Recent Noteworthy Physics Papers

[ZapperZ]

The aim of this thread is to highlight recent (as in within the past 3 months) noteworthy physics papers that have been published in peer-reviewed journals. Anyone can post here, subject to the guidelines listed below:

1. Papers must be from respected peer-reviewed journals. This includes Nature, Science, Physical Review Letters, The Physical Review Journals, European Physical Review, Europhysics Letters, Physics Letters, Applied Physics Letters, and Journal of Applied Physics.

2. Provide the full reference to the paper. If the paper also can be found on the web without requiring subscription (such as on the e-print ArXiv), please provide the url. In any case, the full reference to the paper in the journal must be provided. The format for the reference should be as follows:

First Author's Name et al., Journal Name, Volume number, Page/Article number, (Year).

3. Provide the full abstract to the paper. This is not necessary if the paper is also available for free on the web.

4. [Optional] Please write a brief description why you are highlighting this paper, and why it is noteworthy.

5. [Optional] If this paper is also highlighted by other services, such as AIP's Physics News Update or ScienceDaily, please provide the link to those highlights. This can be added later as they are found.

6. This thread is only meant for highlighting the paper, not as a discussion thread. Please create a different thread if you think the paper deserves some discussion or comments.

Zz.

 

[actionintegral]

Why just a thread? Wouldn't this make a good category?

 

[ZapperZ]

G. Gabrielse et al., Phys. Rev. Lett., 97, 030802 (2006).

New Determination of the Fine Structure Constant from the Electron g Value and QED

Abstract: Quantum electrodynamics (QED) predicts a relationship between the dimensionless magnetic moment of the electron (g) and the fine structure constant ($\alpha$). A new measurement of g using a one-electron quantum cyclotron, together with a QED calculation involving 891 eighth-order Feynman diagrams, determine [itex]\alpha^{-1}=137.035 999 710 (96) [0.70 ppb]. The uncertainties are 10 times smaller than those of nearest rival methods that include atom-recoil measurements. Comparisons of measured and calculated g test QED most stringently, and set a limit on internal electron structure.

Also see: http://www.aip.org/enews/physnews/2006/split/783-1.html

Two obvious reasons why this paper is noteworth:

1. The most accurate determination of the fine structure constant

2. The amazing agreement with QED predictions that takes into account up to the 8th order corrections.

Edit: There is a Perspective article on this paper written by Dan Kleppnner in the latest issue of Science: Science 28 July 2006:Vol. 313. no. 5786, pp. 448 - 449.

Edit2: There is also a review of this work on PhysicsWeb.
http://physicsweb.org/articles/news/10/8/1/1

Zz.

 

[Pythagorean]

Light Transport in Cold Atoms in Thermal Decoherence
Labeyrie, G. Physical Review Letters, v. 97, no. 1, 7 July 2006

http://www.citebase.org/abstract?id=oai:arXiv.org:quant-ph/0603153

Not so much because I know exactly what's going on, but it sounds relevant to the recent experiments in which light was stopped. I'm reading it now...

 

[Gokul43201]

Soft Condensed Matter

I thought this was nice - a recent breakthrough in Soft Cond Mat, this paper reports on what appears to be the first detailed model of pebble erosion, one with significant predictive capability. I'll defer to PhysicsWeb for their introduction to the paper:

A question that has been around since the time of Aristotle - what shape is a pebble?- has now been solved by physicists in France and the US. Douglas Durian of the University of Pennsylvania and colleagues in Strasbourg say that a pebble is "a nearly round object with a near-Gaussian distribution of curvatures". All pebbles, regardless of their original shape, end up with a similar shape that depends solely on how the pebble was eroded over time. The results could help geologists determine the history of a pebble simply by looking at its geometry.

Durian et al, Phys. Rev. Lett. 97, 028001 (2006)

Abstract:

We propose to characterize the shapes of flat pebbles in terms of the statistical distribution of curvatures measured along the pebble contour. This is demonstrated for the erosion of clay pebbles in a controlled laboratory apparatus. Photographs at various stages of erosion are analyzed, and compared with two models. We find that the curvature distribution complements the usual measurement of aspect ratio, and connects naturally to erosion processes that are typically faster at protruding regions of high curvature.

