CiteULike: dchen's library [610 articles]

Trapping an Intensely Bright, Stable Sonoluminescing Bubble

Raúl Urteaga — 2008-05-07T23:29:17-00:00

Physical Review Letters, Vol. 100, No. 7. (2008)

Previous works on single bubble sonoluminescence in sulfuric acid solutions have stressed the fact that the sonoluminescence (SL) emissions are the highest ever found, but at the same time the bubble moves in orbits. We have fixed the SL bubble spatially and at the same time we have reached higher SL emissions using another harmonic acoustic signal to produce the acoustic excitation. Multiple harmonic excitation produces up to a fourfold increase in SL emissions, reaching the peak value of about 40 µW for a moving bubble and 15 µW for a nonmoving bubble. The ability to have a bright stationary bubble also opens new research opportunities. In particular, we develop a new method to measure the absolute radius evolution of the bubble that exploits this stability.

Solvent-Induced DNA Conformational Transition

B Gu — 2008-05-07T23:28:21-00:00

Physical Review Letters, Vol. 100, No. 8. (2008)

Modified water models with scaled charges are used to investigate solvent polarity effects on DNA structure. Several intensive molecular dynamics simulations of the DNA EcoRI dodecamer d(CGCGAATTCGCG) in different model solvents are performed. When the polarity of the solvent molecule decreases, from overpolarized to less polarized, DNA experiences the conformational transitions of constrainedB form(A-B)mixA form. We demonstrate that one important cause of these structure changes is the competition between hydration and direct cation coupling to the free oxygen atoms in the phosphate groups on DNA backbones.

Diffusion-Driven Pattern Formation in Ionic Chemical Solutions

Zsanett Virányi — 2008-05-07T23:25:52-00:00

Phys. Rev. Lett. 100, 088301 (2008)

The driving force in diffusion-driven pattern formation is the difference in the diffusional flux of the key species, which in the case of ionic systems builds up a local electric field at the concentration gradients. The arising additional migrational flux not only decreases but also enhances the instability of the base state, depending on the charge distribution among the components. The opposite charges on the slower diffusing autocatalyst and its reacting counterpart favor pattern formation and shift the onset of instability to a smaller difference in the diffusion coefficients. The same charges, in addition to having the opposite effect, may even lead to the complete stabilization of planar reaction fronts unstable in the neutral system.

Measuring the Kinetics of Biomolecular Recognition with Magnetic Colloids

Cohen Tannoudji — 2008-05-07T23:01:32-00:00

Physical Review Letters, Vol. 100, No. 10. (2008)

We introduce a general methodology based on magnetic colloids to study the recognition kinetics of tethered biomolecules. Access to the full kinetics of the reaction is provided by an explicit measure of the time evolution of the reactant densities. Binding between a single ligand and its complementary receptor is here limited by the colloidal rotational diffusion. It occurs within a binding distance that can be extracted by a reaction-diffusion theory that properly accounts for the rotational Brownian dynamics. Our reaction geometry allows us to probe a large diversity of bioadhesive molecules and tethers, thus providing a quantitative guidance for designing more efficient reactive biomimetic surfaces, as required for diagnostic, therapeutic, and tissue engineering techniques.

Origin of the Complex Molecular Dynamics in Functionalized Discotic Liquid Crystals

MM Elmahdy — 2008-05-07T22:55:49-00:00

Physical Review Letters, Vol. 100, No. 10. (2008)

The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been studied using site-specific NMR techniques and dielectric spectroscopy as a function of temperature and pressure. These probes (i) suggest that the thermodynamic state completely controls the dynamic response, (ii) clarify the origin of two dynamic processes associated with the presence of two glass temperatures, and (iii) provide the first phase diagram for substances of this kind.

In Situ Observation of Fringing-Field-Induced Phase Separation in a Liquid-Crystal--Monomer Mixture

Hongwen Ren — 2008-03-24T17:17:09-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Fringing-field-induced phase separation dynamics in liquid-crystal–(LC-)monomer mixtures is investigated via a microscope. At a low LC concentration, the fringing field converts the randomly dispersed LC droplets to an ordered droplet array, while at a high LC concentration the fringing field converts the amorphous LC-monomer system to a composite film. Because the LC and monomer are immiscible, the converted morphologies are stable even after the voltage is removed. Using the fringing field-induced phase separation, it is possible to prepare different polymer-dispersed LC morphologies.

