Lev Landau

Landau, one of my idols, died 42years ago. He is among the most important physicists in the last century. Here finds a brief tribute to him from Ginzburg.

Landau's occasional abruptness has brought about misconceptions that have become a part (and a rather distorted part) of a legend. I, for instance, have heard people saying, "Landau thinks he is more clever than anyone else." This is completely erroneous, however, and there are many chances for one to see how soberly and modestly Landau assesses his place in science. Many years ago, his love for systematization and clarity led him to classify physicists, by way of a joke, according to the scale of a slide rule. This means that a physicist of second-class standing has achieved ten times less than a physicist of the first class. According to this scale, Albert Einstein, even better than first class, belongs to the 0.5 class, while Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Paul Dirac, Enrico Fermi, and some others belong to the first class. As for himself, Landau used to say that he was in the 2.5 class, and only ten years ago, satisfied with his work, he said he had reached the second class.

Landau's very critical attitude and his opinion that many ideas (or, to be more precise, hints at ideas) are "pathological" spring in a substantial measure from his sober nature and clarity of thinking. His criticism and his denial of certain proposals are always based on scientific grounds and thoroughly deliberated arguments. It is quite a different thing that Landau does not always like to explain his remarks: Often he answers, "Think for yourself." The fact that he is not always ready to answer and explain, however, has nothing in common with conceit or snobbery. A profoundly democratic individual, he is alien to pomposity and respect for rank. Any student could discuss scientific problems with him without difficulty, with the single stipulation that held good for all: The student had to achieve the necessary level of understanding and analyze the problem instead of expecting Landau to think for him and do the things that the student could do himself.

Interview with Roger Penrose

Nature conducted an interview with Professor Roger Penrose, who recently re-captured one's imagination with his consecutive aeons referring to the universe. [http://www.nature.com/nature/journal/v468/n7327/full/4681039a.html]:

You are currently working on a book called Fashion, Faith and Fantasy. What it is about?

I rashly suggested that title for three lectures I gave at Princeton University in 2003. 'Fashion' refers mainly to string theory, which has many merits but is not believable. I don't see how you can make sense of all those extra dimensions. 'Faith' refers to quantum mechanics. It's a wonderful theory and works beautifully, but is self-inconsistent — in my view, when you make a measurement, you violate the Schrödinger equation. At some scale in the Universe, quantum mechanics will have to be replaced by a better theory.

And 'fantasy'?

That's largely directed at cosmic inflation, in which the Universe is supposed to have expanded by an enormous factor just after the Big Bang. I've always been against this — it can only work if you start off in a very special state. In my recent book Cycles of Time, I propose my own fantastical scheme that the entire history of the Universe is just one stage in a succession. What we think of as the Big Bang is not the beginning. It's the continuation of the remote future of a previous aeon.

How might we know if that is true?

The cosmic microwave background — the radiation left over from the Big Bang — would reveal evidence of events taking place in the aeon before ours, mainly encounters between supermassive black holes. When galaxies collide, their central black holes may spiral around and swallow each other up, causing an enormous burst of gravitational radiation. Such a burst from late in the previous aeon would leave its mark as circles around which the temperature is anomalously uniform. My colleague Vahe Gurzadyan sees tentative signs of them [see http://go.nature.com/Lbwiou].

The breakthrough of this year from Science

Near the end of this year, one may ask, what is the breakthrough in science this year ? Every one may have his own answer. The one from Science is the achievement of a tiny quantum vibrator that is less the width of a hair. This article reviews why it is a breakthrough:

A team of American physicists found a quicker route, as they reported in March. Instead of a beam, they fashioned a tiny diving board of aluminum nitride plated with aluminum that vibrated by getting thinner and thicker. As the doohickey hummed away at a very high frequency—a whopping 6 billion cycles per second—the “piezoelectric” material in it produced a warbling electric field that was easy to detect. Most important, through that field, the physicists managed to “couple” the mechanical device to an electronic one called a “phase qubit,” a ring of superconductor that itself has one low-energy and one high-energy quantum state.

Manipulating the qubit with microwaves, the researchers could use it to feed energy quanta into the oscillator or pull them out of it, as one might use an ATM to deposit a $20 bill to a bank account or withdraw one. First they showed that when they cooled the oscillator to a few hundredths of a degree they could get no quanta out of it. That meant it had to be in the cashed-out ground state, jiggling with only zero-point motion. The researchers then put the oscillator in a state with exactly one more quantum of energy. They even coaxed it into both states at once, so that it was literally moving two different amounts simultaneously.

The ingenuity in this scheme lay in the design of the oscillator and the use of a qubit to control it. In fact, in 2009, the team used a phase qubit to feed quanta into a long strip of superconducting metal that would ring with microwaves much as an organ pipe rings with sounds. Once they worked the kinks out, they replaced the microwave cavity with their clever mechanical oscillator, a move that had other physicists slapping their foreheads for not having seen it coming.

Insights of the decade from Science

Now we are coming to the end of not only this year but also the first decade of this century. Science has its list of the insights of this decade in science. In materials physics, the meta-material and the related conformal optics which underlies the operation of these materials are enlisted. The ground breaking papers are as follows: [http://www.sciencemag.org/site/special/insights2010/]

• U. Leonhardt, "Optical Conformal Mapping," Science 312, 1777 (2006).
• J. Pendry et al., "Controlling Electromagnetic Fields," Science 312, 1780 (2006).
• D. Schurig et al., "Metamaterial Electromagnetic Cloak at Microwave Frequencies," Science 314, 977 (2006).
• T. Ergin et al., "Three-Dimensional Invisibility Cloak at Optical Wavelengths," Science 328, 337 (2010).

No evidence for the time before Big Bang

Several years ago, Roger Penrose proposed a different cosmological model, which allows the universe to recycle. His considerations are mainly based on two things: (1) The second law of thermodynamics, which states that, the entropy of the universe as an isolated system must increase with time; (2)The conformal invariance, which states that the metric can be determined up to a scale. Point (1) is strong, but (2) is weak. Recently, Roger and his collaborators claimed evidence for their model. This claimed evidence comes from concentric circular belts discernible on the CMB. However, this claim soon intrigues lots of rebuttals. A good review can be found here and here .

The proton size

In a previous entry, I posted a report on the measurement of the proton size that is based on muons. It gives a smaller size than had been accepted. There came a new measurement, which, nonetheless, disagrees with this smaller proton saying. This new one is based on electrons. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.105.242001]

The charge radius of the proton is one of nature’s fundamental parameters. Its currently accepted CODATA (Committee on Data for Science and Technology) value, 0.8768×10^-15 m, has been determined primarily by measurements of the hydrogen Lamb shift and, to lesser accuracy, by electron-proton scattering experiments. This value has recently been called into question by a research team at the Paul Scherrer Institut (PSI) in Villigen, Switzerland. By measuring the Lamb shift in muonic hydrogen, these researchers obtained a value of 0.8418×10^-15 m for the charge radius, five standard deviations below the CODATA value.

