Showing papers in "Physical Review in 2004"

Sedimentation and multiphase equilibria in suspensions of colloidal hard rods

Abstract: Sedimentation and multiphase equilibria in a suspension of hard colloidal rods are explored by analyzing the (macroscopic) osmotic equilibrium conditions. We observe that gravity enables the system to explore a whole range of phases varying from the most dilute phase to the densest phase, i.e., from the isotropic (I), to the nematic (N), to the smectic (Sm), to the crystal (K) phase. We determine the phase diagrams for hard spherocylinders with a length-to-diameter ratio of 5 for a semi-infinite system and a system with fixed container height using a bulk equation of state obtained from simulations. Our results show that gravity leads to multiphase coexistence for the semi-infinite system, as we observe I, I+N, I+N+Sm, or I+N+Sm+K coexistence, while the finite system shows I, N, Sm, K, I+N, N+Sm, Sm+K, I+N+Sm, N+Sm+K, and I+N+Sm+K phase coexistence. In addition, we compare our theoretical predictions for the phase behavior and the density profiles with Monte Carlo simulations for the semi-infinite system and we find good agreement with our theoretical predictions.

Interplay between shape and magnetocrystalline anisotropies in patterned bcc Fe/Co(001) multilayers

Abstract: Interplay between shape and magnetocrystalline anisotropies in patterned bcc Fe/Co(001) multilayers

Heavy-Quark Symmetry and the Electromagnetic Decays of Excited Charmed Strange Mesons

Abstract: Heavy-hadron chiral perturbation theory ($\mathrm{H}\mathrm{H}\ensuremath{\chi}\mathrm{P}\mathrm{T}$) is applied to the decays of the even-parity charmed strange mesons, ${D}_{s0}(2317)$ and ${D}_{s1}(2460)$. Heavy-quark spin-symmetry predicts the branching fractions for the three electromagnetic decays of these states to the ground states ${D}_{s}$ and ${D}_{s}^{*}$ in terms of a single parameter. The resulting predictions for two of the branching fractions are significantly higher than current upper limits from the CLEO experiment. Leading corrections to the branching ratios from chiral loop diagrams and spin-symmetry violating operators in the $\mathrm{H}\mathrm{H}\ensuremath{\chi}\mathrm{P}\mathrm{T}$ Lagrangian can naturally account for this discrepancy. Finally the proposal that the ${D}_{s0}(2317)$ (${D}_{s1}(2460)$) is a hadronic bound state of a $D({D}^{*})$ meson and a kaon is considered. Leading order predictions for electromagnetic branching ratios in this molecular scenario are in very poor agreement with existing data.

Simple functional form for the n+208Pb total cross section between 5 and 600 MeV

Abstract: The total cross section for neutron scattering from 208Pb with energies between 5 and 600 MeV has been analyzed, extending a previously defined simple function of three parameters to reveal a Ramsauer-like effect throughout the whole energy range. This effect can be parametrized in a simple way so that it may be anticipated that the complete function prescription will apply for total cross sections from other nuclei.

Proximity effect of vanadium on spin-density-wave magnetism in Cr films

Abstract: The spin-density-wave (SDW) state in thin chromium films is well known to be strongly affected by proximity effects from neighboring layers. To date the main attention has been given to effects arising from exchange interactions at interfaces. In the present work we report on combined neutron and synchrotron scattering studies of proximity effects in Cr/V films where the boundary condition is due to the hybridization of Cr with paramagnetic V at the interface. We find that the V/Cr interface has a strong and long-range effect on the polarization, period, and the Neel temperature of the SDW in rather thick Cr films. This unusually strong effect is unexpected and not predicted by theory.