Nonequilibrium Microtubule Fluctuations in a Model Cytoskeleton

Clifford Brangwynne — 2008-05-07T21:19:39-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Biological activity gives rise to nonequilibrium fluctuations in the cytoplasm of cells; however, there are few methods to directly measure these fluctuations. Using a reconstituted actin cytoskeleton, we show that the bending dynamics of embedded microtubules can be used to probe local stress fluctuations. We add myosin motors that drive the network out of equilibrium, resulting in an increased amplitude and modified time dependence of microtubule bending fluctuations. We show that this behavior results from steplike forces on the order of 10 pN driven by collective motor dynamics.

Polyelectrolyte-Compression Forces between Spherical DNA Brushes

Kati Kegler — 2008-05-07T21:14:58-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Optical tweezers are employed to measure the forces of interaction within a single pair of DNA-grafted colloids, dependent on the molecular weight of the DNA chains, and the concentration and valence of the surrounding ionic medium. The resulting forces are short range and set in as the surface-to-surface distance between the colloidal cores reaches the value of the brush height. The measured force-distance relation is analyzed by means of a theoretical treatment that quantitatively describes the effects of compression of the chains on the surface of the opposite-lying colloid. Quantitative agreement with the experiment is obtained for all parameter combinations.

Colloidal Assembly on Magnetically Vibrated Stripes

Pietro Tierno — 2008-05-07T15:36:01-00:00

Physical Review Letters, Vol. 100, No. 14. (2008)

We investigate the collective organization of paramagnetic colloidal particles externally driven above the periodic stripes of a uniaxial ferrimagnetic garnet film. An external field modulation induces vibration of the stripe walls and produces random motion of the particles. Defects in the stripe pattern break the symmetry of the potential and favor particle nucleation into large clusters above a critical density. Mismatch between particle size and pattern wavelength generates assemblies with different morphological order. At even higher field strengths, repulsive dipolar interactions between the particles induce cluster melting. We propose a novel approach to generate and externally control a variety of colloidal assemblies.

Thermal Convection and Emergence of Isolated Vortices in Soap Bubbles

F Seychelles — 2008-05-06T21:07:23-00:00

Physical Review Letters, Vol. 100, No. 14. (2008)

A novel thermal convection cell consisting of half a soap bubble heated at the equator is introduced to study thermal convection and the movement of isolated vortices. The soap bubble, subject to stratification, develops thermal convection at its equator. A particular feature of this cell is the emergence of isolated vortices. These vortices resemble hurricanes or cyclones and similarities between our observed structures and these natural objects are found. This is brought forth through a study of the mean square displacement of these objects showing signs of superdiffusion.

Human Skin as Arrays of Helical Antennas in the Millimeter and Submillimeter Wave Range

Yuri Feldman — 2008-05-07T01:00:17-00:00

Physical Review Letters, Vol. 100, No. 12. (2008)

Recent studies of the minute morphology of the skin by optical coherence tomography showed that the sweat ducts in human skin are helically shaped tubes, filled with a conductive aqueous solution. A computer simulation study of these structures in millimeter and submillimeter wave bands show that the human skin functions as an array of low-Q helical antennas. Experimental evidence is presented that the spectral response in the sub-Terahertz region is governed by the level of activity of the perspiration system. It is also correlated to physiological stress as manifested by the pulse rate and the systolic blood pressure.

Dynamics of Probe Diffusion in Rod Solutions

Victor Pryamitsyn — 2008-05-07T00:55:39-00:00

Physical Review Letters, Vol. 100, No. 12. (2008)

Applications of probe diffusion in polymer matrices typically envision that for particles sizes (R) larger than the correlation length of the polymer solution (), the probe (at long times) diffuses as in a continuum polymer solution. We present simulation results for probe diffusion in rod solutions which challenge this conventional wisdom and indicate a new mechanism of a probe diffusion operative for R>. Our simulation results are rationalized by scaling arguments invoking a novel mechanism of the constraint release motion of the rods, and suggest that the dynamical characteristics of the polymer matrix also proves important in developing a complete description of the probe motion.

