Flow hydrodynamics revealed by MRI

LP13351 - In this work, new advances in magnetic resonance imaging (MRI) are used to give fresh insight into the complex behavior of multiphase flows, such as the reason bubbles wobble from side to side as they rise. Physicists have long sought to understand the complex dynamics of these systems, which are of fundamental importance throughout physics and engineering, in establishing predictive models for systems as diverse as bubbles of gas rising in a glass of beer, to oil droplets rising from the sea floor. While conventional MRI is too slow to image these rapidly changing systems, MRI acquisitions are here accelerated using the science behind image compression to allow the measurement of effectively instantaneous velocity fields around rising single bubbles and swarms of bubbles. These measurements, which have not been previously possible, reveal that vortices generated in the horizontal plane behind rising bubbles are coupled with oscillations of the bubble rise path, and instigate the generation of turbulence in multi-bubble systems.

Liquid projections originate in bubble distortion

LM12997 - Liquid projections and bubbles are often intimately linked, as witnessed by our everyday experience: tiny jets from sparkling wines, moisturizing droplets from ocean spray, and splashes from boiling soup.  Occurring in very ordinary situations, liquid projections also play a key role in the gaseous balance between ocean and atmosphere, in the fabrication process of glassy or metallic melts and even in aroma diffusion from champagne fizz. Although of prime importance for this wide variety of applications, projections are still poorly understood and their occurrence appears somewhat erratic and random. In this paper, we have been able to track down the origin of large projections to the very initial formation and distortion of large bubbles. Studying the formation process of such bubbles, we have uncovered a surprisingly violent deformation of bubbles just after detachment that systematically sparks a fast liquid jet, possible prelude of a liquid projection. This dramatic experimental observation appears to exhibit an unexpected dynamics, and is likely to trigger new research in the field.

Fast liquid jet forming inside a large disconnecting bubble

Experiments demonstrate technique for improved optical gyroscopes

AC10917 - Accurate navigation in an aircraft or other vehicle requires sensitive measurements of rotation of the vehicle. For example, ring laser gyroscopes are commonplace in navigation systems used today. Experiments reported in PRA, [link], demonstrate that the sensitivity of such devices may be increased substantially. These gyroscopes use an optical cavity, a series of mirrors in which laser light bounces in a closed path, which  effectively changes length when the gyroscope undergoes rotation. The frequency of light which may be sustained in the optical cavity changes correspondingly, and a measurement of the change in frequency gives the rotation rate of the cavity. By adding a rubidium vapor cell inside the optical cavity, the new results demonstrate that the frequency change of the cavity can be enhanced by a factor of as much as 15, the largest increase achieved in an experiment to date. The "dispersion-enhanced cavity" can be used for measuring rotation rates with substantially greater sensitivity, leading to more accurate measurements of rotation, and thereby, improving the accuracy of navigation systems.

Rolled-up hyperlens goes broadband

BM11761 - We demonstrate that a rolled-up hyperlens fabricated from thin self-rolling metallic and semi-conducting layers can be used to image sub-wavelength details over a broad frequency range in the visible. We perform transmission and reflection measurements on rolled-up hyperlenses and characterize their optical properties. By means of simulations we demonstrate that two dipoles with a sub-wavelength separation placed at the inner perimeter of the rolled-up hyperlens create a magnified image at the outer perimeter. For our devices this hyperlensing behavior is present in a broad wavelength range in the visible and near-infrared regime making them particularly interesting, e.g., for sub-wavelength imaging of biological cells in a microfluidic system.

