ALBERT

Notes: Nonequilibrium electron distributions and phonons in CdTe have been studied by transient picosecond/subpicosecond Raman spectroscopy at T=300 K. Our experimental results show that for photoexcited electron–hole density of n(approximately-equal-to)1018 cm−3, the electron distributions can be reasonably well described by Fermi–Dirac distribution functions with effective electron temperature substantially higher than the lattice temperature. From an ensemble Monte Carlo analysis of the nonequilibrium phonon population as a function of photoexcited electron–hole pair density, the LO phonon lifetime in CdTe has been deduced to be τ(approximately-equal-to)0.75±0.25 ps. © 1996 American Institute of Physics.

Notes: The vibrational spectrumof acetylene is reported. This is the stimulat emission pumping spectrum obtained by stimulated downward transition from an upper electronic state to the ground electronic surface.Broad band and high resolution spectra were recorded at 25840−27250 wave numbers.(AIP)

Notes: Using the method of Edwards and Singh we calculate the mean square radius 〈R2〉 of the polyelectrolyte chain in solution containing counterions for the temperature greater than the Flory's θ temperature. For the counterion density n we obtain: (a) 〈R2〉∝L2 when n→0, and (b) 〈R2〉∝L6/5 when n→∞. These results are in agreement with the Flory-type calculations and recent Monte Carlo simulations performed by Baumgärtner.

Notes: We develop a self-consistent calculation of the probability of base pair opening from room temperature through the critical behavior at melting for poly(dA)⋅poly(dT). The method uses as input only force constants and interaction potential parameters refined at room temperature plus one parameter describing the open state.

Notes: Heterodyne detection of light scattered from capillary waves is used to study the surface properties of the air–liquid interface. A diffraction grating is employed to select a precise ripplon wave vector k and to provide a stable local oscillator field. The optics are designed to form a real image of the grating on the surface, thus allowing the measurements to be reproducible within 5%. The power spectrum detected in frequency domain is well fitted by a Lorentzian profile for the range of ripplon wave vectors 260≤k≤390 cm−1. The shape of the scattered spectra at each wave vector is corrected for the incident beam profile by effecting a convolution of the Lorentzian and Gaussian functional forms. The surface tensions and the kinematic viscosities of water, anisole, and ethanol are deduced from the ripplon dispersion relation corrected to the first order in damping, and found to be within 5% of the literature values. Thus we confirm the validity of approximations made for the instrumental function and the dispersion relation.

Notes: Liquid-state integral equation methods are employed to study the thermodynamic and structural properties of ideal and repelling rigid rods mixed with hard spheres in the limits when one of the species is dilute. The role of rod aspect ratio and sphere/rod size asymmetry is explored over a wide range of system parameters encompassing the colloid, nanoparticle, and crossover regimes. Novel predictions are found for the polymer (sphere) mediated depletion potentials and second virial coefficients of particles (rods) in dense polymer (sphere) suspensions. The adequacy of the closure approximations employed is tested by comparison with available numerical calculations and more rigorous theories in special limits. The liquid-state theory appears to be accurate for all properties in the nanoparticle regime and for the insertion chemical potential of needles and spherocylinders. However, it significantly underestimates depletion attractions effects in the colloidal regime of short rods and large spheres due to nonlocal entropic repulsion effects between polymers and particles not captured by the classic Percus–Yevick approximation. © 2002 American Institute of Physics.

Notes: In this paper, the occurrence of a linearization phenomenon in the Abelian Higgs model for isotropic solutions will be proved. In this linearizing process, as the parameter λ→∞, the smooth solutions of the nonlinear differential equations will degenerate to the Green's function of the related linearized equation, which is an inhomogeneous linear elliptic equation.

Notes: In this paper we present a tensor analysis of Raman induced polarization spectroscopy (RIPS), a variation of time-resolved rotational coherence spectroscopy (RCS), developed in our laboratory to study coherent rotational motions in fluids over a range of densities. Based on the irreducible tensor algebra, the analysis separates the fundamental molecular response in RIPS from the dependence of the signal on geometric factors (i.e., laser polarizations). Explicit formulas for the femtosecond RIPS response of symmetric and asymmetric tops are derived. The results are in good agreement with experimental RIPS spectra of low pressure CO2 and O3. The tensor analysis is also generalized to other types of four wave mixing experiments, allowing detailed comparison of the various frequency- and time-domain pump–probe experiments. © 1995 American Institute of Physics.

Notes: We study the behavior of a hysteretic current-biased Josephson junction in the vicinity of its return to the zero-voltage states, with primary though not exclusive emphasis on the limit of weak damping (βJ(very-much-less-than)1), and under the assumption that the zero-point and thermal energies are both small compared to Icφ0 so that fluctuation effects are important only very close to the return point. We consider in detail the resistively shunted junction (RSJ) and quasiparticle-tunneling models, and also make predictions for more general models. Denoting the value of imposed current I at which return to the zero-voltage state would take place in the absence of fluctuations by Ir, we study in particular (a) the dc current-voltage characteristic in the running state for I−Ir(very-much-less-than)Ir, and (b) the first-passage-time statistics of the return to the zero-voltage state induced by both classical and quantum fluctuations. With regard to (b), we express our results in the form of a prediction of the width σ of the distribution of retrapping events as a function of imposed current; this prediction extends down to zero temperature and can be compared directly with the experimentally measured widths.Our two principal results are as follows: (a) In the running state, for I(very-much-less-than)Ir, the current-voltage characteristic should be given quite generally by the formula (I−Ir)/Ir =[(AV0/V)+B]exp−V0/V, where A and B are constants specific to the model, and V0 is a characteristic voltage which for the simplest models is given in the weak-damping limit by V0=ωJφ0+0(β2J) , with ωJ the junction plasma resonance frequency at zero current bias. (b) The square σ2 of the width of the retrapping distribution plotted as a function of I/Ir is given to within logarithmic factors by σ2(T)=const μf(T), where μ≡(h-dash-bar)ωJ/Icφ0, the constant is of order 1, and f(T) is a function which tends to 1 as T→0 and is proportional to T in the limit of high T; it is computed explicitly for the RSJ model. We also suggest an explanation (other than lead effects) of the "forbidden voltage regions'' which appear to be a characteristic of many high-quality junctions. We discuss the application of our results to the determination of the parameters of Josephson junctions necessary for the investigation of quantum effects on the macroscopic level.

Notes: We present studies of the compositional dependence of the optical properties of AlxGa1−xN(0〈x〈0.22) alloys by modulation spectroscopy and photoluminescence. The yellow luminescence, which is well known in GaN and is generally assigned to shallow donor–deep acceptor pair recombination has also been observed in AlxGa1−xN. As aluminum concentration increases, the color of the band changes from yellow (2.2 eV) to blue (2.6 eV). The shift was less than that of the band gap. Together with previously published studies, it implies that the deep acceptor level is pinned to a common reference level to both materials, thus the deep level responsible for the yellow emission is used as a common reference level to determine the band alignment in AlxGa1−xN/GaN heterojunctions. Combining with the near-band-edge modulation spectra, the estimated ratio of conduction-to-valence band discontinuity is 65:35. Our results are close to the values obtained from PL measurements on Al0.14Ga0.86N/GaN quantum wells and those calculated by linear muffin-tin orbital method and linearized augmented plane wave method. © 2001 American Institute of Physics.