Other links :

arXiv eprint: http://arxiv.org/PS_cache/cond-mat/pdf/0607/0607061.pdf

PhysicsWeb news report : http://physicsweb.org/articles/news/10/7/7/1

 

[ZapperZ]

B. R. Heckel et al., Phys. Rev. Lett., 97, 021603 (2006)

New CP-Violation and Preferred-Frame Tests with Polarized Electrons

Abstract: We used a torsion pendulum containing $~9×10^{22}$ polarized electrons to search for CP-violating interactions between the pendulum's electrons and unpolarized matter in the laboratory's surroundings or the Sun, and to test for preferred-frame effects that would precess the electrons about a direction fixed in inertial space. We find $|g_P^e g_S^N|/(\hbar c) < 1.7 \times10^{-36}$ and $|g_A^e g_V^N|/(\hbar c) < 4.8 \times 10^{-56}$ for $\lambda > 1$ AU. Our preferred-frame constraints, interpreted in the Kostelecký framework, set an upper limit on the parameter |b|<= $5.0 \times10^{-21}$ eV that should be compared to the benchmark value $m_e^2/M_{Planck} = 2 \times 10^{-17}$ eV.

This paper is from the same Eric Adelberger group at U. of Washington has has made the measurement of the gravititational constant G up to the sub micron scales. The result of this paper puts even more severe constraints on any preferred frame effects and CPT violation.

Zz.

 

[ZapperZ]

Here's something you can show your instructor in your circuit theory class. :)

J. Gabelli et al., Science v.313. p.499 (2006)

Violation of Kirchhoff's Laws for a Coherent RC Circuit.

Abstract:What is the complex impedance of a fully coherent quantum resistance-capacitance (RC) circuit at gigahertz frequencies in which a resistor and a capacitor are connected in series? While Kirchhoff's laws predict addition of capacitor and resistor impedances, we report on observation of a different behavior. The resistance, here associated with charge relaxation, differs from the usual transport resistance given by the Landauer formula. In particular, for a single-mode conductor, the charge-relaxation resistance is half the resistance quantum, regardless of the transmission of the mode. The new mesoscopic effect reported here is relevant for the dynamical regime of all quantum devices.

Of course, the significance of this work is not really to show that Kirchhoff's law doesn't work in this regime, but rather to study the behavior of conduction at the mesoscopic scale. This is important since electronics components are getting to be smaller, and quantum computing and circuits will be relevant in future devices. It is vital to know how such components differ from their classical counterparts.

Zz.

Edit: They have put this up on ArXiv, so if you don't have access to Science, this is how you can get it:

http://arxiv.org/abs/cond-mat/0610572

 

[ZapperZ]

P. Walther et al., Phys. Rev. Lett., 97, 020501 (2006)

Experimental Entangled Entanglement

Abstract: All previous tests of local realism have studied correlations between single-particle measurements. In the present experiment, we have performed a Bell experiment on three particles in which one of the measurements corresponds to a projection onto a maximally entangled state. We show theoretically and experimentally that correlations between these entangled measurements and single-particle measurements are too strong for any local-realistic theory and are experimentally exploited to violate a Clauser-Horne-Shimony-Holt-Bell inequality by more than 5 standard deviations. We refer to this possibility as "entangled entanglement."

This is another paper on the EPR-type experiment (or more specifically, the CHSH-type) from the Zeilinger group. This time, they are measuring the joint properties of 2 or more entangled particles.

Zz.

 

[ZapperZ]

E. Vliegen and F. Merkt, Phys. Rev. Lett. 97, 033002 (2006).

Normal-Incidence Electrostatic Rydberg Atom Mirror

Abstract: A Rydberg atom mirror has been designed and its operational principle tested experimentally. A supersonic expansion containing H atoms moving with a velocity of 720 m/s initially propagates toward a quadrupolar electrostatic mirror. The H atoms are then photoexcited to n=27 Rydberg states with a positive Stark shift and move in a rapidly increasing electric field. The H atom beam is stopped in 4.8 µs, only 1.9 mm away from the photoexcitation spot, and is then reflected back. The reflection process is monitored by pulsed field ionization and imaging.

An interesting paper. They have generated what is essentially an atomic mirror.

A further review of this work can be found here:

http://focus.aps.org/story/v18/st3

Zz.

 

[Gokul43201]

Zz, would you like to post a few comments on JC Davis' http://www.nature.com/nature/journal/v442/n7102/abs/nature04973.html;jsessionid=71D3A118823B0ED589BAEA274E6394C1 on tunneling spectroscopy in BSCCO?

 

[ZapperZ]

Yes, I'm going to once I finish reading it carefully enough. This will be my own review of the paper, especially since they cited two of my papers.

:)

Zz.

