Resistance becomes lower under pressure

The property of matter often changes drastically as external knobs such as temperature (goes lower) and pressure (gets raised) are tuned (http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.108.026403).

Wustite (FeO) is a prototype for the iron-bearing minerals found in the Earth. Though FeO is insulating at ambient conditions, in the late 1980s researchers observed it undergo a transition to a metallic state when compressed by shock waves. The nature of this transition has, however, been unclear.

In a paper in Physical Review Letters, Kenji Ohta of Osaka University, Japan, and colleagues report their combined theoretical and experimental attack on the problem. The research team measured high-temperature resistivity and structural x-ray diffraction patterns of FeO in a diamond anvil cell to simulate conditions in Earth’s mantle. At a temperature of 1900 kelvin and pressure of 70 gigapascals, Ohta et al. were able to watch as FeO in a rocksalt atomic structure became metallic without any structural changes.

To understand these findings, Ohta et al. performed density-functional calculations of electrical conductivity as a function of temperature and pressure. The results suggest that their observations are consistent with a new kind of insulator-metal transition involving fluctuations between a high-spin state to a low-spin state in the FeO. For geophysicists, this makes the picture of conductivity deep in the Earth richer: both insulating and metallic phases must be added to the phase diagram, with potential implications for thermal and electrical conductivity, and in turn models of the planetary magnetic field. –David Voss

A new version of Wheeler's set up

The particle-wave duality seem always under debate and ingenious experiments have been contrived from time to time to violate it. A famous example is the one proposed by John-Wheeler. It is called 'delayed-choice experiment'. In its existing version, a classical switch has been in use. Here comes a new design using quantum switches [http://physics.aps.org/articles/v4/102].

This so-called delayed-choice experiment was performed in 2007 using an interferometer [1]. In the normal setup, a beam splitter creates two separate light beams that later recombine in a second beam splitter. Detectors placed at the two outputs of this beam splitter both register an interference pattern. However, this wave detector can be turned into a particle detector by removing the second beam splitter, so that the two paths no longer interfere. In the experiment, the choice to add or remove the second beam splitter was made after an individual photon had already passed through the first beam splitter. The data showed that particle and wave behavior were unaffected by the delayed choice, as expected from standard quantum mechanics.

Radu Ionicioiu, now at the Institute for Quantum Computing in Waterloo, Canada, and Daniel Terno of Macquarie University in Sydney, Australia, wanted to see what happens in the thought experiment if the delayed choice is made through quantum means. They imagined that the interferometer contains a quantum device—perhaps an atom in a cavity or a micro-mirror placed on a cantilever—that can exist in two possible states. One state selects the particle experiment, and the other selects the wave experiment. This quantum control element can be placed in a combination, or superposition, of its two states, making the whole experiment participate in the wave-particle duality.

“We show you can do both wave and particle experiments at once,” Ionicioiu says. This means the choice of wave vs particle can be delayed indefinitely. The photon can be observed at one of the detectors and still not “know” if it is supposed to be a wave or a particle. It’s only when the observer decides to measure the state of the quantum control that the photon’s behavior can be identified as wavelike or particlelike.

Two pieces of work on cuprates

Just to highlight them here, because they seemingly represent something defying the cliche.
1. Electron-spin excitation coupling in an electron-doped copper oxide superconductor [Nphy, 7:719(2011)]

High-temperature (high-Tc) superconductivity in the copper oxides arises from electron or hole doping of their antiferromagnetic (AF) insulating parent compounds. The evolution of the AF phase with doping and its spatial coexistence with superconductivity are governed by the nature of charge and spin correlations, which provides clues to the mechanism of high-Tc superconductivity. Here we use neutron scattering and scanning tunnelling spectroscopy (STS) to study the evolution of the bosonic excitations in electron-doped superconductor Pr0:88LaCe0:12CuO4􀀀 with different transition temperatures (Tc) obtained through the oxygen annealing process.We find that spin excitations detected by neutron scattering have two distinct modes that evolve with Tc in a remarkably similar fashion to the low-energy electron tunnelling modes detected by STS. These results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometre length scales, and the dominant electron–boson coupling at low energies originates from the electron-spin excitations.