Light-front approach for heavy pentaquark transitions

Abstract: Assuming the two diquark structure for the pentaquark state as advocated in the Jaffe-Wilczek model, there exist exotic parity-even anti-sextet and parity-odd triplet heavy pentaquark baryons. The theoretical estimate of charmed and bottom pentaquark masses is quite controversial and it is not clear whether the ground-state heavy pentaquark lies above or below the strong-decay threshold. We study the weak transitions of heavy pentaquark states using the light-front quark model. In the heavy quark limit, heavy-to-heavy pentaquark transition form factors can be expressed in terms of three Isgur-Wise functions: two of them are found to be normalized to unity at zero recoil, while the third one is equal to 1/2 at the maximum momentum transfer, in accordance with the prediction of the large-Nc approach or the quark model. Therefore, the light-front model calculations are consistent with the requirement of heavy quark symmetry. Numerical results for form factors and Isgur-Wise functions are presented. Decay rates of the weak decays Θ+b → Θcπ(ρ), Θc → Θ+π−(ρ−), Σ 5b → Σ′0 5cπ(ρ) and Σ′0 5c → N 8 π(ρ) with ΘQ, Σ5Q and N8 being the heavy antisextet, heavy triplet and light octet pentaquarks, respectively, are obtained. For weakly decaying Θb and Θ 0 c , the branching ratios of Θ + b → Θcπ, Θc → Θ+π− are estimated to be at the level of 10−3 and a few percents, respectively.

Ni 3d–BN p hybridization at the h-BN/Ni(111) interface observed with core-level spectroscopie

Abstract: Ni 3d–BN p hybridization at the h-BN/Ni(111) interface observed with core-level spectroscopie

Magnetization and domain structure of bcc Fe81Ni19/Co (001) superlattices

Abstract: The effect of compositional modulation on structural and magnetic properties of magnetic heterostructures was explored. The systems under focus were ferromagnetic superlattices Fe81Ni19/Co, metal-insulator multilayers Al2O3/Ni81Fe19, nanoparticles and artificial multilayered pillars. The heterostuctures were grown by magnetron sputtering in a state-of-the-art ultra-high vacuum system. The structural characterization was done by X-ray diffraction and reflectivity, as well as by transmission electron microscopy. Magneto-optical Kerr effect, SQUID and XMCD magnetometry and magnetic force microscopy were used for magnetic characterization. The bilayer thickness, ratio of the constituents and the interface quality influence the magnetic properties (magnetic moments and anisotropy) of metallic heterostructures. In particular, magnetic moments in bcc Fe81Ni19/Co superlattices were found to scale with the interface density thus, implying different magnetic moments at the interfaces as compared to the interior part of the layers. The easy direction of magnetization can be rotated from in-plane to out-of-plane, by increasing the bilayer thicknesses, keeping other parameters unchanged. Consequently, the anisotropy strength is strongly dependent on the repeat distance. Stripe domains appear in the films that possess an out-of-plane magnetization. The average domain period was found to be dependent on the applied in-plane magnetic field and on the total thickness of the films. The structural and magnetic properties of Al2O3/Ni81Fe19 multilayers depend strongly on the individual layers thicknesses. By increasing the amount of the magnetic deposits one can change the obtained film structure, from superparamagnetic nanoparticles to ferromagnetic multilayers. By increasing the oxide layer thickness the magnetic behavior of the nanoparticles can be altered from ferromagnetic, via spin glass like, to a superparamagnetic character.

Observation of finite-size effects on a structural phase transition of 2D nanoislands

Abstract: We study a reversible, temperature-driven structural surface phase transition of Pb/Si(111) nanoislands with a variable-temperature scanning tunneling microscope. Our quantitative measurements indicate that the transition temperature decreases with decreasing island and domain size. The boundaries of the nanoislands also influence the transition. Careful examination of the change in the interior structure of nanoislands near the transition temperature allows us to image the effects of the thermal fluctuations.

Simple function forms and nucleon-nucleus total cross sections

Abstract: Total cross sections for neutron scattering with energies between 10 and 600 MeV and from nine nuclei spanning the mass range from 6Li to 238U have been analyzed using a simple function of three parameters. The values of those parameters with which neutron total cross-section data are replicated vary smoothly with energy and target mass and may themselves be represented by functions of energy and mass.

Profile of Resonant Photoelectron Spectra versus the Spectral Function Width and Photon Frequency Detuning

Abstract: Profile of Resonant Photoelectron Spectra versus the Spectral Function Width and Photon Frequency Detuning