The prominence of O sites in copper oxides

People almost unanimously make use of t-J model as derived by Zhang and Rice in 1988 to investigate the electronic properties of cuprate superconductors (CSC). The insufficiency of this model at low doping, i.e., in the pseudogap regime, has been pointed out theoretically[1]. In addition, experimental data up to now has been well documented, prompting the abandoning Zhang-Rice model in examining pseudo-gap behaviors. These experiments directly images the spatial profile on both Cu and O sites and revealed the prominence of O sites. It was actually perceived that, the C-4 rotation symmetry with a CuO2 unit cell has been broken in pseudo-gap states, down to a C2 bond symmetry[2]. More importantly, it was observed that, weak magnetic moment appears on O sites[2]. According to [1], this weak moment can be interpreted very well. There the authors claimed that, the pseudo-gap state actually consists of weakly mobile double-plaquette magnetic states that has projections on O sites. Another conclusion from [1] is that, electron-doped cuprates should differ essentially from hole-doped ones. In the former, no spin glass intervenes and AFM destruction occurs via spin dilution. In the latter, it is the spin glass that destroy AFM. This has been confirmed in this experiment [3]. This compels people to go beyond the too simple Zhang-Rice model. 'Physics should be made as simple as possible, but not simpler.'

[1]J. Phys.: Condens. Matter 21 (2009) 075702;
[2]NATURE| Vol 466| 15 July 2010.
[3]DOI: 10.1038/NPHYS1717

[2]Our studies reveal intra-unit-cell, C2 symmetric excitations at the pseudogap energy and that these effects are associated primarily with electronic inequivalence at the two O sites within the CuO2 unit cell. Given the many common characteristics observed by these diverse techniques, it is reasonable to consider whether ARPES, neutron diffraction and spectroscopic-imaging STM are detecting the same excitations with the samebroken symmetries. If so, the pseudogap excitations of underdoped copper oxides would represent weakly magnetic states at the O sites within each CuO2 unit cell, the electronic structure of which breaks C4 symmetry. Then, the electronic symmetry breaking that occurs on entering the pseudogap phase would be due to the electronic nematic state visualized here, for the first time to our knowledge. Finally, the nematicity found in electronic transport27, thermal
transport28 and the spin excitation spectrum29 of YBa2Cu3O61 x could then occur because the Ising domains of OQ n (r, e) become aligned by the strong orthorhombicity of its crystal structure30.


[3]A Mott insulator is a material that is insulating because
of strong Coulomb repulsions between electrons. Doping
charge carriers, electrons or holes into a Mott insulator can
induce high-temperature superconductivity. Thus, what exactly
happens when a charge carrier is doped into a Mott insulator
is a key question in many-body physics1–4. To address this
issue, ideally one should start from a zero-doping state5–7
and be able to introduce both holes and electrons in the
dilute limit. However, such an idealized experiment has been
impossible because of the lack of suitable materials. Here
we show that a new ‘ambipolar’ cuprate makes it possible
for the first time to cross the zero-doping state in the same
material, which in turn allows us to address the physics of
the extremely low-doping region. Surprisingly, we found that
the antiferromagnetic ground state sharply changes between
electron- and hole-doped sides, and this change is dictated by
the existence of only 0.1 ppm of charge carriers. Moreover, we
observed that the Néel temperature TN shows an unexpected
reduction in a narrow range centred at the zero-doping state,
across which the system exhibits asymmetric behaviours in
transport measurements. Our findings reveal the inherently
different nature of electron and hole doping in the dilute limit
of a Mott-insulating cuprate.

Topological insulators: next gold rush ?

At this year’s APS meeting, however, the hallways were filled with talk of a promising newcomer— an eccentric class of materials known as topological insulators. The most striking characteristic of these insulators is that they conduct electricity only on their surfaces. The reasons are mathematically subtle — so much so that one physicist, Zahid Hasan of Princeton University in New Jersey, tried to explain the behaviour using ‘simpler’ concepts such as superstring theory. (“It’s awfully beautiful stuff,”
he said reassuringly.) Yet the implications are rich, ranging from practical technology for quantum computing to laboratory tests of advanced particle physics.

