The current

I think I have missed some very funny things these days. I would like to pile their links below and I'll come back to them as I get time.
1. Physics, 4:26 (2011), by I.I.Mazin. This is a review on the puzzles and surprised conferred by the iron-based superconductors, which shows quasi-3D structure rather than a 2D one, quite different from their cuprate counter-parts.
2. Physics, 4:25 (2011), by P.Recher et al. This is an analog of spin Hall effect. It reviews a work that shows how a line defect could be utilized to filter valley-featured carriers in graphene.
3. Science, 331: 1579 (2011), by R.-H. He et al. This is a sequel to an earlier article by these authors. They have previously argued that, two gaps of distinct origins should exist in cuprates instead of one. Here they further explore their work and show the opening of the pseudogap might be related to a phase transition.

LambdaDCM or MOND ?

Yesterday I encountered an article [http://physics.aps.org/articles/v4/23] and today I attended a public lecture, both on dark matter, which, if it exists, seems invisible to most of the forces known to us. But it does participate in gravitation and plausibly in weak interaction. Their nature is crying to be unveiled. In the article, the author explains the observed v-R curve in galaxies, which is the most direct motivation for dark matter, using the so-called MOND (Modified Newtonian Dynamics) and the dark matter is subsidiary. On the other hand, Harry Nelson, the today's lecturer, dismisses the MOND as unnecessary complications. He cited the "Bullet Cluster" [http://en.wikipedia.org/wiki/Bullet_Cluster] as evidences. All in all, I feel that, the MOND idea can hardly withstand this. The point is that, a good idea should explain not only the v-R curve but also many other relevant things, such as the "Bullet Cluster". On the other hand, what is dark matter ? This is regarded as one of the biggest questions in today's science.

Physics and Physicists: One Clear Difference Between a "Myth" and "Science"

Physics and Physicists: One Clear Difference Between a "Myth" and "Science"

Twist impacts graphene greatly

It is reported that, inter-layer rotation of a stack of graphene drastically changes the electronic properties. For large rotation, the inter-layer effects disappear.

" Stacks of graphene are expected to possess drastically different electronic properties compared to their single-layer components, as a result of interlayer coupling. Theory dictates that graphene’s massless low-energy excitations should disappear, even for the simple bilayer structure. However, experiments have shown that, paradoxically, the electronic properties of single layers somehow survive in layered structures. Theorists have suggested that a relative rotation of one layer with respect to the one below it—a “twist” in the stacking—might be the resolution of the paradox.

Now, in a paper appearing in Physical Review Letters, Adina Luican and collaborators from Rutgers University and the Massachusetts Institute of Technology, both in the US, and the University of Manchester, UK, provide experimental proof of the influence of twisting on the band structure of bilayer graphene. Using scanning tunneling microscopy and spectroscopy, the researchers find that for twist angles above 20°, the electronic properties of the twisted layers are practically indistinguishable from those of single-layer graphene. Based on their measurements, Luican et al. conclude that at small angles, the massless Dirac fermion picture of graphene breaks down and the electronic spectrum is dominated by a saddle point feature, while at large rotation angles, the intersection point in the spectrum shifts to irrelevant high energies and the familiar Dirac energy dispersion is recovered. – Alexios Klironomos " [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.126802]

How gels sediment

This experiment studies how gel particles move during the course of sedimentation.

"Depending on the kind of colloidal particles it contains, a gel will sediment in a matter of minutes or days. Understanding how shifts in the positions of the typically submicron sized particles affect the more macroscopic sedimentation process (and vice versa) could be helpful in designing industry-use gels. So far, however, no experiments have provided simultaneous access to these vastly different length scales.

Now, a group of scientists in France and Italy report in Physical Review Letters the use of light scattering to capture both the microscopic and macroscopic pictures of a gel collapsing under its own weight.

Giovanni Brambilla of the Université Montpellier, France, and colleagues filled a tall glass column with about 10 mm of a water-based gel. The sticky, colloidal particles in the gel slowly rearranged as the gel started to sediment, altering the specklelike pattern of laser light that the team scattered through a vertical slice of the gel. Over the course of ten days, Brambilla et al. captured this speckle pattern at various heights along the column and used an algorithm to extract such parameters as the particle relaxation rate, sedimentation velocity and density.

The team finds, at least in the slowly settling gels they studied, that both the microscopic and macroscopic dynamics mimic what is found in glassy polymers. Brambilla et al.’s data should thus provide a solid basis on which to test the theory of gels. – Jessica Thomas " [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.118302]

Celebrating 100 years of superconductivity

This year marks the 100th anniversary since superconductivity was discovered in Leiden, The Netherlands, by Heike Kamerlingh Onnes and co-workers on 8 April 1911. Yielding no less than seven Nobel Prizes, the study of superconductors remains more active than ever in terms of forming a fundamental understanding of their underlying mechanism, and in seeking new and novel applications that already extend to digital electronics, sensors, medicine and metrology.

In recognition of this centennial year we are pleased to present a collection of superconductivity-themed review articles published in Reports on Progress in Physics over the last 10 years. Reflecting the wide-reaching impact of superconductors across many areas of physics, each article will be free to read until the end of 2011.

Tim Smith
Senior Publisher

A One-Way Wall of Silence

In a previous entry I highlighted this work. Here is another citation of it [http://www.sciencemag.org/content/331/6022/twil.full?sa_campaign=Email/toc/3316022twil]:

The recent development of metamaterials and photonic crystals has provided a route to control the propagation of electromagnetic waves through the engineered structure of a material. Combined with transformation optics, such control is rewriting the expected rules of behavior governing the propagation of electromagnetic waves, and offers myriad possibilities ranging from imaging to communications and stealth applications. Sound is also a wave, and so the manipulation of acoustic waves may be expected to carry over by analogy to their electromagnetic counterparts. Li et al. present a sonic crystal composed of a periodic array of steel rods, the geometry of which was selected to give rise to a band gap, whereby the transmission of sound waves in a specific frequency range is prohibited in one direction but allowed in the opposite direction. The authors also show that by mechanically changing the spacing of the array (by rotating the square steel rods), the diode-like behavior can be switched on and off. A range of applications might be expected to follow, from acoustic isolation and filtering to ultrasound imaging.

Phys. Rev. Lett. 106, 84301 (2011).