2. Intense paramagnon excitations in a large family of high-temperature superconductors [Nphy 7:725(2011)]

In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum on doping from the antiferromagnetic insulating to the superconducting state of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic X-ray scattering to show that a large family of superconductors, encompassing underdoped YBa2Cu4O8 and overdoped YBa2Cu3O7, exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. The comprehensive experimental description of this surprisingly simple spectrum enables quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa2Cu3O7 reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to that of Eliashberg theories of conventional superconductors.

Another simple and universal role in high Tc ?

These authors presented a very simple rule that seems validated by their analysis of experimental data [J. Phys.: Condens. Matter 23 (2011) 295701 (17pp)]. In this rule, the Tc of optimal compounds is essentially set by two length scales and the electron charge, i.e., Tc~e^2/l\times l'. What is striking is that, this rue was argued to cover a wide range of materials, including cuprates, pnictides and ruthenates. They proposed a paring mechanism via Compton scattering: e.g., the holes in the conducting layer is scattered by the electrons in the charge reservoir layer. Instead of forming excitons, superfluid forms. The following is a brief sojourn over this work [http://iopscience.iop.org/0953-8984/labtalk-article/46706]:

High-T_C superconductors have layered crystal structures, where T_C depends on bond lengths, ionic valences, and Coulomb coupling between electronic bands in adjacent, spatially separated layers. Analysis of 31 high-T_C materials—cuprates, ruthenates, rutheno-cuprates, iron pnictides and organics—has revealed that the optimal transition temperature T_CO is given by the universal expression k_B^-1e²Λ / ℓζ. Here, ℓ is the spacing between interacting charges within the layers, ζ is the distance between interacting layers, Λ is a universal constant, equal to about twice the reduced electron Compton wavelength, k_B is Boltzmann's constant and e is the elementary charge. Non-optimum compounds in which sample degradation is evident typically exhibit T_C below T_CO. Figure 1 shows T_CO versus (ση/A)^1/2/ζ—a theoretical expression determining 1 / ℓζ, where σ is the charge fraction, η is the layer number count and A is the formulaic area. The diagonal black line represents the theoretical T_CO. Coloured data points falling within ± 1.4 K of the line constitute validation of the theory.

Figure 1. Experimental test of theory.

Figure 2. Proposed pairing interaction.

The elemental building block of high-T_C superconductors comprises two adjacent and spatially separated charge layers. The factor e² / ℓζ, determining T_CO arises from Coulomb forces between them. Remarkably an explicit dependence on phonons, plasmons, magnetism, spins, band structure, effective masses, Fermi-surface topologies and pairing-state symmetries in high-T_C materials is absent. The magnitude of Λ suggests a universal role of Compton scattering in high-T_C superconductivity, as illustrated in figure 2 that considers pairing of carriers (h) mediated by electronic excitation (e) via virtual photons (ν). Several other important predictions are given. A conducting charge sheet is non-superconducting without a second mediating charge layer next to it, and a charge structure representing a room-temperature superconductor yet to be discovered is presented.

Pseudogap does not twin with Superconducting gap: another evidence

I only have time to quickly graze over this interesting paper for the moment.

In underdoped cuprate superconductors, phase stiffness is low
and long-range superconducting order is destroyed readily by
thermally generated vortices (and anti-vortices), giving rise to
a broad temperature regime above the zero-resistive state in
which the superconducting phase is incoherent1–4. It has often
been suggested that these vortex-like excitations are related to
the normal-state pseudogap or some interaction between the
pseudogap state and the superconducting state5–10. However,
to elucidate the precise relationship between the pseudogap
and superconductivity, it is important to establish whether
this broad phase-fluctuation regime vanishes, along with the
pseudogap11, in the slightly overdoped region of the phase
diagram where the superfluid pair density and correlation
energy are both maximal12. Here we show, by tracking
the restoration of the normal-state magnetoresistance in
overdoped La2􀀀xSrxCuO4, that the phase-fluctuation regime
remains broad across the entire superconducting composition
range. The universal low phase stiffness is shown to be
correlated with a low superfluid density1, a characteristic of
both underdoped and overdoped cuprates12–14. The formation
of the pseudogap, by inference, is therefore both independent
of and distinct from superconductivity.