 

[ZapperZ]

As requested by Gokul, here it is:

J. Lee et al., Nature v.442, p.546 (2006)

Interplay of electron-lattice interactions and superconductivity in $Bi_2Sr_2CaCu_2O_{8+\delta}$

Abstract:Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current $d^2I/dV^2$ corresponds to each phonon mode. For high-transition-temperature (high-Tc) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron–boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in $d^2I/dV^2$ spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report $d^2I/dV^2$ imaging studies of the high-Tc superconductor $Bi_2Sr_2CaCu_2O_{8+\delta}$. We find intense disorder of electron–boson interaction energies at the nanometre scale, along with the expected modulations in $d^2I/dV^2$. Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of $^{18}O$ for $^{16}O$ throughout the material reduces the average mode energy by approximately 6 per cent—the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.

A review of this paper can be found in both the same issue of Nature, and in Science of the same week. A short overview of it can also be found here:

http://physicsweb.org/articles/news/10/8/4/1

And now, my take on this. This work encompasses both areas of my expertise - tunneling and angle-resolved photoemission (i.e. check out the reference to the "mode" energy). It also continues the on-going battle between phonons and magnetic fluctuations as the mechanism for superconductivity in these cuprate compounded. The phonon scenario has gotten a lot of bruises lately with a number of rather interesting experimental results. However, with this paper, they seem to make a comeback - but have they?

One of the continuing issues in tunneling spectroscopy in these material is the origin of what is known as the "dip-hum structure" in practically ALL of high-Tc tunneling spectroscopy (see the arrows in Fig. 1b of the paper). This is a structure that is at an energy larger than the superconducting gap. A similar structure is seen quite clearly in ARPES measurement, especially at or near the antinodal direction of the crystal momentum space. In conventional superconductors, these structures have been extracted from the tunneling data (the $d^2I/dV^2$ spectrum) using the McMillan-Rowell inversion and the resulting "modes" matches exactly with the phonon modes for that material. This was one of the most convincing evidence that phonons were responsible for the superconducting mechanism in these materials.

Doing this for high-Tc superconductors isn't that easy. The phonon modes for these materials are still not that well-known. Furthermore, the material is very complicated. To be able to know of phonons are responsible, you can't just do one measurement - you need to do this for different types of phonon spectra and see if the changes in superconductivity follows that trend. The isotope effect is a good example. This is essentially what is done in this paper. They doped the high Tc superconductor with an isotope of oxygen (doping this family of high-Tc superconductors with oxygen introduces holes, which are the charge carriers in this "hole-doped" superconductors). So doping with O-18 means you are introducing a heavier hole as the charge carrier. This changes the phonons spectrum, and in particular, they found that the "mode" energy reduced by the expected amount.

[This "mode" energy is roughly the strength of the coupling between the charge carrier (in this case, the holes) and the boson that is the "force carrier". If you believe in phonons as the mechanism, then this boson is a phonon. If you believe in magnetic fluctuations, then this boson could be a spinon or a magnon. This is the QFT description of interactions in this scenario.]

So is this a slam-dunk evidence for phonons? Nope. There are two issues that are still left dangling:

1. Even by changing the doping oxygen isotope and changing the mode energy, the value of Tc doesn't change! One would expect, as in conventional superconductor, that as one changes the strength of the mode coupling, that Tc would also change. This didn't occur (which is why the isotope effect is still vague in high-Tc compounds).

2. In performing tunneling experiments in this particular material, the "cleave surface" is not the Cu-O plane (where it is believed all superconductivity is occurring), but rather the insulating Bi-O layer. So the charge carrier has to first pass through these insulating barrier. Now, there are many tantalizing evidence that when one dopes this material, not all of the oxygen actually does get doped into the Cu-O later, but rather some gets into the insulating later. The charge carriers making the tunneling adventure can be affected by such a barrier. So the signature that was seen in this paper cannot rule out the insulating layer as the origin of the effects they witnessed.

While this paper certainly gives a strong "straw" to the phonon camps, I still don't see how it can explain a series of other experimental results that it could not be consistent with. This is an on-going battle that will require other results to settle.

Edit: They have uploaded the paper to the e-print ArXiv. You may find it here:

http://arxiv.org/abs/cond-mat/0608149

Zz.

 

[ZapperZ]

I. R. Hooper et al., Phys. Rev. Lett. 97, 053902 (2006).

Making Tunnel Barriers (Including Metals) Transparent

Abstract: The classical "brick wall," which may, according to quantum mechanics, leak via tunneling, is here shown to be completely transparent when appropriate impedance matching media are placed both in front of and behind the "wall." Optical experiments involving beyond-critical-angle-tunnel barriers in the frustrated total internal reflection scheme which mimic quantum mechanical systems provide convincing proof of this remarkable effect. The same mechanism also allows vastly enhanced transmission through unstructured thin metal films without the need for surface wave excitation.