Reversible order-disorder transition

Usually order-disorder transitions are not exactly reversible. Nevertheless, here [1] comes a report just on this seldom reversibility. It occurs to an organic overlayer on a metal surface upon cooling.
[1] Science 329, 303 (2010)

Inverse melting or disordering, in which the disordered phase forms upon cooling, is known for a few cases in bulk systems under high pressure. We show that inverse disordering also occurs in two dimensions: For a monolayer of 1,4,5,8-naphthalene-tetracarboxylic dianhydride on Ag(111), a completely reversible order-disorder transition appears upon cooling. The transition is driven by strongly anisotropic interactions within the layer versus with the metal substrate. Spectroscopic
data reveal changes in the electronic structure of the system corresponding to a strengthening of the interface bonding at low temperatures. We demonstrate that the delicate, temperature dependent balance between the vertical and lateral forces is the key to understanding this unconventional phase transition.

Thermal Hall effect of magnons

Thermal Hall Effects refer to a class of phenomena in which thermal current carried by elementary excitations of a material is deflected by applying magnetic field. Magnons, as magnetic excitations have been predicted to show such behaviors when both magnetic field and temperature gradient are present. This has been just observed in Lu2V2O7, whose V element is magnetic and magnetic ordering occurs below about 100K[1].

[1]Science 329, 297 (2010)

The Hall effect usually occurs in conductors when the Lorentz force acts on a charge current in the presence of a perpendicular magnetic field. Neutral quasi-particles such as phonons and spins can, however, carry heat current and potentially exhibit the thermal Hall effect without resorting to the Lorentz force. We report experimental evidence for the anomalous thermal Hall effect caused by spin excitations (magnons) in an insulating ferromagnet with a pyrochlore lattice structure. Our theoretical analysis indicates that the propagation of the spin waves is
influenced by the Dzyaloshinskii-Moriya spin-orbit interaction, which plays the role of the vector potential, much as in the intrinsic anomalous Hall effect in metallic ferromagnets.

Stephen Hawking on PI

In communicating his excitement and enthusiasm for scientific progress, Prof. Hawking said, “The recipe is simple: Bring brilliant people together, in an inspiring and free intellectual environment, where they are encouraged to pursue ambitious and timely research. The importance of special places and special times, where magical progress can happen, cannot be overstated… It seems to me, the same ingredients are being assembled here, at Perimeter Institute. Perimeter's chosen scientific focus, connecting quantum theory and spacetime, is central to new insights, which are emerging, concerning not only black holes and the beginning of the universe, but also nuclear and particle physics, quantum computers, and the science of new materials. Perimeter is a grand experiment in theoretical physics. I am hoping, and expecting, great things will happen here.”

http://www.perimeterinstitute.ca/News/In_The_Media/Stephen_Hawking_on_Perimeter_Institute_and_Special_Places_&_Times_for_Scientific_Progress/

A history of cosmology

I found this nice video and this one as an account of the history of cosmology, from the 2 and half facts epoch to the very present.

GOD PARTICLES ELUSIVE YET

Irresponsible scientists (are they really scientists ?) and media agencies spread a rumor about the Higgs boson, which are dismissed by Fermi Lab.

On Tuesday, the laboratory's Twitter feed said: "Let's settle this: the rumours spread by one fame-seeking blogger are just rumours. That's it."

Rumors dismissed by Fermi Lab

Folks at Fermilab, the national physics lab in Batavia, are dismissing rumors that they have found evidence pointing to the existence of the "God particle."

Folks at Fermilab, the national physics lab in Batavia, are dismissing rumors that they have found evidence pointing to the existence of the "God particle."

"Pure rumor. There is no factual basis to it whatsoever," Judy Jackson, director of public affairs at Fermi, said Tuesday of a blogger's report that worked its way all the way to Britain's Daily Telegraph online.