In a paper appearing in Physical Review Letters, the A1 Collaboration has determined the electric and magnetic form factors of the proton with higher statistics and precision than previously known, using the Mainz (Germany) electron accelerator MAMI (Mainz Microtron) to measure the electron-proton elastic-scattering cross section. Both form factors show structure at Q²≈ m_π² that may indicate the influence of the proton’s pion cloud. But, in addition, the collaboration’s extracted value for the charge radius agrees completely with the CODATA value. The discrepancy between “electron-based” measurements and the recent PSI “muon-based” measurement thus remains a puzzle. – Jerome Malenfant

Single band or double band ?

It is frequently disputed in the field of cuprate superconductors that, the essential physics may be encapsulated more adequately by a single (let's call it d) or double (s-d) band model. Fairly speaking, throughout the whole span of this arena, from the early days till now, as initialized by Anderson and pushed by Zhang and Rice et al., the d-model has been at the center and unchallenged. This situation was further corroborated by the thinking that, the s-d model can be mapped onto the d-model rigorously [see Ref.1 for a review]. Nevertheless, this situation ha to change for two reasons: (1) the d-model can not address the intervening spin glass phase; (2) the d-model can hardly explain the checkerboard pattern observed at low doping.

Why cant the spin glass phase exist within the d-model ? Suppose one has a half-filled single-band Hubbard model. Now add an extra electron to it. What can this electron do ? It shall try to hop from one site to another. Due to Pauli's principle, to render this hopping, the electrons on the two sites must have their spins aligned parallel. This means that, this extra electron tends to align the already existing electrons. On the other hand, the already existing electrons also try to hop and require their neighbors anti-parallel. And these two effects cancel exactly, because the two electrons on the same site have exactly the same hopping amplitude.

Obviously, in the s-d model, the effects don't cancel. This is where the difference gets in.

[1]SPIN POLARONS AND HIGH-Tc SUPERCONDUCTIVITY, A. L. Chernyshev†,* and R. F. Wood Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831;

Magnetic fluctuations at high energies in parent La2CuO4

The LCO is a quasi-2D anti-ferromagnetic insulator which can be doped into a superconductor. Seemingly simple, its 2D magnetic behaviors have recently displayed some new features, which were published in a PRL days ago [PRL 105, 247001 (2010)]. They used INS as the probes. They found some unusual traits with the zone corner fluctuations with momenta around (pi,0). The dispersion around this point is much broader and with long tail in comparison with, let's say, the (pi/2, pi/2) region. Moreover, significant spectral weight were disclosed over there. The authors proposed spinon as the cause of the broad peak: a magnon decays into spinons mostly at zone corners. The decay shortens its life time and increases the peak width.

Photon generates upthrust

I think this is a very funny work [Nature Photon. doi:10.1038/nphoton.2010.266 (2010)], which demonstrates that photons carry energy and momentum, just as other forms of matter. This working mechanism is quite straightforward, much the way as air supports air crafts.

Grover Swartzlander at the Rochester Institute of Technology in New York and his colleagues shone a weakly focused laser beam through the roughly semi-cylindrical rods, which refracted the light rays. This refraction changed the direction of the rays' momentum, causing an equal and opposite momentum change on the rods themselves. Because of the rods' asymmetrical shape, the momentum shift was directed more towards one side, driving the rods upwards at around 2.5 micrometres per second.
[Nature, 468:734]

quantum entanglement observable at high temperatures

Suppose there is a bulk system at thermodynamic equilibrium. Now push it away from this equilibrium states to non-equilibrium one. Due to the second law of thermodynamics, it is expected this system shall after some time relax to equilibrium eventually. This time is called the relaxation time, which serves a time scale to judge the importance of pure mechanical dynamics. Now if the latter occurs at a much shorter time than the relaxation time, it is hoped that the thermal noises will not take big role, which means the behavior is governed by the pure dynamics. This being so, a system even very high temperatures can show quantum phenomena on the short time scale. This idea was recently written in PRL [Phys. Rev. Lett. 105, 180501 (2010)]. A review is as follows [Nature, 468:769]:

The basic intuition behind this result is as follows. When a system is not in thermal equilibrium, the temperature no longer provides the relevant energy scale against which to compare the system's quantum behaviour. What matters instead is an effective temperature, which can be much lower than the absolute one. This effective temperature is obtained by multiplying the absolute temperature by the rate at which the system approaches equilibrium divided by the driving frequency, the frequency of the signal with which the system is made to oscillate. Galve and colleagues demonstrate that this new condition for entanglement — that the interaction between subsystems should be compared with the thermal energy at the effective temperature — holds quite generally and is intuitively pleasing. It says that if we can drive the system to oscillate within a shorter timescale than the time it takes to reach thermal equilibrium, then an entangled steady state can be attained at higher temperatures than the absolute one.

The Big Bang Picture of the universe

http://en.wikipedia.org/wiki/File:Reion_diagram.jpg

Time for Scientists to underpin Wikipedia

Wikipedia, the world's largest online encyclopaedia, is regarded with suspicion by some in the scientific community — perhaps because the wiki model is inconsistent with traditional academic scholarship (Nature 468, 359–360; 2010). But the time has come for scientists to engage more actively with Wikipedia.

Type any scientific term into any search engine and it is likely that a Wikipedia article will be the first hit. Ten years ago, it would have been inconceivable that a free collaborative website, written and maintained by volunteers, would dominate the global provision of knowledge. But Wikipedia is now the first port of call for people seeking information on subjects that include scientific topics. Like it or not, other scientists and the public are using it to get an overview of your specialist area.

Wikipedia's user-friendly global reach offers an unprecedented opportunity for public engagement with science. Scientists who receive public or charitable funding should therefore seize the opportunity to make sure that Wikipedia articles are understandable, scientifically accurate, well sourced and up-to-date.

Many in the scientific community will admit to using Wikipedia occasionally, yet few have contributed content. For society's sake, scientists must overcome their reluctance to embrace this resource. [Nature, 468:765]

Tinkham passed away

Tinkham is famous for his work and his book in superconductivity. He passed away last month. Here is an obituary from Nature: [Nature: 468, 766]

This spectacular role in the early history of the BCS theory behind him, Tinkham continued to work with far-infrared spectroscopy but also began to study the macroscopic quantum behaviour of superconductors. Quantum mechanics is normally thought of as important only in the microscopic world of atoms, but in superconductors it manifests itself in very large objects, such as in the superconducting magnets used in magnetic resonance imaging. After Tinkham took up a professorship in 1966 at Harvard University in Cambridge, Massachusetts, one question emerged that remained of central interest to him: what is the nature and origin of resistance in a superconductor? Or put more simply, when is a superconductor really a superconductor?

As it turns out, when superconductors are carrying a current, they don't stay in a fixed macroscopic quantum state, but cascade down from one energy level to another. As energy is lost with each transition, this is equivalent to saying that superconductors have resistance, although it is extremely small under most conditions. In the latter stages of his career, Tinkham was examining the conditions under which these transitions happen and how they happen in very thin wires of a superconductor.

Despite all these achievements, being elected to the US National Academy of Sciences and winning the prestigious Oliver E. Buckley prize of the American Physical Society, Mike was a modest man with an exceptional sense of humour. The same legions of students who witnessed his alchemy with data will remember how they first knocked nervously on his door to be greeted by a somewhat gruff “Come in”, only to learn that he was a very warm and witty mentor.