Subwavelength focus of sound

In focusing waves, one is often faced with the so-called diffraction limit as a result of the wave nature, which limits the resolution when seeing objects using waves. Now there came an interesting study beating this limit by focusing sound into a 1/25th wave length spot. Remarkably, this is attained with Coke cans !

Sound, like light, can be tricky to manipulate on small scales. Try to focus it to a point much smaller than one wavelength and the waves bend uncontrollably — a phenomenon known as the diffraction limit. But now, a group of physicists in France has shown how to beat the acoustic diffraction limit — and all it needs is a bunch of soft-drink cans.

Scientists have attempted to overcome the acoustic diffraction limit before, but not using such everyday apparatus. The key to controlling and focusing sound is to look beyond normal waves to 'evanescent' waves, which exist very close to an object's surface. Evanescent waves can reveal details smaller than a wavelength, but they are hard to capture because they peter out so quickly. To amplify them so that they become detectable, scientists have resorted to using advanced man-made 'metamaterials' that bend sound and light in exotic ways.

Some acoustic metamaterials have been shown to guide and focus sounds waves to points that are much smaller than a wavelength in size. However, according to Geoffroy Lerosey, a physicist at the Langevin Institute of Waves and Images at the Graduate School of Industrial Physics and Chemistry in Paris (ESPCI ParisTech), no one has yet been able to focus sound beyond the diffraction limit away from a surface, in the 'far field'. "Without being too enthusiastic, I can say [our work] is the first experimental demonstration of far-field focusing of sound that beats the diffraction limit," Lerosey says.

Lerosey and his colleagues took a similar approach to an experiment they performed in 2007 and later described theoretically for electromagnetic waves^1,2. The group generated audible sound from a ring of computer speakers surrounding the acoustic 'lens': a seven-by-seven array of empty soft-drink cans. Because air is free to move inside and around the cans, they oscillate together like joined-up organ pipes, generating a cacophony of resonance patterns. Crucially, many of the resonances emanate from the can openings, which are much smaller than the wavelength of the sound wave, and so have a similar nature to evanescent waves.

To focus the sound, the trick is to capture these waves at any point on the array. For this, Lerosey and his team used a method known as time reversal: they recorded the sound above any one can in the resonating array, and then played the recording backwards through the speakers. Thanks to a quirk of wave physics, the resultant waveform cancels out the resonance patterns everywhere — except above the chosen can.

After the playback, the can continues to resonate by itself, scattering out the sound energy left inside. Normal waves scatter efficiently, so they disappear quickly. However, the evanescent-like waves are less efficient at scattering, and take roughly a second to make it out of the can — a prolonged emission that allows the build up of a narrow, focused spot. In fact, Lerosey's group found that the focused spot could be as small as just 1/25th of one wavelength, way beyond the diffraction limit. The results are due to be published in Physical Review Letters³.

There is some debate among acoustic scientists as to whether this is the first time anyone has truly beaten the acoustic diffraction limit. Mechanical engineer Nicholas Fang at the Massachusetts Institute of Technology in Cambridge thinks that the results are a first because the focal point is away from the lens, in the far field. But John Page, a physicist at the University of Manitoba in Winnipeg, Canada, who has published evidence for sub-wavelength focusing in the near field⁴, disagrees. "Super-resolution is super-resolution, no matter in what regime it is obtained," he says.

Still, Page calls the Lerosey group's work "a very important accomplishment" and believes it could find many applications, such as feeding energy to tiny electromechanical devices so they can operate.

Lerosey himself thinks that the simplicity of the apparatus is what bodes so well for applications. "To me, this experiment says, 'we can do it easily, even with Coke cans,' and it opens a door."

[http://www.nature.com/news/2011/110708/full/news.2011.406.html]

Space is much smooth on Planck scale

This finding might be the most stunning and most fundamentally interesting and important in the past tens of years. Many garish, dazzling yet gaudy, speculations can hardly withstand this finding.
The space is just so smooth ! [http://www.physorg.com/news/2011-06-physics-einstein.html]

Einstein’s General Theory of Relativity describes the properties of gravity and assumes that space is a smooth, continuous fabric. Yet quantum theory suggests that space should be grainy at the smallest scales, like sand on a beach.

One of the great concerns of modern physics is to marry these two concepts into a single theory of quantum gravity.