A very good review of this paper can be found here:

http://focus.aps.org/story/v18/st4

Zz.

 

[ZapperZ]

This time, there are two independent papers published at the same time in the same issue of PRL, both arriving at the same result/conclusion.

Paper 1
K. Beloy et al., Phys. Rev. Lett. 97, 040801 (2006).

High-Accuracy Calculation of the Blackbody Radiation Shift in the $^{133}$Cs Primary Frequency Standard.

Abstract: The blackbody radiation (BBR) shift is an important systematic correction for the atomic frequency standards realizing the SI unit of time. Presently, there is controversy over the value of the BBR shift for the primary $^{133}$Cs standard. At room temperatures, the values from various groups differ at the 3×10$^{-15}$ level, while modern clocks are aiming at 10$^{-16}$ accuracies. We carry out high-precision relativistic many-body calculations of the BBR shift. For the BBR coefficient beta at T=300 K, we obtain $\beta$ = -(1.710±0.006)×10$^{-14}$, implying 6×10$^{-17}$ fractional uncertainty. While in accord with the most accurate measurement, our 0.35% accurate value is in a substantial (10%) disagreement with recent semiempirical calculations. We identify an oversight in those calculations.

Paper 2
E. J. Angstmann et al., Phys. Rev. Lett. 97, 040802 (2006).

Frequency Shift of the Cesium Clock Transition due to Blackbody Radiation

Abstract: We perform ab initio calculations of the frequency shift induced by a static electric field on the cesium clock hyperfine transition. The calculations are used to find the frequency shifts due to blackbody radiation. Our result [$\delta\nu/E^2$ = -2.26(2)×10$^{-10 }$Hz/(V/m)$^2$] is in good agreement with early measurements and ab initio calculations performed in other groups. We present arguments against recent claims that the actual value of the effect might be smaller. The difference (~10%) between ab initio and semiempirical calculations is due to the contribution of the continuum spectrum in the sum over intermediate states.

These two papers presented a more precise calculation of the hyperfine transition in Cs, which makes the atomic clock more precise as well. They also have explained the 10% discrepency in earlier calculations of this transition.

A summary of both papers can be found here:

http://physicsweb.org/articles/news/10/8/9/1

Zz

 

[Gokul43201]

Long-Lived Bloch Oscillations with Bosonic Sr Atoms and Application to Gravity Measurement at the Micrometer ScaleAbstract:

We report on the observation of Bloch oscillations on the unprecedented time scale of several seconds. The experiment is carried out with ultracold bosonic ⁸⁸Sr atoms loaded into a vertical optical standing wave. The negligible atom-atom elastic cross section and zero angular momentum in the ground state makes ⁸⁸Sr an almost ideal Bose gas, insensitive to typical mechanisms of decoherence due to thermalization and external stray fields. The small size of the system enables precision measurements of forces at micrometer scale. This is a challenge in physics for studies of surfaces, Casimir effects, and searches for deviations from Newtonian gravity predicted by theories beyond the standard model.

URL: Ferrari, et al, Phys. Rev. Lett. 97, 060402 (2006)

What are the unique features of this experiment? First of all, although Bloch oscillations have been observed before, they have never been sustained for as long as 10 seconds, which is the case here. Experiments that mix gravity and quantum mechanics are rare.

Furthermore, even though the cloud of Sr atoms in use do not exist in the form of a Bose-Einstein condensate (BEC), the atoms do absorb the trapping laser light in a coherent way; that is, they absorb the light in a stimulated (not random) way. They quickly re-emit the light and then absorb still another photon. The number of photons per atom transferred in this way -- in the thousands rather than tens -- is the largest ever for a physics experiment.

Finally, close observation of the Bloch oscillations allows you to measure the strength of the static force, gravity, with high precision -- in this case to measure gravity with an uncertainty of a part in a million.

AIP News: http://www.aip.org/pnu/2006/split/788-1.html

Link to e-print: http://www.lens.unifi.it/tinogroup/Sr/doc/0605018.pdf

 

[ZapperZ]

S. Sasaki et al. Superfluidity of Grain Boundaries and Supersolid Behavior, Science v.313, p.1098 (2006)

Abstract: When two communicating vessels are filled to a different height with liquid, the two levels equilibrate because the liquid can flow. We have looked for such equilibration with solid $^4$He. For crystals with no grain boundaries, we see no flow of mass, whereas for crystals containing several grain boundaries, we detect a mass flow. Our results suggest that the transport of mass is due to the superfluidity of grain boundaries.