Invisible gateway

Manipulating light with meta-materials is under intensive study recently. In a work published in PRL, the researchers realized an invisible gateway, which is an open channel but appears not there for certain frequency light. The produces optical illusions. [Phys. Rev. Lett. 105, 233906 (Published December 2, 2010)]

In 2009, a team of researchers led by Che-Ting Chan at the Hong Kong University of Science and Technology theorized on using transformation optics and complementary media to produce optical illusion devices that change the optical response of an object into that of another object. Illusion optics, the science of making an object appear as something else, or reappear elsewhere in space, or even disappear altogether (cloaking) is full of exciting possibilities, pending experimental realization.

In a paper in Physical Review Letters, Chao Li and co-workers at the Chinese Academy of Sciences, Beijing, and colleagues at Soochow University, China, and Hong Kong University of Science and Technology, experimentally demonstrate the first illusion-optics device. They trick light to miss an open channel across a slab at a frequency range of interest, rendering the channel into an electromagnetically invisible gateway. Li et al.’s design involves carving out an open channel across a metamaterial slab that behaves as a perfect electric conductor, then replacing a trapezoidal region of the slab adjacent to the channel with another metamaterial having the exact opposite dielectric properties. This “double-negative” region complements the dielectric space inside the channel into an optically equivalent region that behaves as a perfect electric conductor, thereby giving the appearance of a blocked gateway to light that attempts to go through.

Li et al. use a transmission-line approach that allows them to design metamaterials with the desired optical properties and with minimal losses. Their illusion-optics prototype works at around ~50 MHz and has a ~15 MHz bandwidth. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.105.233906]

Chandra, a great physcist

Mr. Chandra is a groundbreaking physicist, who contributed significantly in various aspects in physics. He was born in 1915 and passed away in 1995. Physics Today in this edit has four articles talking about his life and legacy:

(1)http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_63/iss_12/38_1.shtml
(2)http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_63/iss_12/44_1.shtml
(3)http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_63/iss_12/49_1.shtml
(4) http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_63/iss_12/57_1.shtml

The last one is written by himself.

A perspective on high Tc superconductors

The CuO2 plane is supposed to underlie the basic physics of cuprate superconductors. Such a plane is made of three square Braviss lattices, two of which are occupied by O ions and one by Cu ions. In a work published last year, both the O- (Zhang-Rice orbital) and Cu-sub-lattice contribute one orbital per site. The inter-electron correlations are negligible on O ions but essential on Cu ions. The correlation is signified by a Hubbard repulsion on Cu sites. The low energy sector thus consists of one orbital from O and one from the lower-band of Cu, per site. What results is a theory that might be able to explain a heap of perplexing phenomena discovered recently. I would say, in many aspects this model resembles the single-band Hubbard model. The difference, which is critical in producing the p-n asymmetry, is that, this model is two-band.

In the latest Nature Physics, a perspective comes up highlighting some aspects of the single-band model. Particularly, the authors focus on two things: (1) the strange metal phase with linear T-dependence resistivity and (2) the spectral weight transfer that suggests electron correlations. By analyzing relevant experiments, they advocate composite, rather than fractionalized, excitations, which they call doublons. These doublons constitute the freedom degrees from the upper Hubbard band. Again, such model ignores the p-n asymmetry, which is, I think, a key to the understanding of cuprate SC physics. [nature physics | VOL 6 | DECEMBER 2010 | www.nature.com/naturephysics]

Topological insulators reviewed

In Review Of Modern Physics this month, an article appears to cover the fundamentals and frontiers in the research in topological insulators, to which more information can be found in earlier blog entries. [REVIEWS OF MODERN PHYSICS, VOLUME 82, OCTOBER–DECEMBER 2010]

Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducting states on their edge or surface. These states are possible due to the combination of spin-orbit interactions and time-reversal symmetry. The two-dimensional 2D topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum
Hall state. A three-dimensional 3D topological insulator supports novel spin-polarized 2D Dirac fermions on its surface. In this Colloquium the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topological insulators have been observed. Transport experiments on HgTe/CdTe quantum wells are described that demonstrate the existence of the edge states predicted for the quantum spin Hall
insulator. Experiments on Bi1−xSbx, Bi2Se3, Bi2Te3, and Sb2Te3 are then discussed that establish these materials as 3D topological insulators and directly probe the topology of their surface states. Exotic states are described that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana
fermions and may provide a new venue for realizing proposals for topological quantum computation. Prospects for observing these exotic states are also discussed, as well as other potential device applications of topological insulators.

Polar thin film is not ferroelectric

Epitaxial thin film of strontium titanate on a silicon substrate is now demonstrated non-ferroelectric, despite its spontaneous polarization [PRL 105, 217601 (2010)]. This polarization is not due to spontaneous symmetry breaking, rather it is a property of the interface ground state. The symmetry is absent from the outset in the presence of the substrate, which strains the film. Therefore, the polarization is not switchable. This finding is made by DFT computations and STEM techniques.

We use SrTiO3=Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed
(pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.

Life is physics

For whoever are interested in biophysics, this review article should be useful.
http://arxiv.org/PS_cache/arxiv/pdf/1011/1011.4125v1.pdf

Uncertainty and Nonlocality

The uncertainty principle states that, two non-commutable physical quantities cannot be measured
with perfect accuracy at the same time. The non-locality concept means that, two subsystems can be strongly correlated even if they are distant. It is reasonable to think that, these two notions may be connected. This is the case, as demonstrated in this report [Science 330, 1072 (2010)]:

Two central concepts of quantum mechanics are Heisenberg’s uncertainty principle and a subtle form of nonlocality that Einstein famously called “spooky action at a distance.” These two fundamental features have thus far been distinct concepts. We show that they are inextricably and quantitatively linked: Quantum mechanics cannot be more nonlocal with measurements that respect the uncertainty principle. In fact, the link between uncertainty and nonlocality holds for all physical
theories. More specifically, the degree of nonlocality of any theory is determined by two factors: the strength of the uncertainty principle and the strength of a property called “steering,” which determines which states can be prepared at one location given a measurement at another.

Supersolidity ?

The so-called supersolidity has fascinated experimentalists for several years. Suppose one cools down an amount of Helium4 under huge pressure to solidify it. Some dizzying phenomena might happen: the as-solidified crystal shows a considerable decrease in friction. A a group from Japan and Korea did recently an experiment that supports the existence of supersolidity [DOI: 10.1126/science.330.6007.1033-a ].

Now, Kim, Kimitoshi Kono of Japan's research institute RIKEN in Wako, and colleagues have performed a torsional oscillator experiment in a specialized refrigerator, or “cryostat,” in which they can spin the whole experiment at speeds up to a revolution per second. Working in Kono's lab, they found that as the rate of rotation increased, the shift in the frequency that supposedly tracks the resistance-free flow decreased and eventually vanished.

That's what should happen if the flow is real. Thanks to quantum mechanics, a superfluid abhors rotation. Spin a bucket of superfluid liquid helium, and the liquid will sprout tiny whirlpools called “vortices” spinning to counteract the rotation. Put a torsional oscillator in a spinning fridge, and vortices will tie up the superfluid, leaving less to stand still and reducing the frequency shift seen as superfluid flow sets in. Kim's result suggests that rotation stirs up vortices in solid helium, too, says Sébastien Balibar, a physicist at the École Normale Supérieure in Paris.

