Helical structures through simple mechanical rules

Structures of this kind commonly occurs in nature. How do we generate them artificially? Here is a review of a work that gives a simple thumb rule.

Macroscopic helical structures formed by organisms include seashells, horns, plant tendrils, and seed pods (see the figure, panel A). The helices that form are chiral; like wood screws, they have a handedness. Some are helicoids, twisted helices with saddle-like curvature and a straight centerline; others are cylindrical helices with cylindrical curvature and a helical centerline. Studies of the mechanisms underlying the formation of helicoid or helical ribbons and of the transitions between these structures (1–4) have left an important question unanswered: How do the molecular organization of the material and its global geometrical features interact to create a diversity of helical shapes? On page 1726 of this issue, Armon et al. (5) explore the rich phenomenology associated with slender strips made of mutually opposing “molecular” layers, taking a singular botanical structure—the Bauhinia seed pod—as their inspiration. They show that a single component, namely a flat strip with a saddle-like intrinsic curvature, is sufficient to generate a wide variety of helical shapes.

An introduction to dimensional analysis

This is a nice article giving a brief account of dimensional analysis, which is an extremely powerful tool for scientists. Still, many postgraduates don't know how to use it !

Converting mechnical energy by surface tension

Hey, someday you may charge your phone by just walking around !
[http://www.physicstoday.org/daily_edition/physics_update/a_microfluidics_path_to_harvesting_mechanical_energy]

Imagine a drop of water on a solid, pulled up into a ball by surface tension. The simple application of voltage between liquid and solid alters the interfacial energy and prompts the droplet to spread out, an effect known as electrowetting; the greater the voltage, the greater the spread. In the past decade, researchers exploiting the effect have developed, among other applications, liquid lenses with voltage-tunable focal lengths and microfluidic circuits that store and steer droplets without the need for pumps or mixers. Tom Krupenkin and Ashley Taylor at the University of Wisconsin–Madison have now developed an approach that runs the process in reverse—converting the mechanical energy of liquid motion into electrical current. In one implementation, they pressurized a fluidic channel to force a train of mercury droplets past dielectric-coated electrodes connected to a bias voltage on the order of tens of volts. As the overlap area between the droplets and electrodes changed, so did the charge stored at their interface, giving rise to an alternating current that can drive a load. The researchers measured a few milliwatts from a channel containing 22 droplets. But from their model of the process they calculate that average powers of 1 W or more could easily be generated in a fluidic device with 1000 flowing droplets. The devices are small enough to fit into a pair of shoes; with each step, fluid is squirted back and forth between the heel and toe. (T. Krupenkin, J. A. Taylor, Nat. Commun. 2, 448, 2011.)—R. Mark Wilson

Tips for delightful partnership with editors

These tips should be interesting to anyone who are as fresh as me !

I do

• Read the magazine's official author guidelines. It might seem obvious, but many people are surprised that the tone for journals differs from that of magazines when it comes to writing style. Save yourself a lot of headaches and hair follicles by reading the "prenup" so that you're prepared for the first editing round.
• Meet your deadlines. If you're out of town or won't be available, tell your editor. Many editorial types work on multiple publications, and it's critical that they schedule their time. If the deadline is unreasonable, let them know. They're willing to work with you, but they don't know what you don't tell them.
• Use "Track Changes" in Word documents. It's much harder to collaborate on an article when nobody can tell how it's evolved. Editors know when you're trying to slip them a mickey. If you don't like how "Track Changes" looks, don't turn it off—hide it. Turning it off just gives the editor the extra work of using "Compare Documents." And nobody likes that particular "feature."
• Watch your tone in email. Your editor isn't out to get you—his or her goal is to work closely with you to produce a technically accurate, timely, useful, and readable article. Insulting your editor, threatening his or her job, or intimating that a future lawsuit could be in order won't endear you to the editorial staff, and word travels fast around the office. Let's put it this way—the wise diner doesn't berate a waiter before that person brings out the soup.
• Realize that all magazines edit their content. Nobody—but nobody—gets his or her text printed on glossy paper without someone changing it somehow. Use of contractions won't kill you or your career. Just because The New York Times doesn't do it, does that mean you can't? No! If you threaten to take your 2000-word opinion piece over to The Atlantic because its editors won't dare insert headers in your wall of text, you're so very, very wrong.
• Give us high-resolution images. The JPEGs and GIFs that are great for the Web, don't always work for print. Low-resolution images print fuzzy and are unreadable; no one wants that.
• Give us a recent, decent picture of yourself. No one looks like they did in their college yearbook picture 20 years later. You look great just the way you are—nobody needs or wants to see a picture of you dancing in a chef's hat, swimming in a pool shirtless, or sitting there drunk in your $40-a-night hotel room ordering room service in a dirty Hawaiian shirt. One author noted that he didn't know how to get a recent picture. His 10-year-old son, however, did. Kids can be useful sometimes.