Now, Integral has placed stringent new limits on the size of these quantum ‘grains’ in space, showing them to be much smaller than some quantum gravity ideas would suggest.

According to calculations, the tiny grains would affect the way that gamma rays travel through space. The grains should ‘twist’ the light rays, changing the direction in which they oscillate, a property called polarisation.

High-energy gamma rays should be twisted more than the lower energy ones, and the difference in the polarisation can be used to estimate the size of the grains.

Philippe Laurent of CEA Saclay and his collaborators used data from Integral’s IBIS instrument to search for the difference in polarisation between high- and low-energy gamma rays emitted during one of the most powerful gamma-ray bursts (GRBs) ever seen.

GRBs come from some of the most energetic explosions known in the Universe. Most are thought to occur when very massive stars collapse into neutron stars or black holes during a supernova, leading to a huge pulse of gamma rays lasting just seconds or minutes, but briefly outshining entire galaxies.

GRB 041219A took place on 19 December 2004 and was immediately recognised as being in the top 1% of GRBs for brightness. It was so bright that Integral was able to measure the polarisation of its gamma rays accurately.

Dr Laurent and colleagues searched for differences in the polarisation at different energies, but found none to the accuracy limits of the data.

Some theories suggest that the quantum nature of space should manifest itself at the ‘Planck scale’: the minuscule 10-35 of a metre, where a millimetre is 10^-3 m.

However, Integral’s observations are about 10 000 times more accurate than any previous and show that any quantum graininess must be at a level of 10^-48 m or smaller.

“This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories,” says Dr Laurent.

Integral made a similar observation in 2006, when it detected polarised emission from the Crab Nebula, the remnant of a supernova explosion just 6500 light years from Earth in our own galaxy.

This new observation is much more stringent, however, because GRB 041219A was at a distance estimated to be at least 300 million light years.

In principle, the tiny twisting effect due to the quantum grains should have accumulated over the very large distance into a detectable signal. Because nothing was seen, the grains must be even smaller than previously suspected.

“Fundamental physics is a less obvious application for the gamma-ray observatory, Integral,” notes Christoph Winkler, ESA’s Integral Project Scientist. “Nevertheless, it has allowed us to take a big step forward in investigating the nature of space itself.”

Now it’s over to the theoreticians, who must re-examine their theories in the light of this new result.

Provided by European Space Agency (news : web)

The mysterious Moire'- pattern-based electronic properties

Plenty of attention has been diverted to studying the bilayer graphene and hybrid structures consisting of patched mono-bi-layer graphene. A very fundamental problem in bi-layer graphene is how the electronic properties depend on the twisted angle. Theoretical study has been challenging. An interesting review in Nature on a recent PRL paper[http://www.nature.com/nature/journal/v474/n7352/full/474453a.html?WT.ec_id=NATURE-20110623#/references]:

In their study, Luican et al.⁴ find that, at small rotation angles, the local density of electronic states develops a dependence on position within the moiré-pattern unit cell and no longer exhibits the Dirac-like, decoupled-layer, Landau-level pattern. Layer coupling becomes strong in this sense for rotation angles less than about 2°, corresponding to moiré-pattern periods longer than about 10 nanometres. Here it is tempting to conjecture — from the spatial dependence of the density of electronic states — that bilayer wavefunctions have become localized, so that an STM measurement at one position reflects the stacking arrangement only at that position.
...
The extraordinary sensitivity of the electronic properties of few-layer graphene systems to the relative orientations of their layers could prove useful in various applications, for example in ultra-sensitive strain gauges, pressure sensors or ultra-thin capacitors. Further progress requires an improved understanding of both large and small rotation-angle limits, and also improved experimental control of rotation angles.

Noteworthy papers from latest issue of Science

1. Disorder-Enhanced Transport in Photonic Quasicrystals, 332:1541(2011);

Quasicrystals are aperiodic structures with rotational symmetries forbidden to conventional periodic crystals; examples of quasicrystals can be found in aluminum alloys, polymers, and even ancient Islamic art. Here, we present direct experimental observation of disorder-enhanced wave transport in quasicrystals, which contrasts directly with the characteristic suppression of transport by disorder. Our experiments are carried out in photonic quasicrystals, where we find that increasing disorder leads to enhanced expansion of the beam propagating through the medium. By further increasing the disorder, we observe that the beam progresses through a regime of diffusive-like transport until it finally transitions to Anderson localization and the suppression of transport. We study this fundamental phenomenon and elucidate its origins by relating it to the basic properties of quasicrystalline media in the presence of disorder.

