Scientific Journals

On the world, only two kinds of journals claiming publishing science can be found: (1) rubbish journals; (2) scientific journals.

Class (1) features papers filled with errors, trivial results or unjustified conclusions and claims, without rigorous peer review process;
Class (2) publishes papers of real scientific value (see the PRL acceptance criteria for reference).

One should always avoid class (1). Don't throw your thoughts in rubbish can.

In class (2), two categories can be seen: in (a) publish work on mainstream and hot topics, while in (b) publish work in non-mainstream subjects. Both are of scientific values, but interesting to different readers.

The mysterious Moire'- pattern-based electronic properties

Plenty of attention has been diverted to studying the bilayer graphene and hybrid structures consisting of patched mono-bi-layer graphene. A very fundamental problem in bi-layer graphene is how the electronic properties depend on the twisted angle. Theoretical study has been challenging. An interesting review in Nature on a recent PRL paper[http://www.nature.com/nature/journal/v474/n7352/full/474453a.html?WT.ec_id=NATURE-20110623#/references]:

In their study, Luican et al.⁴ find that, at small rotation angles, the local density of electronic states develops a dependence on position within the moiré-pattern unit cell and no longer exhibits the Dirac-like, decoupled-layer, Landau-level pattern. Layer coupling becomes strong in this sense for rotation angles less than about 2°, corresponding to moiré-pattern periods longer than about 10 nanometres. Here it is tempting to conjecture — from the spatial dependence of the density of electronic states — that bilayer wavefunctions have become localized, so that an STM measurement at one position reflects the stacking arrangement only at that position.
...
The extraordinary sensitivity of the electronic properties of few-layer graphene systems to the relative orientations of their layers could prove useful in various applications, for example in ultra-sensitive strain gauges, pressure sensors or ultra-thin capacitors. Further progress requires an improved understanding of both large and small rotation-angle limits, and also improved experimental control of rotation angles.

All organic molecule spin valve

I remember in my previous entry I mentioned a spin filtering effect of DNA molecules. Here comes another molecule with similar effect and can operate as a spin valve at low temperatures[http://www.nature.com/nmat/journal/v10/n7/full/nmat3061.html?WT.ec_id=NMAT-201107].

Now, writing in Nature Materials, Urdampilleta and co-workers¹ report that a single-walled carbon nanotube decorated with magnetic molecules can act in just the same way as a conventional spin valve, albeit only at low temperature.
...............
How is it possible for a single molecule to perform as efficiently as 10 nm of iron? The key is the ability of a chemical bond to modify the magnetic properties of a surface, which has been studied under the suggestive name of 'spinterface science'⁵. It has already been shown that an attached molecule can alter the spin-polarization of the electrons emerging from a magnetic surface^{6, 7}; the experiments of Urdampilleta and co-workers now prove the opposite effect — namely that a magnetic molecule can alter the spin polarization of the current flowing in a non-magnetic material. Two particular features make this possible. First, the magnetic centre must be sufficiently close to the conduction channel. In this respect, the case of bis-phthalocyaninato-terbium(III) is rather peculiar, because the Tb³⁺ ion (Tb³⁺ carries a total angular momentum, J = 6) is sandwiched between two phthalocyanine ligands, and it is at least 1 nm away from the nanotube — too far to transfer any magnetic information. However, there is a second source of spin in this molecule, namely a S = 1/2 radical delocalized over the two phthalocyanine ligands. These are likely to participate in the bond and help to spin-polarize the electron current. Second, the conduction channel must be sufficiently sensitive to the local magnetic moment. All the atoms in a single-walled carbon nanotube reside on the surface, so that a surface modification results in an alteration of the entire electronic structure. It is an extreme surface sensitivity that makes this spin valve work.

