Observation of confinment in condensed matter systems

It is well known that baryons are made of quarks. However, these quarks can not be directly observed due to a phenomenon called 'aymptotic freedom' or say 'confinement', which arises becasuse of increasing force strength with separation. It is interesting that, such confinement is not exclusive to high energy physics. It was recently observed occuring to condensed matter systems. This gives another example on how concepts are shared between various fields in physics.

a, The region between two spinons (domain walls) on a chain consists of reversed spins (coloured in red); if this chain is coupled antiferromagnetically to another chain, as in a spin ladder, these reversed spins cost energy owing to their parallel alignment with the spins on the neighbouring chain. This energy cost, which is proportional to the separation of the spinons, acts to confine the spinons. b,c, The structure of CaCu₂O₃ for the a–b plane (b) and the a–c plane (c). CaCu₂O₃ has orthorhombic symmetry with space group Pmmn and lattice parameters a=9.949 Å, b=4.078 Å and c=3.460 Å at T=10 K. The magnetic Cu²⁺ ions have spin=1/2 and are represented by the red spheres; they are coupled to each other by superexchange interactions through the O²⁻ ions (blue spheres) and the Cu–O bonds are represented by the solid black lines; the Ca²⁺ ions are not shown. The lattice parameters are shown in grey as well as the rung distance d_rung, which is approximately one third of the a lattice parameter. The structure consists of copper oxide layers stacked along the c direction, the ladders lie within this plane running parallel to b and neighbouring ladders are shifted by half a unit cell in a. The dotted black lines indicate the separate ladder units and the inter- and intraladder exchange interactions are labelled. The coupling along the legs, J_leg, occurs through superexchange interactions mediated by oxygen; the Cu–O–Cu bond angle is 180^°, giving rise to strong antiferromagnetic coupling (according to the Goodenough–Kanamori–Anderson rules). In contrast, the Cu–O–Cu bond along the rungs is 123^° and therefore J_rung is expected to be substantially weaker although still antiferromagnetic. In addition, a weak antiferromagnetic interaction, J_diag, is predicted between opposite copper ions within each plaquette of the ladder. The ladders are coupled together by a number of weaker interactions. Within the a–b plane, Cu²⁺ ions on neighbouring ladders are connected through Cu–O–Cu bonds that are 90^°, giving rise to a weak ferromagnetic J_inter. Note that J_inter is frustrated and competes with the much stronger J_leg; thus, its energy cancels in the Hamiltonian to first order. Weak interladder couplings J_c1 and J_c2 are also expected between ladders in the c direction. Finally, in common with other planar copper oxide materials, CaCu₂O₃ is expected to have a four-spin cyclic exchange interaction, J_cyclic, coupling the four copper ions that form each plaquette. Quantum chemistry calculations give the following exchange constants for CaCu₂O₃: J_leg=−147 to −134 meV; J_rung=−15 to −11.3 meV; J_cyclic=4 meV; J_inter<24 meV; J_diag=−0.2 meV; J_c1=0.1 meV; J_c2=0.8 meV (refs 19, 20). Susceptibility data fitted to a spin-1/2 Heisenberg chain model without other interactions provide good agreement with the data and suggest that J_leg is indeed the dominant interaction and has a value of −168 meV (ref. 22).

Nature Physics 6, 50 - 55 (2009)

Curved space acts as gauge field in graphene

a, Distortion of a graphene disc which is required to generate uniform B_S. The original shape is shown in blue. b, Orientation of the graphene crystal lattice with respect to the strain. Graphene is stretched or compressed along equivalent crystallographic directions 100. Two graphene sublattices are shown in red and green. c, Distribution of the forces applied at the disc’s perimeter (arrows) that would create the strain required in a. The uniform colour inside the disc indicates strictly uniform pseudomagnetic field. d, The shown shape allows uniform B_S to be generated only by normal forces applied at the sample’s perimeter. The length of the arrows indicates the required local stress.

