Improved search for $B_{s}^{0} - \overline{B}_{s}^{0}$ oscillations

An improved search for B _s ⁰ oscillations is performed in the ALEPH data sample collected during the first phase of LEP, and reprocessed in 1998. Three analyses based on complementary event selections are presented. First, decays of B _s ⁰ mesons into hadronic flavour eigenstates are fully reconstructed. This selection yields a small sample of candidates with excellent decay length and momentum resolution and hi gh average B _s ⁰ purity. Semileptonic decays with a reconstructed D _s ^- meson provide a second sample with larger statistics, high average B _s ⁰ purity, but a poorer momentum and decay length resolution due to the partial decay reconstruction. Finally, semileptonic b-hadron decays are inclusively selected and yield the data sample with the highest sensitivity to B _s ⁰ oscillations, as the much higher statistics compensate for the low average B _s ⁰ purity and poorer time resolution. A lower limit is set atps^-1 at 95% C.L., significantly lower than the expected limit of 15.2 ps^-1. Received: 21 February 2003 / Published online: 11 June 2003     

Time-resolved photoelectron spectroscopy of $\mathrm{Al}_{3}\mathrm{O}_{3}^{-}$: photoisomerization versus photofragmentation

In general, clusters are unstable and in many cases several metastable isomers exist even at low temperature. Therefore, a cluster may react with a dramatic geometry change to a small disturbance such as a weak field or to the absorption of a low-energy photon. Here, we study the response of Al₃O₃^-\mathrm{Al}_{3}\mathrm{O}_{3}^{-} to photoexcitation using time-resolved photoelectron spectroscopy. Earlier experimental and theoretical studies suggested that this cluster anion undergoes a geometry change after photoexcitation. In contrast, our time-resolved spectra indicate that photoexcitation triggers ultra-fast fragmentation. This example demonstrates that ultra-fast processes in clusters are not well understood and that it is still difficult to gain reliable experimental data about such processes.     

Structural instability and superconductivity in B{a_{1 - x}}{K_x}Bi{o_3}

We report results of the ultrasonic investigation of Ba_1-xK_xBiO₃ superconducting (SC) single crystals for two potassium concentrations and in a wide temperature range including the normal and the SC states. An instability of the crystal lattice that develops above the superconducting phase transition leads to a softening of both the transverse c₄₄ and the longitudinal c₁₁ modes at temperatures between 200 K and 50 K. In the case of Ba_0.65K_0.35BiO₃ a pronounced hysteresis was discovered. Low temperature X-ray powder diffraction analysis does not reveal any change of the cubic structure in the samples within a resolution of our X-ray technique. The softening of the elastic moduli, the hysteresis, the maximum in the attenuation of sound along with the possible short- (or long-) range structural distortion can be explained qualitatively in a simple model by assuming a coupling of the acoustic modes with the anharmonic oscillations of BiO₆ octahedra. Some weak anomalies are discovered in the velocity of the longitudinal sound in the vicinity of the superconducting phase transition. Received 25 June 1999 and Received in final form 14 February 2000     

Hyperfine splitting constants in the optical transition \boldsymbol{{4{f}^{7}} {6{s}^{2}} \;{^{8}{\rm S}_{7/2}} \to {4{f}^{7}} {6{s}6{p}}\; {^{6}{\rm P}_{5/2}} \ {\rm of} \ {^{151-155}{\rm Eu}}} isotopes and hyperfine anomaly

Using the method of laser-induced fluorescence in an atomic beam we have measured the hyperfine splitting constants, A and B, of the ground and excited states of the optical transition 4f ⁷6s ^{2 8}S $_{1/2}\to 4f^{7}$ 6s6p ⁶P_5/2 (564.58 nm) for ^151???155Eu isotopes. For all isotopes, the magnetic dipole constants of the ⁶P_5/2 atomic level are determined to a precision better than 0.04%. The A and B constants for the ground state ⁸S_7/2 of the radioactive ^152,154,155Eu were obtained for the first time with a precision better than 0.5%. Our data along with previous ground state hyperfine structure measurements for the stable europium isotopes allow us to determine the hyperfine anomaly for mentioned Eu isotopes.     