Noteworthy papers from latest issue of Science

1. Disorder-Enhanced Transport in Photonic Quasicrystals, 332:1541(2011);

Quasicrystals are aperiodic structures with rotational symmetries forbidden to conventional periodic crystals; examples of quasicrystals can be found in aluminum alloys, polymers, and even ancient Islamic art. Here, we present direct experimental observation of disorder-enhanced wave transport in quasicrystals, which contrasts directly with the characteristic suppression of transport by disorder. Our experiments are carried out in photonic quasicrystals, where we find that increasing disorder leads to enhanced expansion of the beam propagating through the medium. By further increasing the disorder, we observe that the beam progresses through a regime of diffusive-like transport until it finally transitions to Anderson localization and the suppression of transport. We study this fundamental phenomenon and elucidate its origins by relating it to the basic properties of quasicrystalline media in the presence of disorder.

2.Carbon-Based Supercapacitors Produced by Activation of Graphene, 332:1537(2011)

Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp²-bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

3. The Limits of Ordinary Matter, 332:1513(2011)

All ordinary matter consists of protons and neutrons, collectively called nucleons, which are bound together in atomic nuclei, and electrons. The elementary constituents of protons and neutrons, the quarks, almost always remain confined inside nucleons (or any other particle made up of quarks, called hadrons). The fundamental force that binds quarks together—the strong, or “color” force—cannot be overcome unless extremely high-energy conditions are created, such as through heavy-particle collisions. Theoretical simulations based on quantum chromodynamics (QCD) predict that the transition temperature for the appearance of free quarks should occur at 2.0 × 10¹² K (an energy of 175 million eV) (1, 2). Since 2000, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has created the necessary conditions to form quark matter in particle collision, but determining the transition temperature under these conditions is challenging. On page 1525 of this issue, Gupta et al. (3) show that the relevant temperature and energy scales can be extracted from recent experimental studies and find that the transition temperature is in remarkable agreement with theory.

4 This paper is not published in Science, but highlighted in it: Nano Lett. 11, 10.1021/nl200928k (2011).

It has long been known from ex situ studies that metal nanoparticles can catalyze reaction of oxygen with graphite surfaces and create grooves or channels. Such reactions could be used for patterning graphene sheets. Booth et al. have studied the dynamics of silver nanoparticles on suspended monolayer and bilayer graphene sheets in a transmission electron microscope. They imaged these samples at temperatures from 600 to 850 K and partial pressures of oxygen over the sample from about 30 to 100 millitorr. The nanoparticles cut channels along <100> crystallographic directions, but some fluctuations of motion normal to the channel direction were also observed. The nanoparticles did not move at a constant speed. Instead, their velocity profile was erratic, and the start-stop motion was better described by a Poisson distribution.

News from this issue of Science Magazine

1. Batteries are going to break out from old forms:

Now he's on his way. Last month, Chiang and his colleagues reported in Advanced Energy Materials that they've created a new battery design called a semisolid flow cell that's like a battery with a fuel tank. Like today's batteries, the device contains lithium ions that shuttle back and forth either storing or releasing electrical charges on demand. But instead of packaging those ions along with the electrodes and other apparatus all together, as in a typical battery, Chiang's semisolid flow cell separates the energy-delivery apparatus from energy storage. In this battery, the storage medium is a pair of gooey, black liquids, the consistency of yogurt, that contain nanoscale particles of materials commonly used as anodes and cathodes in lithium-ion cells. These particles are suspended in an electrolyte and separated by a porous membrane. When power is needed, a bolus of each goo is pumped from external tanks into a network of current collectors that extract electrons while lithium ions shuttle through the membrane from the anode particles to the cathode particles. The spent slurries can then be reenergized, as in a normal rechargeable battery, or pumped out and replaced. Other types of flow batteries have been made in the past, but Chiang says the new setup can store up to 30 times as much energy as previous versions. He has launched another company, called 24M, to commercialize the technology.[http://www.sciencemag.org/content/332/6037/1494.full]

2.