Among many remarkable qualities of graphene, its electronic properties attract particular interest owing to the chiral character of the charge carriers, which leads to such unusual phenomena as metallic conductivity in the limit of no carriers and the half-integer quantum Hall effect observable even at room temperature^{1, 2, 3}. Because graphene is only one atom thick, it is also amenable to external influences, including mechanical deformation. The latter offers a tempting prospect of controlling graphene’s properties by strain and, recently, several reports have examined graphene under uniaxial deformation^{4, 5, 6, 7, 8}. Although the strain can induce additional Raman features^{7, 8}, no significant changes in graphene’s band structure have been either observed or expected for realistic strains of up to ~15% (refs 9, 10, 11). Here we show that a designed strain aligned along three main crystallographic directions induces strong gauge fields^{12, 13, 14} that effectively act as a uniform magnetic field exceeding 10 T. For a finite doping, the quantizing field results in an insulating bulk and a pair of countercirculating edge states, similar to the case of a topological insulator^{15, 16, 17, 18, 19, 20}. We suggest realistic ways of creating this quantum state and observing the pseudomagnetic quantum Hall effect. We also show that strained superlattices can be used to open significant energy gaps in graphene’s electronic spectrum.

Is time in a hurry?

By Marcelo Gleiser

Well, 2009 is almost over. To me at least, and I bet to most of you, it went way too fast. On average, it was a year like any other, with some new things to celebrate and others to lament. (I'll abstain from listing them. Each person has her own list.) But it's hard to shake off the feeling that everything happened faster, that time seems to be in a hurry to get somewhere. Sometimes, people ask me if it's possible, from a physics perspective, for time to be passing faster. It can't.

According to the theory of relativity, time can slow down but not speed up. There are a few ways to do this. For example, you may move faster than other people. If you get to speeds close to the speed of light, time will slow down for you relative to the others. Hard to do, as the speed of light is a whopping 186,400 miles per second, in round numbers. Or, you may go live on the surface of the Sun. Time there would tick slower than here as well. But that's really not what people have in mind when they wonder about time. The question is about our psychological perception of time. And I am sure many of you would agree that sometimes it does feel like time is on a roller coaster.

Time is a measure of change. If nothing happens, time is unnecessary. So, at a personal level, we perceive the passage of time in the changes that happen around and within us. What's interesting is that--as anyone who has tried to meditate knows--even if you shut off all your senses, time keeps ticking away. As our thoughts unfold, our brains give us time. To "quiet the chatter" is the big challenge for going deeper into a meditative state, to be in the now.

The passage of time is about the ordering of events, things that happen one after another. Numbers, some say, are devices that were created to help us order time. Maybe, although counting chicks is also very useful if you are a hen. However, if we are to order events, we must remember them. Ergo, the perception of time is deeply related to memory. If our memories were to be erased, we would revert to the wonder of babyhood, where time extends forever. The more we have to learn, the more memories we make, the slower time passes. Routine, sameness, makes time speed up. Since routine is not usually equated with fun, this seems to go contrary to the "time flies when you're having fun" dictum. What's going on here?

The answer may be in the level of mindful engagement, that is, in how tuned-in your brain is to what you are doing. Newness, as in fun newness, works as a flood of information and places the focus on the immediate. There is no ordering between events yet and not sense of the passage of time. I have felt this disengagement when lost in a calculation for hours or trying out a new trout stream with my fly rod. This is the opposite of routine, where new memories are not being made and the now is all there is. But maybe someone will prove me wrong.

In physics, things are simpler. Time is a fundamental quantity, something that cannot be defined in terms of anything else. There are some issues with this, that we will address some other time. (Sorry...) The second is the universal unit, and it's defined as 9,192,631,770 oscillations between two levels of the cesium-133 atom. Very different from the tick-tack of old mechanical clocks, which are not very reliable.

Einstein had a colloquial definition of the relativity of time: by the side of a pretty girl an hour feels like a second; if you burn your hand on the stove, a second feels like an hour. His special theory of relativity showed that the simultaneity of two events depends on how they are observed: what may be simultaneous for one observer will not be for another moving with respect to the first. Be that as it may, even in physics the ordering of time is essential: that's causality, causes preceding effects so that the present vanishes into the past and the future becomes the present.