This is another example on why, when something is published, we need to give it some time to be digested and tested by independent groups to verify that the discovery, or the result, is valid. A report earlier regarding the discovery of "superfluidity" in solid He appears to be due to superfluid flow in the grain boundaries, i.e. it wasn't a supersolid.

Zz.

 

[ZapperZ]

M. Steffen et al. Measurement of the Entanglement of Two Superconducting Qubits via State Tomography, Science v.313,p.1423 (2006)

Abstract: Demonstration of quantum entanglement, a key resource in quantum computation arising from a nonclassical correlation of states, requires complete measurement of all states in varying bases. By using simultaneous measurement and state tomography, we demonstrated entanglement between two solid-state qubits. Single qubit operations and capacitive coupling between two super-conducting phase qubits were used to generate a Bell-type state. Full two-qubit tomography yielded a density matrix showing an entangled state with fidelity up to 87%. Our results demonstrate a high degree of unitary control of the system, indicating that larger implementations are within reach.

A review of this paper can be found here:
http://physicsweb.org/articles/news/10/9/3/1

Besides the obvious importance to quantum computing, this work is significance because I believe this is the first time the tomography methodology has been applied to characterize the quantum state.

Zz.

 

[Maxwell]

Here's something you can show your instructor in your circuit theory class. :)

J. Gabelli et al., Science v.313. p.499 (2006)

Violation of Kirchhoff's Laws for a Coherent RC Circuit.

Abstract:What is the complex impedance of a fully coherent quantum resistance-capacitance (RC) circuit at gigahertz frequencies in which a resistor and a capacitor are connected in series? While Kirchhoff's laws predict addition of capacitor and resistor impedances, we report on observation of a different behavior. The resistance, here associated with charge relaxation, differs from the usual transport resistance given by the Landauer formula. In particular, for a single-mode conductor, the charge-relaxation resistance is half the resistance quantum, regardless of the transmission of the mode. The new mesoscopic effect reported here is relevant for the dynamical regime of all quantum devices.

Of course, the significance of this work is not really to show that Kirchhoff's law doesn't work in this regime, but rather to study the behavior of conduction at the mesoscopic scale. This is important since electronics components are getting to be smaller, and quantum computing and circuits will be relevant in future devices. It is vital to know how such components differ from their classical counterparts.

Zz.

Zapper, you don't happen to know where I can find this article on-line, do you?

 

[nbo10]

Zapper, you don't happen to know where I can find this article on-line, do you?

http://www.sciencemag.org/cgi/content/abstract/313/5786/499

 

[ZapperZ]

Zapper, you don't happen to know where I can find this article on-line, do you?

The online access to most of the journal articles being listed here requires subscription, either personal or institutional. You didn't indicate if you have either. If you don't, then the best thing to do is go to a library or nearby university. Most tend to get site-wide access to prominent journals such as Science and Nature.

Zz.

 

[Maxwell]

The online access to most of the journal articles being listed here requires subscription, either personal or institutional. You didn't indicate if you have either. If you don't, then the best thing to do is go to a library or nearby university. Most tend to get site-wide access to prominent journals such as Science and Nature.

Zz.

I'll stop by my university library later today. Thanks for the heads up about the article.

 

[ZapperZ]

A. Naik et al. Cooling a nanomechanical resonator with quantum back-action, Nature v.443, p.193 (2006).

Abstract: Quantum mechanics demands that the act of measurement must affect the measured object. When a linear amplifier is used to continuously monitor the position of an object, the Heisenberg uncertainty relationship requires that the object be driven by force impulses, called back-action. Here we measure the back-action of a superconducting single-electron transistor (SSET) on a radio-frequency nanomechanical resonator. The conductance of the SSET, which is capacitively coupled to the resonator, provides a sensitive probe of the latter's position; back-action effects manifest themselves as an effective thermal bath, the properties of which depend sensitively on SSET bias conditions. Surprisingly, when the SSET is biased near a transport resonance, we observe cooling of the nanomechanical mode from 550 mK to 300 mK—an effect that is analogous to laser cooling in atomic physics. Our measurements have implications for nanomechanical readout of quantum information devices and the limits of ultrasensitive force microscopy (such as single-nuclear-spin magnetic resonance force microscopy). Furthermore, we anticipate the use of these back-action effects to prepare ultracold and quantum states of mechanical structures, which would not be accessible with existing technology.

Zz.