Directed Motion of Proteins along Tethered Polyelectrolytes

Katja Henzler — 2008-05-06T17:57:50-00:00

Physical Review Letters, Vol. 100, No. 15. (2008)

We present the first time-resolved investigation of motions of proteins in densely grafted layers of spherical polyelectrolyte brushes. Using small-angle x-ray scattering combined with rapid stopped-flow mixing, we followed the uptake of bovine serum albumin by poly(acrylic acid) layer with high spatial and temporal resolution. We find that the total amount of adsorbed protein scales with time as t1/4. This subdiffusive behavior is explained on the basis of directed motion of the protein along the polyelectrolyte chains.

Heating Mechanism Affects Equipartition in a Binary Granular System

Hong Wang — 2008-05-06T14:42:31-00:00

Physical Review Letters, Vol. 100, No. 15. (2008)

Two species of particles in a binary granular system typically do not have the same mean kinetic energy, in contrast to the equipartition of energy required in equilibrium. We investigate the role of the heating mechanism in determining the extent of nonequipartition of kinetic energy. In most experiments, different species are unequally heated at the boundaries. We show by event-driven simulations that differential boundary heating affects nonequipartition even in the bulk of the system. This conclusion is fortified by studying numerical and solvable stochastic models without spatial degrees of freedom. In both cases, even in the limit where heating events are rare compared to collisions, the effect of the heating mechanism persists.

Complexity in Strongly Correlated Electronic Systems

Elbio Dagotto — 2008-05-05T22:04:34-00:00

Science, Vol. 309, No. 5732. (8 July 2005), pp. 257-262.

A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that are not spatially homogeneous. This occurs in cases in which several physical interactions--spin, charge, lattice, and/or orbital--are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior. 10.1126/science.1107559

Hexagonal Order in Crystalline and Columnar Phases of Hard Rods

Eric Grelet — 2008-05-05T21:53:20-00:00

Physical Review Letters, Vol. 100, No. 16. (2008)

We report a study of colloidal suspensions of highly monodisperse semiflexible chiral rodlike viruses, denoted fd, in the range of high concentrations. Small angle x-ray scattering experiments reveal the existence of two hexagonal phases: the first one is crystalline and the second one is hexatic columnar, as shown by its short-range positional order. The suspension of rodlike viruses is the first experimental system showing the whole phase sequence with increasing particle concentration theoretically predicted for systems of hard rods, ranging from the chiral nematic via the smectic to columnar and crystalline phases.

Narrow-Escape Time Problem: Time Needed for a Particle to Exit a Confining Domain through a Small Windowb

O Bénichou — 2008-05-05T21:45:19-00:00

(2008)

The time needed for a particle to exit a confining domain through a small window, called the narrow-escape time (NET), is a limiting factor of various processes, such as some biochemical reactions in cells. Obtaining an estimate of the mean NET for a given geometric environment is therefore a requisite step to quantify the reaction rate constant of such processes, which has raised a growing interest in the past few years. In this Letter, we determine explicitly the scaling dependence of the mean NET on both the volume of the confining domain and the starting point to aperture distance. We show that this analytical approach is applicable to a very wide range of stochastic processes, including anomalous diffusion or diffusion in the presence of an external force field, which cover situations of biological relevance.

Analysis of DNA Elasticity

RP Linna — 2008-05-05T21:17:56-00:00

Physical Review Letters, Vol. 100, No. 16. (2008)

With a model that incorporates hydrodynamics directly, we show that flow experiments can be used for detecting some characteristics of the DNA elasticity which manifest themselves clearly at large length scales but cannot be observed by mechanical forcing experiments even at very small length scales. By systematic analysis, the conclusiveness of different experimental methods is evaluated. For the wormlike chain, confirmed as the correct model for DNA, we find an underlying scaling relation between its extension and flow velocity of the form Lp~v0.155, which emphasizes the significance of hydrodynamics.

