Electronic density oscillations in gold atomic chains assembled atom by atom

Linear Au chains two to 20 atoms long were constructed on a NiAl(110) surface via the manipulation of single atoms with a scanning tunneling microscope. Differential conductance (dI/dV) images of these chains reveal one-dimensional electronic density oscillations at energies 1.0 to 2.5 eV above the Fermi energy. The origin of this delocalized electronic structure is traced to the existence of an electronic resonance measured on single, isolated Au atoms. Variations in the wavelength in dI/dV images of an eleven-atom chain taken at different energies revealed an effective electronic mass of 0.4+/-0.1 times the mass of a free-electron.     

Influence of the substrate voltage on the random telegraph signal parameters in submicron n-channel metal-oxide-semiconductor field-effect transistors under a constant inversion charge density

In this paper, the impact of the substrate bias U_BS on the parameters of a repulsive random telegraph signal in an n-channel metal-oxide-semiconductor field-effect transistor is studied. Particular attention is paid to the variation of the capture time constant F_c with the channel current I in linear operation. It is shown that the strong reduction of F_c with I can be explained by the Coulomb blockade effect. The corresponding Coulomb energy (E of the charged-near-interface oxide trap is shown to be a strong function of the substrate bias. From the analysis of the experimental results considering surface quantization effects follows that the variation of (E with U_BS is caused by the change in both the inversion layer surface charge density N_s and in the surface electric field F_s that influences the distance between the centroid of the inversion layer and the interface. In fact, it will be demonstrated that (E can be expressed in function of a single parameter (N_sF_s²). Finally, the impact of the substrate bias on the other parameters, i.e., the amplitude (I, the emission time constant F_e and the distance d of the trap from the interface, will also be addressed.     

Al grain boundary segregations in doped intermetallic NiAl and their effect on brittleness at room temperature

The nature of intermetallic NiAl brittleness is investigated by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy-loss fine structure (EELFS) means. It is found that the reason for this phenomenon is the ordering of intermetallic NiAl, accompanied by the formation of aluminum structural segregations at grain boundaries. Doping with 0.1 wt % La transforms the atomic and electronic structures of grain boundaries, eliminating NiAl intergranular fracturing at room temperature. Doping is accompanied by Fermi level shifts and an increase in conduction electron density n_eff. These factors are responsible for the lower covalence of interatomic bonds in pure intermetallic NiAl. The effect A1 segregations have on the critical deformation of the generating grain boundary fracture in NiAl is discussed.     

The W-boson mass and precision tests of the Standard Model

We have examined the electroweak radiative corrections in the LEP precision data in view of the new measurements of M_W and m_t as well as the recent progress in the higher order radiative corrections. From the minimal L²-fit to the experimental Z-decay parameters (with the aid of a modified ZFITTER program), we predict that M_W=80.29(4)(2)rGeV where the first error is due to the uncertainty in the fitted m_t for a fixed m_H and the second error comes from the m_H in the range 60􊖸rGeV, which is to be compared with the current world average M_W=80.23(18)rGeV. The current world average value of M_W and the 1994 LEP data definitely favor nonvanishing electroweak radiative corrections and are consistent with a heavy m_t as measured by the recent CDF report but with a heavy Higgs scalar of about 400rGeV within the context of the minimal standard model. The sensitivity of and the errors in the best fit solutions due to the uncertainties in the gluonic coupling !_s(M_Z) and !(M_Z) are also studied carefully. In addition we discuss how the future precision measurements of M_W can provide a decisive test for the standard model with radiative corrections and give a profound implication for the measurement of t-quark and Higgs masses.     

Atomic engineering of photon emission with a scanning tunneling microscope

We present measurements of photon emission from individual several-atom silver chains on the NiAl(110) surface, excited by tunneling electrons in a scanning tunneling microscope (STM). The chains were assembled by manipulating single silver atoms on the NiAl(110) surface with the STM. The photon energy of this emission can be tuned by appending a single atom to the chain. These changes in photon emission result from changes in the electronic structure of the silver chain, each electronic state inside the chain being associated with a distinct channel of emission.     

Deuteron electromagnetic form factors in the light-front dynamics

The deuteron form factors are calculated in the framework of the relativistic nucleon-meson dynamics, by means of the explicitly covariant light-front approach. The inflluence of the nucleon electromagnetic form factors is discussed. At Q² < 3 (GeV/c)² the prediction for the structure function A(Q²) and for the tensor polarization observable t₂₀ are in agreement with the recent data of CEBAF/TJNAF.     