A novel approach to cuprate SC ?

Here is a seemingly very interesting paper recently published in PRX. It contains an exotic treatment of the superconducting gap (not the pseudogap) in LSCO.

A generic theory of the quasiparticle superconducting gap in underdoped cuprates is derived in the strong-coupling limit, and found to describe the experimental ‘‘second gap’’ in absolute scale. In drastic contrast to the standard pairing gap associated with Bogoliubov quasiparticle excitations, the quasiparticle gap is shown to originate from anomalous kinetic (scattering) processes, with a size unrelated to the
pairing strength. Consequently, the k dependence of the gap deviates significantly from the pure dx2 y2 wave of the order parameter. Our study reveals a new paradigm for the nature of the superconducting gap, and is expected to reconcile numerous apparent contradictions among existing experiments and point
toward a more coherent understanding of high-temperature superconductivity.

Physics and Physicists: Current Research on High-Tc Superconductivity

Physics and Physicists: Current Research on High-Tc Superconductivity

Physics and Physicists: Hendrik Schon Loses His PhD

Physics and Physicists: Hendrik Schon Loses His PhD

The h-index

Obviously, it cannot be fair to evaluate an individual scientist by a single number. So, I object to any kind of misuse of such numbers, especially the so-called impact factors, which were originally invented to rank journals but now widely misused in a silly way to rank scientists. The best way to refute possible confuse is perhaps to have a clear understanding of the contents of certain numbers. Here I would like to highlight two papers on this topic:
1. The most influential journals: Impact Factor and Eigenfactor, PNAS, 106:6883
2. An index to quantify an individual’s scientific research output, PNAS, 102:16569

Two pieces of work on cuprates

Just to highlight them here, because they seemingly represent something defying the cliche.
1. Electron-spin excitation coupling in an electron-doped copper oxide superconductor [Nphy, 7:719(2011)]

High-temperature (high-Tc) superconductivity in the copper oxides arises from electron or hole doping of their antiferromagnetic (AF) insulating parent compounds. The evolution of the AF phase with doping and its spatial coexistence with superconductivity are governed by the nature of charge and spin correlations, which provides clues to the mechanism of high-Tc superconductivity. Here we use neutron scattering and scanning tunnelling spectroscopy (STS) to study the evolution of the bosonic excitations in electron-doped superconductor Pr0:88LaCe0:12CuO4􀀀 with different transition temperatures (Tc) obtained through the oxygen annealing process.We find that spin excitations detected by neutron scattering have two distinct modes that evolve with Tc in a remarkably similar fashion to the low-energy electron tunnelling modes detected by STS. These results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometre length scales, and the dominant electron–boson coupling at low energies originates from the electron-spin excitations.

2. Intense paramagnon excitations in a large family of high-temperature superconductors [Nphy 7:725(2011)]

In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum on doping from the antiferromagnetic insulating to the superconducting state of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic X-ray scattering to show that a large family of superconductors, encompassing underdoped YBa2Cu4O8 and overdoped YBa2Cu3O7, exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. The comprehensive experimental description of this surprisingly simple spectrum enables quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa2Cu3O7 reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to that of Eliashberg theories of conventional superconductors.

More analogy in Graphene

More work on the analogy between garphene and high energy physics. In this case, the focus the running coupling constants [ Nature Phys. 7, 701–704 (2011). ]:

The best times in Physics are those when physicists of different expertise meet around a problem of common interest. And this is now happening in the case of graphene. From the early days of the isolation of single sheets of graphene, the relativistic nature of its charge carriers was clear¹. These carriers, known as Dirac fermions, are described by equations similar to those that describe the quantum electrodynamic (QED) interactions of relativistic charged particles. A meticulous study performed by Elias and co-workers² of the electronic structure of graphene shows that at very low energies reaching a few meV of graphene's Dirac point, where its cone-like valence and conduction bands touch, the shape of the conduction and valence bands diverge from a simple linear relation. The result implies that the analogy between graphene and high-energy physics is deeper than first expected. In particular, it implies that the electromagnetic coupling of graphene does renormalize, as occurs in quantum field theory [http://www.nature.com/nphys/journal/v7/n9/full/nphys2066.html?WT.ec_id=NPHYS-201109].