Wikipedia goes to Grad

Science, Volume 330, Number 6006, Issue of 12 November 2010:

Education:

Wikipedia Goes to Grad School

Melissa McCartney

Very few graduate-level science curricula include training in communicating advanced concepts to a general audience. Moy et al. report a class project that addressed this by having chemistry students edit an entry in Wikipedia.org collaboratively. Students selected topics that were related to the course and were minimally covered on Wikipedia. Student entries contained references, an introduction aimed at the general public, and figures to enhance the explanation of the topic. Student feedback collected at the end of the project revealed increased knowledge of their topic. A specialist in writing and rhetoric concluded that the students' entries were more engaging to general readers because of the attention to real-world applications and clear explanations of vocabulary. Course professors noted that students appeared to assess the material they added to the entry more critically than when they were simply studying for the class, which is consistent with the notion of students' developing a higher level of explanatory knowledge when teaching the material is a goal.

J. Chem. Educ. 87, 1159 (2010).

The physics in skateboarding

Here is a video that talks about how to improve skateboarding tricks by the help of simple physics,
especially the so-called "Ollie":
http://www.sciencedaily.com/videos/2007/0701-science_of_skateboarding.htm

Toward engineering the color of metals by carving rings on the surface

The electrons in the metal are nimble and mobile and able to conduct electricity. The behaviors are controlled by two things: the band structure and the coulomb interactions. The elementary excitations of this sea of electrons may not be simply fermionic quasi-particles that partially resemble the original electrons. They can also be bosonic, for example, plasma. Such plasmons are hardly excitable by low energy probes, such as visible light. But they can indeed be created by X-ray. What controls the colors are basically visible light. To understand the colors of a particular metal, one needs know how the visible light interacts with which kind of elementary excitations of the similar energy scales. To describe this interaction, one may assume quantum mechanics, but the usual Maxwell equations will suffice, because the visible light has a wave length between 400nm to 760nm, which are indeed very long in comparison with the metallic band gaps of the order of nm (and hence, only the single partially filled band needs be considered). Basically, one has to treat an entangled system of light and electrons, the exact solution of which is a considerable problem. Usually, one treats the metal as a medium that is characterized by a complex dielectric function of frequency. This function determines which photon will be absorbed and which can be transmitted and which will be reflected. The reflected light decides the color. Most naturally occurring metals bear silver color. This is because, the spectrum encoded in the imaginary part of the dielectric function is a continuum in the visible light energy window, rather than a discrete set of resonances. Is it possible to tune the color of a metal without affecting its conductivity? The answer is yes. Due to the complex part of the dielectric function, visible light can hardly enter the bulk metal and can penetrate only a very thin layer near the surface, an effect called "skinning effect". Thus, the colors are actually controlled by the skin. By manipulating the surface electron spectrum, one should be able to tune the color. This has been achieved in a latest work by Jianfa Zhang at the University of Southampton and a few pals [arXiv:1011.1977v1 ]. See a review from Arxiv Blog [http://www.technologyreview.com/blog/arxiv/]:

Their idea is to carve a different type of repeating pattern on to the surface of a metal.

These patterns are smaller than the wavelength of visible light. Instead of causing the light to interfere, they work by changing the properties of the sea of electrons in the metal--in particular its resonant frequency. This alters the frequency of light it absorbs and reflects.

This is the same technique that researchers have been using for some time to build invisibility cloaks . The idea is that by carefully building repeating patterns of subwavelength structures, researchers can tailor the way a "metamaterial" can steer light.

But instead of creating 3D structures that steer light as it passes through the material, Zhang and co carve the relevant structures onto the surface to control the way light is absorbed and reflected.

The structures that do the trick are tiny rings carved into the surface. The team calculate that they can make gold or aluminium appear almost any colour simply by varying the size and depth of these rings. They've even demonstrated the technique on a thin layer of gold.

The cosmic history

How to retrieve the early universe ?
This is a review article published in Nature:
http://www.nature.com/nature/journal/v468/n7320/pdf/nature09527.pdf

Star-forming galaxies trace cosmic history. Recent observational progress with the NASA Hubble Space Telescope has led to the discovery and study of the earliest known galaxies, which correspond to a period when the Universe was only 800 million years old. Intense ultraviolet radiation from these early galaxies probably induced a major event in cosmic history: the reionization of intergalactic hydrogen.

How charge is renormalized by gravity

Electrical charge (the bare one), g, measures the coupling strength between electrons and photons. In QED, g is a constant. However, if interactions of QED fields with other fields (particles) are taken into account, the g shall be renormalized in the sense of renormalization group theory. In this article [doi:10.1038/nature09506], the author looks at how gravitational field renormalizes the g. In his treatment, there assumes a cutoff, below which the Einstein's theory is a reasonable starting point for quantization. Going through the usual RG procedures, he arrives at the statement that, gravity results in QED asymptotic freedom at high energy scales: g tends to zero at very large energy.

The first term on the right hand side of equation (12) is that present in the absence of gravity (found by letting kR0) and results in the electric charge increasing with energy. The second termis the correction due to quantum gravity. For pure gravity with L50, or for a small value of L as suggested by present observational evidence40, the quantum gravity contribution to the renormalization group b-function is negative and therefore tends to result in asymptotic freedom, in agreement with the
original calculation13.

The Coulomb Interactions In the Graphene as measured in Graphite

As a 2D Dirac physics simulators, graphene harbors very efficiently mobile electrons and may find wide applications in electronics and other arena. Most experiments detect these electrons as if they were free and independent. Nevertheless, a simple estimation [1] suggests that, the ratio of U, the electrostatic energy to K, the kinetic energy, is about 2.2, which is very large. So, why has it been unseen yet ? The reason is ascribed to screening or say shielding effects. Such effects are very strong for nimble electrons, which is true for graphene. On the other hand, the shielding should not be on all scales. In fact, a simple Yukawa potential modeling this shielding suggests that, such effects becomes pronounced only for distances beyond a critical value. Inside this value, screening can be neglected and strong repulsions should reveal itself. Put in math, the shielding function depends on energy and momentum scales that are looked at. Now these authors [2] did nice experiments and confirmed this saying. They measured the shielding in graphite, which consists of loosely layered graphene.

Figure Caption: The effective, screened fine-structure constant, , as defined in the text. (A) The magnitude of , plotted against momentum and energy. The Dirac dispersion is indicated by the white line. In the low momentum region, is larger above this line than below. (B) The phase of , in radians. [2]

[1] The estimation is done as ;
[2] DOI: 10.1126/science.1190920

The Theoretical Group As Founded

It is a pleasure to announce that, with some friends I have founded a theoretical group of physics in Hong Kong. This is a very small one, resembling the Olympia Academy and intended for very motivated young peers to communicate their scientific activities. The meeting is on every Tuesday and informal. No money is needed to run this. All are just like minds. Basically, we

(1) Invite peers to present their latest studies or something they find stunning and then discuss the topics;
(2) Learn some new topics through a presentation by one of the participants.