Hadronic weak decays of heavy mesons and nonfactorization

The parameters L_1,2, which measure nonfactorizable soft gluon contributions to hadronic weak decays of mesons, are updated by extracting them from the data of D,rBMPP,r VP decays (P: pseudoscalar meson, V: vector meson). It is found that L₂ ranges from т.36 to т.60 in the decays from DMK¥? to D⁺MJ?⁺,r DMK¥^*?, while it is of order 10% with a positive sign in BMNK,r D?,r D^*?,r DA decays. Therefore, the effective parameter a₂ is process dependent in charm decay, whereas it stays fairly stable in B decay. This is in accordance with the picture that nonfactorizable soft gluon effects become stronger when the relative momentum of the decay particles becomes smaller. As for D,r BMVV decays, the presence of nonfactorizable terms in general prevents a possible definition of effective a₁ and a₂. This is reinforced by the observation of a large longitudinal polarization fraction in BMNK^* decay, implying nonfactorizable effects contributing differently to S-, Pand D-wave amplitudes. We found that A^nf₁/A₁>0> A^nf₂/A₂,r V^nf/V (nf standing for nonfactorization) for BMNK^* decay and 0>A^nf₁/A₁>A^nf₂/A₂,r V^nf/V for DMK¥^*A decay. A measurement of longitudinally and transversely polarized decay rates &_L and &_T in colorsuppressed decay modes B¥⁰MD^*0A⁰, D^*0P and D⁺M JA⁺ is urged.     

Ergodic Actions of Universal Quantum Groups on Operator Algebras

We construct ergodic actions of compact quantum groups on C^*-algebras and von Neumann algebras, and exhibit phenomena of such actions that are of different nature from ergodic actions of compact groups. In particular, we construct: (1) an ergodic action of the compact quantum A_u(Q) on the type III_u Powers factor R_u for an appropriate positive Q ] GL(2, Â); (2) an ergodic action of the compact quantum group A_u(n) on the hyperfinite II₁ factor R; (3) an ergodic action of the compact quantum group A_u(Q) on the Cuntz algebra _boxclose_boxclose{\cal O}_n for each positive matrix Q ] GL(n, ³); (4) ergodic actions of compact quantum groups on their homogeneous spaces, as well as an example of a non-homogeneous classical space that admits an ergodic action of a compact quantum group.     

Photoluminescence spectra of MOCVD-grown P-doped GaAS/Alx Ga1-x As MQW

PhotoLuminescence (PL) measurement techniques have been used to investigate on MOCVD grown P-doped GaAs/Al_xGa_1mx As (x=0.3) Multiple Quantum Wells (MQW). The spectra reveal extrinsic luminescence characteristics of e-A⁰ transitions for interface and centre of well acceptors in addition to both bound and free exciton emissions.     

First identification of excited states in the N = Z nucleus 70Br

Excited states in the T_z = 0 nucleus ⁷⁰Br have been investigated using the reaction ⁵⁸Ni(¹⁶O,1p3n). % rays were detected with one EUROBALL CLUSTER detector and three single HPGe detectors. Charged particles and neutrons were registered with the Rossendorf silicon ball and six modules of the EUROBALL neutron wall, respectively. The identification of % transitions in ⁷⁰Br is based on the analysis of %%-proton-neutron coincidences. A level scheme of ⁷⁰Br has been established for the first time. It shows a multiplet-like structure of probably isospin T = 0 while T = 1 isobaric analogue states are not observed.     

Influence of the size of the interpolated cation upon the CMR properties of the perovskites Nd0.7Ca0.3−xSrxMnO3−δ

The systematic study of the manganese perovskite Nd_0.7Ca_0.3-xSr_xMnO_3-' has allowed colossal magnetoresistance effects (CMR) with resistance ratios (RR), R_B=0/R_B=5T, up to 10⁴ at 60rK in a field of 5 tesla to be evidenced. The influence of the mean size of the interpolated cation (Nd, Ca, Sr), on the magnitude of the RR ratios, has been demonstrated for the first time: RR goes through a maximum for a particular value of the mean radius of the interpolated cation. This effect seems to be correlated with the evolution of the cell volume that exhibits a singular point around x=0.085. A second important feature deals with the irreversibility of the CMR effect at low temperature that decreases as T increases and disappears at T_max. It is also remarkable that for A(T) curves characterized by a maximum, T_max not only increases as the size of the interpolated cation increases, but also as the magnitude of the applied magnetic field increases.     