At the cosmic level, there is a well-defined direction of time: the expansion of the universe, which has been going on for 13.7 billion years, pointing resolutely forward. Link it to our own passage through life, and we have a well-defined asymmetry of time, what's sometimes called time's arrow . There is not much we can do to escape this at the physical level. But at the psychological level, to slow down time we have to engage our minds, create more memories, absorb knowledge. Perhaps I will leave my guitar aside for a while and start playing the piano.

Although we use clocks to count time, it is impossible to know if the clock is in hurry or not. Because, to know if the clock goes differently, you need refer to another clock. Actually, time is defined by uniform periodic motions. But, how do you know if it is uniform or not ? You would need another motion to define uniform. So, this is a cyclic logic: time itself needs be used to define time. Eventually, it seems falling upon our sense to make a decision on 'uniform or not' !

bastardization of physics, an example

Are Human Ideas And Opinions Quantum Objects?

Are human ideas and opinions quantum objects? Does it show properties of superposition? Does it have a wavefunction and observable operators? Really!

In the continuing effort to "bastardize" quantum mechanics, many people who don't have a clue of what it is routinely cite the various aspects of quantum mechanics and then applying it to situations where it may not even apply. Crackpots do this all the time, especially in areas of pseudoscience where QM has been used as a justification for all the new-age mumbo jumbo. They do this while forgetting that various aspects of QM have been experimentally tested and verified, whereas their applications to other things have not.

And that brings us to this "delightful" discussion. The writer applied both QM and SR (the physics-bastardization double-coupon) to make amazing justification regarding opposite opinions.

So what's physics got to do with it? First, it allows two contradictory descriptions of nature to be true. So both my friends both could be (and were) right. As Neils Bohr put it: The opposite of a shallow truth is false, but the opposite of a deep truth is also true.

Particles are waves and waves are particles. Whether they show one face or the other depends on what you look for in your experiment, on what kind of question you ask. In other words, the context.

Each of my friends is a complex, warm, caring, passionate and much-loved individual. Each is also nothing but a bunch of quarks and electrons. Two contradictory statements. Both true. Different contexts.

This, of course, isn't new. Extreme post-modernists have done this already, with hysterical and nonsensical conclusions. One only needs to follow the situation surrounding the Sokal Hoax.

The problem in all of this is, of course, that if you understand only a very small and superficial portion of something, and then you apply it, you've essentially ignore the majority of what you applied. For example, if contradictory ideas like that can be represented or justified as "waves" or having such duality, then ALL the other consequences of such analogy should also follow. What happened when they "interfere" with each other, or underwent rapid decoherence? If one makes an "observation", shouldn't the other ideas essentially goes away since the wavefunction has collapsed? Now what?

Bastardization of physics produces nonsensical results. I don't know why people need to grasp onto something they don't even understand to justify something.

Has dark matter been detected ?

For 80 years, it has eluded the finest minds in science. But tonight it appeared that the hunt may be over for dark matter, the mysterious and invisible substance that accounts for three-quarters of the matter in the universe.

In a series of coordinated announcements at several US laboratories, researchers said they believed they had captured dark matter in a defunct iron ore mine half a mile underground. The claim, if confirmed next year, will rank as one the most spectacular discoveries in physics in the past century.

Tantalising glimpses of dark matter particles were picked up by highly sensitive detectors at the bottom of the Soudan mine in Minnesota, the scientists said.

Dan Bauer, head of the Cryogenic Dark Matter Search (CDMS), said the group had spotted two particles with all the expected characteristics of dark matter. There is a one in four chance that the result is due to some other effect in the underground detectors, Bauer told a seminar at the Fermi National Accelerator Laboratory, near Chicago.

Rumours that Bauer's group was on the verge of making an announcement surfaced on physicists' blogs a few weeks ago. Though tentative, tonight's results triggered an immediate wave of excitement in the science community.

"If they have a real signal, it's a seriously big deal. The scale on which people are looking for dark matter is vast," said Gerry Gilmore at Cambridge University's institute of astronomy. "Dark matter is what created the structure of the universe and is essentially what holds it together. When ordinary matter falls into lumps of dark matter it turns into galaxies, stars, planets and people. Without it, we wouldn't be here," Gilmore said.

Scientists have debated the existence of dark matter since 1933, when the Swiss astronomer Fritz Zwicky argued that a distant cluster of galaxies would fall apart were it not for the gravitational pull of some vast but invisible cosmic substance. It was named dark matter because it does not reflect or absorb light, making it impossible to observe with telescopes.