Move over, China. Japan's “K Computer” is now the fastest supercomputer in the world. On 20 June, K was ranked number one in the TOP500 list of the world's supercomputers, performing three times as fast as its Chinese rival and the previous champion, Tianhe-1A.

The TOP500 list is updated twice a year and ranks how quickly computers solve a standard mathematical equation. K, built by Fujitsu and located at the RIKEN Advanced Institute for Computational Science in Kobe, can perform 8.2 quadrillion calculations per second, equivalent to linking about 1 million desktop computers. That performance is still shy of the target kei, or 10 quadrillion, calculations for which the supercomputer was named. This is the first time Japan has topped the list since 2004.[http://www.sciencemag.org/content/332/6037/1488.2.full]

3. Education is not a rece:

In the United States and elsewhere, the competitive pressures placed on young people in school are damaging many otherwise promising lives. In addition to generating debilitating anxiety and encouraging a culture of cheating, this competition takes the joy out of learning. The film Race to Nowhere, which continues to receive attention since its release a year ago, documents the unhealthy consequences of the competitive “teach to the test” climate that many U.S. students experience. The film, in which I was interviewed, puts in clear relief the pressures that youth are under to amass large numbers of Advanced Placement (college-equivalent) classes, win science fairs, excel in the arts and sports, and in other ways distinguish themselves from the competition for admission into a few select universities that parents and schools believe are critical for future success. Research on motivation makes it clear that focusing attention entirely on performance, whether grades or test scores, destroys whatever intrinsic interest the subject matter might have had.* There are certainly students whose passions spur them to realize their full potential in rigorous academic courses and other impressive activities. But how many potential Nobel Prize winners have written off science before the end of high school because their only exposure to the subject had been in test preparation courses rather than in classes that delved into meaningful questions? It doesn't have to be this way, but change will require coordinated efforts at many levels.

Success in life does not require a degree from one of 10 universities. We need to evaluate U.S. high schools (pre-college education) on how well they help students find a college that matches their interests and goals, not on the proportion of students that they send to elite institutions. And the coveted universities need to demonstrate that they are interested in students who have a genuine passion for extending their educational experience, not merely in tallying items on resumés.

Many U.S. teachers also must change their approach to teaching. Extensive research shows that students will become more emotionally engaged (and even passionate) if simple principles are followed: if the subject matter is connected to students' personal lives and interests; if students have opportunities to be actively involved in solving or designing solutions to novel and multidimensional problems, doing experiments, debating the implications of findings, or working collaboratively; if students have multiple opportunities to earn a good grade (by rewriting papers or retaking tests); if attention is drawn to the knowledge and skills that students are developing, not to grades or scores; and if all learning and skill development is celebrated, whatever the level.

Schools must create homework policies to ensure that diligent students aren't kept up late into the night; schedule some spacing between major tests and offer ample opportunities for students to get extra help; make sure that at least one adult is paying attention to every student's emotional needs; provide parent education on the advantages of a broad array of potential colleges; survey students regularly on the sources of their stress and make sure that this feedback informs policies; and offer opportunities for students to pursue academic interests unencumbered by performance concerns, such as in independent studies or clubs.

The world is rapidly changing. Problem-solving skills and critical analysis have become infinitely more important than being able to answer the typical questions given on standardized tests. A valuable science of teaching and learning exists that should guide efforts to improve students' interest, engagement, and intellectual skills, as well as reduce the debilitating stress that is becoming epidemic.** Only by paying attention to what we know can we make the changes that youth need to lead healthy and productive lives. [http://www.sciencemag.org/content/332/6037/1481.full]

Einstein's Theory STANDS UP

Usually quoted as the most refined brain child ever seen in human history, Einstein's general relativity theory has been frequently under test and it passed. The latest test was initiated some half a century ago and eventually came up with a result: it passed again ! [http://www.physicstoday.org/daily_edition/physics_update/gravity_probe_b_concludes_its_50-year_quest]