I must say, the presentations are really very theoretical and contain many difficult math. So, we are indeed serious in doing this.

Still Quiet is Dark Matter

Cosmological observations suggest the existence of dark matter, which has not shown any traces of interacting with known baryonic matter. Yet, dark matter comprises over 80% of the total matter needed to explain the space-time structure. Scientists have not a clue regarding the nature of these matter. One proposal says they may be made of sort of particles, the so-called WIMPs (weakly interacting massive particles). Various experiments have been devised to detect them. NO positive results exist up to now. A latest effort came in PRL, still no activities of these particles detected. They are really quiet, should they be there. [Phys. Rev. Lett. 105, 131302 (2010)]

The XENON100 experiment, in operation at the Laboratori Nazionali del Gran Sasso in Italy, is designed to search for dark matter weakly interacting massive particles (WIMPs) scattering off 62 kg of liquid xenon in an ultralow background dual-phase time projection chamber. In this Letter, we present first dark matter results from the analysis of 11.17 live days of nonblind data, acquired in October and
November 2009. In the selected fiducial target of 40 kg, and within the predefined signal region, we observe no events and hence exclude spin-independent WIMP-nucleon elastic scattering cross sections above 3:4 10 44 cm2 for 55 GeV=c2 WIMPs at 90% confidence level. Below 20 GeV=c2, this result
constrains the interpretation of the CoGeNT and DAMA signals as being due to spin-independent, elastic, light mass WIMP interactions.

The Compositions of Neutron stars

Does a neutron star comprise primarily of neutrons and protons or there are some other particles ? Both options have been used to construct models to describe the behaviors of neutron stars. A great difference between these two options is that, they yield different maximum star masses. For a star of largely protons and neutrons, the mass can be larger, because including other matter will soften the star in response to gravitational field. Recently, a group studied a pulsar, which is a neutron star and has a companion [doi:10.1038/4671057a]. This group measured the so-called Shapiro delay and has determined with high precision the masses of both the pulsar and its companion. The as-measured mass is 1.97+/-0.04 times the solar mass. Such a massive star can hardly be harbored by models containing matter other than protons and neutrons [Lattimer, J. M. & Prakash, M. Nucl. Phys. A 777, 479–496 (2006). ].

The Shapiro delay is caused by the gravitation of the companion: the spinning pulsar emits pulses regularly and this pulse passes by the companion on the journey to the earth, and the companion distorts the space-time nearby and makes a time delay. This delay is expected periodic, since the pulsar is moving around the companion. This enables the determination of the masses.

Visualizing the edge states in graphene

(1) Band bending and the associated spatially inhomogeneous population of Landau levels play a central role in the physics of the quantum Hall effect (QHE) by constraining the pathways for charge-carrier transport and scattering¹. Recent progress in understanding such effects in low-dimensional carrier gases in conventional semiconductors has been achieved by real-space mapping using local probes^{2, 3}. Here, we use spatially resolved photocurrent measurements in the QHE regime to study the correlation between the distribution of Landau levels and the macroscopic transport characteristics in graphene. Spatial maps show that the net photocurrent is determined by hot carriers transported to the periphery of the graphene channel, where QHE edge states provide efficient pathways for their extraction to the contacts. The photocurrent is sensitive to the local filling factor, which allows us to reconstruct the local charge density in the entire conducting channel of a graphene device. [doi:10.1038/nphys1745]

(2) Spintronics, where the spin of electrons is used to carry information, is a rapidly growing area of research^{1, 2}. There are several techniques for generating pure spin currents^{3, 4, 5, 6, 7, 8, 9, 10}; however, there is no method that can directly detect them, largely because they carry no net charge current and no net magnetization. At present, studies of pure spin currents rely on measuring the induced spin accumulation with either optical techniques^{5, 11, 12, 13} or spin-valve configurations^{14, 15, 16, 17}. However, spin accumulation does not directly reflect the spatial distribution or temporal dynamics of the pure spin current, and therefore does not give a real-time or real-space measurement. Here we demonstrate a second-order nonlinear optical effect of the pure spin current that has never been explored before, and show that it can be used for the non-invasive, non-destructive and real-time imaging of pure spin currents. The detection scheme can be applied in a wide range of materials with different electronic band structures because it does not rely on optical resonances. Furthermore, the control of nonlinear optical properties of materials with pure spin currents may have potential applications in photonics integrated with spintronics. [doi:10.1038/nphys1742]

Loss of quasi-particle weight upon doping in cuprates

[doi:10.1038/nphys1763]
a, YBCO6.34 nodal dispersion and MDCs at E_F (±15 meV integration, shaded region), for light polarization parallel to Γ–S. b, YBCO7 MDCs for polarization along Γ–Y (note the strong polarization dependence). c, Evolution of k_F,N^B (down triangles) and k_F,N^AB (up triangles); below p≃0.15 the B–AB splitting vanishes and only one single k_F,N is detected (diamonds). d, Z_N as determined from the B–AB splitting with and the rescaled low-energy spectral-weight ratio . Also shown are spline guides-to-the-eye and the 2p/(p+1) relation (dashed red line). For the splitting-derived data, error bars are determined from the B–AB MDC fits when splitting is detected, and from the experimental resolutions otherwise; for the spectral weight ratio (SWR), they are calculated from the spread in SWR values for integration windows smaller than

Pinwheel magnetic structure

Solid black lines are magnetic exchange interactions with three different strengths. The ellipses show the main spin correlations of the pinwheel valence-bond solid state found by Matan and co-workers in Rb₂Cu₃SnF₁₂. Spin singlets form between spin pairs linked by the dominant exchange interactions.


[Nature Physics Volume:6 ,Pages:837–838 Year published: 2010]

Zhang-Rice singlets fall apart

A quarter of century has already passed since the discovery of cuprate superconductors. The theoretical understanding has been a central problem in condensed matter physics. Perhaps the most frequently utilized model to model their behaviors is based on the so-called Zhang-Rice singlets. Such singlet is spin less and consists of an O hole and a Cu hole. Although, this picture has been prevailing in literature over so many years, loopholes gradually show up in both experimental and theoretical studies (please see previous blog entries). Here I mention another study that reveals this fallacy. It was published last year. [PRL 103, 087402 (2009)]

X-ray absorption spectra on the overdoped high-temperature superconductors Tl2Ba2CuO6þ and La2 xSrxCuO4 reveal a striking departure in the electronic structure from that of the underdopedregime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygenK edge, while the lowest-energy prepeak gains less intensity than expected above p 0:21. This suggests a breakdown of the Zhang-Rice singlet approximation and a loss of correlation effects or a significant shift in the most fundamental parameters of the system, rendering single-band Hubbard models inapplicable. Such fundamental changes suggest that the overdoped regime may offer a distinct route to understanding in the cuprates.

