Two-dimensional, partially flexible polymers interacting with each other through the Onsager-like excluded-volume interaction are shown to exhibit a second-order, isotropic-nematic transition at a sufficiently high de...
Two-dimensional, partially flexible polymers interacting with each other through the Onsager-like excluded-volume interaction are shown to exhibit a second-order, isotropic-nematic transition at a sufficiently high density. The instability of the free energy under the influence of a nematic state is investigated and the critical density at which the isotropic-nematic phase transition takes place is determined analytically. It is shown that both second- and fourth-rank order parameters must be included in order to deduce a Landau expansion of the free energy.
A numerical solution is obtained for the generalized Onsager problem for the isotropic-nematic interface of rigid rods, which accounts for the biaxial effect that was ignored in previous calculations. It is found that...
A numerical solution is obtained for the generalized Onsager problem for the isotropic-nematic interface of rigid rods, which accounts for the biaxial effect that was ignored in previous calculations. It is found that the biaxial effect on interfacial tension is indeed weak, as previously expected. The biaxiality becomes significant only near the isotropic side of the interface.
Infrared absorption spectra from metastable helium atoms (2s 3S, 2s 1S) and molecules (a Σu+3) were previously acquired by irradiating dense helium gas near 4.2 K with a pulsed proton beam [R. L. Brooks, J. L. Hunt, ...
Infrared absorption spectra from metastable helium atoms (2s 3S, 2s 1S) and molecules (a Σu+3) were previously acquired by irradiating dense helium gas near 4.2 K with a pulsed proton beam [R. L. Brooks, J. L. Hunt, and D. W. Tokaryk, J. Chem. Phys. 91, 7408 (1989)]. The molecular spectrum was unusual because the observed rovibrational distribution within the a Σu+3 state was far from thermal equilibrium. Three different rovibrational groups were observed: (i) v=0, N=1 (‘‘thermal’’ molecules); (ii) v=0, 9≤N≤21 (rotationally excited molecules); and (iii) 10≤v≤12, N=1 (vibrationally excited molecules). In this work, the time evolutions of members of these three molecular populations were studied both during irradiation and in the subsequent afterglow. In addition, the evolution of the 2s 3S–2p 3P atomic line was investigated. This study quantitatively explores the reaction dynamics of the metastable molecule in the gas phase near 4.2 K. Gas pressures between 100 and 750 Torr were used. Time-resolved data were taken with a transient-digitizer system and summed for several thousand cycles of the pulsed proton beam. The absorption measurements were converted to time-resolved number densities with the aid of theoretical transition moments. The analysis required that the data be fit to the solutions of sets of coupled differential equations with a nonlinear least-squares-fit routine. The results provide insight into the complicated reactions involved in generating the unusual molecular distribution and into the reactions between the metastable molecules, metastable atoms, and the background helium gas.
Angular distributions of L3 x-ray transitions, including electric-dipole-forbidden ones, were measured for 1-, 2-, and 3-MeV proton impact on a thorium target, where the excitation and detection systems were cylindric...
Angular distributions of L3 x-ray transitions, including electric-dipole-forbidden ones, were measured for 1-, 2-, and 3-MeV proton impact on a thorium target, where the excitation and detection systems were cylindrically symmetric. A sophisticated spectrum-analysis technique was applied, where both the Lorentzian broadening of the transitions and the Si(Li) detector response function with various tailing features were taken into account. The anisotropy-parameter ratios are expected to be independent of the ionization process, and to be characteristic of the x-ray transitions in the independent-particle model. The ratios of the anisotropy parameters of the electric-dipole-allowed transitions were in disagreement with this expectation, even when the higher-order multipole contributions were taken into account. These results follow the same trend as earlier angular distribution and angular correlation measurements. Additionally, the electric-dipole-forbidden LtL3M2) and Ls(L3M3) transitions had a P4(cos(θ)) term, which is not expected within the framework of the single-particle-model predictions. Possible causes for this discrepancy are explored.
Angular correlations between Kα1 x rays and subsequent L3 x-ray transitions were measured using a Pa233 radionuclide source and high-resolution x-ray detectors. The results provide separately the values of A22(Kα1−L...