2.Carbon-Based Supercapacitors Produced by Activation of Graphene, 332:1537(2011)

Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp²-bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

3. The Limits of Ordinary Matter, 332:1513(2011)

All ordinary matter consists of protons and neutrons, collectively called nucleons, which are bound together in atomic nuclei, and electrons. The elementary constituents of protons and neutrons, the quarks, almost always remain confined inside nucleons (or any other particle made up of quarks, called hadrons). The fundamental force that binds quarks together—the strong, or “color” force—cannot be overcome unless extremely high-energy conditions are created, such as through heavy-particle collisions. Theoretical simulations based on quantum chromodynamics (QCD) predict that the transition temperature for the appearance of free quarks should occur at 2.0 × 10¹² K (an energy of 175 million eV) (1, 2). Since 2000, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has created the necessary conditions to form quark matter in particle collision, but determining the transition temperature under these conditions is challenging. On page 1525 of this issue, Gupta et al. (3) show that the relevant temperature and energy scales can be extracted from recent experimental studies and find that the transition temperature is in remarkable agreement with theory.

4 This paper is not published in Science, but highlighted in it: Nano Lett. 11, 10.1021/nl200928k (2011).

It has long been known from ex situ studies that metal nanoparticles can catalyze reaction of oxygen with graphite surfaces and create grooves or channels. Such reactions could be used for patterning graphene sheets. Booth et al. have studied the dynamics of silver nanoparticles on suspended monolayer and bilayer graphene sheets in a transmission electron microscope. They imaged these samples at temperatures from 600 to 850 K and partial pressures of oxygen over the sample from about 30 to 100 millitorr. The nanoparticles cut channels along <100> crystallographic directions, but some fluctuations of motion normal to the channel direction were also observed. The nanoparticles did not move at a constant speed. Instead, their velocity profile was erratic, and the start-stop motion was better described by a Poisson distribution.

Einstein's Theory STANDS UP

Usually quoted as the most refined brain child ever seen in human history, Einstein's general relativity theory has been frequently under test and it passed. The latest test was initiated some half a century ago and eventually came up with a result: it passed again ! [http://www.physicstoday.org/daily_edition/physics_update/gravity_probe_b_concludes_its_50-year_quest]

Results of an experiment conceived around 1960 to test general relativity and launched in 2004 were announced at a NASA press conference earlier this month: Albert Einstein’s theory passed. The experiment featured four (for redundancy) gyroscopes—spinning, niobium-covered spheres—orbiting 642 km above Earth (see the figure). The goal was to measure the precession induced in the gyroscopes by two general relativistic effects. The easier-to-measure geodetic effect influences any spinning object orbiting a mass. The second effect, frame dragging, arises when the spacetime-distorting mass, here Earth, is itself spinning. Gravity Probe B was not the first to measure the two effects, but it was designed to measure them independent of each other and to extraordinary precision. The gyroscopes are the most perfectly spherical objects ever fabricated. They needed to be, lest the general relativistic precessions be swamped by those arising from Newtonian torques. To measure the spin of those featureless spheres, the experimenters cooled them below niobium’s superconducting transition; the superconducting metal then produces a magnetic field, parallel to its spin axis, that can be measured with a superconducting quantum interference device. In the end, the experiment was a qualified success. It measured the geodetic effect to 0.3% precision, but stray charges on the gyroscopes and their housings limited the precision of the frame-dragging measurement to 20%. In both cases other efforts have achieved comparable results. (C. W. F. Everitt et al., Phys. Rev. Lett., in press.)—Steven K. Blau

No physical signal travels faster than c

Einstein's special relativity theory stipulates that no physical signal (i.e., anything that carries energy and obeys physical laws and is measurable) can not go faster than the vacuum light speed. There have been many 'dissidents' (mostly crackpots) don't like this and want to disprove this law, but all have been defied. Now an experiment that was recently done in HKUST demonstrated that, even a single photon cannot break it.

