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)].