E-index to measure individual's impacts

h-index is the usual measure of a scientist's impact. However, it produces unrealistic results in many cases. For example, the h-index of Einstein turns out to be 27, much lower than Edward Witten's 125. This can hardly be acceptable. So, this man came up with a new method of counting citations, the quantity of which he calls E-index.
http://ptonline.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTOAD000063000011000012000001&idtype=cvips

The breakdown of Born-Oppenheimer approximation

In dealing with electrons attached to nuclei, the Born-Oppenheimer approximation is often invoked, in which the motions of the nuclei are treated adiabatically relative to that of electrons. However, it will breakdown if the nuclei is light and the electron-nuclei coupling is very strong. In this case, the evolution of such electron-nuclei systems become coherent and entangled. A direct experimental demonstration of this breakdown was recently attained [Nature Physics, 1802(2010)].

Cloud Chamber

A cloud chamber (CC) is used to detect tiny radiation particles. It was invented by Wilson and won him a Nobel Prize. The operating principle is simple: soak a chamber with alcohol and then seal it, and cool it down. The supercooled air shall enter a metastable state but the alcohol evaporation get ready to condense, which can be ignited by a small perturbation (nucleation centers). When a particle passes through this chamber, liquid drops shall track it and make it detectable.

This is a Video demonstrating how to make a simple cloud chamber:
http://education.jlab.org/frost/cloud_chamber.html

Note: a metastable state is a state that is stable but not robust against any perturbations.

Inventing new physical systems

Particle physicists can hardly invent new physical systems but only to discover the already existing matter and energy world. Condensed matter physicists tend to create their own systems to meet fundamental intelligent challenges and practical ends. The as-created new physical systems are surely immense and the underlying physics are also diverse, but the mathematical models seem not so diverse: it is frequently the case that the model constructed for one system may be transplanted to describe another system, with proper interpretation of the symbols. In other words, there exists some kind of universality. This scenario offers opportunities for both theorists and experimentalists: (1) the theorists can make predictions about system A via the knowledge of system B if A and B are found sharing the same mathematical structure; while (2) the experimentalists can simulate system A by measuring system B. Such possibility drives the emergence of a host of artificial systems. The following is a list:
(1) p-n junction and transistors;
(2) 2DEG;
(3) Optical lattice and Ultra-cold atoms;
(4) Photonic crystals;
(5) Metamaterials;
(6) Circuit QED;
(7) Cavity QED;
(8) Trapped ions;
(9) Graphene, and CNTs;
(10) Topological insulators;
(11) ...

Edison and his carbon filament bulbs

Harnessing electricity for light had been an ambition over a century ago. Edison had caught that rush for incandescent bulbs. A lasting incandescent bulb must have a filament that won't be burnt quickly and thus bear a long life. To have this, the filament has to be protected from chemical reactions that damage its property. At that time, vacuum is the choice. Edison first figured out the way to pump air and then looked for the desired filament and eventually found one, the bamboo ! Interestingly, it is also the same bamboo under the same arc that led Ijishima to his carbon nanotubes !
[http://www.wired.com/thisdayintech/2009/10/1021edison-light-bulb/]

Edison’s lab put a lot of effort into making a bulb with a platinum filament, but that work went nowhere, because platinum has a relatively low resistance. But gas bubbles in the platinum had led Edison to develop an efficient vacuum pump to remove the air from the inside of his bulbs. And that created a new opportunity: carbon.

Carbon conducts electricity, has a high resistance and can be shaped into thin filaments. And it’s cheap. But it burns easily — unless there’s no oxygen around. The vacuum bulbs Edison had created for platinum were ideal for carbon.

Edison pushed hard on his research assistants, whom he more or less affectionately called “muckers.” After testing hundreds of materials, they baked a piece of coiled cotton thread until it was all carbon. Inside a near-vacuum bulb, it stayed alight for more than half a day. The “three or four month” project had taken 14 months.

Soon, the lab got a carbon-filament bulb to last 40 hours. It had cost $40,000 (about $850,000 in today’s money) and taken 1,200 experiments, but was ready at last for a public debut.

comic physics

Don't let yourself miss this funny stuff !

Thermofluidic effects in nanochannels

When a fluid is subject to a temperature gradient, a velocity field can be generated, a phenomenon known as convection. It should be noticed that, such phenomena parallel what happens to electrons in a metal: temperature gradient drives electrical current under the name of thermoelectric effect. Now as things can be made smaller and smaller, it becomes an interesting subject to investigate the so-called micro- or nano-fluidic flow: liquid flow through a micro-size or nano-size channel. In this study by researchers from Hong Kong [PRL 105, 174501 (2010)], they used molecular dynamics simulations to examine a nano fluid housed in a nano-channel with particularly designed walls: the wall consists of two parts, the left and the right one, with respective surface energies, and a temperature gradient is held symmetric with respect to the border between the left and right wall. Their study showed that, an asymmetric flow can be generated with this temperature gradient provided the variance of surface energies is big enough. This is funny and many possibilities can be imagined to broaden their studies.

Quantum grativity in its present status

This is a nice article that was originally used as lecture notes in the Stockholm workshop. It gives a concise account of the present understanding of quantum gravity and and the arguments go at the heart. It also maps the planing for experimental search. In addition, rich references can be found.
[http://arxiv.org/PS_cache/arxiv/pdf/1010/1010.3420v1.pdf]

Curved space generating mass ?

Since Newtonian times, physicists have to talk of mass, a quantity having its origin a deep myth. In Newtonian mechanics, mass is impedes the change of velocity. In relativity, mass (in the conventional sense) is no more than the static energy. In non-relativistic quantum mechanics, mass plays to make the entity more of a particle. In relativistic quantum mechanics, mass is the energy required to generate a pair of electron and positron. In condensed matter physics, mass is the minimum energy to excite a system. Besides, mass also measures some correlation length.

Although we know many things about mass, we don't have a clear clue where mass comes from. In the standard model, all masses are produced by Higss mechanism: every mass-less particle moves in some kind of ether that is the Higgs clouds and acquires mass. Another idea is, mass can be generated by curved space, or more accurately, compactified dimension. Compactifying a dimension yields finite motion, one that is confined. According to quantum mechanics, finite motion implies discrete levels and finite gaps, so comes the mass. Yet, a clear regime is missing.

Graphene provides a playground for studying this regime. These authors roll the graphene and obtain a massive 1D system from a 2D massless Dirac system [http://arxiv.org/ftp/arxiv/papers/1010/1010.3437.pdf]. This is no surprising and actually was known before. But this is an example showing how mass might be generated this way. However, back to elementary particle physics, where is the hidden dimension in addition to the 4D space-time we are all used to ? Another question is, how the as-obtained masses interact in a gravitational way ? Anyway, mass should be gravitationally active !!!!

Twitter predicts Stock Market ?

This study is really astounding. The researchers from Indiana University found likely correlations between the calmness index that can be tallied by twitter data and the price walk of stock market [http://www.technologyreview.com/blog/arxiv/25900/].

Today, Johan Bollen at Indiana University and a couple of pals say they've found just such a predictor buried in the seemingly mindless stream of words that emanates from the Twitterverse.

For some time now, researchers have attempted to extract useful information from this firehose. One idea is that the stream of thought is representative of the mental state of humankind at any instant. Various groups have devised algorithms to analyse this datastream hoping to use it to take the temperature of various human states.

One algorithm, called the Google-Profile of Mood States (GPOMS), records the level of six states: happiness, kindness, alertness, sureness, vitality and calmness.