Galaxy Cluster Motions Detected for the First Time

LQ13028 - A team of astronomers and physicists have for the first time detected motions
of galaxy clusters, the largest objects in the universe. In 1972, Rashid Sunyaev and
Yakov Zel'dovich predicted that a moving cluster of galaxies should slightly
shift the temperature of microwave background radiation passing
through it, in analogy with a Doppler shift. This effect has now been
observed statistically by combining a catalog of 27000 luminous galaxies
from the Sloan Digital Sky Survey's Baryon Oscillation Spectroscopic
Survey (which trace cluster locations on the sky) with overlapping
maps of the microwave background temperature from the Atacama
Cosmology Telescope. The collaboration finds that pairs of galaxy
clusters are slightly more likely to be moving towards each other
rather than away from each other, as expected from the attractive
nature of the gravitational force. This is the first direct measurement
of motions of objects at cosmological distances. Future improvements
of this measurement can give precisely characterize the growth of
structure in the universe and the nature of dark energy. The result also
demonstrates the power of combining large astronomical surveys,
allowing new statistical measurements of subtle physical effects that no
single survey could detect.

Dying stars have a Halo

LQ13224 - We have discovered that within the heart of an exploding star (core-collapse supernova) there is a "Halo" of neutrinos.  The neutrinos in this halo, although very few in number, may nevertheless control the way that neutrinos change their flavors, e.g., converting from "electron-type neutrinos" into, say, "muon-type neutrinos".  This will force a new paradigm in the way that neutrino flavor is  handled in supernovae, and, in turn, could lead to a new understanding of supernova explosions and the origin of the elements.

Uncovering the secrets of splashing

LP12846 - Drop impact splashing has fascinated and challenged scientists for over a century. In a surprise development, super-high-resolution numerical simulations and ultra-high-speed video imaging suggest a new mechanism responsible for splashing. In a similar way as telephone lines hum in the wind, the liquid jet below a drop starts to oscillate at higher velocities, thus breaking into micro-droplets and shedding vortices into the drop. Systematic experiments showed the diversity and complexity of drop impacts, while numerical results were used to investigate the flow inside the drop. Direct comparison between experimental results and numerical simulations showed the high accuracy achieved to reproduce some of the finest details. Familiar to photographers for their aesthetic appeal, drop splashing has applications as diverse as combustion, inkjet printing, coating, crop spraying and aerosol production. Understanding these dynamics could be important to improve or control splashes

Cooper pairs of an electron and a hole

LL13172B - Superconductors conduct current without resistance, because many-body effects create pairs of two electrons, called “Cooper pairs.” Now, a paper appearing in Physical Review B reports the observation of similar pairs in a bulk semiconductor, but now of an electron and a hole. Rather than causing superconductivity, these electron-hole Cooper pairs give rise to strong light emission.

Physicists used ultrashort laser pulses to highly excite a bulk zinc oxide crystal. Upon cooling down the crystal to 4 kelvin, they discovered a strong new peak in the light emission spectrum. Both wavelength and intensity of the light excellently agreed with predictions from theory. Their conclusion is that the observed light emanates from electron-hole Cooper pairs in an uncondensed state. Since there are indications that high-temperature superconductivity might also be related to uncondensed Cooper pairs, these so-called “preformed electron-hole Cooper pairs” can be important in the quest to understand high-temperature superconductivity.

Quantum mechanics on a Mobius ring: Energy levels, symmetry, optical transitions, and level splitting in a magnetic field

BQ12069 - In this paper, the authors explore the quantum mechanical properties of an electron constrained to move on the one-sided surface of a nanoscale Möbius ring. The results are of more than theoretical interest, as recent advances in the production of graphene sheets suggest that it may be only a matter of time before it will be possible to create twisted graphene ribbons. By solving the Schrödinger equation on the Möbius surface, the authors show that the quantum numbers for the energy spectra correspond to those for cylindrical rings of the same dimension while their degeneracies are lifted. Rings with odd numbers of twists have quantum numbers with both whole and half integer values. Because the twisted rings lack the rotational symmetry of the cylindrical rings, the values for the orbital angular momentum component vary slightly from the integer and half-integer values seen in the cylindrical ring. Also, the non-zero variance in angular momentum permits the transition of photons from half-integral to integral angular momentum states, something normally prohibited by the requirement for the conservation of angular momentum. The Zeeman splitting in an external magnetic field shows level anti-crossing in the lowest two levels. Using high-accuracy finite element methods, the authors investigate rings with 1, 2, ...5 twists to identify a pattern in the level splitting that they explain using group representation theory. Beautiful wavefunctions with 2p and also 4p periodicity are also shown.