Last year, the Hubble telescope photographed indirect evidence in the form of a ghostly halo around a distant galaxy, caused by clumps of dark matter bending light from stars as it passed by. A year before that, scientists led by the British astronomer Richard Massey, at the California Institute of Technology, published the first 3D map of dark matter, which revealed how it clung around galaxies and held clusters of them together.

Dark matter is likely to be made up of a variety of invisible particles that not only explain the missing mass of the universe, but shed light on some of the most profound mysteries in science.

Some dark matter particles could explain why ordinary matter is not radioactive, while others may help scientists understand why time – so far as we know – always runs forward.

"The real impact of this is psychological, in that it shows we're getting close to being able to do a whole new kind of physics," Gilmore said. "We know there are properties of the universe that should correspond to new families of particles. One of the great mysteries is why time only goes in one direction, and one candidate to explain that is a dark matter particle."

Many scientists believe dark matter particles will turn out to be proof of a theory called supersymmetry, which predicts that every kind of particle in the universe is paired with a heavier twin. Finding evidence for supersymmetry is one of the major goals of the Large Hadron Collider at Cern, in Switzerland.

Dark matter particles are peculiar because they pass through objects as if they were not there. Their aloof nature has led scientists to name them weakly interacting massive particles, or Wimps. Vast amounts of these are thought to be constantly moving through the Earth and everything on it, us included, as the solar system spins around our galaxy.

The detectors at the Soudan mine are buried underground to shield them from other kinds of particles that bombard Earth from space. To detect dark matter, scientists have to wait for the extremely rare occasion when a dark matter particle knocks into an atomic nucleus in the detector and makes it vibrate.

Detectors in the mine will be upgraded in the new year before the search for more dark matter continues, Bauer said.

The hunt for dark matter

What is dark matter?

The night sky might seem full of stars and planets, but what we see is only 4% of the stuff of the universe. Some three-quarters is dark matter, an invisible substance that scientists believe is there because of the gravitational force it exerts.

What does dark matter do?

Dark matter stretches throughout space where it attracts ordinary matter that coalesces into galaxies of billions of stars and planets. It forms a kind of cosmic skeleton that gives the universe its structure. Many scientists believe they will find a family of invisible dark matter particles, each of which plays a different role in nature. Some may even explain why time always goes in the same direction.

Who came up with the idea?

The Swiss astronomer Fritz Zwicky postulated dark matter in 1933. He noticed that a distant cluster of galaxies would fall apart were it not for the extra gravitational pull of some mysterious unseen mass in space. Astronomers verified his prediction by showing that stars swirling around distant galaxies zipped around so fast they must be held in place by extra gravitational forces.

Does everyone believe in dark matter?

A minority of astronomers and physicists dismiss dark matter as a fudge. Instead, they suspect that the strength of gravity varies from place to place, in a way that explains why stars do not hurtle out of spinning galaxies. The theory is known as Modified Newtonian Dynamics (Mond).

• This article was amended on Friday 18 December 2009. We said dark matter accounts for three-quarters of the mass of the universe; we meant to say three-quarters of the matter of the universe. This has been corrected.

Covalency makes a smaller form factor !

Theories involving highly energetic spin fluctuations are among the leading contenders for explaining high-temperature superconductivity in the cuprates¹. These theories could be tested by inelastic neutron scattering (INS), as a change in the magnetic scattering intensity that marks the entry into the superconducting state provides a precise quantitative measure of the spin-interaction energy involved in the superconductivity^{2, 3, 4, 5, 6, 7, 8, 9, 10, 11}. However, the absolute intensities of spin fluctuations measured in neutron scattering experiments vary widely, and are usually much smaller than expected from fundamental sum rules, resulting in 'missing' INS intensity^{2, 3, 4, 5, 12, 13}. Here, we solve this problem by studying magnetic excitations in the one-dimensional related compound, Sr₂CuO₃, for which an exact theory of the dynamical spin response has recently been developed. In this case, the missing INS intensity can be unambiguously identified and associated with the strongly covalent nature of magnetic orbitals. We find that whereas the energies of spin excitations in Sr₂CuO₃ are well described by the nearest-neighbour spin-1/2 Heisenberg Hamiltonian, the corresponding magnetic INS intensities are modified markedly by the strong 2p–3d hybridization of Cu and O states. Hence, the ionic picture of magnetism, where spins reside on the atomic-like 3d orbitals of Cu²⁺ ions, fails markedly in the cuprates.