Visible Fluorescence Spectroscopy of Single Proteins at Liquid-Helium Temperature

Satoru Fujiyoshi — 2008-05-05T20:53:11-00:00

Physical Review Letters, Vol. 100, No. 16. (2008)

Fluorescence spectroscopy of single proteins at liquid-helium temperatures reveals a relation between structural dynamics and biological functions of the proteins. The technical difficulties in detecting visible fluorescence are chromatic aberration and optical background. They were overcome by a new optical design using reflective optics and employing two-photon excitation. The fluorescence spectrum of single green-fluorescent proteins taken at a temperature of 1.5 K makes a distinction between different metastable conformations that last for tens of seconds.

Energy Landscape, Antiplasticization, and Polydispersity Induced Crossover of Heterogeneity in Supercooled Polydisperse Liquids

Sneha Abraham — 2008-05-05T20:40:53-00:00

Physical Review Letters, Vol. 100, No. 16. (2008)

Polydispersity is found to have a significant effect on the potential energy landscape; the average inherent structure energy decreases with polydispersity. Increasing polydispersity at a fixed volume fraction decreases the glass transition temperature and the fragility of glass formation analogous to the antiplasticization seen in some polymeric melts. An interesting temperature dependent crossover of heterogeneity with polydispersity is observed at low temperature due to the faster buildup of dynamic heterogeneity at lower polydispersity.

Magic Angles and Cross-Hatching Instability in Hydrogel Fracture

T Baumberger — 2008-05-05T20:13:26-00:00

Physical Review Letters, Vol. 100, No. 17. (2008)

The full 2D analysis of roughness profiles of fracture surfaces resulting from quasistatic crack propagation in gelatin gels reveals an original behavior characterized by (i) strong anisotropy with maximum roughness at V-independent symmetry-preserving angles and (ii) a subcritical instability leading, below a critical velocity, to a cross-hatched regime due to straight macrosteps drifting at the same magic angles and nucleated on crack-pinning network inhomogeneities. Step height values are determined by the width of the strain-hardened zone, governed by the elastic crack blunting characteristic of soft solids with breaking stresses much larger than low strain moduli.

Density Dependent Interactions and Structure of Charged Colloidal Dispersions in the Weak Screening Regime

Rojas Ochoa — 2008-05-05T20:12:24-00:00

Physical Review Letters, Vol. 100, No. 17. (2008)

We determine the structure of charge-stabilized colloidal suspensions at low ionic strength over an extended range of particle volume fractions using a combination of light and small angle neutron scattering experiments. The variation of the structure factor with concentration is analyzed within a one-component model of a colloidal suspension. We show that the observed structural behavior corresponds to a nonmonotonic density dependence of the colloid effective charge and the mean interparticle interaction energy. Our findings are corroborated by similar observations from primitive model computer simulations of salt-free colloidal suspensions.

Formation of a New Dynamical Mode in alpha-Uranium Observed by Inelastic X-Ray and Neutron Scattering

ME Manley — 2008-05-05T16:32:03-00:00

Physical Review Letters, Vol. 96, No. 12. (2006)

Phonon dispersion curves were obtained from inelastic x-ray and neutron scattering measurements on -uranium single crystals at temperatures from 298 to 573 K. Both measurements showed a softening and an abrupt loss of intensity in the longitudinal optic branch along [00] above 450 K. Above the same temperature a new dynamical mode of comparable intensity emerges along the [01] zone boundary with energy near the top of the phonon spectrum. The new mode forms without a structural transition but coincides with an anomaly in the mechanical deformation behavior. We argue that the mode is an intrinsically localized vibration and formed as a result of a strong electron-phonon interaction.

Delaying Transition to Turbulence by a Passive Mechanism

Jens Fransson — 2008-05-05T16:29:56-00:00

Physical Review Letters, Vol. 96, No. 6. (2006)

Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the underlying linearized operator, these perturbations are highly amplified and able to modify the mean velocity profiles at leading order. We report results of wind-tunnel experiments in which we implement this concept by using suitably designed roughness elements placed on the skin to enforce nearly optimal perturbations. We show that by using this passive control technique it is possible to sensibly delay transition to turbulence.