Results of an experiment conceived around 1960 to test general relativity and launched in 2004 were announced at a NASA press conference earlier this month: Albert Einstein’s theory passed. The experiment featured four (for redundancy) gyroscopes—spinning, niobium-covered spheres—orbiting 642 km above Earth (see the figure). The goal was to measure the precession induced in the gyroscopes by two general relativistic effects. The easier-to-measure geodetic effect influences any spinning object orbiting a mass. The second effect, frame dragging, arises when the spacetime-distorting mass, here Earth, is itself spinning. Gravity Probe B was not the first to measure the two effects, but it was designed to measure them independent of each other and to extraordinary precision. The gyroscopes are the most perfectly spherical objects ever fabricated. They needed to be, lest the general relativistic precessions be swamped by those arising from Newtonian torques. To measure the spin of those featureless spheres, the experimenters cooled them below niobium’s superconducting transition; the superconducting metal then produces a magnetic field, parallel to its spin axis, that can be measured with a superconducting quantum interference device. In the end, the experiment was a qualified success. It measured the geodetic effect to 0.3% precision, but stray charges on the gyroscopes and their housings limited the precision of the frame-dragging measurement to 20%. In both cases other efforts have achieved comparable results. (C. W. F. Everitt et al., Phys. Rev. Lett., in press.)—Steven K. Blau

How do wings work ?

This is an interesting article from the wonderful journal I posted in my last entry. It tries to poke the usual (even textbook) explanation of how wings work [2003 Phys. Educ. 38: 497].

Now Bernoulli’s equation is quoted, which states that larger velocities imply lower pressures and thus a net upwards pressure force is generated. Bernoulli’s equation is often demonstrated by blowing over a piece of paper held between both hands as demonstrated in figure 2. As air is blown along the upper surface of the sheet of paper it rises and, it is said, this is because the average velocity on the upper surface is greater (caused by blowing) than on the lower surface (where the air is more
or less at rest). According to Bernoulli’s equation this should mean that the pressure must be lower above the paper, causing lift. The above explanation is extremely widespread. It can be found in many textbooks and, to my knowledge, it is also used in the RAF’s instruction manuals. The problem is that, while it does contain a grain of truth, it is incorrect in a number of key places.

What’s wrong with the ‘popular’ explanation?
The distance argument;
The ‘equal time’ argument;
The Bernoulli demonstration.

Next, examine a particle moving along a curved streamline as shown in figure 7. For simplicity we can assume that the particle’s speed is constant3. Because the particle is changing direction there must exist a centripetal force acting normal to the direction of motion. This force can only be generated by pressure differences (all other forces are ignored), which implies that the pressure on one side of the particle is greater than that on the other. In other words, if a streamline is curved, there must be a pressure gradient across the streamline, with the pressure increasing in the direction away from the centre of curvature.

Physics Education Journal

I like this journal very much ! It is nice, filled with wonderful and provoking 'mundane' experiments ! I love finding out the physics behind these 'mundane' things and puzzles. That is really something that can easily occupy my mind. I love them, to say it again! I will use it to educate my children, Aha !
http://iopscience.iop.org/0031-9120/

Classical Not Always Lose

It was shown that, classical physics does as efficiently as quantum physics in energy transfer in biological systems [http://physics.aps.org/synopsis-for/10.1103/PhysRevE.83.051911].

A prominent goal of quantum information and computing is to be able to exploit quantum entanglement in qualitatively new devices, such as massively parallel computers. Has biological evolution already harnessed entanglement for its own purposes? Recent studies have indeed suggested that electronic excitation transfer (EET) in photosynthesis benefits from quantum entanglement. Now, a paper appearing in Physical Review E is likely to stimulate further investigation and controversy on this question. Based on calculations, John Briggs and Alexander Eisfeld, of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany, assert that under the conditions prevailing in photosynthesis (in particular, in the so-called Fenna-Matthews-Olson complex that lies at the heart of the process), energy transfer in a classical system is just as efficient as in its quantum counterpart.