Magnetic control of Leidenfrost drops

LQ12932E - Oxygen is a common gas but its liquid state has interesting properties. Since its boiling point is at –183°C, a drop of liquid oxygen placed on a solid at room temperature levitates on a cushion of its own vapour (the so-called Leidenfrost effect, also observed with water on a hot pan at 300°C), which makes it ultra-mobile since it does not contact its substrate. In addition, oxygen is attracted by the poles of a magnet so that, as shown in our paper, magnets properly placed can be used to deviate, capture or accelerate these ultra-mobile and elusive drops. Figure 1 shows top views of trajectories observed when a drop of liquid oxygen travels across a horizontal glass plate below which a magnet is placed (grey circle). Beyond the understanding of these trajectories, this simple system allows to probe the dynamics of a liquid drop without touching it and it provides new possibilities for controlled experiments with levitating liquids.

The sound of an electron spin

LK13359 - The spin of an electron is a property separate from its motion which can be used for high-precision magnetic sensing and, because it is intrinsically quantum-mechanical, as a "quantum bit" for a quantum computer.  The separation of spin and motion is not perfect, however:  the theory of relativity predicts a "spin-orbit" coupling which can influence how electrons move in a solid.  In this paper, we show that this effect strongly couples the spin of an electron trapped on a carbon nanotube suspended over a small trench to the vibrations the nanotube itself.  The nanotube acts like  a tiny guitar string, with a sound mode that can ring for a very large number of oscillation periods.  When the nanotube is tuned to ring in unison with the spin, a quantum of sound emitted by the electron can be reabsorbed and reemitted many times before it is lost.  This strong coupling has many interesting consequences and potential applications: e.g., for studying the quantum nature of nanoscale mechanical motion or for enabling long distance communication between quantum bits.

Super Rogue Waves: Observation of a Higher-Order Breather in Water Waves

Click the image for a video of an experimentally-generated
rogue wave capsizing a Lego boat.

Phys. Rev. X 2, 011015
(2012) -
Oceanic rogue waves are relatively large surface waves that appear spontaneously far at sea. They can suddenly develop from very calm and apparently safe sea states, cause serious damage to ships or offshore structures, and then disappear without a trace. Where these apparent anomalies come from is still a puzzle for scientists. A simple theoretical model for describing the evolution of these waves is a nonlinear Schrödinger equation. This equation, due to its nonlinearity, has a set of hierarchically ordered solutions known as rational breathers, or evolving solitons growing out of, and amplifying, a small localized wave perturbation. Recently, we created the lowest order rational breather, also known as the Peregrine soliton, in a laboratory-scale water tank. One open question was then: Could higher-order strongly amplifying breathers, or super rogue waves, be generated also in such a water tank? In this paper, we combine an experiment with the theory to show that the answer is an affirmative “yes.”

To generate these solutions in an open water tank, we start with a carrier wave that is a wave-tank analogue of the stable small ocean waves. The mathematical initial conditions describing small localized perturbations for the generation of the higher-order breathers are simulated exactly with a computer controlled paddle and stage-wise experiments are carefully designed to remove artifacts or get around the constraints imposed by the limited size of the tank. Indeed, large localized waves with an amplification factor of 5—super rogue waves in this laboratory setting—grow out of the small perturbation in the manner predicted by the equation.

We believe that our work not only suggests an easily accessible platform for exploring extreme water-wave dynamics, but may also stimulate similar experimental studies on high-order breather solutions in other fields such as optics, plasma physics, and superfluidity where nonlinear dynamics rules.