The rumored, highly elusive particle known formally at the Higgs boson — in honor of British physicist Peter Higgs — is believed to give mass to all things, a status that has led to its nickname.

http://www.chicagotribune.com/news/opinion/ct-talk-god-particle-0714-20100713,0,6952453.story

Upper bound on neutrino mass ; friction

(1) This new bound is now estimated to be of 0.28ev, summed over all three neutrino types.
The estimate is based on the data of cosmic microwave background. The reasoning should be very interesting (I have not gone through the original paper yet).

(2)As this study suggests, the sources of friction are still an open question.

What may come out of a two-species-fermionic systems ?

The following is from a recent paper: Physics 3, 58 (2010) DOI: 10.1103/Physics.3.58, which is a comment on the work [Phys. Rev. A 82, 011605 (2010) ].

Caption of the above figure:

Parallel layers of fermionic atoms offer rich new physics. One species of fermionic atoms (A) is constrained to move only on two thin layers separated by distance d. Another species (B) is free to move in three-dimensional space. (a) At large interlayer separation, the A atoms only interact within a layer as they are dominated by p-wave interaction, forming a BCS-type pairing as shown by the rotating atoms. (b) If the interlayer spacing is small, the A atoms will pair up across layers. (c) If the interaction between A and B species is strong, they will form “molecules” or (d) three-body Efimov states involving A atoms in each layer and a B atom.

What is wrong with the world ?

Is it really the trees of knowledge ruining the trees of life ?
The daily world seemingly gets more and more incomprehensible under living pressure.
http://physicsandphysicists.blogspot.com/2010/07/no-higgs-yet.html

Sir M. Berry

Professor S.M.Berry is usually known for the geometric phase after his name. But actually, his contributions are rather broad and massive. One can download all his work from his website for free. Very nice ! This big mind is encouragingly strange: he seems having no intention to publish his works in those so-called A-class journals, in sharp contrast with those scholors who can live no where but in mundane toggled by money. It is my pleasure to have a link of one of his papers here.

In one of my recent blogs, I said something on possible enhancement of photon-photon interactions. I suggested two basic elements: (1)BEC and (2) small polarization energy. An easy calculation has been just done to verify the idea. Here is a GoogleTech Talk also going under the name of photon-photon interactions, which lays their emphasis on frequency change, rather than momentum change as in my considerations, after effective interactions.

What does physics say about the Jobulani ?

The ball created by Adidas for this year's World Cup has its name as 'Jobulani' ? What is special about it ? Listen to the audio of this link.

Spin orbit coupling of light

Light is described by a vector potential and thus has internal degrees of freedom, which represents its intrinsic spin carrying an internal momentum. Now, if this beam of light travels along a curved trajectory, it shall also posses an orbital momentum, which can be manifested in the way it transforms under coordinate rotation. These two momenta may interact with each other. Differing from the situation with electrons, the spin-orbit coupling of light is geometric by origin. Here is a description of the expriments observing this coupling.

Fluid close to a wall

The turbulence phenomena have remained a puzzle for hundreds of years. Efforts to resolve it never stop.

The behavior of turbulent fluid motion, particularly in the thin chaotic fluid layers immediately adjacent to solid boundaries, can be difficult to understand or predict. These layers account for up to 50% of the aerodynamic drag on modern airliners and occupy the first 100 meters or so of the atmosphere, thus governing wider meteorological phenomena. The physics of these layers is such that the most important processes occur very close to the solid boundary—the region where accurate measurements and simulations are most challenging. We propose a mathematical model to predict the near-wall turbulence given only large-scale information from the outer boundary layer region. This predictive capability may enable new strategies for the control of turbulence and may provide a basis for improved engineering and weather prediction simulations.

