CiteULike: dchen's focus

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.

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.

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.

Multiferroicity Induced by Dislocated Spin-Density Waves

Joseph Betouras — 2008-05-04T00:09:06-00:00

Physical Review Letters, Vol. 98, No. 25. (2007)

We uncover a new pathway towards multiferroicity, showing how magnetism can drive ferroelectricity without relying on inversion symmetry breaking of the magnetic ordering. Our free-energy analysis demonstrates that any commensurate spin-density-wave ordering with a phase dislocation, even if it is collinear, gives rise to an electric polarization. Because of the dislocation, the electronic and magnetic inversion centers do not coincide, which turns out to be a sufficient condition for multiferroic coupling. The novel mechanism explains the formation of multiferroic phases at the magnetic commensurability transitions, such as the ones observed in YMn2O5 and related compounds. We predict that in these multiferroics an oscillating electrical polarization is concomitant with the uniform polarization. On the basis of our theory, we put forward new types of magnetic materials that are potentially ferroelectric.

Increasing Magnetoplasticity in Polycrystalline Ni-Mn-Ga by Reducing Internal Constraints through Porosity

Yuttanant Boonyongmaneerat — 2008-05-04T00:05:33-00:00

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

Foams with 55% and 76% open porosity were produced from a Ni-Mn-Ga magnetic shape-memory alloy by replication casting. These polycrystalline martensitic foams display a fully reversible magnetic-field-induced strain of up to 0.115% without bias stress, which is about 50 times larger than nonporous, fine-grained Ni-Mn-Ga. This very large improvement is attributed to the bamboolike structure of grains in the foam struts which, due to reduced internal constraints, deform by magnetic-field-induced twinning more easily than equiaxed grains in nonporous Ni-Mn-Ga.

Time-Resolved Shadowgraphs of Material Ejection in Intense Femtosecond Laser Ablation of Aluminum

Nan Zhang — 2008-05-03T23:46:49-00:00

Physical Review Letters, Vol. 99, No. 16. (2007)

The dynamic process of intense 50 fs laser ablation of aluminum is investigated by ultrafast time-resolved microscopy. A stripe pattern preceding phase explosion is clearly seen in the shadowgraph of 1 ns time delay. Intermittent material ejections are observed within the ejected plume after 2.5 and 7 ns time delay, respectively, which may be attributed to the material response to the generation of an extremely strong thermoelastic wave. Similar processes are also recorded in the ablation of silicon and glass samples, except for the glass samples, the intermittent material ejections are not found.

Limited Path Percolation in Complex Networks

Eduardo — 2008-05-03T23:43:59-00:00

We study the stability of network communication after removal of a fraction q=1-p of links under the assumption that communication is effective only if the shortest path between nodes i and j after removal is shorter than a[script-l]ij(a>=1) where [script-l]ij is the shortest path before removal. For a large class of networks, we find analytically and numerically a new percolation transition at p-tildec=(kappa0-1)(1-a)/a, where kappa0[equivalent]<k2>/<k> and k is the node degree. Above p-tildec, order N nodes can communicate within the limited path length a[script-l]ij, while below p-tildec, Ndelta (delta<1) nodes can communicate. We expect our results to influence network design, routing algorithms, and immunization strategies, where short paths are most relevant.

Spatial Self-Organization of Surface Structure during an Oscillating Catalytic Reaction

MP Cox — 2008-05-03T20:12:24-00:00

Physical Review Letters, Vol. 54, No. 15. (15 April 1985), 1725.

Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange

G Binasch — 2008-05-03T20:09:10-00:00

Physical Review B, Vol. 39, No. 7. (1 March 1989), 4828.

Speckle Evolution of Diffusive and Localized Waves

Sheng Zhang — 2008-05-03T19:57:29-00:00

Physical Review Letters, Vol. 99, No. 6. (2007)

We show that while the statistics of static speckle patterns are generic, fluctuations in the change within speckle patterns are greatly enhanced in the localization transition. The probability distributions of the displacement of phase singularities and the standard deviations of the changes of phase and intensity with frequency shift of incident microwave radiation are given in terms of the same expression which describes the probability distribution of total transmission. This function depends only upon a single parameter, the variance of the corresponding variable. The changing statistics in the localization transition reflects the number of underlying electromagnetic modes with which the incident wave interacts.

Strong Photon Nonlinearities and Photonic Mott Insulators

Michael Hartmann — 2008-05-03T19:49:30-00:00

Physical Review Letters, Vol. 99, No. 10. (2007)

We show that photon nonlinearities in an electromagnetically induced transparency can be at least 1 order of magnitude larger than predicted in all previous approaches. As an application we demonstrate that in this regime they give rise to very strong photon-photon interactions which are strong enough to make an experimental realization of a photonic Mott insulator state feasible in arrays of coupled ultrahigh-Q microcavities.