Edit: Cornel press release: http://www.news.cornell.edu/stories/Sept06/schwab.quantum.html

 

[Gokul43201]

Kramer et al, Tests of General Relativity from Timing the Double Pulsar, Science, 313, 1556 (2006)

Pulsars prove Einstein right (nearly)

19 September 2006

An international team of astronomers has used an unusual double pulsar to provide the strongest confirmation yet of Einstein’s general relativity -- the theory that physicists believe best explains gravity.

Michael Kramer at Jodrell Bank Observatory and colleagues carried out four separate tests on the pair of rotating neutron stars, verifying general relativity to an astonishing accuracy of 99.5% (Science 313 1556). The team now hopes to improve the precision so that they can eventually probe the internal structure of these superdense stars and perhaps even see the first hints of quantum gravity.

http://physicsweb.org/articles/news/10/9/10/1

 

[ZapperZ]

S. Banna et al., "Experimental Observation of Direct Particle Acceleration by Stimulated Emission of Radiation", Phys. Rev. Lett. 97, 134801 (2006)

Abstract: We report the first experimental evidence for direct particle acceleration by stimulated emission of radiation. In the framework of this proof-of-principle experiment, a 45 MeV electron macrobunch was modulated by a high-power CO2 laser and then injected into an excited CO2 gas mixture. The emerging microbunches experienced a 0.15% relative change in the kinetic energy, in a less than 40 cm long interaction region. According to our experimental results, a fraction of these electrons have gained more than 200 keV each, implying that such an electron has undergone an order of magnitude of 2×106 collisions of the second kind.

http://www.aip.org/pnu/2006/split/792-1.html

Levi Schacter also broke the news to the accelerator community for the very first time during the Advanced Accelerator Workshop this year that I co-organized. His viewgraphs presentation on this can be found here:

http://www.hep.anl.gov/aac06/plenary-talk/tuesday/Morning%20I/SCHACHTER.pdf

Zz.

 

[ZapperZ]

M. Kramer et al., "Tests of General Relativity from Timing the Double Pulsar", Science v.314, p.97.

Abstract: The double pulsar system PSR J0737-3039A/B is unique in that both neutron stars are detectable as radio pulsars. They are also known to have much higher mean orbital velocities and accelerations than those of other binary pulsars. The system is therefore a good candidate for testing Einstein's theory of general relativity and alternative theories of gravity in the strong-field regime. We report on precision timing observations taken over the 2.5 years since its discovery and present four independent strong-field tests of general relativity. These tests use the theory-independent mass ratio of the two stars. By measuring relativistic corrections to the Keplerian description of the orbital motion, we find that the "post-Keplerian" parameter s agrees with the value predicted by general relativity within an uncertainty of 0.05%, the most precise test yet obtained. We also show that the transverse velocity of the system's center of mass is extremely small. Combined with the system's location near the Sun, this result suggests that future tests of gravitational theories with the double pulsar will supersede the best current solar system tests. It also implies that the second-born pulsar may not have formed through the core collapse of a helium star, as is usually assumed.

The more they test it...

Zz.

 

[ZapperZ]

H. Sakai et al. "Spin Correlations of Strongly Interacting Massive Fermion Pairs as a Test of Bell's Inequality" Phys. Rev. Lett. 97, 150405 (2006).

Abstract: We report the results of the first-time test of the local hidden variable theories (Bell-Clauser-Horne-Shimony-Holt) involving strongly interacting pairs of massive spin 1/2 hadrons from the decay of short-lived (tau<10-21sec) 2He spin-singlet state, populated in the nuclear reaction 2H+1H-->2He+n. The novel features of this experiment are (a) the use of an "event body" detector of nearly 100% efficiency to prepare an unbiased sample and (b) a focal-plane polarimeter of full 2pi sr acceptance with a random "post selection" of the reference axes. The spin-correlation function is deduced to be Sexp(pi/4)=2.83±0.24stat±0.07sys. This result is in agreement with nonlocal quantum mechanical prediction and it violates the Bell-CHSH inequality of |S|<=2 at a confidence level of 99.3%.

Zz.