Science 9 July 2010:
Vol. 329. no. 5988, pp. 193 - 196
DOI: 10.1126/science.1188765

Spin-triplet pairs in the proximity of a supercondutor and a ferromagnet

Electrons in a supercondutor below Tc are described by a single wave function, which satifies Schrodinger's equation (dBG equation). The value of this function relates to the effective potential felt by the Copper pairs. The potential is negative in the superconductor but positive in a normal material. Thus, normal mateials make a energy barrier that prevents the peameability of electron pairs , and hence supercurrent to spill much, especially when the normal material is ferromagnetic. A ferromagnet favors spin-triplet, while Copper pairs are singlets. Nevertheless, there are some recent experiments observing a longer-range spilling of supercurrent, which was being put under the context of spin-triplet paring that may arise the perephery of an S-N interaface. Now, it came a paper aiming at this problem[1]. They found further evidences of such paring.

The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors.

[1]Science 2 July 2010: Vol. 329. no. 5987, pp. 59 - 61; DOI: 10.1126/science.1189246

Muonic hydrogen indicating a smaller proton

Muons are conceived as point-like elementary particles living a few microseconds (shall live longer when it moves fast) under lab conditions. They are heavier counterparts (200 times heavier) of electrons. When they are caught by a proton, a muonic hydrogen forms. Now they are used to investigate some fundamental questions concerning QED.

1. Muonic hydrogen is an exotic hydrogen atom, where a muon (instead of an electron) orbits the proton. Because the muon is 200 times heavier than the electron, the muon's orbit is 200 times closer to the proton in muonic hydrogen than that of the electron in regular hydrogen. This 200 times smaller orbit means that the muon "feels" the size of the proton: certain muon orbits are significantly perturbed by the size of the positive charge distribution of the proton. By measuring the perturbation of the muon orbit using a laser, it is possible to determine the size of the proton.
(https://muhy.web.psi.ch/wiki/index.php/Main/Introduction)
2. Our measurement of the muonic hydrogen Lamb shift has to be conceived as a progress in the investigation of the hydrogen atom. In fact, when combined with the the measured transition frequencies in hydrogen, the proton radius inferred from the measurement of the muonic hydrogen Lamb shift will provide the most precise test of bound-state QED in the hydrogen atom to this date. Our measurement is thus likely to spur additional investigations of the fundamental theory of the electromagnetic interaction (quantum electrodynamics), a theory that links charged particles and photons (and hence light), which are some of the most important building blocks of our universe. (https://muhy.web.psi.ch/wiki/index.php/Main/Introduction)
3. The μp Lamb shift, ΔE(2P-2S) ≈ 0.2 eV, is dominated by vacuum polarization which shifts the 2S binding energy towards more negative values (see figure). The μp fine- and hyperfine splittings are an order of magnitude smaller than the Lamb shift. The relative contribution of the proton size to ΔE(2P-2S) is as much as 1.8%, two orders of magnitude more than for normal hydrogen atoms. The muonic Lamb shift ΔE(2P-2S) was recalculated recently by several authors [6-8] considering all QED contributions on the ppm level, including three-loop vacuum polarization, hadronic vacuum polarization, light-by-light scattering, and recoil corrections to the order α6. The uncertainty in the calculated proton polarization shift will ultimately limit the calculated ΔE(2P-2S)-value to the 0.3 ppm precision level (disregarding terms which depend on the proton radius). The theoretical prediction of the muonic hydrogen Lamb shift is

ΔE(2P^F=2 - 2S^F=1)=205.952 (3)(4)(137) meV

where the first error is the uncertainty of the calculated QED-terms, the second one the uncertainty from the proton polarization, and the third one the uncertainty given by the poor knowledge of the proton radius. A measurement of the muonic Lamb shift with 30 ppm precision will hence determine the proton radius with 0.1% precision.(https://muhy.web.psi.ch/wiki/index.php/Main/Motivation)

4. The aim of our laser spectroscopy experiment is to measure the Lamb shift in muonic hydrogen (μp):

ΔE(2P - 2S) (with 30 ppm precision)

and to deduce the rms proton charge radius r_p (with 1000 ppm, 10 times more precise than presently known):

ΔE(2P -2S) = 209.98 - 5.23 r_p² [meV]

where r_p is given in fm (r_p ≈ 0.9 fm).