Angular correlations between Kα1 x rays and subsequent L3 x-ray transitions were measured using a Pa233 radionuclide source and high-resolution x-ray detectors. The results provide separately the values of A22(Kα1−Lα1) and A22(Kα1−Lα2) as opposed to the compound quantity A22(Kα1−Lα). For the Ll and Lα2 transitions, the A22 values agreed closely with those based upon theoretical (Hartree-Fock) E1 and M2 transition rates. For the less intense Lβ6 and Lβ2,15 transitions, agreement was also observed, although within larger uncertainties. In contrast, the value of A22(Kα1−Lα1) was 0.085±0.007, which is somewhat larger than the predicted value of 0.073. Possible causes for this discrepancy are explored.
Fermion mean-field solutions for the Heisenberg and the t-J models have been obtained by considering particle-hole correlations and carrying out the mean-field-theory calculations in momentum space. At half-filling th...
Fermion mean-field solutions for the Heisenberg and the t-J models have been obtained by considering particle-hole correlations and carrying out the mean-field-theory calculations in momentum space. At half-filling the solutions have mixed (s±d)-wave-like symmetry and have a lower ground-state energy than the s (uniform phase) and the d phases. On doping, the solutions with mixed s and d symmetry still have lower ground-state energy than the uniform phase. In the presence of doping, the density of states at the Fermi level in the mixed phases is depressed from its value in the uniform phase and is in good agreement with the experimental data.
The influence of a laterally inhomogeneous electrostatic potential of an adsorbate-covered surface on the resonant charge transfer in ion-surface scattering is investigated by means of the time-dependent Anderson-Newn...
The influence of a laterally inhomogeneous electrostatic potential of an adsorbate-covered surface on the resonant charge transfer in ion-surface scattering is investigated by means of the time-dependent Anderson-Newns Hamiltonian. The adsorbate-induced random modulation of the projectile orbital energy level is treated as a stochastic process, and its role in the charge transfer is evaluated analytically, in the Gaussian-process approximation, for low projectile speeds and low surface coverage by alkali atoms.
Non-hydrogenated amorphous silicon nitride (a-SiNx) films have been prepared using ion-beam-assisted reactive deposition and N2for the ion source gas. For a fixed ion beam voltage, a fixed Si deposition rate and a fix...
Non-hydrogenated amorphous silicon nitride (a-SiNx) films have been prepared using ion-beam-assisted reactive deposition and N2for the ion source gas. For a fixed ion beam voltage, a fixed Si deposition rate and a fixed substrate temperature, the N concentration in the film is determined by the ion beam current. At low N concentrations, the dark conductivities are dominated by hopping transport via the gap states. When the N content is increased, the gap state density is reduced and activated conductivities are observed. Room-temperature dark conductivities varying by ten orders of magnitude and optical bandgaps from 1.3 to 3.5 eV were obtained by changing the N concentration. Films with a ratioNN/NSiof N to Si content greater than 0.7 were photoconducting and this degraded slowly over time with exposure to a mercury light source. Since care was taken to eliminate H from these films and no H was detected in the films by infrared absorption, this would suggest that H may not be needed for the photoinduced degradation mechanism in a-SiNx. In addition, evidence is given that suggests sample polarization occurs and this could explain the photoconductivity fatigue observed.
Transport properties of antiferromagnetic superconductors with T(N) < T(c) have been investigated. Detailed numerical results are given for SmRh4B4 by using the following model. The paramagnetic phase (T(N) < T ...
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Transport properties of antiferromagnetic superconductors with T(N) < T(c) have been investigated. Detailed numerical results are given for SmRh4B4 by using the following model. The paramagnetic phase (T(N) < T < T(c)) is described by using Abrikosov-Gorkov theory of magnetic ions in an ordinary superconductor. In the AF phase (T less-than-or-equal-to T(N)), the effects of the molecular field H(Q)(T) and the elastic scattering of conduction electrons from spin fluctuations are included. Expressions for H(Q)(T) and the scattering rate from spin fluctuations have been derived. The aim has been to see if properties are enhanced or depressed by the AF ordering occurring below T(N). It is found that whereas the electronic thermal conductivity K(s), nuclear spin relaxation rate R(s), and the longitudinal ultrasonic attenuation alpha(s), are depressed by the AF ordering, the inverse of the magnetic penetration depth, [lambda(T)]-1, is enhanced below T(N). The effect of all types of impurities is included in the study. Theoretical results for K(s) and [lambda(T)]-1 agree with the experimental values for SmRh4B4 (experimental data for other properties are not available in literature). In the above, T(N) is the Neel temperature and T(c) is superconducting transition temperature.
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