Spin Proximity Effect in Ultrathin Superconducting Be-Au Bilayers

XS Wu — 2008-05-05T16:25:55-00:00

Physical Review Letters, Vol. 96, No. 12. (2006)

We present a detailed study of the effects of interface spin-orbit coupling on the critical field behavior of ultrathin superconducting Be/Au bilayers. Parallel field measurements were made in bilayers with Be thicknesses in the range of d=2–30 nm and Au coverages of 0.5 nm. Though the Au had little effect on the superconducting gap, it produced profound changes in the spin states of the system. In particular, the parallel critical field exceeded the Clogston limit by an order of magnitude in the thinnest films studied. In addition, the parallel critical field unexpectedly scaled as Hc||/01/d, suggesting that the spin-orbit coupling energy was proportional to 0/d2. Tilted field measurements showed that, contrary to recent theory, the interface spin-orbit coupling induces a large in-plane superconducting susceptibility but only a very small transverse susceptibility.

High-Rayleigh-Number Convection in a Vertical Channel

M Gibert — 2008-05-05T16:13:23-00:00

Physical Review Letters, Vol. 96, No. 8. (2006)

We measure the relation between convective heat flux and temperature gradient in a vertical channel filled with water, the average vertical mass flux being zero. Compared to the classical Rayleigh-Bénard case, this situation has the advantage of avoiding plates and, thus, their neighborhood, in which is usually concentrated most of the temperature gradient. Consequently, inertial processes should control the convection, with poor influence of the viscosity. This idea gives a good account of our observations, if we consider that a natural vertical length, different from the channel width, appears. Our results also suggest that heat fluxes can be deduced from velocity measurements in free convective flows. This confers to our results a wide range of applications.

Imaging Spin-Reorientation Transitions in Consecutive Atomic Co Layers on Ru(0001)

Farid Gabaly — 2008-05-05T16:10:21-00:00

Physical Review Letters, Vol. 96, No. 14. (2006)

By means of spin-polarized low-energy electron microscopy, we show that the magnetic easy axis of one to three atomic-layer thick cobalt films on Ru(0001) changes its orientation twice during deposition: One-monolayer and three-monolayer thick films are magnetized in plane, while two-monolayer films are magnetized out of plane. The Curie temperatures of films thicker than one monolayer are well above room temperature. Fully relativistic calculations based on the screened Korringa-Kohn-Rostoker method demonstrate that only for two-monolayer cobalt films does the interplay between strain, surface, and interface effects lead to perpendicular magnetization.

Optical Spin-to-Orbital Angular Momentum Conversion in Inhomogeneous Anisotropic Media

L Marrucci — 2008-05-05T16:01:46-00:00

Physical Review Letters, Vol. 96, No. 16. (2006)

We demonstrate experimentally an optical process in which the spin angular momentum carried by a circularly polarized light beam is converted into orbital angular momentum, leading to the generation of helical modes with a wave-front helicity controlled by the input polarization. This phenomenon requires the interaction of light with matter that is both optically inhomogeneous and anisotropic. The underlying physics is also associated with the so-called Pancharatnam-Berry geometrical phases involved in any inhomogeneous transformation of the optical polarization.

Soft Nanopolyhedra as a Route to Multivalent Nanoparticles

Jiunn Roan — 2008-05-05T15:43:40-00:00

Physical Review Letters, Vol. 96, No. 24. (2006)

Computer simulations show that end-grafted immiscible homopolymers can confer multivalence to nanoparticles, resulting in soft nanopolyhedra with structures identical to those found in small clusters of colloidal microspheres. Unprecedented structure tunability is demonstrated by several structure transition sequences, including a reentrant transition, induced by varying composition, polymer lengths, or grafting patterns. These results suggest a new method for fabricating nanoparticles with precisely controlled numbers and locations of functional sites (i.e., multivalent nanoparticles).