The question that Bollen and co ask is whether any of these states correlates with stock market prices. After all, they say, it is not entirely beyond credence that the rise and fall of stock market prices is influenced by the public mood.

So these guys took 9.7 million tweets posted by 2.7 million tweeters between March and December 2008 and looked for correlations between the GPOMS indices and whether Dow Jones Industrial Average rose of fell each day.

Their extraordinary conclusion is that there really is a correlation between the Dow Jones Industrial Average and one of the GPOMS indices--calmness.

In fact, the calmness index appears to be a good predictor of whether the Dow Jones Industrial Average goes up or down between 2 and 6 days later. "We find an accuracy of 87.6% in predicting the daily up and down changes in the closing values of the Dow Jones Industrial Average," say Bollen and co

That's an incredible result--that a Twitter mood can predict the stock market--but the figures appear to point that way.

Is it really possible that the calmness index is correlated with the stock market? Maybe. Back in April we looked at some work showing how tweets about films can be used to predict box office takings.

But there are at least two good reasons to suspect that this result may not be all it seems. The first is the lack of plausible mechanism: how could the Twitter mood measured by the calmness index actually affect the Dow Jones Industrial Average up to six days later? Nobody knows.

The second is that the Twitter feeds Bollen and co used were not just from the US but from around the globe. Although it's probably a fair assumption that a good proportion of these tweeters were based in the US in 2008, there's no way of knowing what proportion. By this reckoning, tweeters in Timbuktu somehow help predict the Dow Jones Industrial Average.

If so, what might happen to the way people play with stocks ?

Wet dog shaking

This piece of study is something that should never be missed. It is perhaps one of the best examples exemplifying what science is all about: Just being curious and trying to find out how things actually happen. This work looks at how fast a wet dog wriggles its body to shake off the water sticking to its fur. These authors from Geogia Insttitute of Technology set up a model and compare the results out of this model to reality. They photographyed a range of animals and figure out the wriggling frequencies. Their model predicted the frequency should be proportional to the square root of the belly radius of the animals, close to the observations that yield an exponent of 0.75 rather than 0.5. [http://www.wired.co.uk/news/archive/2010-10/20/physicists-find-perfect-speed-for-wet-dogs-to-shake-at] In this link, there is a video that sumarizes their interesting and provoking work.

Physics assailing cancers

This must be a very useful resource compilation on the stories of attacking problems of cells by physics. [http://www.nature.com/nphys/journal/v6/n10/pdf/nphys-insight-physics-cell.pdf]

Principles and methods from the physical sciences have long been applied to questions in biology; however, the application of such principles to the study of cancer biology has only begun to flourish. In the latter part of the 20th century, and especially the last decade, advanced technologies have fueled an unprecedented period of discovery and progress in the molecular
sciences that promises to revolutionize cancer medicine. In 1999, the National Institutes of Health Director, Harold Varmus highlighted this point in his speech at the Centennial Meeting of the American Physical Society by stating, “Biology is rapidly becoming a science that demands more intense mathematical and physical analysis than biologists have been accustomed to, and such analysis will be required to understand the workings of cells.” This issue of Nature Physics Insight – Physics and the Cell reviews a number of areas in which physical scientists are tackling biological problems relating to cells and their interaction with their surroundings.

Failed theories of SC

In one of my previous entries, I mentioned the failed theories that were briefly reviewed in a preprint. Here in Nature Physics [Nature Physics, 6: 715, 2010], another one based on this review came out. I especially like this story about Laudau:

Yet it turns out that Landau first proposed these ideas in the context of superconductivity, thinking not of magnetization but of electrical current. He expanded the free energy F around the state of zero current, j = 0, and argued that as the direction of the current shouldn't affect F, the odd terms should vanish. This gives an equation of the form F(j) = F(0) + aj² + bj⁴. Assuming b > 0 and that a passes through zero at a critical temperature T_c, he showed that there could be an abrupt transition from zero to non-zero current below T_c.

This early theory conflicted with observations — it erroneously predicted j ~ (T_c − T)^1/2 just below the critical temperature — and Landau went back to the drawing board. Yet here already were the seeds of the later Ginzburg–Landau theory of phase transitions. And Landau's introduction of the notion of an 'order parameter' as a convenient handle on order and how it changes has influenced physics ever since, even if it did appear in a failed theory.

This idea is crazy: when one expands free energy in current, one has in his mind that he is dealing with an equilibrium state. However, current usually exists in a non-equilibrium state. This gives a glimpse of the aberration of superconductivity !

A circuit that beats Jaynes–Cummings model

In circuit quantum electrodynamics1–10 (QED), where superconducting
artificial atoms are coupled to on-chip cavities, the exploration of fundamental quantum physics in the strongcoupling regime has greatly evolved. In this regime, an
atom and a cavity can exchange a photon frequently before coherence is lost. Nevertheless, all experiments so far are well described by the renowned Jaynes–Cummings model11. Here, we report on the first experimental realization of a circuit QED system operating in the ultrastrong-coupling limit12,13, where the atom–cavity coupling rate g reaches a considerable fraction of the cavity transition frequency !r. Furthermore, we present direct evidence for the breakdown of the Jaynes–Cummings model.We reach remarkable normalized coupling rates g=!r of up to 12% by enhancing the inductive coupling14 of a flux qubit to a transmission line resonator. Our circuit extends the toolbox of quantum optics on a chip towards exciting explorations of ultrastrong light–matter interaction. [DOI: 10.1038/NPHYS1730]

Does noise entail decoherence ?

Every system is in contact with its surroundings and is thus an open system. It is not possible to have exact information about its surroundings, implying a statistical treatment of the interactions between the system and its surrounding. In equilibrium thermodynamics, a statistical weight can be assigned to effect the surrounding. However, in non-equilibrium cases, where dynamics are crucial, noises of certain type are supposed to capture the physics. Usually, white noises, which are the noises from, let's say, a heat bath, cause de-coherence of quantum states. How about 1/f noises ? This kind of noise is pointing and may not be so detrimental. In this piece of work, the authors theoretically examined the fate of a quantum critical state under such 1/f noise. They found that, the criticality can be preserved. To investigate the nature of a system, one may either look at its ground state (by considering static response to certain stimuli) or its excited states (by inspecting its dynamic response to certain stimuli) or both. Basically, these methods should be the same: from G-state one could have some idea (just some idea based on clever guess) what the E-states might be and vice-versa. In many cases, the G-state is preferred, because once the G-state is known, it shows at least how to construct low energy excitations (again, vice versa). In this work, they of course have to study the excitations, since it is a non-equilibrium system. [NATURE PHYSICS, VOL 6, OCTOBER 2010 ,www.nature.com/naturephysics]

Quantum critical points are characterized by scale-invariant correlations and therefore by long-range entanglement. As such, they present fascinating examples of quantum states of matter and their study is an important theme in modern physics.
However, little is known about the fate of quantum criticality under non-equilibrium conditions. Here we investigate the effect of external noise sources on quantum critical points. It is natural to expect that noise will have a similar effect to
finite temperature, that is, destroying the subtle correlations underlying the quantum critical behaviour. Surprisingly, we find that the ubiquitous 1=f noise does preserve the critical correlations. The emergent states show an intriguing interplay of
intrinsic quantum critical and external-noise-driven fluctuations.We illustrate this general phenomenon with specific examples describing solid-state and ultracold-atoms systems. Moreover, our approach shows that genuine quantum phase transitions can exist even under non-equilibrium conditions.