Simple determination of Na2 scattering lengths using observed bound levels at the ground state asymptote

A simple model is developed to determine a scattering length from the experimental values of the energy of the last, least bound, vibrational levels of the ground state, either ¹S_g⁺^1\Sigma _g^{+} or ³S_u⁺^3\Sigma _u^{+},of an alkali dimer. It is based on an extrapolation of the positions of the few outermost nodes of the bound vibrational wave functions towards the dissociation threshold. It uses the asymptotic part of the molecular potential only. The method is applied to recently measured levels at the asymptote (3s+3s) of Na₂. We obtain precise values for the two scattering lengths respectively associated to the dissociation limits F₁=F₂=1,F=2 (55.1±\pm 1.6 a₀) and F₁=F₂=1,F=0 (50.0±\pm 1.6 a₀); the first value corresponds to the a_1,-1 scattering length usually considered in cold collisions. The extrapolation procedure is also applied to Li₂, using existing experimental data.     

Enhanced quantum-well photoluminescence in InGaAs/InGaAsP heterostructures following excimer-laser-assisted surface processing

The influence of excimer-laser-assisted surface processing on quantum-well photoluminescence (QW PL) has been investigated in InGaAs/InGaAsP heterostructures capped with a thin layer of In_0.53Ga_0.47As. The PL mapping measurements carried out on samples before they were processed in a rapid thermal annealer indicated no significant differences in the QW PL signal intensity measured at the excimer-laser-processed sites and in their vicinity. However, a large difference in the QW PL signal, with its intensity significantly enhanced at the laser-processed sites, was observed after 10 s of rapid thermal annealing (RTA) at 750 °C. The largest contrast in the PL signal (an intensity difference up to about 100%) was obtained for a site processed with 50 pulses and a laser fluence of 123 mJ/cm². The changes in the QW PL signal intensity have been found to coincide with the changes in the surface chemical composition that were detected with Auger electron spectroscopy. The main difference concerns the development of a GaO_x layer on the laser-processed In_0.53Ga_0.47As surface.     

A scaling for the c-axis resistivity of La1.86Sr0.14CuO4 as a function of temperature, field and field orientation

The c-axis resistivity, A_c(T,rH,r/), of La_1.86 Sr_0.14CuO₄ is experimentally studied as a function of temperature T, magnetic field H and angle / between H and ab-plane. It is argued that the experimental findings cannot be accounted for by previously considered mechanisms. By contrast, they can be explained by a phenomenological model, which is developed by replacing H in the dissipation model for HÁIÁc with the reduced field h=H(sin²/+cos²//%²)^1/2, where % is the anisotropic parameter in magnetic fields. Based on this phenomenological model, it is shown that all the measured A_c(T,rH,r/) curves could consistently map onto a single curve.     

Collisional broadening and shift of Doppler-free two-photon thallium lines perturbed by rare-gas atoms

The extended Omont-Ueda-Kaulakys treatment of collisional effects on quasi-Rydberg states, in which the perturbation of the lower state is taken into account is applied to thallium-rare gas systems. The pressure broadening and shift coefficients of two-photon transitions in thallium involving the 6P_1/2 -n P_1/2,3/2 (n = 9-14) states are calculated and compared with experimental data obtained by Hermann et al. [Eur. Phys. J. D 1, 129 (1998)].     

N Dependence of Upper Bounds of Critical Temperatures of 2D O(N) Spin Models

We investigate critical temperature of the classical O(N) spin model in two dimensions. We show that if N is large and there is a phase transition in the system, the critical inverse temperature g_c obeys the bound g_c(N)> const. N log N.     

The E1 and E2 capture amplitudes in 12C(α,γ0)16O

Excitation functions of the %₀ capture transition in ¹²C(!,%)¹⁶O at /_% = 90^§ were obtained using a 4 2 4' BGO crystal in close geometry (E = 0.94 to 3.39 MeV) and a 2 2 2' BGO crystal in far geometry (E = 1.69 to 3.29 MeV), where the study of the reaction was initiated in inverse kinematics involving a windowless gas target. The small crystal detected essentially the E1 multipole component in the %₀ capture transition, while the large crystal observed approximately the angle-integrated sum of the E1 and E2 multipole components. Analysis of the two data sets together with data from previous work provided strong evidence that both multipoles are of equal importance at the relevant stellar energy E₀ = 0.3 MeV.     

Comparative analysis of the growth of SmBa2Cu3O7-x thin films by off-axis pulsed laser deposition on different substrates

The growth of SmBa₂Cu₃O_7-x superconducting thin films by off-axis pulsed laser deposition on different substrates (SrTiO₃, MgO, LaAlO₃, and YSZ) has been analyzed by means of resistance vs. temperature and X-ray diffraction measurements. The onset and width of the resistive transition depend on the substrate type and are in the ranges (89-80) K and (1-9) K, respectively. X-ray diffraction spectra show only the 00l reflections, from which the lattice parameter c can be estimated. Moreover, the rocking curves of the 005 peaks give an indication of the films' crystallinity and oxygen stoichiometry.