 

[Gokul43201]

Oganessian, Moody, et al. http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVCAN000074000004044602000001&idtype=cvips&gifs=yes , Phys. Rev. C 74, 044602 (2006)

Abstract:
The decay properties of ²⁹⁰116 and ²⁹¹116, and the dependence of their production cross sections on the excitation energies of the compound nucleus, ²⁹³116, have been measured in the ²⁴⁵Cm (⁴⁸Ca, xn)^293-x116 reaction. These isotopes of element 116 are the decay daughters of element 118 isotopes, which are produced via the ²⁴⁹Cf+⁴⁸Ca reaction. We performed the element 118 experiment at two projectile energies, corresponding to ²⁹⁷118 compound nucleus excitation energies of E*=29.2±2.5 and 34.4±2.3 MeV. During an irradiation with a total beam dose of 4.1×1019 ⁴⁸Ca projectiles, three similar decay chains consisting of two or three consecutive alpha decays and terminated by a spontaneous fission (SF) with high total kinetic energy of about 230 MeV were observed. The three decay chains originated from the even-even isotope ²⁹⁴118 (E_alpha=11.65±0.06 MeV, T_alpha=0.89_-0.31^+1.07 ms) produced in the 3n-evaporation channel of the ²⁴⁹Cf+⁴⁸Ca reaction with a maximum cross section of 0.5_-0.3^+1.6 pb.

http://link.aps.org/abstract/PRC/v74/e044602

Researchers discover element 118

19 October 2006

...The definitive discovery of element 118, which is expected to be a noble gas that lies right below radon in the periodic table, was a collaborative effort between researchers at the Livermore lab and the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. Based on data taken early last year in experiments at the JINR cyclotron, in which a target of californium (atomic number 98) is bombarded with a beam of calcium-48 ions, the team observed alpha decay chains that could only occur if element 118 exists. In these decay chains, previously observed element 116 is produced via the alpha decay of element 118.

http://physicsweb.org/articles/news/10/10/11/1

 

[ZapperZ]

Ambarish Ghosh and Peer Fischer "Chiral Molecules Split Light: Reflection and Refraction in a Chiral Liquid" Phys. Rev. Lett. 97, 173002 (2006).

Abstract: A light beam changes direction as it enters a liquid at an angle from another medium, such as air. Should the liquid contain molecules that lack mirror symmetry, then it has been predicted by Fresnel that the light beam will not only change direction, but will actually split into two separate beams with a small difference in the respective angles of refraction. Here we report the observation of this phenomenon. We also demonstrate that the angle of reflection does not equal the angle of incidence in a chiral medium. Unlike conventional optical rotation, which depends on the path-length through the sample, the reported reflection and refraction phenomena arise within a few wavelengths at the interface and thereby suggest a new approach to polarimetry that can be used in microfluidic volumes.

A brief review of this work can be found here:

http://focus.aps.org/story/v18/st14

Zz.

 

[ZapperZ]

M. Meschke et al. "Single-mode heat conduction by photons" Nature 444, 187 (2006).

Abstract: The thermal conductance of a single channel is limited by its unique quantum value GQ, as was shown theoretically in 1983. This result closely resembles the well-known quantization of electrical conductance in ballistic one-dimensional conductors. Interestingly, all particles—irrespective of whether they are bosons or fermions—have the same quantized thermal conductance when they are confined within dimensions that are small compared to their characteristic wavelength. The single-mode heat conductance is particularly relevant in nanostructures. Quantized heat transport through submicrometre dielectric wires by phonons has been observed, and it has been predicted to influence cooling of electrons in metals at very low temperatures due to electromagnetic radiation. Here we report experimental results showing that at low temperatures heat is transferred by photon radiation, when electron–phonon8 as well as normal electronic heat conduction is frozen out. We study heat exchange between two small pieces of normal metal, connected to each other only via superconducting leads, which are ideal insulators against conventional thermal conduction. Each superconducting lead is interrupted by a switch of electromagnetic (photon) radiation in the form of a DC-SQUID (a superconducting loop with two Josephson tunnel junctions). We find that the thermal conductance between the two metal islands mediated by photons indeed approaches the expected quantum limit of GQ at low temperatures. Our observation has practical implications—for example, for the performance and design of ultra-sensitive bolometers (detectors of far-infrared light) and electronic micro-refrigerators9, whose operation is largely dependent on weak thermal coupling between the device and its environment.

Read the News and Views article by Keith Schwab in the same issue.

Zz.

 

[Gokul43201]

E. Bustarret et al, "Superconductivity in doped cubic silicon" Nature 444, 465-468 (2006)

Abstract:
Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature Tc approximately 0.35 K, with a critical field of about 0.4 T.Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron–phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.

http://physicsweb.org/articles/news/10/11/19/1

Who ever thought you could possibly dope silicon up to 9% B? Wow!

 

[Manchot]

9%? How does it even keep its crystalline structure?

 

[ZapperZ]

Nico, J. S. et al., "Observation of the radiative decay mode of the free neutron, Nature v.444, p.1059 (2006).