Microevaporators for Kinetic Exploration of Phase Diagrams

Jacques Leng — 2008-05-05T00:02:09-00:00

Physical Review Letters, Vol. 96, No. 8. (2006)

We use pervaporation-based microfluidic devices to concentrate species in aqueous solutions with spatial and temporal control of the process. Using experiments and modeling, we quantitatively describe the advection-diffusion behavior of the concentration field of various solutions (electrolytes, colloids, etc.) and demonstrate the potential of these devices as universal tools for the kinetic exploration of the phases and textures that form upon concentration.

The Positive Electron

Carl Anderson — 2008-05-04T23:52:49-00:00

Physical Review, Vol. 43, No. 6. (15 March 1933), 491.

Torque Detection using Brownian Fluctuations

Giovanni Volpe — 2006-11-27T18:32:15-00:00

Physical Review Letters, Vol. 97, No. 21. (2006)

We report the statistical analysis of the movement of a submicron particle confined in a harmonic potential in the presence of a torque. The absolute value of the torque can be found from the auto- and cross-correlation functions of the particle's coordinates. We experimentally prove this analysis by detecting the torque produced onto an optically trapped particle by an optical beam with orbital angular momentum.

Strong far-field coherent scattering of ultraviolet radiation by holococcolithophores

Quintero Torres — 2008-05-04T23:04:15-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 74, No. 3. (2006)

By considering the structure of holococcoliths (calcite plates that cover holococcolithophores, a haploid phase of the coccolithophore life cycle) as a photonic structure, we apply a discrete dipolar approximation to study the light backscattering properties of these algae. We show that some holococcolith structures have the ability to scatter the ultraviolet radiation. This property may represent an advantage for holococcolithophores possessing it, by allowing them to live higher in the water column than other coccolithophores.

Diffraction of Electrons by a Crystal of Nickel

C Davisson — 2008-05-04T22:40:03-00:00

Physical Review, Vol. 30, No. 6. (1 December 1927), 705.

Probing “Cosmological” Defects in Superfluid [sup 3]He-B with a Vibrating-Wire Resonator

CB Winkelmann — 2008-05-04T22:37:24-00:00

Physical Review Letters, Vol. 96, No. 20. (2006)

We report on the observation of an anomalously high damping measured by a vibrating-wire resonator (VWR) immersed into superfluid 3He-B at ultralow temperatures. The observed dissipation is orders of magnitude above that corresponding to friction with the dilute normal fraction and superfluid vortices. A clear pinning behavior is also observed, as well as a strong magnetic field dependence. Our analysis points to the interaction of the VWR with a planar topological defect, analogue to cosmological vacua defects, as proposed by Salomaa and Volovik.

Controllable Snail-Paced Light in Biological Bacteriorhodopsin Thin Film

Pengfei Wu — 2008-05-04T19:36:43-00:00

Physical Review Letters, Vol. 95, No. 25. (2005)

We observe that the group velocity of light is reduced to an extremely low value of 0.091 mm/s in a biological thin film of bacteriorhodopsin at room temperature. By exploiting unique features of a flexible photoisomerization process for coherent population oscillation, the velocity is all-optically controlled over an enormous span, from snail-paced to normal light speed, with no need of modifying the characteristics of the incident pulse. Because of the large quantum yield for the photoreaction in this biochemical system, the ultraslow light is observed even at low light levels of microwatts, indicating high energy efficiency.

Crystallization of a Quasi-Two-Dimensional Granular Fluid

PM Reis — 2007-12-10T22:23:17-00:00

Physical Review Letters, Vol. 96, No. 25. (2006)

We experimentally investigate the crystallization of a uniformly heated quasi-2D granular fluid as a function of the filling fraction. Our experimental results for the Lindemann melting criterion, the radial distribution function, the bond order parameter, and the statistics of topological changes at the particle level are the same as those found in simulations of equilibrium hard disks. This direct mapping suggests that the study of equilibrium systems can be effectively applied to study nonequilibrium steady states such as those found in our driven and dissipative granular system.

Resonance Absorption by Nuclear Magnetic Moments in a Solid

EM Purcell — 2007-05-01T07:36:30-00:00

Physical Review, Vol. 69, No. 1-2. (1 January 1946), 37.