The principle of the experiment is to irradiate μp atoms in the 2S state by a short pulse of infrared laser radiation whose wavelength (of about 6 micrometer) corresponds to the small energy difference of the binding energies of the 2S and 2P states. What can be measured is the number of 2P-1S transitions which occurs in time-coincidence with the laser pulse when its wavelength is tuned over the 2S-2P resonance.

• The PiE5 beam at the PSI proton accelerator provides 2 x 10⁸ s^-1 negative pions with a momentum of 100 MeV/c.
• Pions are injected into the cyclotron trap where they decay and produce negative muons of low kinetic energy.
• From the cyclotron trap the muons are axially extracted and transported to a solenoid with 5 T magnetic field where the hydrogen target is placed.
• Before entering the target, each muon is detected, and this is used to trigger the laser and the data acquisition system.
• About 500 s^-1 low energy muons (3 - 6 keV) enter the target and slow down in 1 mbar of hydrogen gas by ionization. When an electron from the hydrogen atom is replaced by the muon, a muonic atom μp is formed in a highly excited state (n ≈ 14).
• 99% of the muonic hydrogen atoms deexcite to the 1S state within 100 ns and produce "prompt" x-rays of 2 keV energy. The remaining 1% reach the metastable 2S state which has a lifetime of about ≈ 1 μs at 1 mbar.
• A laser pulse with a wavelength tunable around 6 μm is injected in the target and, when in resonance (hν = ΔE(2P - 2S)), induces a 2S to 2P transition.
• Atoms in the 2P state decay to the 1S ground state emitting 1.9 keV x-rays , which are delayed relative to the prompt x-rays, and occur in coincidence with the laser pulse.
• Detection of a delayed 1.9 keV x-ray followed by an electron (originated from the muon decay reaction) is a signature of the laser transition.
• By measuring the rate of delayed 1.9 keV x-rays as a function of the laser frequency, the resonance frequency (corresponding to ΔE(2P - 2S)), and hence the proton charge radius r_p can be determined.
  (https://muhy.web.psi.ch/wiki/index.php/Main/Experiment)

Microscopic phase separation in thin films

Matter can exist in various phases. These phases are frequently found separated by interfaces, if they can coexist. For example, liquid H2O can not be mixed uniformly with gas H2O, due to their differing densities. Thus, phase separation is a very common phenomena that are encountered every day. Of course, there may also exist systems that exhibit multiple coexisting phases without spatial phase boundaries. One example is multiferroics: BiFeO3 is at the same time an anti-ferromagnet and an ferroelectric, without electrical polarization aliened from magnetic one. They are both homogeneously distributed in the entire sample.

Phase separation can happen on not only large scales, but also on small ones. Examples include multi-domain crystal, in which down magnetization regions are spaced from up magnetization regions by domain walls. Such multi-domain structure is due to long range interactions. Short-range interactions are usually not supposed to raise multi-domain structures. Another example is pnictide superconductors, in which some authors claimed evidences of coexistence of superconductivity and AFM, spatially divorced.

Here[1], the author studied phase separation in thin films of magnese substrated upon STO. Strain caused at the interface is found of important role. The phases are complex and rich.

[1]Science 329, 190 (2010)

Spin hall effect of light.

I cant help keeping a record of this piece of work, which shows a fast light split. By incidenting a beam of linearly polarized light upon the interface between two media with differing refraction indice, they obtain two closely separated beams of light with oppositely polarized circular helicity.

We experimentally demonstrate a general optical pump-probe technique to observe the spin Hall effect of light SHEL in an absorbing medium. In essence, a locally confined pump-induced modification of a material’s absorptivity can effectively be used as an induced aperture allowing one to detect the transversely displaced
circular polarization components of an incident beam through differential transmission techniques. We consider linear absorption mechanisms such as free-carrier absorption and Pauli blocking as well as nonlinear absorption processes such as two-photon absorption. For absorption mechanisms that do not depend on light polarization, the SHEL of the probe beam is obtained directly, while polarization-dependent properties give an effective SHEL displacement that depends on the action of the SHEL on both pump and probe beams. Using 150 fs pump, 820 nm pump and probe pulses we observe SHEL effects in silicon via free-carrier absorption. SHEL effects are also observed via Pauli blocking at 820 nm and two-photon absorption at 1550 nm in GaAs using 150 fs pump and probe pulses.