Adsorption and desorption of no from Rh{111} and Rh{331} surfaces

The adsorption and desorption chemistry of NO on the clean Rh{111} and Rh{331} single crystal surfaces was followed with SIMS, XPS, and LEED. Results suggest dissociative NO adsorption occurs at step and/or defect sites. At saturation coverage there was ～ 10 times more dissociated species on the Rh{331} surface at 300 K than on the Rh{111} surface. On both surfaces two molecular states of NO_ads have been identified as β₁, and β₂ which possess different chemical reactivity. Under the condition of saturation coverage the β₁ and β₂ states are populated on the Rh{111} surface in a different proportion than on the Rh{331} surface. Further, their population on both surfaces is coverage and temperature dependent. When the sample is heated to desorb the saturation overlayer formed on the Rh{111} and Rh{331} crystal surfaces, approximately 50% of the overlayer is found to desorb below ? 400 K primarily from the β₂ state, molecularly as NO(g). Between 300 and 400 K the β₁ state dissociates as binding sites necessary to coordinate N_ads and O_ads are freed by desorption of NO(g).     

A stable and easily sintering {\text{BaCe}}_{0.7} {\text{Sn}}_{0.1} {\text{Gd}}_{0.2} {\text{O}}_{3 - \delta } electrolyte for solid oxide fuel cells

${\text{BaCe}}_{0.7} {\text{Sn}}_{0.1} {\text{Gd}}_{0.2} {\text{O}}_{3 - \sigma } $ (BCSG) and ${\text{BaCe}}_{0.8} {\text{Gd}}_{0.2} {\text{O}}_{3 - \sigma } $ (BCG) powders were prepared by solid-state reaction method. After exposure in 5% CO₂?+?5% H₂O?+?90% N₂ at 500 °C for 5 h, the BCSG powders were hardly affected while the BCG powders decomposed into CeO₂ and BaCO₃ phases. Moreover, the relative density of BCSG reaches 97%, while the BCG just displays 91% after sintering at 1,400 °C. The BCSG displays a conductivity of 0.01 S/cm at 700 °C in humid hydrogen, which is quite close to 0.012 S/cm for BCG. A fuel cell with BCSG exhibits 1.02 V for open circuit voltage, 420 mW/cm² for peak performance and 0.23 Ω cm² for interfacial resistance at 700 °C, respectively.     

Study of ^{201} Tl\xrightarrow{\varepsilon }^{201} Hg decay properties

The intensities of the transitions to the ground-state and the first excited level of ²⁰¹Hg in ²⁰¹Tl decay and the values of the constant log ft are estimated on the basis of an analysis of the relative intensities of the mercury M ₄ N _6,7 N _6,7 Auger electron line induced by ? capture and the mercury conversion O ₃ line caused by the 1.56-keV nuclear transition.     

High-speed planar laser-induced fluorescence of the CH radical using the C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi \left( {0,0} \right) band

The potential for kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the Methylidyne (CH) radical using its \(C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi\) (v′ = 0, v′′ = 0) band was investigated. We show that due to its strength and the excitation wavelength (~314 nm), which is conveniently generated by a dye laser operating with a red dye, the C–X(0,0) band is a good choice for CH LIF studies wherein suppression of background scattering is not required. While interference from polycyclic aromatic hydrocarbons is small, that caused by hydroxyl (OH) can be significant. Nonetheless, the OH lines can be avoided, and we observe good CH image fidelity. Most importantly, we show that due to the favorable properties of the CH C–X band (i.e., good signal strength and convenient wavelength), it can be used for PLIF at kHz acquisition rates using a continuously pulsing laser system. This is demonstrated in laminar and turbulent CH₄-air flames with a laser system operating at 10 kHz and delivering ~0.2 mJ/pulse at 314 nm.     