Luttinger Liquid in the Core of a Screw Dislocation in Helium-4

M Boninsegni — 2008-05-03T19:31:03-00:00

Physical Review Letters, Vol. 99, No. 3. (2007)

On the basis of first-principles Monte Carlo simulations we find that the screw dislocation along the hexagonal axis of an hcp 4He crystal features a superfluid (at T0) core. This is the first example of a regular quasi-one-dimensional supersolid—the phase featuring both translational and superfluid orders, and one of the cleanest cases of a Luttinger-liquid system. In contrast, the same type of screw dislocation in solid H2 is insulating.

Imaging Surface Topography using Lloyd's Mirror in Photoemission Electron Microscopy

DE Jesson — 2008-05-03T18:26:53-00:00

Physical Review Letters, Vol. 99, No. 1. (2007)

We use Lloyd's mirror to modulate electron photoemission in photoemission electron microscopy. This results in the projection of Lloyd's fringes on to three-dimensional (3D) surface objects. An iterative reconstruction method is used to correct for distortions in the fringe pattern due to the cathode immersion lens, thereby providing a quantitative interpretation of surface shape. It is therefore possible to extract 3D height information directly from a two-dimensional, plan-view image. The technique is of sufficient intensity and contrast to study real-time changes in surface topography and we apply the method to study unusual contact-line dynamics during the reactive wetting of metal droplets.

Architecture of Columnar Nacre, and Implications for Its Formation Mechanism

Rebecca Metzler — 2008-05-03T18:21:34-00:00

Physical Review Letters, Vol. 98, No. 26. (2007)

We analyze the structure of Haliotis rufescens nacre, or mother-of-pearl, using synchrotron spectromicroscopy and x-ray absorption near-edge structure spectroscopy. We observe imaging contrast between adjacent individual nacre tablets, arising because different tablets have different crystal orientations with respect to the radiation's polarization vector. Comparing previous data and our new data with models for columnar nacre growth, we find the data are most consistent with a model in which nacre tablets are nucleated by randomly distributed sites in the organic matrix layers.

Spontaneous Formation of Complex Micelles from a Homogeneous Solution

Xuehao He — 2008-05-03T17:52:28-00:00

Physical Review Letters, Vol. 100, No. 13. (2008)

We present an extensive computer simulation study of structure formation in amphiphilic block copolymer solutions after a quench from a homogeneous state. By using a mesoscopic field-based simulation method, we are able to access time scales in the range of a second. A “phase diagram” of final structures is mapped out as a function of the concentration and solvent philicity of the copolymers. A rich spectrum of structures is observed, ranging from spherical and rodlike micelles and vesicles to toroidal and net-cage micelles. The dynamical pathways leading to these structures are analyzed in detail, and possible ways to control the structures are discussed briefly.

Universality behind Basquin's Law of Fatigue

F Kun — 2008-05-03T17:26:06-00:00

Physical Review Letters, Vol. 100, No. 9. (2008)

Basquin's law of fatigue states that the lifetime of the system has a power-law dependence on the external load amplitude, tf~0-alpha" align="middle">, where the exponent has a strong material dependence. We show that in spite of the broad scatter of the exponent , the fatigue fracture of heterogeneous materials exhibits universal features. We propose a generic scaling form for the macroscopic deformation and show that at the fatigue limit the system undergoes a continuous phase transition. On the microlevel, the fatigue fracture proceeds in bursts characterized by universal power-law distributions. We demonstrate that the system dependent details are contained in Basquin's exponent for time to failure, and once this is taken into account, remaining features of failure are universal.

Current-Induced Stabilization of Surface Morphology in Stressed Solids

Vivek Tomar — 2008-05-03T17:23:45-00:00

Physical Review Letters, Vol. 100, No. 3. (2008)

We examine the surface morphological evolution of a conducting crystalline solid under the simultaneous action of an electric field and mechanical stress based on a fully nonlinear model and combining linear stability theory with self-consistent dynamical simulations. We demonstrate that electric current, through surface electromigration, can stabilize the surface morphology of the stressed solid against cracklike surface instabilities. The results also have more general implications for the morphological response of solid surfaces under the simultaneous action of multiple external forces.