Abstract: The theory of quantum electrodynamics (QED) predicts that beta decay of the neutron into a proton, electron and antineutrino should be accompanied by a continuous spectrum of soft photons. While this inner bremsstrahlung branch has been previously measured in nuclear beta and electron capture decay, it has never been observed in free neutron decay. Recently, the photon energy spectrum and branching ratio for neutron radiative decay have been calculated using two approaches: a standard QED framework and heavy baryon chiral perturbation theory (an effective theory of hadrons based on the symmetries of quantum chromodynamics). The QED calculation treats the nucleons as point-like, whereas the latter approach includes the effect of nucleon structure in a systematic way. Here we observe the radiative decay mode of free neutrons, measuring photons in coincidence with both the emitted electron and proton. We determined a branching ratio of (3.13 +/- 0.34) x 10^-3 (68 per cent level of confidence) in the energy region between 15 and 340 keV, where the uncertainty is dominated by systematic effects. The value is consistent with the predictions of both theoretical approaches; the characteristic energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the calculated spectrum. This result may provide opportunities for more detailed investigations of the weak interaction processes involved in neutron beta decay.

Also read the News and Views on this work in the same issue of Nature. This certainly would qualify as another one of those predictions from QED that has finally been verified.

Zz.

 

[ZapperZ]

Valla et al, "The Ground State of the Pseudogap in Cuprate Superconductors", Science v.314, p.1914 (2006).

Abstract: We present studies of the electronic structure of La2-xBaxCuO4, a system where the superconductivity is strongly suppressed as static spin and charge orders or "stripes" develop near the doping level of x=1/8. Using angle-resolved photoemission and scanning tunneling microscopy, we detect an energy gap at the Fermi surface with magnitude consistent with d-wave symmetry and with linear density of states, vanishing only at four nodal points, even when superconductivity disappears at x=1/8. Thus, the non-superconducting, "striped" state at x=1/8 is consistent with a phase incoherent d-wave superconductor whose Cooper pairs form spin/charge ordered structures instead of becoming superconducting.

The arxiv version of the paper, along with suppliment materials, can be found here:

http://arxiv.org/abs/cond-mat/0612672

This paper is significant because it adds another substantiative argument that the pairs in the pseudogap state of high-Tc superconductors are the pre-cursor of superconductivity without the long-range coherence. This is crucial in separating out on whether the pseudogap is an intrinsic part of the superconductivity mechanism for these materials, or simply a red herring.

However, at the same time that this paper makes that assertion, a competing preprint appears that contradicts its conclusion. I will simply point to a link to it since it has not been published yet.

http://arxiv.org/abs/cond-mat/0612048

This shows that this issue, while several issues do get resolved and become clearer, is still being hotly debated.

I will make a brief analysis of the Valla et al. paper in a thread in thehttps://www.physicsforums.com/showthread.php?p=1200954#post1200954", since this thread isn't meant for in-depth discussion of highlighted papers.

Zz.

 

[FUNKER]

Why just a thread? Wouldn't this make a good category?

just want to say I fully AGREE! :!)

 

[ZapperZ]

T. Jeltes et al., Comparison of the Hanbury Brown–Twiss effect for bosons and fermions, Nature v.445, p.402 (2007).

Abstract: Fifty years ago, Hanbury Brown and Twiss (HBT) discovered photon bunching in light emitted by a chaotic source1, highlighting the importance of two-photon correlations and stimulating the development of modern quantum optics. The quantum interpretation of bunching relies on the constructive interference between amplitudes involving two indistinguishable photons, and its additive character is intimately linked to the Bose nature of photons. Advances in atom cooling and detection have led to the observation and full characterization of the atomic analogue of the HBT effect with bosonic atoms. By contrast, fermions should reveal an antibunching effect (a tendency to avoid each other). Antibunching of fermions is associated with destructive two-particle interference, and is related to the Pauli principle forbidding more than one identical fermion to occupy the same quantum state. Here we report an experimental comparison of the fermionic and bosonic HBT effects in the same apparatus, using two different isotopes of helium: 3He (a fermion) and 4He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible; therefore, the contrasting bunching and antibunching behaviour that we observe can be fully attributed to the different quantum statistics of each atomic species. Our results show how atom–atom correlation measurements can be used to reveal details in the spatial density or momentum correlations in an atomic ensemble. They also enable the direct observation of phase effects linked to the quantum statistics of a many-body system, which may facilitate the study of more exotic situations.

Also read the News and Views of this work in the same issue of Nature, and a review of it in http://physicsweb.org/articles/news/11/1/21/1".

Zz.