To model the photosynthesis that occurs in plants, Briggs and Eisfeld study a collection of monomers, each possessing a single electronic state and coupled to its neighboring units by a dipolar interaction. The authors find that for dipolar interactions similar to those found in real molecular aggregates, the coherences in quantum transport (from the Schrödinger equation) are identical to those occurring in classical transport according to Newton’s equation. Although their analysis neglects the influence of the environment, the authors report that calculations including dephasing processes in the quantum and classical equations lead to the same conclusion. – Ron Dickman

Closed Line Of dislocations in graphene

Defects are valuables in materials science. They are purported to generate novel physics in unexpected ways. Low dimensional systems are more prone to defects. In graphene, structural defects can exist in zero or one D, called point defect or line defect, respectively. A line defect can have tremendous impact upon the transport properties of graphene. Very common is the dislocations. In this new work[PHYSICAL REVIEW B 83, 195425 (2011)], closed line of dislocations were observed. They look like flowers. They are topologically different from open dislocations. What kind of effects can be expected on electronic properties ? Let's see.

Topological defects can affect the physical properties of graphene in unexpected ways. Harnessing their influence may lead to enhanced control of both material strength and electrical properties. Here we present a class of topological defects in graphene composed of a rotating sequence of dislocations that close on themselves,
forming grain boundary loops that either conserve the number of atoms in the hexagonal lattice or accommodate vacancy or interstitial reconstruction, while leaving no unsatisfied bonds. One grain boundary loop is observed as a “flower” pattern in scanning tunneling microscopy studies of epitaxial graphene grown on SiC(0001).We show that the flower defect has the lowest energy per dislocation core of any known topological defect in graphene, providing a natural explanation for its growth via the coalescence of mobile dislocations.

Light passes through without reflection

This is not an old concept: a light beam may not be reflected if the thickness of the glass it shines upon is carefully chosen so that the reflected wave from the second surface goes out of phase with the one from the first surface. Now that the proper thickness is proportional to the wavelength of the incident light, it is not possible to use a single piece of glass for reflectionless control of light with various colors. But, nature offers much more. One can make a more delicate refractive index profile of medium so that it invisible to a wide spectrum [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.193903].

Abrupt interfaces disrupt wave propagation. For example, light passing from air into a sheet of glass will partially reflect backwards. When the light exits back into air, there is a second reflection that can cancel the first for light of just the right frequency, given the refractive index and thickness of the sheet. The wave nature of electrons creates similar effects when they encounter a region with a changing electrostatic potential. But early in the history of quantum mechanics, theorists realized that certain smoothly varying potential profiles could eliminate the reflection of electrons over a wide range of frequencies.

As it turns out, the same concepts work for light: intense light pulses known as solitons create precisely this kind of profile in the refractive index of the surrounding medium, eventually becoming trapped. Creating permanent versions of such “reflectionless potentials” has, however, proved difficult. In Physical Review Letters, Alexander Szameit of the Technion in Haifa, Israel, and colleagues in Germany and Australia at last implement the lack of light reflection in the laboratory.

In their experiments, a beam of light travels along an array of closely spaced, parallel waveguides created in a glass sample through direct laser-writing. By changing the spacing between some of the waveguides, the researchers construct a stripe along the length of the array that has a different refractive index modulation relative to the rest of the array. For almost any change in spacing, light traveling diagonally across the stripe is partially reflected, as usual. But a stripe having the special variation suggested by theory generates almost no reflection. The technique adds to the bag of tricks that researchers have for manipulating light. – Don Monroe

No physical signal travels faster than c

Einstein's special relativity theory stipulates that no physical signal (i.e., anything that carries energy and obeys physical laws and is measurable) can not go faster than the vacuum light speed. There have been many 'dissidents' (mostly crackpots) don't like this and want to disprove this law, but all have been defied. Now an experiment that was recently done in HKUST demonstrated that, even a single photon cannot break it.

