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.