Temperature Dependence of the Structure of Langmuir Films of Normal-Alkanes on Liquid Mercury

The temperature dependent phase behavior of Langmuir films of n-alkanes [CH3(CH2)n−2CH3, denote Cn] on mercury was studied for chain lengths 19 ⩽ n ⩽ 22 and temperatures 15 ⩽ T ⩽ 44 °C, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where molecules invariably orient roughly surface normal, alkanes on mercury are always oriented surface parallel and show no long-range in-plane order at any surface pressure. A gas and several condensed phases of single, double, and triple layers of lying-down molecules are found, depending on n and T. At high coverages, the alkanes studied here show transitions from a triple to a double to a single layer with increasing temperature. The transition temperature from a double to a single layer is found to be ∼ 5 °C, lower than the bulk rotator-to-liquid melting temperature, while the transition from a triple to a double layer is about as much below the double-to-single layer transition. Both monolayer and bulk transition temperatures show a linear increase with n with identical slopes of ∼ 4.5 °C/CH2 within the range of n values addressed here. It is suggested that the film and bulk transitions are both driven by a common cause: the proliferation of gauche defects in the chain with increasing temperature.Engineering and Applied Science

Implementing universal multi-qubit quantum logic gates in three and four-spin systems at room temperature

In this paper, we present the experimental realization of multi-qubit gates $$ in macroscopic ensemble of three-qubit and four-qubit molecules. Instead of depending heavily on the two-bit universal gate, which served as the basic quantum operation in quantum computing, we use pulses of well-defined frequency and length that simultaneously apply to all qubits in a quantum register. It appears that this method is experimentally convenient when this procedure is extended to more qubits on some quantum computation, and it can also be used in other physical systems.Comment: 5 Pages, 2 Figure

Atomic-scale surface demixing in a eutectic liquid BiSn alloy

Resonant x-ray reflectivity of the surface of the liquid phase of the Bi$_{43}$Sn$_{57}$ eutectic alloy reveals atomic-scale demixing extending over three near-surface atomic layers. Due to the absence of underlying atomic lattice which typically defines adsorption in crystalline alloys, studies of adsorption in liquid alloys provide unique insight on interatomic interactions at the surface. The observed composition modulation could be accounted for quantitatively by the Defay-Prigogine and Strohl-King multilayer extensions of the single-layer Gibbs model, revealing a near-surface domination of the attractive Bi-Sn interaction over the entropy.Comment: 4 pages (two-column), 3 figures, 1 table; Added a figure, updated references, discussion; accepted at Phys. Rev. Let

Surface Layering in Liquid Gallium: An X-Ray Reflectivity Study

Surface-induced atomic layering in liquid gallium has been observed using x-ray reflectivity, ultrahigh vacuum conditions, and sputtered clean surfaces. Reflectivity data, collected on a supercooled liquid sample to momentum transfers as large as $q_z$ = 3.0 Å$^{-1}$, exhibit a strong maximum near 2.4 Å$^{-1}$ indicating a layer spacing that is comparable to its atomic dimensions. The amplitude of the electron density oscillations decays with a characteristic length of 6 Å. This is unexpectedly twice that of recent results for Hg, and the difference may be related to covalent bonding or supercooling.Engineering and Applied Science

Integrated X-Ray-Scattering Intensity Measurement of the Order Parameter at the Nematic-to-Smectic-A Phase Transition

The temperature dependence of the square of the smectic order parameter, $\mid \psi \mid^2$, was determined from the integrated x-ray scattering intensity. The data are described very well by the form $I(t)/I(0)=1 \mp A^{(\pm)}\mid t \mid^x$, where $t=(T-T_{NA})/T_{NA}$. The determined values for x do not agree with the theoretically expected value $x=1-\alpha$, where $\alpha$ is the heat-capacity exponent. This disagreement raises questions concerning the quantitative validity of the Landau–de Gennes free energy.Engineering and Applied Science

Polyhedral Cosmic Strings

Quantum field theory is discussed in M\"obius corner kaleidoscopes using the method of images. The vacuum average of the stress-energy tensor of a free field is derived and is shown to be a simple sum of straight cosmic string expressions, the strings running along the edges of the corners. It does not seem possible to set up a spin-half theory easily.Comment: 15 pages, 4 text figures not include

Photonic band gaps and defect states induced by excitations of Bose-Einstein condensates in optical lattices

We study the interaction of a Bose-Einstein condensate, which is confined in an optical lattice, with a largely detuned light field propagating through the condensate. If the condensate is in its ground state it acts as a periodic dielectric and gives rise to photonic band gaps at optical frequencies. The band structure of the combined system of condensed lattice-atoms and photons is studied by using the concept of polaritons. If elementary excitations of the condensate are present, they will produce defect states inside the photonic band gaps. The frequency of localized defect states is calculated using the Koster-Slater model.Comment: 10 pages, 1 figure, RevTe

X-Ray Specular-Reflectivity Study of the Liquid-Vapor Density Profile of 4^4He

The helium liquid-vapor interfacial density profile has been measured with x-ray specular reflectivity. Measurements were performed on thick films of helium adsorbed onto atomically flat silicon substrates. Both the amplitude and the phase of the complex scattering amplitude of the helium-vapor interface were obtained from measured interference between reflections from the helium liquid-vapor interface and the silicon-helium interface. Films whose thickness varied from 15 Å to 220 Å over a range of temperatures from 1.1 K to 3.0 K were studied. At T=1.13 K the film thickness is 215 Å and the interfacial width is 9.2 $\pm$ 1 Å. No significant variation was seen in the interfacial widths measured at temperatures between 1.1 K and 1.8 K. Analysis of these measurements indicates that the interface is asymmetric, with the decay of the density into the vapor having the sharper falloff. The zero-K interfacial width extrapolated from the finite-temperature measurements with a quantized capillary-wave theory is 7.6$\pm^{1}_{2}$ Å.Engineering and Applied Science

Liquid-Vapor Density Profile of Helium: An X-Ray Study

The average liquid-vapor density profiles 〈ρ(z)〉 of thick $^4$He films adsorbed onto a silicon substrate were measured using x-ray reflectivity. The results are well represented by a 90%-10% interfacial width of 9.2$\pm$1 Å at 1.13 K which extrapolates to a T=0 K, 90%-10% interfacial width of 7.6$\pm^{1}_{2}$ Å. The sensitivity of the measurement to the width, shape, and asymmetry of the density profile is discussed.Engineering and Applied Science

Who Produces for Whom in the World Economy?

International audienceFor two decades, the share of trade in inputs, also called vertical trade, has been dramatically increasing. In reallocating trade flows to their original input-producing industries and countries, this paper suggests a new measure of international trade: "value-added trade" and makes it possible to answer the question "who produces for whom?". In 2004, 27% of international trade was vertical trade. The industrial and geographic patterns of value-added trade are very different from those of standard trade. Value-added trade is relatively less important in regional trade but the difference is not more important for Asia than for AmericaLa part du commerce en produits intermédiaires dans le commerce international, appelé aussi "commerce vertical", n'a cessé d'augmenter depuis vingt ans. Cet article propose une nouvelle mesure du commerce international "le commerce en valeur ajoutée" qui réalloue les flux commerciaux aux pays et aux secteurs produisant les intrants. Les répartitions géographique et sectorielle du commerce en valeur ajoutée sont très différentes de celles du commerce " standard ". La différence entre le commerce en valeur ajoutée et le commerce standard est plus importante dans le cas du commerce régional mais ce n'est pas plus le cas en Asie qu'en Amérique