A recent high Tc model seems promising in solving the underestimated INS intensity. This model explicitly covers the spin-spin interaction between the spin of O holes and the spin of Cu holes. Such interaction effectively makes a smaller scattering form factor, which may give a good fit into observations. Details to be worked out !

[1]Nature Physics 5, 867 - 872 (2009);
[2]J.Phys.:Condens.Matter, 21:075702(2009)

A review on cloaking theory

Scientists and novelists have been intrigued for centuries by the possibility of hiding an object so completely that neither trace of the object nor of its cloak is to be found. Recent theoretical developments show that cloaking is, in principle, possible for electromagnetic waves and to a limited extent for other types of wave, such as acoustic waves. An energetic program of experimental research has shown some of the schemes to be realizable in practice.

We have a touching faith in the ability of our eyes to tell the truth. No other sense has such confidence invested in it, so when our eyes deceive us the result is bewilderment, giving rise to appeals to magic or even the supernatural. This explains the enormous interest aroused by recent work on invisibility and the cloaking of objects from electromagnetic radiation. In this article we review the theories and experiments behind the hype and suggest what devices might realistically be expected in the near future and what is likely to prove impossible.

Hard wired into our brains is the expectation that light travels in straight lines. Mostly this is true, but there are well-known exceptions, such as mirages, which occur when a hot surface heats the air above, reducing its density and hence creating a refractive index gradient immediately above the surface (Fig. 1, top). Such a gradient bends the trajectories of light rays so that an observer misinterprets where the light is coming from. Typically, light from the sky is refracted by the gradient, giving the appearance of water shimmering in the distance—hence a cruel illusion seen by a thirsty traveler in the desert or, more prosaically, the appearance of a wet road on a hot day.

It is the ability of refractive index gradients to bend light that the invisibility engineer exploits. Light is steered around the hidden object by a cloaking device, and then returned to the same straight line trajectory, rather as a skier would make a chicane around a tree (Fig. 1, bottom). The observer’s brain is unaware of the possibility of chicanes and sees only that which is behind the cloak and nothing of the cloak itself or of its contents. The real challenge of cloaking lies in deriving a theoretical prescription for the optical properties of the cloak and, even more challenging, realizing these properties in a material. Transformation optics provides the theoretical background and metamaterials provide the means of achieving the prescribed parameters.

This year's Nobel Prize: CCD

Getting a digital camera for Christmas? Before you fire it up to capture Uncle Wally's fateful fifth trip to the punch bowl, take a moment to picture this: You've got a genuine scientific marvel in your mitts. In fact, it took nothing less than two Nobel prizes and a revolution in physics in order for you to point and shoot.

Why? Because to take a filmless picture, your camera or camcorder relies on, um, quantum mechanics. In particular, it exploits the fact -- revealed by Albert Einstein himself -- that a beam of light, which behaves like a wave in some circumstances, acts like a bunch of separate particles in other circumstances. (If that seems infuriatingly contradictory, suck it up. It's just how we do things in this cosmos. Or go complain to the management.)

The individual particles, called photons, come in a wide range of energies. Visible light has enough so that when its photons slam into something, such as a sheet of specially fabricated semiconductor material in a digital camera, they kick electrons right out of the stuff, producing an electrical charge at the crash site. Explaining this phenomenon, known as the photoelectric effect, got Einstein his Nobel.

In most consumer cameras, the photoelectric action takes place back behind the lens, when the light reflected from Uncle Wally hits a "charge-coupled device," or CCD. A typical CCD contains a light-sensitive semiconductor rectangle, usually smaller than a fingernail, crisscrossed by a grid of tiny channels that divide it into several million separate picture elements, or pixels.