Einstein taught us that the speed of light was the traffic law of the universe—nothing could go faster. The development of media in which atomic gases can slow down or speed up the passage of light pulses initially caused a stir, at least until the difference between phase velocity and group velocity could be carefully explained. But what about the behavior of single photons, the fundamental quanta of light? Reporting in Physical Review Letters, Shanchao Zhang and colleagues at the Hong Kong University of Science and Technology have shown that photons obey the law too.

Zhang et al. study optical precursors, which are signals preceding the main wave packet in a light pulse with a sharply rising leading edge (as in a step function pulse). Past work has shown that even in “superluminal” media where the group velocity may be faster than light speed, the precursor is always in front of the pulse. The authors extend this work to the single-photon level with the help of cold atomic gases: a photon generated in one rubidium gas traverses a second collection of rubidium atoms. With careful use of electromagnetically induced transparency, the researchers can separate the precursor from the main pulse and confirm it travels at the speed of light. The results add to our understanding of how single-photon signals propagate but also confirm the upper bound on how fast information travels. – David Voss [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.243602]

Spin diffuses through interacting fermi gas

This is definitely a typical non-equilibrium problem. In their experiment, "A spin current is induced by spatially separating two spin components and observing their evolution in an external trapping potential." [Nature, 472:401(2011)] They found that, "interactions can be strong enough to reverse spin currents, with components of opposite spin reflecting off each other. Near equilibrium, we obtain the spin drag coefficient, the spin diffusivity and the spin susceptibility as a function of temperature on resonance and show that they obey universal laws at high temperatures. In the degenerate regime, the spin diffusivity approaches a value set by /m, the quantum limit of diffusion, where /m is Planck’s constant divided by 2π and m the atomic mass. For repulsive interactions, our measurements seem to exclude a metastable ferromagnetic state^{9, 10, 11}." For a review, click here.

More on the pseudogap in cuprate

This article [http://www.nature.com/nphys/journal/v7/n4/full/nphys1921.html?WT.ec_id=NPHYS-201104#/affil-auth] definitely refreshes one's mind in thinking about the nature of the pseudogap of cuprates. The review is here [http://www.nature.com/nphys/journal/v7/n4/full/nphys1973.html?WT.ec_id=NPHYS-201104]. The authors measured the specific heat of UD YBCO under magnetic field. Oscillations were found above H* ~27T. But not only that, there is a background scaling as the square root of H.
What does this imply ?

SC fluctuations not so strong as previously thought

Here is a wonderful Letter [Nature Physics, 7:298(2011)] which, in a sense, denies the pseudogap as precursor of SC in cuprate. These authors probes LSCO, a very typical p-type high Tc, by Terahertz spectroscopy. They found that, such SC fluctuations persist up to 16 K above Tc, much weaker than former speculations. This is actually quite consistent with a latest work with ARPES [see my yesterday's entry].

The nature of the underdoped pseudogap regime of the high-temperature copper oxide superconductors has been a matter of long-term debate^{1, 2, 3}. On quite general grounds, we expect that, owing to their low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties in this part of the phase diagram^{4, 5}. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap and their significance for understanding high-temperature superconductivity^{6, 7, 8, 9, 10}. Here we use THz time-domain spectroscopy to probe the temporal fluctuations of superconductivity above the critical temperature (T_c) in La_2−xSr_xCuO₄ (LSCO) thin films over a doping range that spans almost the entire superconducting dome (x=0.09–0.25). Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, at most 16 K above T_c. Our measurements show that superconducting correlations do not make an appreciable contribution to the charge-transport anomalies of the pseudogap in LSCO at temperatures well above T_c.

Two Consecutive Thermal phase transitons make a High Tc supercoductor !

This is claimed in this wonderful article [Science, 331:1579(2011)], which I have already mentioned in my yesterday's entry.

In cuprate superconductors, people observe not only one d-wave SC gap but an additional gap, which opens up at the zone boundary and at a temperature T* far above Tc. A natural question is how these two gaps are connected. Two proposed scenarios are common in the market: one takes both originating from the same source while the other associates them with respective orders. Clarifying this question paves the way to the ultimate theory of high Tc copper oxides.