Spin-wave excitations in finite chain segments of the diluted one-dimensional Heisenberg antiferromagnet CsM{n_{1 - x}}M{g_x}B{r_3}

The inelastic neutron scattering technique was employed to study the magnetic excitation spectra in the diluted one-dimensional Heisenberg antiferromagnet CsMn_1-xMg_xBr₃ (x =0, 0.05, 0.10, 0.25, 0.50). The spectral response is interpreted in terms of spin-wave excitations in finite chain segments of Mn²⁺ ions, which are found to exist as long as the chain length exceeds twice the wavelength of the spin excitation. This limit determines the crossover into the mesoscopic regime. Received 31 December 1999     

Precursor diamagnetism above the superconducting transition in YNi_\mathsf{2}B_\mathsf{2}C

High-resolution SQUID magnetization measurements in a single crystal of YNi₂B₂C around the superconducting transition are reported. The diamagnetic magnetization -M _fl at constant field H as a function of temperature and isothermal magnetization curves -M _fl vs. H are used to derive insights on precursor phenomena approaching the bulk transition temperature K. The precursor diamagnetism is found strongly enhanced with respect to the conventional Ginzburg-Landau value for Gaussian fluctuations and the curves -M _fl vs. H exhibit an upturn with the field and hysteretic effects up to T ^* = 15.4 K. These results are interpreted in terms of a non-zero order parameter in superconducting droplets above the bulk T _c . These droplets are likely to be related to inhomogeneities resulting from small amount of boron to carbon substitutions.Received: 23 April 2003, Published online: 15 October 2003PACS: 74.40.+k Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.) - 74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.) - 74.25.Ha Magnetic properties     

Contribution of vector resonances to the \bar{B}_{d}^{0}\to\bar {K}^{*0}\mu^{+}\mu^{-} decay

The fully differential angular distribution for the rare flavor-changing neutral current decay $\bar{B}_{d}^{0} \to\bar{K}^{*0} (\to K^{-} \pi^{+}) \mu^{+}\mu^{-} $ is studied. The emphasis is placed on accurate treatment of the contribution from the processes $\bar{B}_{d}^{0} \to\bar{K}^{*0} (\to K^{-} \pi^{+}) V $ with intermediate vector resonances V=??(770),??(782),?(1020),J/??,??(2S),?? decaying into the ?? ⁺ ?? ^? pair. The dilepton invariant-mass dependence of the branching ratio, longitudinal polarization fraction f _L of the $\bar{K}^{*0}$ meson, and forward?Cbackward asymmetry A _FB is calculated and compared with data from Belle, CDF and LHCb. It is shown that inclusion of the resonance contribution may considerably modify the branching ratio, calculated in the SM without resonances, even in the invariant-mass region far from the so-called charmonia cuts applied in the experimental analyses. This conclusion crucially depends on values of the unknown phases of the B ⁰??K ^?0 J/?? and B ⁰??K ^?0 ??(2S) decay amplitudes with zero helicity.     

The photoassociative spectroscopy, photoassociative molecule formation, and trapping of ultracold \mathsf{^{39}K^{85}Rb}