Einstein taught us that the speed of light was the traffic law of the universe—nothing could go faster. The development of media in which atomic gases can slow down or speed up the passage of light pulses initially caused a stir, at least until the difference between phase velocity and group velocity could be carefully explained. But what about the behavior of single photons, the fundamental quanta of light? Reporting in Physical Review Letters, Shanchao Zhang and colleagues at the Hong Kong University of Science and Technology have shown that photons obey the law too.

Zhang et al. study optical precursors, which are signals preceding the main wave packet in a light pulse with a sharply rising leading edge (as in a step function pulse). Past work has shown that even in “superluminal” media where the group velocity may be faster than light speed, the precursor is always in front of the pulse. The authors extend this work to the single-photon level with the help of cold atomic gases: a photon generated in one rubidium gas traverses a second collection of rubidium atoms. With careful use of electromagnetically induced transparency, the researchers can separate the precursor from the main pulse and confirm it travels at the speed of light. The results add to our understanding of how single-photon signals propagate but also confirm the upper bound on how fast information travels. – David Voss [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.243602]

Three-body states in dipolar molecules

Two-body bound states (Efimov states) are common: the earth-sun, the hydrogen atom, the double-atom molecu;es, cooper pairs, and so on. But three-body bound systems seem not that abundant (Why? I don't know yet). Now due to a theoretical work, the prospect is bright. According to it, such states should widely exist in dipolar molecules.

Tractable three-body problems are rare, which is why Vitaly Efimov’s study in 1970 proposing that bound states could exist between three interacting bosons was so intriguing. It took more than 30 years, though, to observe Efimov states in an ultracold gas of cesium atoms, in which interactions could be controlled with a magnetic field. Now, writing in Physical Review Letters, theorists suggest similar states should also exist between dipolar molecules.

In his prediction, Efimov assumed the interacting bosons were spherically symmetric. In their new work, Yujun Wang and colleagues at JILA, at the University of Colorado, Boulder, use numerical methods to look for bound states between molecules that have an electric dipole—an extended structure that greatly complicates the calculations. The group shows that such dipolar Efimov states are in fact long-lived and “universal,” meaning they don’t depend on the molecules’ detailed structure. (The states only exist when the separation between the molecules is large compared with the length of their dipole moment.)

Wang et al.’s prediction is timely, as it is only in the last two to three years that experimentalists have been able to cool the molecules in a gas to their absolute ground state and study and manipulate the dipole interactions between them. – Jessica Thomas [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.233201]

Nesting not so holy in pnictides

This [Phys. Rev. B 83, 220504 (2011)] might be call theories solely based on nesting into question !

Despite intense study, researchers have not yet uncovered the secrets behind the peculiar properties of iron-based (pnictide) superconductors. Many theories that try to explain the driving mechanism of superconductivity in these materials suggest it is tied to so-called nesting of the electron and hole Fermi surfaces. This geometric feature of the Fermi surface, where one portion of the surface maps to another if it is translated by a suitable reciprocal-lattice vector, is common to the structure of many families of pnictides. Nesting often implies the existence of collective electron behavior, so if it is present in the host materials of the pnictides, it would have significant implications for their properties.

In a Rapid Communication appearing in Physical Review B, Brendan Arnold at the University of Bristol, UK, and colleagues use the de Haas-van Alphen effect, where electrons and holes orbit the extrema of the Fermi surface in response to a magnetic field, to map out the electron and hole Fermi surface sheets of BaFe₂P₂, the parent material of an important family of pnictide materials. Besides providing highly detailed information about the geometry of the Fermi surfaces, they find, rather surprisingly, that the nesting present in the superconducting doped compounds BaFe₂(As_1-xP_x)₂ persists in BaFe₂P₂, which is not superconducting. This finding agrees with a growing list of experiments that conclude nesting does not play a dominant role in the development of superconductivity, at least in one family of pnictide compounds. – Alex Klironomos

The UNiverse seems less smooth than theory

If so, there will need some new understanding. Since Einstein's application of his grand theory, GRT, to comprehending the cosmos, a lot of observations have been achieved during the past years, especially about the cosmological structure on large scales. This allows one to make relatively accurate estimate about the mass and energy distribution, by calibration with GRT. Great fitting has been found if dark matter and dark energy are presumed up till now, when people find that, the universe is much more clumpier than expected on larger scale.