Now this article provides convincing evidence, combining ARPES, Polor Ker Effect and Time Resolved Reflectivity, that, T* signals a true but somewhat rounded thermal phase transition into a non-Sc phase. Actually, their work revealed three temperatures: Tc, T* and Tg. Here the Tg is a bit higher than Tc but far below T*, indicating the pairing fluctuations. Their mean-field calculations some candidate orders suggest that the paring energy and the pseudogap energy are of the same order, raising the question if they are connected in a deeper manner.

The current

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

Dark Matter Particles Remain Dark

Here is a controversy about dark matter particles, whose properties are definitely quite elusive. Most we know about them are just speculative. Bear in mind how a scientific conclusion has to go through scrutinies !

Couterflow

This Letter addresses a practical problem that is encountered in, for example, a recent oil gushing in Mexico Gulf. To 'top kill' the oil well, one may pump "mud" from above to suppress the welling from bottom. However, in this "top kill", one needs consider an issue as described in the mentioned Letter, the so-called "Helmholtz instability", namely, "Dense fluids, i.e., mineral suspensions called ‘‘mud’’ [1,2], are introduced into oil wells to provide hydrostatic pressure to offset hydrocarbon (oil and gas) fluid pressure in deep formations, stopping upward flow and reducing the fluid pressure at the surface to near ambient. If hydrocarbon is flowing upward in the well, there is a counterflow between the upwelling hydrocarbon and the descending mud. Successful top kill requires that the mud descend despite this counterflow. However, upwelling at speeds > 1m/s, as in the uncontrolled Macondo well ‘‘blowout’’ in the Gulf of Mexico in 2010, may lead to a Kelvin- Helmholtz instability [3]."[PRL 106, 058301 (2011)] How to avoid the instability ? Suggested to use viscoelastic materials.

BEC is like laser, with perfect coherence: from an experimental point of view

In this work, BEC has been shown possessing the same kind of coherence properties of laser, as expected. The researchers measure directly the third order correlation functions regarding atom bunching (for bosons), g(0,t1,t2), which shows the likelihood of having three particles detected at moments 0, t1 and t2, respectively. For thermal bosons, this function features a broad peak, while for BEC it is rather uniform. This was just observed [Science, 331:3046(2011)] !

A major advance in understanding the behavior of light was to describe the coherence of a light source by using correlation functions that define the spatio-temporal relationship between pairs and larger groups of photons. Correlations are also a fundamental property of matter. We performed simultaneous measurement of the second- and third-order correlation functions for atoms. Atom bunching in the arrival time for pairs and triplets of thermal atoms just above the Bose-Einstein condensation (BEC) temperature was observed. At lower temperatures, we demonstrated conclusively the long-range coherence of the BEC for correlation functions to third order, which supports the prediction that like coherent light, a BEC possesses long-range coherence to all orders.

Competing gaps scenario for underdoped cuprates

The normal state properties are definitely more exotic than the superconducting properties of the engimatic quasi-2D d-wave cuprate superconductors. Under special inspection is the psudogap state, which is signalled with a loss in quasi-particle spectral weight above a typical temperature T*, which is much higher than Tc, the SC critical temperature. A basic question concerns the nature of this gap opening. One idea precasts that the pseudogap senses the preformed cooper pairs, which will condense into a coherent superfluid at Tc. Another idea relates this gap to a different order, which might compete with the SC order. Up to now, evidences regarding these two ideas are elusive. This terrific work [Nature Physics, 7: 21–25 (2011) doi:10.1038/nphys1851] comes to give a real boost. They study Bi2212 with various dopings. They focus on the temperature dependence of the weight loss associated with the antinodal point in the 1BZ. As the sample is cooled across T*, a linear tempeature dependence was spotted around T*, but a very fast deviation sets in at lower temperatures, T**>Tc. This deviation motivates the authors to conceive a two-gap scenario: the proper pseudogap opens at T*, but the cooper pair forms at T**, and SC coherence comes at Tc. According to this proposition, the pseudogap has a separate origin that should be concerned with a different order. They are able to find out a remarkably accurate scaling behaviors regarding the dependence of the cooper paring spectral weight loss on the T**. This scaling law holds for all samples they prepared. Read this paper !! Very inspiring !!