We have observed the photoassociative spectra of colliding ultracold ³⁹K and ⁸⁵Rb atoms to produce KRb* in all eight bound electronic states correlating with the ³⁹K (4s) + ⁸⁵Rb(5p _1/2 and 5p _3/2) asymptotes. These electronically excited KRb* ultracold molecules are detected after their radiative decay to the metastable triplet (a state and (in some cases) the singlet (X ground state. The triplet (a ultracold molecules are detected by two-photon ionization at 602.5 nm to form KRb ⁺ , followed by time-of-flight mass spectroscopy. We are able to assign a majority of the spectrum to three states (2(0 ⁺ ), 2(0^-), 2(1)) in a lower triad of states with similar C ₆ values correlating to the K(4s) + Rb (5p _1/2) asymptote; and to five states in an upper triad of three states (3(0 ⁺ ), 3(0^-), 3(1)) and a dyad of two states (4(1), 1(2)), with one set of similar C ₆ values within the upper triad and a different set of similar C ₆ values within the dyad. We are also able to make connection with the short-range spectra of Kasahara et al. [J. Chem. Phys. 111, 8857 (1999)], identifying three of our levels as v = 61, 62 and 63 of the 1 4(1) state they observed. We also argue that ultracold photoassociation to levels between the K(4s) + Rb (5p _3/2) and K(4s) + Rb (5p _1/2) asymptotes may be weakly or strongly predissociated and therefore difficult to observe by ionization of a (or X molecules; we do know from Kasahara et al. that levels of the 1 4(1) and 2 5(1) states in the intra-asymptote region are predissociated. A small fraction ( 1/3) of the triplet (a ultracold molecules formed are trapped in the weak magnetic field of our magneto-optical trap (MOT).Received: 22 September 2004, Published online: 23 November 2004PACS: 33.20.Fb Raman and Rayleigh spectra (including optical scattering) - 34.20.Cf Interatomic potentials and forces - 33.80.Ps Optical cooling of molecules; trapping     

Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d₁₂ = 0.70 ± 0.03 Å and for the inner potential V₀ = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).     

Magnetic structure of L{i_{1 - x}}N{i_{1 + x}}{o_2}

We study the magnetic structure of layered Li_1-xNi_1+xO₂ and propose a new scheme: the AF interaction between the excess Ni²⁺ in the Li layers and the Ni³⁺ ions in the Ni planes, gives rise to the formation of ferrimagnetic clusters, which control the physics of these systems. The values of the different interactions are estimated from a mean field calculation in the high temperature limit. For the small x samples studied here the method does not yield an accurate value of , but it is very sensitive to the intralayer interactions, allowing to conclude that they are ferromagnetic. The recent proposal of a quantum spin-orbital liquid in this system is discussed and the comparison with Jahn-Teller distorted NaNiO₂ is made. Received 8 December 1999     

Effect of temperature on $$\mathbf{In}_{{\varvec{x}}} \mathbf{Ga}_{1-{{\varvec{x}}}} \mathbf{As}/\mathbf{GaAs}$$ quantum dots

Analytical solution of the Dirac equation for the modified Pöschl–Teller potential and trigonometric Scarf II non-central potential for spin symmetry is studied using asymptotic iteration method. One-dimensional Dirac equation consisting of the radial and angular parts can be obtained by the separation of variables. By using asymptotic iteration method, the relativistic energy equation and orbital quantum number (l) equation can be obtained, where both are interrelated. Relativistic energy equation is calculated numerically by the Matlab software. The increase in the radial quantum number n _r causes a decrease in the energy value, and the wave functions of the radial and the angular parts are expressed in terms of hypergeometric functions. Some thermodynamical properties of the system can be determined by reducing the relativistic energy equation to the non-relativistic energy equation. Thermodynamical properties such as vibrational partition function, vibrational specific heat function and vibrational mean energy function are expressed in terms of error function.     

Magnetic interactions and electronic structure of $$\hbox {Pt}_{2}\hbox {Mn}_{1-x}\hbox {Y}_{x} \hbox {Ga (Y = Cr and Fe)}$$ system: An ab-initio calculation

Structural, optical, electrical conductivity and dielectric relaxation properties of bulk 4-amino-3-mercapto-6-(2-(2-thienyl)vinyl)-1,2,4-triazin-5(4H)-one donor (AMT) are studied. The structure of AMT in its powder form was analysed by X-ray diffraction (XRD), infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). AC measurements (impedance, capacitance and phase angle) are done over the temperature range 303–373 K and in the frequency range from 42 Hz to 5 MHz. Analytical approaches for the experimental results of the σ _AC(ω, T) and the temperature behaviour of the frequency exponent show that the correlated barrier hopping (CBH) model is a good model to explain the AC electrical conductivity of bulk AMT organic semiconductor material. Application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The activation energy from the DC conductivity and the relaxation time are quite similar suggesting a hopping mechanism for AMT. The optical band gap of AMT is investigated using spectrophotometric measurement of transmittance at normal incidence of light in the wavelength range 300–1100 nm.     