Each pixel emits a different number of electrons, depending on how many photons struck it, and it stores those electrons in a gizmo called a capacitor, which functions like a bucket. After the exposure is over, the CCD circuitry empties the millions of pixel buckets one by one, records the amount of charge in each, and transfers the resulting mosaic to a processor that converts it into digital form -- all in a fraction of a second. Not surprisingly, the guys from Bell Labs who invented the CCD won a 2009 Nobel Prize in Physics.

Of course, if that were all that happened, you'd only have a black-and-white picture. But to photograph your gift from Aunt Myrna, who somehow found a sweater so lurid that it can be seen from space, you want color. There are a few ways to get hues you can use, and they all rely on the convenient truth that all the shades we recognize can be represented by various proportions of red, green and blue, the "RGB" of computer monitor fame.

Unless you've got a high-end camcorder, your gear probably has a single CCD whose grid is covered by an exactly matching grid of color filters arranged in a repeating pattern. For every two-by-two set of four pixels, one is covered by a blue filter, one by a red filter and two (at opposite corners) by green filters. Doubling up on green is needed because the human eye evolved to be disproportionately sensitive to that color, which is right in the middle of the sun's visible spectrum.

The CCD records the electron count on each set of four pixels, and then the camera's on-board computer compares the value of each pixel in the foursome to that of its three neighbors to calculate the "true" color of each one. Considering that these are software-generated approximations and not actual measured colors, the accuracy is astonishing. And the range is equally impressive: customarily at least 256 levels of R, G and B in each pixel, for a total of 16.7 million different colors.

If you've got a still camera that cost more than a case of cat food, it probably has 6 million to 25 million pixels, or six to 25 megapixels in photo argot. How does that stack up to film? The finest 35-mm film in the best cameras using incomparable lenses produces images that can "resolve" (that is, show the difference between) somewhere around 90 million separate spots. A lot of that detail, however, would never be noticed by the human eye unless the photo was blown up to drive-in movie dimensions. A reasonable benchmark is that a good film picture is equivalent to about 20 megapixels.

But the whole megapixel mania that is used to market digital cameras can be awfully misleading, especially in the case of the pocket-size models. For one thing, if you don't have a good enough lens or a CCD sophisticated enough to capture fine differences in contrast and tone, it doesn't matter how many megapixels you've supposedly got. You'll just get a more expensive blur.

For another, most people don't enlarge their photos to the point at which the difference between six and 10, or 12 and 16 megapixels is important. And if you pass your pictures around on the Internet, they probably won't display at much over 100 dots per inch anyway -- about one-third the resolution of an ordinary print. For most folks, gross pixel count is more about self-image than photographic image. But who needs an ego boost when you've mastered quantum mechanics?

2DEG switchable by electric field ?

Perovskite materials are cool as they frequently exhibit exotic properties and thus offer opportunities to fabricate new electronic components.

Here i talk about a perovskite-based interface structure that traps electrons within a few layers (2DEG). 2DEG has been the focus of extensive investigations for many years, examples concerning cuprate superconductors and transistors.

This structure consists of a NbO2 layer sanwitched by strontium STO on one end and KNO on the other. Electrons shall pool around that NbO2 sheet. As we know, the d orbitals on every Nb atom in bulk KNO are nominally empty. So does the pure NbO2 sheet. As one incorperates this sheet into that structure, due to electronic reconstruction that happens often at interfaces, the d orbitals shall be taken up by electrons, but only partially, which forms the so-called Hubbard layer. For partial filling, these electrons shall conduct electricity, with conductivity proportional to the electron density.

Now that KNO is a ferroelectric (STO is only incipient), one may wonder if the spontaneous polarization appearing in it shall affect the electron density and hence the conductivity. Yes, it is, as recently demonstrated by first-principles computations [1]. The physics is simple: the electric field produced by this polarization shall deplet or enrich electrons (screening effect), depending on the field direction, resembling what takes place to a conventional p-n jucntion in the presence of an ecternal electric field. Hence, by inverting the spontaneous polarization in KNO, one is able to switch the conduction states of the NbO2 layer.

For the moment, it may be interesting to see how this prediction will be confirmed experimentally and to undrstand the switch time required for the polarization reversal. Obviously, this time shall be crucial for applications.

[1]PRL, 103:016804(2009)