Chiral Molecules Split Light: Reflection and Refraction in a Chiral Liquid

Ambarish Ghosh — 2006-11-14T23:28:28-00:00

Physical Review Letters, Vol. 97, No. 17. (2006)

A light beam changes direction as it enters a liquid at an angle from another medium, such as air. Should the liquid contain molecules that lack mirror symmetry, then it has been predicted by Fresnel that the light beam will not only change direction, but will actually split into two separate beams with a small difference in the respective angles of refraction. Here we report the observation of this phenomenon. We also demonstrate that the angle of reflection does not equal the angle of incidence in a chiral medium. Unlike conventional optical rotation, which depends on the path-length through the sample, the reported reflection and refraction phenomena arise within a few wavelengths at the interface and thereby suggest a new approach to polarimetry that can be used in microfluidic volumes.

Theory of Superconductivity

J Bardeen — 2006-06-19T20:17:17-00:00

Physical Review, Vol. 108, No. 5. (1 December 1957), 1175.

A theory of superconductivity is presented; based on the fact that the interaction between electrons resulting from virtual exchange of phonons is attractive when the energy difference between the electrons states involved is less than the phonon energy; âÏ. It is favorable to form a superconducting phase when this attractive interaction dominates the repulsive screened Coulomb interaction. The normal phase is described by the Bloch individual-particle model. The ground state of a superconductor; formed from a linear combination of normal state configurations in which electrons are virtually excited in pairs of opposite spin and momentum; is lower in energy than the normal state by amount proportional to an average (âÏ) 2 ; consistent with the isotope effect. A mutually orthogonal set of excited states in one-to-one correspondence with those of the normal phase is obtained by specifying occupation of certain Bloch states and by using the rest to form a linear combination of virtual pair configurations. The theory yields a second-order phase transition and a Meissner effect in the form suggested by Pippard. Calculated values of specific heats and penetration depths and their temperature variation are in good agreement with experiment. There is an energy gap for individual-particle excitations which decreases from about 3.5 k T c at T =0Â°K to zero at T c . Tables of matrix elements of single-particle operators between the excited-state superconducting wave functions; useful for perturbation expansions and calculations of transition probabilities; are given.

Anti-Stokes Laser Cooling in Bulk Erbium-Doped Materials

Joaquin Fernandez — 2008-05-04T18:58:31-00:00

Physical Review Letters, Vol. 97, No. 3. (2006)

We report the first observation of anti-Stokes laser-induced cooling in the Er3+:KPb2Cl5 crystal and in the Er3+:CNBZn (CdF2-CdCl2-NaF-BaF2-BaCl2-ZnF2) glass. The internal cooling efficiencies have been calculated by using photothermal deflection spectroscopy. Thermal scans acquired with an infrared thermal camera proved the bulk cooling capability of the studied samples. The implications of these results are discussed.

Nonlinearity Management in Optics: Experiment, Theory, and Simulation

Martin Centurion — 2008-05-04T18:54:49-00:00

Physical Review Letters, Vol. 97, No. 3. (2006)

We conduct an experimental investigation of nonlinearity management in optics using femtosecond pulses and layered Kerr media consisting of glass and air. By examining the propagation properties over several diffraction lengths, we show that wave collapse can be prevented. We corroborate these experimental results with numerical simulations of the (2+1)-dimensional focusing cubic nonlinear Schrödinger equation with piecewise constant coefficients and a theoretical analysis of this setting using a moment method.

Capillary Origami: Spontaneous Wrapping of a Droplet with an Elastic Sheet

Charlotte Py — 2008-05-04T18:50:09-00:00

Physical Review Letters, Vol. 98, No. 15. (2007)

The interaction between elasticity and capillarity is used to produce three-dimensional structures through the wrapping of a liquid droplet by a planar sheet. The final encapsulated 3D shape is controlled by tailoring the initial geometry of the flat membrane. Balancing interfacial energy with elastic bending energy provides a critical length scale below which encapsulation cannot occur, which is verified experimentally. This length is found to depend on the thickness as h3/2, a scaling favorable to miniaturization which suggests a new way of mass production of 3D micro- or nanoscale objects.