Guiding superconducting vortices with magnetic domain walls

Vlasko Vlasov — 2008-05-03T16:28:20-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 77, No. 13. (2008)

We demonstrate a unique prospect for inducing anisotropic vortex pinning and manipulating the directional motion of vortices by using the stripe domain patterns of a uniaxial magnetic film in the superconducting/ferromagnetic hybrid. Our observations can be described by a model, which considers interactions between magnetic charges of vortices and surface magnetic charges of domains resulting in the enhanced pinning of vortices on domain walls.

Ghost-imaging experiment by measuring reflected photons

Ron Meyers — 2008-05-03T16:01:55-00:00

Physical Review A (Atomic, Molecular, and Optical Physics), Vol. 77, No. 4. (2008)

A CCD array is placed facing a chaotic light source and gated by a photon counting detector that simply counts all randomly scattered and reflected photons from an object. A “ghost” image of the object is then observed in the gated CCD. Differing from all published ghost-imaging experiments, this setup captures ghosts from scattered and reflected light of an object, instead of the transmitted ones. This new feature is not only useful for practical applications, but is also important fundamentally. It further explores the nonclassical interference nature of thermal light ghost imaging.

Ring-diffusion mediated homogeneous melting in the superheating regime

Xian Bai — 2008-05-03T15:47:24-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 77, No. 13. (2008)

Homogeneous melting in the superheating regime is investigated by using molecular dynamics simulation of a Lennard-Jones model system. We show that the commonly observed catastrophic melting at the superheating limit is caused by fast heating rate. By keeping the system isothermally at temperatures below the superheating limit, we observe intense self-diffusion motions as the precursor of melting. The highly correlated atomic motions are related to the self-diffusion loops or rings. Two types of loops are observed, closed loop and open loop, where the latter is directly related to the homogeneous nucleation of the liquid phase. Homogeneous melting occurs when the number density of diffusion loops reaches a critical value. Our results suggest that homogeneous melting in the superheating regime is a first-order thermodynamic phase transition triggered by the self-diffusion loops when the kinetic constraint imposed by heating rate is lessened.

Finger Rafting: A Generic Instability of Floating Elastic Sheets

Dominic Vella — 2007-02-26T21:01:49-00:00

Physical Review Letters, Vol. 98, No. 8. (2007)

Colliding ice floes are often observed to form a series of interlocking fingers. We show that this striking phenomenon is not a result of some peculiar property of ice but rather a general and robust mechanical phenomenon reproducible in the laboratory with other floating materials. We determine the theoretical relationship between the width of the resulting fingers and the material's mechanical properties and present experimental results along with field observations to support the theory. The generality of this “finger rafting” suggests that analogous processes may be responsible for creating the large-scale structures observed at the boundaries between Earth's convergent tectonic plates.

Molecular Clock on a Neutral Network

Alpan Raval — 2008-03-18T20:17:34-00:00

Physical Review Letters, Vol. 99, No. 13. (2007)

The number of fixed mutations accumulated in an evolving population often displays a variance that is significantly larger than the mean (the overdispersed molecular clock). By examining a generic evolutionary process on a neutral network of high-fitness genotypes, we establish a formalism for computing all cumulants of the full probability distribution of accumulated mutations in terms of graph properties of the neutral network, and use the formalism to prove overdispersion of the molecular clock. We further show that significant overdispersion arises naturally in evolution when the neutral network is highly sparse, exhibits large global fluctuations in neutrality, and small local fluctuations in neutrality. The results are also relevant for elucidating aspects of neutral network topology from empirical measurements of the substitution process.

Observation of Immobilized Water Molecules around Hydrophobic Groups

YLA Rezus — 2007-10-24T12:22:45-00:00

Physical Review Letters, Vol. 99, No. 14. (2007)

We have used femtosecond midinfrared spectroscopy to study the orientational mobility of water molecules in the hydration shells of hydrophobic groups. Our results show that hydrophobic groups are surrounded by a number of water molecules that display much slower orientational dynamics than the bulk liquid and that are therefore effectively immobilized. It turns out that each methyl group is surrounded by four immobilized water OH groups.

New Insight into Cataract Formation: Enhanced Stability through Mutual Attraction

A Stradner — 2008-03-18T02:02:58-00:00

Physical Review Letters, Vol. 99, No. 19. (2007)

Small-angle neutron scattering experiments and molecular dynamics simulations combined with an application of concepts from soft matter physics to complex protein mixtures provide new insight into the stability of eye lens protein mixtures. Exploring this colloid-protein analogy we demonstrate that weak attractions between unlike proteins help to maintain lens transparency in an extremely sensitive and nonmonotonic manner. These results not only represent an important step towards a better understanding of protein condensation diseases such as cataract formation, but provide general guidelines for tuning the stability of colloid mixtures, a topic relevant for soft matter physics and industrial applications.