matrix. Next, this matrix is explicitly evaluated for independent Fermions in a spherically symmetrical potential, subject to spin-orbit forces. Finally a brief discussion is given about the nature of this matrix and possible uses are proposed.     

Radiostability of Allantoin by Using Electron Spin Resonance Spectroscopy

The radiosterilization potential and dosimetric feature of allantoin were investigated through the molecular degradations produced after ultraviolet and gamma irradiation using electron spin resonance, infrared, and ultraviolet spectroscopies and thermal measurement techniques (differential thermal analysis and the glass transition temperature). Although ultraviolet-irradiated allantoin presents no electron spin resonance signal, gamma irradiation exhibited an electron spin resonance signal of triplet appearance. Room temperature and high-temperature line intensity and spectrum area data and their variations with applied microwave power, storage time, annealing time, annealing temperature, and applied radiation dose were analyzed by assuming the production of two different types of radicals having different spectroscopic and decay characteristics. Based on its relatively stable nature toward gamma and ultraviolet radiations, it was concluded that allantoin itself and the products containing it can be sterilized by gamma and/or ultraviolet radiations without creating a great loss in its beneficial effects in the allowed radiosterilization dose limits.     

Measurement of the generalized forward spin polarizabilities of the neutron

The generalized forward spin polarizabilities gamma(0) and delta(LT) of the neutron have been extracted for the first time in a Q2 range from 0.1 to 0.9 GeV2. Since gamma(0) is sensitive to nucleon resonances and delta(LT) is insensitive to the Delta resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on delta(LT) show significant disagreement with chiral perturbation theory calculations, while the data for gamma(0) at low Q2 are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.     

Bifurcation phenomenon in spin relaxation

Spin relaxation in a strong-coupling regime (with respect to the spin system) is investigated in detail based on the spin-boson model in a stochastic limit. We find a bifurcation phenomenon in temperature dependence of relaxation constants, which is never observed in the weak-coupling regime. We also discuss inequalities among the relaxation constants in our model and show the well-known relation 2gamma(T)>or=gamma(L), for example, for a wider parameter region than before.     

Nuclear orientation study of the decay of{}^{205}Bi\underline {Gd}

Angular distribution of gamma radiation from the decay of²⁰⁵Bi oriented at low temperatures in gadolinium host was investigated. The orientation parameters B2 and B4 were determined. The spin 9/2 could be uniquely assigned to the positive-parity level at 2252.3 keV. No attenuation was observed for the isomeric level at 1013.8 keV. Seven new values of multipole mixing ratios of E2/M1 gammatransitions in²⁰⁵Pb were determined, two earlier measured values were confirmed.