General Probabilistic Approach to the Filtration Process

N Roussel — 2008-05-04T18:21:59-00:00

Physical Review Letters, Vol. 98, No. 11. (2007)

We show experimentally that clogging is basically a matter of the probability of the presence of particles. We describe this process as a function of the main variables of the process, namely, the ratio of particle to mesh hole size, the solid fraction, and the number of grains arriving at each mesh hole during one test, with the help of a simple model, the predictions of which are in very good agreement with our experimental data.

Dynamics of the Solid and Liquid Phases in Dilute Sheared Brownian Suspensions: Irreversibility and Particle Migration

Jennifer Brown — 2008-05-04T18:06:26-00:00

Physical Review Letters, Vol. 99, No. 24. (2007)

Magnetic resonance measurements of migration and irreversible dynamics in the capillary shear flow of a Brownian suspension are presented. The results demonstrate the presence of phenomena typically associated with concentrated noncolloidal systems and indicate the role of many body hydrodynamics in dilute Brownian suspension transport. The application of concepts from chaos theory and nonequilibrium statistical mechanics is demonstrated.

Direct Imaging of Stochastic Domain-Wall Motion Driven by Nanosecond Current Pulses

Guido Meier — 2008-05-04T16:33:53-00:00

Physical Review Letters, Vol. 98, No. 18. (2007)

Magnetic transmission x-ray microscopy is used to directly visualize the influence of a spin-polarized current on domain walls in curved permalloy wires. Pulses of nanosecond duration and of high current density up to 1.0×1012 A/m2 are used to move and to deform the domain wall. The current pulse drives the wall either undisturbed, i.e., as composite particle through the wire, or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current-induced domain-wall motion.

Polymeric Quasicrystal: Mesoscopic Quasicrystalline Tiling in ABC Star Polymers

Kenichi Hayashida — 2008-05-04T16:23:48-00:00

Physical Review Letters, Vol. 98, No. 19. (2007)

A mesoscopic tiling pattern with 12-fold symmetry has been observed in a three-component polymer system composed of polyisoprene, polystyrene, and poly(2-vinylpyridine) which forms a star-shaped terpolymer, and a polystyrene homopolymer blend. Transmission electron microscopy images reveal a nonperiodic tiling pattern covered with equilateral triangles and squares, their triangle/square number ratio of 2.3 (4/), and a microbeam x-ray diffraction pattern shows dodecagonal symmetry. The same kind of quasicrystalline structures have been found for metal alloys (~0.5 nm), chalcogenides (~2 nm), and liquid crystals (~10 nm). The present result (~50 nm) confirms the universal nature of dodecagonal quasicrystals over several hierarchical length scales.

Superstability of Surface Nanobubbles

Bram Borkent — 2008-05-04T16:08:51-00:00

Physical Review Letters, Vol. 98, No. 20. (2007)

Shock wave induced cavitation experiments and atomic force microscopy measurements of flat polyamide and hydrophobized silicon surfaces immersed in water are performed. It is shown that surface nanobubbles, present on these surfaces, do not act as nucleation sites for cavitation bubbles, in contrast to the expectation. This implies that surface nanobubbles are not just stable under ambient conditions but also under enormous reduction of the liquid pressure down to -6 MPa. We denote this feature as superstability.

Universality of Random Close Packing ?

RD Kamien — 2008-05-04T15:50:08-00:00

Dipolar Correlations and the Dielectric Permittivity of Water

Manu Sharma — 2008-05-04T15:35:50-00:00

Physical Review Letters, Vol. 98, No. 24. (2007)

The static dielectric properties of liquid and solid water are investigated within linear response theory in the context of ab initio molecular dynamics. Using maximally localized Wannier functions to treat the macroscopic polarization we formulate a first-principles, parameter-free, generalization of Kirkwood's phenomenological theory. Our calculated static permittivity is in good agreement with experiment. Two effects of the hydrogen bonds, i.e., a significant increase of the average local moment and a local alignment of the molecular dipoles, contribute in almost equal measure to the unusually large dielectric constant of water.