PHYSICAL REVIEW B 82, 045303 2010

Type-2 superconductors used as tweezers

A just published work (PRA, 81, 063408, 2010) did calculations on how type-2 superconductors might be used to trap atoms or other tiny particles. In comparison with other choices, the scheme there-proposed has less loss of coherences. Their calculations proved type-2 SC as a very versatile device in that direction. What holds the key as tweezers is the formation of vortice, which allows the existence of local magnetic field. Such vortice appear in type-2 SC, but not in type-1.

An aternative to Einstein's GRT

Einstein constructed his general relativity theory about a century ago. Adhering to two basic principles, namely, (1)general relativity principle and (2)Equivalence principle, he arrived at his celebrated action, which had also been proposed by Hilbert and led to Einstein's equation. However, there are some seemingly inherent singularity that has been plaguing physicists. Such singularity concerns the beginning of time and space. Now, a very interesting paper [1] has just appeared in PRL, addressing such issues by a different GRT, as first suggested by Eddington. This new action is also invariant under general coordinate transformations. And, in this paper, it was shown that, due to some extra terms particularly relevant in large density regions, this new theory may get around those long standing issues.

Eddington’s Theory of Gravity and Its Progeny
Ma´ximo Ban˜ados, P. Universidad Cato´lica de Chile, Avenida Vicuna Mackenna 4860, Santiago, Chile Astrophysics, University of Oxford, DWB, Keble Road, Oxford, OX1 3RH, United Kingdom Pedro G. Ferreira, Astrophysics, University of Oxford, DWB, Keble Road, Oxford, OX1 3RH, United Kingdom
(Received 21 March 2010; published 2 July 2010)
We resurrect Eddington’s proposal for the gravitational action in the presence of a cosmological constant and extend it to include matter fields. We show that the Newton-Poisson equation is modified in the presence of sources and that charged black holes show great similarities with those arising in Born-Infeld electrodynamics coupled to gravity. When we consider homogeneous and isotropic space-times, we
find that there is a minimum length (and maximum density) at early times, clearly pointing to analternative theory of the big bang.We thus argue that the modern formulation of Eddington’s theory, Born-Infeld gravity, presents us with a novel, nonsingular description of the Universe.
DOI: 10.1103/PhysRevLett.105.011101 PACS numbers: 04.50. h, 98.80. k

[1]PRL 105, 011101 (2010)

Negative specific heat

It is usually thought that, specific heat of a thermodynamic system is positive, that is, energy must be invested to heat the system. How, these authors demonstrated that[1], such a common sense shall be violated if the system does not obey Boltzman statistics or goes out of equilibrium in the presence of long range interactions. They have considered a particular example to illustrate the statement.

[1]PRL 105, 010601 (2010)

Sodium atoms

Here is a summary of the properties of the D lines of sodium atoms, which are frequently used in quantum optics.

A new superconductor: LaNiC

I knew nothing about this supercondutor until I had just a small poster about it. I found this poster on the following blog site:
http://blogs.kent.ac.uk/strongcorrelations/

This superconductor seems abberent in its symmetry, as perfectly summarized in that poster, which I'd like to share in my blog.

soccer balls spinning in the air

The World Cup this round is now near its climax. I'm not a soccer fan, though it is my interest to know how the soccer balls traverse the air. One finds here a good and terse essay on this. It talks of how the trajectory of a soccer through the air is affected by the drag force and Magnus force, both of which are of course exterted by the air. An exact solution of such problem is only numerically or experimentally possible. Even today, after nearly three centuries since Euler equation, some very fundamental issues as regards the transition from laminar and turbulent flow are not concluded.