Two Atom Transistor

LN12557 - The fabrication of single-atom transistors is a breakthrough which is now possible both by top-down and bottom up approaches. Using standard microelectronics techniques we made a transistor where the current is controlled by the fine tuning of the ionization of two phosphorus atoms connected in series. Following single-atom devices, this 2-atom transistor is the next step towards controlled atomic functionalities in electronics. Here we use the discrete ground level of one donor as a sharp energy filter to probe the levels of a second donor. Our results show that the energy spectrum for electrons bounded to a phosphorus atom in a silicon nano device is reminiscent but differs slightly from the spectrum of an isolated donor in a bulk silicon crystal. The large energy separation (10 meV) between the ground and first excited state arises from the sharp atomic confinement potential is a good opportunity for clean atomic orbital's manipulation.

Particle-based model for skiing traffic

EM10763 - We develop and investigate a particle-based model for ski slope traffic. Skiers are modeled as particles with a mass that are exposed to social and physical forces, which define the riding behavior of skiers during their descents on ski slopes. We also report on position and speed data of 21 skiers recorded with GPS-equipped cell phones on two ski slopes. A comparison of these data with the trajectories resulting from computer simulations of our model shows a good correspondence. A study of the relationship between the density, speed and flow of skiers reveals that congestion does not occur even with arrival rates of skiers exceeding the maximum ski lift capacity. In a sensitivity analysis, we identify the kinetic friction coefficient of skis on snow, the skier mass, the range of repelling social forces and the arrival rate of skiers as the crucial parameters influencing the simulation results. Our model allows for the prediction of speed zones and skier densities on ski slopes, which is important in the prevention of skiing accidents.

Water-driven patterns on stalactites retain paleoclimatic records

LN12591 - The stunning variety of patterns shaped by water in karst caves always awakes admiration and wonder. However, although geochemistry of calcite deposition is well-known, the reasons of the astonishing morphological regularity exhibited by the superficial drape-like undulations remained unexplored so far. The short-scale wavelets on the surface of stalactites demonstrate that their scale-invariance is due to a very subtle interplay between fluid dynamics and geochemistry. The perturbations of the water film flowing on the stalactite in fact interact with the calcite deposition rate, thus originating upstream migrating corrugations with a well-selected wavelength. Stalactite accretion occurs by successive calcite stratifications shaped by waves remounting the stalactite itself. This work also shows the wavelength and speed of such waves depend on the hydraulic and chemical characteristics of the cave environment. Therefore, the geometry of the undulations imprinted in the stalactite cores retains the record of the local paleoclimate evolution. These theoretical findings could pave the road to novel morphological analyses of stalactite specimens, aiming to acquire data about paleo-flows in karst environments.

High-temperature plasma target to enhance nuclear reactions

CM10336 - The first experimental evidence that demonstrates a high-temperature plasma
formation (higher than 1 million K) in liquid metal with ultrasonic waves is
given. The key aspect of this experiment deals with a cavitating target in
the liquid Li formed by ultrasonic waves, and the target is bombarded with
deuteron beams (Ed ~ 50 keV) to explore nuclear environments where fusion
reactions can be enhanced very strongly. The striking result is that the
yield of the 2H(d,p)3H reaction increases substantially when cavitation is
turned on and the proton peak becomes broader with a small tail on the
high-energy side. Kinematical analyses indicate a temperature around 600 eV
(~7 million K) of deuterons in the interior of the bubble. The experimental
results obviously show strong reaction enhancement, although no bubble
fusion (or sonofusion) events are observed.

Walking with coffee: Why does it spill?

LM13443 - In our busy lives, almost all of us have to walk with a cup of coffee. While often we spill that precious liquid, this familiar phenomenon has never been explored systematically. Here we report on the results of an experimental study of the conditions under which coffee spills for various walking speeds and initial liquid levels in the cup. These observations are analyzed from the dynamical systems and fluid mechanics viewpoints as well as with the help of a model developed here. Particularities of the common cup sizes, coffee properties, and biomechanics of walking proved to be responsible for the spilling phenomena. The studied problem represents an example of the interplay between the complex motion of a cup, due to biomechanics of a walking individual, and the low viscosity liquid dynamics in it.