Molecular superfluidity ?

Bosons could become superfluid at low temperatures: it flows without feeling the friction. This is so due to the opening of an energy gap as bosons condense into a so-called macro-molecule in the presence of interactions. It is expected that such condensation happens at a number of bosons. Now it was demonstrated that, this number can be down to 9 pH2 molecules.

Clusters of para-hydrogen (pH2) have been predicted to exhibit superfluid behavior, but direct observation of this phenomenon has been elusive. Combining experiments and theoretical simulations, we have determined the size evolution of the superfluid response of pH2 clusters doped with carbon dioxide (CO2). Reduction of the effective inertia is observed when the dopant is surrounded by the pH2 solvent. This marks the onset of molecular superfluidity in pH2. The fractional occupation of solvation
rings around CO2 correlates with enhanced superfluid response for certain cluster sizes. [PRL 105, 133401 (2010)]

Better surface Dirac states

Topological insulators are insulators that house gapless surface states. These states provide playground for a host of Dirac physics. Although a couple of such materials have been found, this newly examined one has some better features:

In summary, our ARPES experiment of TlBiSe2 has revealed three important aspects: First, the surface state Dirac cone is confirmed to be present at the Gamma point. Second, the Dirac cone is practically ideal, especially near the DP, and its velocity is larger than for Bi2Se3. Finally, according to both experiment and theory, there are no bulk continuum states that energetically overlap with the DP. This means that the scattering channel from the topological surface state to the bulk continuum is suppressed. Our experimental results favor the realization of the topological spin-polarized transport with high mobility and long spin lifetime in TlBiSe2. [PRL 105, 146801 (2010)]

Switching polarization in FE in a continuous way

Due to symmetry, domains usually occur in any ferroics. Each domain represents one episode of possible low energy configurations that are allowed by symmetry. Domains are separated by domain walls. Every wall has a definite thickness, across which the order parameter changes continuously but rather steeply. By applying an external field, the polarization of a domain can be switched, either continuously or abruptly. In this experiment [PRL 105, 167601 (2010)] on a PbTiO3 thin film, the switching takes place in a continuous way: the original polarization gradually diminishes to zero as the field increases and grows up in the other direction from zero. There is no threshold value required. It might be a very interesting question to study the general conditions for continuous and abrupt switching.

Magnetic oscillations observed in Cuprates and their implications

There have been the very prolonged debate on the nature of the fermi surface measured of cuprate superconductors. Magnetic oscillations are the conventional probe in mapping out the surface. Here is an overview [Physics 3, 86 (2010)] of what has been learned in this field, with a useful list of resource references.

CO2 as the climate control knob

Ample physical evidence shows that carbon dioxide (CO₂) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO₂, like ozone, N₂O, CH₄, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO₂ and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.

http://www.sciencemag.org/cgi/content/full/330/6002/356

Topological Insulators used to determine fundamental constants

Topological phenomena (TP) play crucial role in determining the precise values of fundamental constants such as elementary charge, Planck constant and speed of light. This is so because of the robustness of topological phenomena against local variation in samples and also weak disorder and interactions between particles. Some famous TP have already been in use to this end: the quanta of magnetic flux has been measured with circular superconducting devices; the electrical conductance has been measured with the help of quantum Hall effect. Recently, a new TP was discovered in materials now known as topological insulators (TI). These materials are characterized by their bulky band gap and gapless surface states that are topologically robust. Examples include Te-Bi type compounds. It is expected that, such TP may also be employed to improve the precision of measurement. This came to realization in a latest publication [PRL 105, 166803 (2010)]:

Fundamental topological phenomena in condensed matter physics are associated with a quantized electromagnetic response in units of fundamental constants. Recently, it has been predicted theoretically that the time-reversal invariant topological insulator in three dimensions exhibits a topological magnetoelectric effect quantized in units of the fine structure constant ¼ e2=@c. In this Letter, we propose an optical experiment to directly measure this topological quantization phenomenon, independent of material details. Our proposal also provides a way to measure the half-quantized Hall conductances on the two surfaces of the topological insulator independently of each other.

Friction not so simple

Friction is certainly a standard part of middle school physics courses. It is observed that, to move an object in contact with another one, a force must be applied larger than the static friction, which is supposed to be uniform across the interface. However, this picture is inadequate. Actually, it was perceived that non-uniformity occurs at least locally. Understanding the nature of friction and how to model it better is not only theoretically interesting but practically imperative, because friction is relevant to a plenty of phenomena, such as rampant earthquakes and snow ruptures. Friction is the force that holds those events from bursting out. On the hand, it is also desirable to gain insight into how slip occurs locally when friction fails. This is key to modeling. This latest publication investigated this problem.

The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip.
Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.

Molecules filtering spins

Using STM with a magnetic tip can be used to probe the magnetic feature of a surface. The tunneling current shall be sensitive to the alignment (collimation) between the spin orientation of the surface and that of the tip. If the tunneling, as is usually the case, is non-magnetic, then parallel alignment yield a bigger current. A very valuable aspect of STM is that this device probes the local properties of a material. This makes it especially useful in investigating defects or impurities of a surface. Now these authors [Phys. Rev. Lett. 105, 066601 (2010)] came to examine what will happen to the signal if the electrons tunnel from the Fe surface into the tip through a single organic molecule with Beneze rings. The result is this: this molecule allows more spin-up electrons to pass. So, it works as a selective valve, which may be tailored to specific applications that needs manipulate spin current. This phenomenon was predicted 3 years ago in Ref.[3], where the computation was implemented in the aid of DFT. However, it may prove more elucidating if a simple model description is prescribed.
For convenience, some references are attested on this subject:

1. Atodiresei, N. et al. Phys. Rev. Lett. 105, 066601 (2010).
2. Brede, J. et al. Phys. Rev. Lett. 105, 047204 (2010).
3. Rocha, A. R. & Sanvito, S. J. Appl. Phys. 101, 09B102 (2007).
4. Barraud, C. et al. Nature Phys. 6, 615–620 (2010).
5. Sanvito, S. Nature Phys. 6, 562–564 (2010).
6. Cinchetti, M. et al. Nature Mater. 8, 115–119 (2009).
7. Drew, A. J. et al. Nature Mater. 8, 109–114 (2009).
8. Szulczewski, G., Sanvito, S. & Coey, J. M. D. Nature Mater. 8, 693–695 (2009)

Pioneering papers on Graphene

1. Novoselov, K. S. et al. Science 306, 666-669 (2004). | Article | PubMed | ISI | OpenURL | | ChemPort |
2. Zhang, Y. et al. Nature 438, 201-204 (2005). | Article | PubMed | ISI | OpenURL | | ChemPort |
3. Novoselov, K. S. et al. Nature 438, 197-200 (2005). | Article | PubMed | ISI | OpenURL | | ChemPort |
4. Kim, K. S. et al. Nature 457, 706-710 (2009). | Article | PubMed | OpenURL | | ChemPort |