Thomas et al. use publicly-released catalogs from the Sloan Digital Sky Survey to select more than 700,000 galaxies whose observed colors indicate a significant redshift and are therefore presumed to be at large cosmological distances. They use the redshift of the galaxies, combined with their observed positions on the sky, to create a rough three-dimensional map of the galaxies in space and to assess the homogeneity on scales of a couple of billion light years. One complication is that Thomas et al. measure the density of galaxies, not the density of all matter, but we expect that fluctuations of these two densities about their means to be proportional; the constant of proportionality can be calibrated by observations on smaller scales. Indeed, on small scales the galaxy data are in good agreement with the standard model. On the largest scales, the fluctuations in galaxy density are expected to be of order a percent of the mean density, but Thomas et al. find fluctuations double this prediction. This result then suggests that the universe is less homogeneous than expected. [http://physics.aps.org/articles/v4/47]

AC field driven population inversion

Non-equilibrium physics can offer much richer phenomena than equilibrium physics. One such example is the population inversion: high energy states are occupied while the low energy ones become emptied, effectively a negative temperature phenomena. It is expected that, many exotic things might be explored with this. For instance, bandwidth vanishing. A new paper reports how to achieve this using non-adiabatic switching of ac field [PRL 106, 236401 (2011)].

We show theoretically that the sudden application of an appropriate ac field to correlated lattice fermions flips the band structure and effectively switches the interaction from repulsive to attractive. The nonadiabatically driven system is characterized by a negative temperature with a population inversion. We
numerically demonstrate the converted interaction in an ac-driven Hubbard model with the nonequilibrium dynamical mean-field theory solved by the continuous-time quantum Monte Carlo method. Based on this, we propose an efficient ramp-up protocol for ac fields that can suppress heating, which leads to an effectively attractive Hubbard model with a temperature below the superconducting transition temperature of the equilibrium system.

Universality in constrained thin sheets

Suppose you hang a curtain down before your window. Set the x-axis and y-axis in the vertical and horizontal direction, respectively. The upper edge (at x=0) of the curtain is somewhat constrained by the reel bar. Certain kind of creases develop that can be described by the out-of-plane deformation, a function z(x,y). Very generally, this function is largely sinusoidal along y given x, with a wavelength lambda(x). More interestingly, such crumpling pattern is also seen in much smaller thin sheets, such as graphene. Is there any universal manner to all thin sheets with constrained boundaries ? The answer is yes. The work has just been done to demonstrate it [PRL 106, 224301 (2011)]. A key result is that, lambda follows a simple power law, namely, lambda~x^m, where m for light sheet (2/3) is different from that for heavy sheet (1/2), thus showing a hierarchy. The amazing thing is the universality.

We show that thin sheets under boundary confinement spontaneously generate a universal self-similar hierarchy of wrinkles. From simple geometry arguments and energy scalings, we develop a formalism based on wrinklons, the localized transition zone in the merging of two wrinkles, as building blocks of the global pattern. Contrary to the case of crumpled paper where elastic energy is focused, this transition is
described as smooth in agreement with a recent numerical work [R. D. Schroll, E. Katifori, and B. Davidovitch, Phys. Rev. Lett. 106, 074301 (2011)]. This formalism is validated from hundreds of nanometers for graphene sheets to meters for ordinary curtains, which shows the universality of our description. We finally describe the effect of an external tension to the distribution of the wrinkles.

Singapore strengthes its science

As a small country, it has made really amazing progress in science and technology.
http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_64/iss_6/20_1.shtml?type=PTALERT