Holographic Storage of Biphoton Entanglement

LP13000 - Coherent and reversible storage of multi-photon entanglement with a multimode quantum memory is essential for scalable all-optical quantum information processing. Although single photon has been successfully stored in different quantum systems, storage of multi-photon entanglement remains challenging because of the critical requirement for coherent control of photonic entanglement source, multimode quantum memory, and quantum interface between them. Here we demonstrate a coherent and reversible storage of biphoton Bell-type entanglement with a holographic multimode atomic- ensemble-based quantum memory. The retrieved biphoton entanglement violates Bell's inequality for 1 microsecond storage time and a memory-process fidelity of 98% is demonstrated by quantum state tomography. Our work makes the first step towards holographic storage of multi-photon entanglement.

Yellowing of Ancient Paper

LK13320 - The hue of the yellowish color in ancient paper may be placed in precise relationship with the environmental conditions to which the artefacts were exposed during their life. This important outcome is the result of our experimental and theoretical investigation on the optical degradation of 15th century papers. 

Paper degradation results in a yellowing of the sheets mainly as a consequence of the oxidation of cellulose fibres. The oxidized products act as chromophores capable of selectively absorbing light and give rise to the yellow coloration of ancient paper sheets. The complex chemical and physical properties of cellulose have prevented a detailed identification of chromophores up to now.

To solve this problem, we measured the optical properties of both ancient and, as comparison, modern paper samples artificially-aged in several environmental conditions and interpreted the collected data by means of ab-initio theoretical calculations based on time-dependent density functional theory. Through this approach the relative concentrations of chromophores responsible for the yellowing of the ancient paper were clearly identified and quantified. A given set of chromophores produced upon aging is strongly related to the environmental conditions to which the artefacts were exposed during their life, such as dry, humid or closed storage.

Our work contributes to a contemporary Preservation Science which attempts to provide conservators a rationale based methods of artefacts analysis and treatment.

Sonic Screwdrivers Serve up Fundamental Physics

LN13325 -  When the scriptwriters for Doctor Who imagined a futuristic device, they came up with the Sonic Screwdriver. Now we have taken equipment designed for MRI-guided focused ultrasound surgery and demonstrated a real Sonic Screwdriver, used to lift and spin a free-floating 10 cm rubber puck.  Energy from an ultrasound array forms a beam that carries momentum and can push objects away objects in its path. If the beam is designed as a vortex, then rotation is also possible.

The helical, phased spiral wavefronts of vortex beams mean there is a rotating, angular component of momentum that can exert torque on an object.  In this paper, we show how to generate vortex beams with many intertwined helices using a 1000-element ultrasound transducer array. These beams are strong enough to levitate and spin the 90 g puck made of ultrasonic absorber in water.

We use the screwdriving effect to test the theory that the ratio of angular momentum to energy in a vortex beam is equal to the ratio of the number of intertwined helices to the frequency of the beam.  This theory is used in topics from quantum physics to biophotonics but it has not previously been proved in a single experiment.  For the first time, our experimental results confirm directly the validity of this fundamental theory.

It may not be the Higgs Boson, but for those working on related topics, it's just as important!

Trouble with the Lorentz Law of Force

LN12869 - Using a simple thought experiment involving a magnetic dipole in the vicinity of an electrically-charged particle, we show that the Lorentz law of force, in conjunction with the Amperian current loop model of a magnetic dipole, violates the fundamental tenets of special relativity. This fact, taken together with the well-known phenomenon of "hidden momentum" in certain magnetic systems (first pointed out by William Shockley nearly 50 years ago), calls into question the applicability of the standard form of the Lorentz law to problems involving magnetic materials, in general, and to radiation pressure problems in magnetic media, in particular. There exists, however, a variant of the Lorentz law, proposed by Albert Einstein and Jakob Laub in 1908, which not only conforms with special relativity, but also is consistent with the conservation laws of energy, momentum, and angular momentum. We argue that a complete and consistent basis for classical electrodynamics is provided by (i) Maxwell's macroscopic equations, (ii) the Poynting postulate for energy, (iii) the Einstein-Laub force and torque densities, and (iv) the Abraham postulate for electromagnetic momentum density.