New clue toward paramagnons as the glue

Certainly lots of doubts are over the magnetic fluctuations as the paring source of carriers in cuprate superconductors. A central issues concerns if there is sufficient paramagnons in the Sc region, since experiments have so far detected only a limited volume of such stuff. Now this gets changed due to this work [Nature Phys. 7, 725–730 (2011). ] reviewed below [http://www.nature.com/nphys/journal/v7/n9/full/nphys2077.html?WT.ec_id=NPHYS-201109]:

However, the observed excitations were restricted to a narrow window in both energy and momentum and furthermore carried relatively little spectral weight, posing a challenge to theoretical ideas about magnetic fluctuations being the source of Cooper pairing in these superconductors. Some researchers have suggested that the experimental limitations inherent in neutron scattering were partially responsible for this state of affairs — and only now has a breakthrough occurred.

In Nature Physics, Le Tacon and colleagues² report the application to various copper oxides of an alternative technique to map magnetic excitations: resonant inelastic X-ray scattering (RIXS)³. Here, an electron is transferred, by a high-energy photon, from a deep core level into an unoccupied low-energy state; subsequently, an electron from a different low-energy state fills the core hole and emits a high-energy photon. Thus, a net excitation is generated in a low-energy band, the energy and momentum of which can be measured by examining the scattered photon.

Among the advantages of RIXS, compared with neutron scattering, is the large cross-section for the scattering of photons (which eliminates the need for large samples) and the possibility to probe essentially the entire Brillouin zone. There are disadvantages as well: in contrast to neutron scattering, the cross-section is not simply related to a dynamic susceptibility, which complicates the data analysis, and the energy resolution is at present limited to about 100 meV (it's far below 1 meV in state-of-the-art neutron-scattering experiments). Despite these limitations, the past decade has seen exciting progress in RIXS³ such that investigations of elementary spin excitations have now become feasible.

Le Tacon et al.² have investigated magnetic excitations using RIXS in a family of copper-oxide materials, covering a range of hole dopings from the undoped insulator to the slightly overdoped superconductor. In all doped materials, they identified damped spin excitations with high intensity over a large part of momentum space. These excitations, in both their overall dispersion and their intensity, seem to show surprisingly little variation with doping.

These findings are important for a number of reasons. First, together with similar recent experiments^{3, 4, 5}, they establish RIXS as a powerful tool for the investigation of complex correlated-electron materials. Second, they show that previous neutron-scattering studies have indeed missed a significant part of the spectral weight of spin fluctuations in copper oxides. This implies that theories of electron pairing based on the exchange of magnetic fluctuations can be considered on safer ground. In fact, Le Tacon et al. provide a sample calculation of a superconducting critical temperature (T_c), in which they use the measured spin-fluctuation spectrum and electronic bands as input and obtain a T_c value comparable to the experimental one.

Third, and perhaps most importantly, their data indicate that key features of the spin fluctuations in doped copper oxides are strikingly similar to that of their undoped counterparts (Fig. 1): at the elevated energies probed by RIXS, the only significant effect of doping is an energy broadening of the excitations, probably arising from damping due to electron-hole excitations. (One should note that the present energy resolution of RIXS is insufficient to resolve fine structures on scales below 100 meV; therefore the similarity of doped and undoped spectra refers to gross features, and the details may well differ.)

Morphology

Recently there have appeared some interesting works on biological morphology development. They help us understanding how a particular pattern, e.g., fingerprints and intestine, comes about under basic mechanical laws [http://www.nature.com/nphys/journal/v7/n9/full/nphys2088.html?WT.ec_id=NPHYS-201109].

Thierry Savin and colleagues refer to Thompson's tome in their investigation, published in Nature, of the elaborate looped morphology that arises in the vertebrate gut (Nature 476, 57–62; 2011). Using experiment, simulation, and an innovative physical mock-up comprising rubber tubing stitched to latex, they have examined the forces arising from relative growth between the gut tube and a neighbouring sheet of tissue known as the dorsal mesentery. The study reveals a mechanism for the formation of loops based on differential strain between the two tissues.

© SPL

This is a timely nod to Thompson's century-old ideas, given the recent surge of physicists and mathematicians into the biological sciences, problem-solving artillery engaged. In another paper, published in Physical Review Letters, Edouard Hannezo, Jacques Prost and Jean-Francçois Joanny adopt a similarly mechanical approach to understanding the complex structures seen lining the small intestine (pictured), invoking an analogy with the buckling of metallic plates under compression (Phys. Rev. Lett. 107, 078104; 2011). They have developed a model that implicates cellular division and death as sources of internal stress, which in turn influences morphology and induces mechanical feedback on organ and tissue development.