Characterization of the current-induced resistive spots in superconducting \hbox {YBa}_{2} \hbox {Cu}_{3} \hbox {O}_{7} strips

For over a decade, ultrathin superconducting films have been developed for the detection of single photons at optical or near infrared frequencies, with competitive performances in terms of quantum efficiency, speed, and low dark count rate. In order to avoid the requirement of helium refrigeration, we consider here the use of high temperature materials, known to achieve very fast responsiveness to laser irradiation. We excite thin filaments of the cuprate \(\hbox {YBa}_{2} \hbox {Cu}_{3} \hbox {O}_{7}\) by rectangular pulses of supercritical current so as to produce either a phase-slip centre (PSC) or a normal hot spot (HS), according to the temperature and the current amplitude selected. That procedure provides information about the maximum bias current to be used in a particle detector, about the return current back to the quiescent state after excitation, and about the rate of growth and decay of a HS. We also measure the time of PSC nucleation. A unique feature of that approach is to provide the rate of heat transfer between the film and its substrate at whatever temperature, in the superconducting state, in the practical conditions of operation.     

\mathrm{CO}_{2}^{*} chemiluminescence study at low and elevated pressures

Chemiluminescence experiments have been performed to assess the state of current $\mathrm{CO}_{2}^{*}$ kinetics modeling. The difficulty with modeling $\mathrm{CO}_{2}^{*}$ lies in its broad emission spectrum, making it a challenge to isolate it from background emission of species such as CH^? and CH₂O^?. Experiments were performed in a mixture of 0.0005H₂+0.01N₂O+0.03CO+0.9595Ar in an attempt to isolate $\mathrm{CO}_{2}^{*}$ emission. Temperatures ranged from 1654 K to 2221 K at two average pressures, 1.4 and 10.4 atm. The unique time histories of the various chemiluminescence species in the unconventional mixture employed at these conditions allow for easy identification of the $\mathrm{CO}_{2}^{*}$ concentration. Two different wavelengths to capture $\mathrm{CO}_{2}^{*}$ were used; one optical filter was centered at 415 nm and the other at 458 nm. The use of these two different wavelengths was done to verify that broadband $\mathrm{CO}_{2}^{*}$ was in fact being captured, and not emission from other species such as CH^? and CH₂O^?. As a baseline for time history and peak magnitude comparison, OH^? emission was captured at 307 nm simultaneously with the two $\mathrm{CO}_{2}^{*}$ filters. The results from the two $\mathrm{CO}_{2}^{*}$ filters were consistent with each other, implying that indeed the same species (i.e., $\mathrm{CO}_{2}^{*}$ ) was being measured at both wavelengths. A first-generation kinetics model for $\mathrm{CO}_{2}^{*}$ and CH₂O^? was developed, since no comprehensively validated one exists to date. CH₂O^? and CH^? were ruled out as being present in the experiments at any measurable level, based on calculations and comparisons with the data. Agreement with the $\mathrm{CO}_{2}^{*}$ model was only fair, which necessitates future improvements for a better understanding of $\mathrm{CO}_{2}^{*}$ chemiluminescence as well as the kinetics of the ground state species.     

Electrical resistivity of the Ti\mathsf{_4}O\mathsf{_7} Magneli phase under high pressure

The electronic and positronic properties of the pentanary semiconductor alloys Ga_xIn_1-xP_ySb_zAs_1-y-z lattice matched to GaSb have been studied. The electron wave function is calculated semiempirically using the pseudopotential band model under the virtual crystal approximation. The positron wave function is evaluated under the point core approximation for the ionic potential. Electronic and positronic quantities namely, electronic structure and band gaps, positron band structure, effective mass and affinity, and electron-positron momentum densities have been predicted and their dependence on the phosphorus composition has been discussed. Received 30 August 2002 / Received in final form 12 February 2003 Published online 24 April 2003