Demonstration of the Interaction between Two Stopped Light Pulses

LH12745 - This study reports the first experimental demonstration that two light pulses were made motionless and interacted with each other through a medium. The scheme with motionless light pulses maximizes the interaction time and can achieve a considerable efficiency even below single-photon level. To demonstrate the enhancement of optical nonlinear efficiency, the experiment in this study used the process of one optical pulse switched by another based on the effect of electromagnetically induced transparency. Moving light pulses activate switching at an energy per area of 2 photons per atomic absorption cross section as discussed in [Phys. Rev. Lett. 82, 4611 (1999)]. This study demonstrates that motionless light pulses can activate switching at 0.56 photons per atomic absorption cross section, and that the light level can be further reduced by increasing the optical density of the medium. The result of this work enters a new regime of low light physics.

Heaviest Oxygen Discovered: 26O

LK13350 - 26O, consisting of 8 protons and 18 neutrons has been predicted to beta-decay to 26F, however, extensive searches have found no evidence for the existence of 26O nor for this decay mode. In this paper we demonstrated for the first time that 26O after it is formed breaks up into two neutrons and 24O. The data suggest that the neutrons are emitted simultaneously, possibly as a di-neutron, an exotic decay mode which was discovered only a month ago in the decay of 16Be. The measured mass of 26O is a critical milestone towards the understanding of the question why oxygen can bind only 14 neutrons (24O) while fluorine with only one additional proton can bind at least six more neutrons (31F has 22 neutrons and 9 protons).

Lasers available in 2015 will tear apart the vacuum to create a new state of matter

LJ13498 - In this paper we have shown that next-generation lasers will tear apart the vacuum to generate a new state of matter: a quantum electrodynamic (QED)-plasma.  This will usher in a revolution in laser plasma physics leading to many exciting new applications from the generation of large quantities of antimatter, to the being the basis of the world's most intense gamma-ray source.  Quantum electrodynamics (QED) underpins our understanding of what happens when elementary particles such as electrons are accelerated to very high energies.  QED predicts that a strong electromagnetic field applied to the vacuum can be converted into mass, tearing the vacuum into electron-positron pairs.  We have shown that this will be possible by firing 10PW lasers (10PW = 10,000 times the electrical generating capacity of the US), due to be completed in 2015, at solid targets. When the laser strikes the solid the electrons are rapidly stripped away from their atoms, the solid is ionised and a dense plasma is created.  An entirely new state of matter is generated in the laser focus, defined by a complex interplay of the QED processes and classical plasma physics: a 'QED-plasma'.

Electric-field control of switchable and non-volatile magnetization at room temperature

LJ12852 - With the fast development of information storage, exploiting new concepts for dense, fast, and non-volatile random access memory with reduced energy consumption is a significant and challenging task. To realize this goal, electric-field control of magnetism is crucial. A promising way to control magnetism via electric fields is using the converse magnetoelectric effect, which is important for realizing high speed and low power writing memories. We report the first example of a large, switchable and non-volatile bipolar-electric-field-controlled magnetization at room temperature in a Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 ferromagnetic/ferroelectric (FM/FE) two-phase structure. Through investigations of the ferroelectric domains and crystal structures with in situ electric fields, we demonstrate a novel mechanism for electric-field control of magnetization involving the combined action of 109¡ã ferroelastic domain switching and absence of magnetocrystalline anisotropy in Co40Fe40B20. This work provides a route to realize electric-field control of large, switchable and non-volatile magnetism at room temperature, and similar phenomena should occur for other comparable FM/FE structures.