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Recent Noteworthy Physics Papers

by ZapperZ
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nbo10
#19
Sep12-06, 07:02 AM
P: 415
Quote Quote by Maxwell
Zapper, you don't happen to know where I can find this article on-line, do you?
http://www.sciencemag.org/cgi/conten...t/313/5786/499
ZapperZ
#20
Sep12-06, 07:20 AM
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Quote Quote by Maxwell
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
#21
Sep12-06, 07:35 AM
P: 531
Quote Quote by ZapperZ
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
#22
Sep13-06, 01:28 PM
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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/...b.quantum.html
Gokul43201
#23
Sep21-06, 02:15 PM
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Kramer et al, Tests of General Relativity from Timing the Double Pulsar, Science, 313, 1556 (2006)

Quote Quote by Physicsweb
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
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#24
Oct3-06, 12:09 PM
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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.../SCHACHTER.pdf

Zz.
ZapperZ
#25
Oct6-06, 05:49 AM
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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
#26
Oct13-06, 07:12 AM
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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
#27
Oct23-06, 08:01 AM
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Oganessian, Moody, et al. "Synthesis of the isotopes of elements 118 and 116 in the 249Cf and 245Cm+48Ca fusion reactions" , 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<sub>-0.31</sub><sup>+1.07</sup> ms) produced in the 3n-evaporation channel of the 249Cf+48Ca reaction with a maximum cross section of 0.5<sub>-0.3</sub><sup>+1.6</sup> 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
#28
Nov6-06, 07:44 PM
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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
#29
Nov10-06, 11:52 AM
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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
#30
Nov23-06, 09:39 AM
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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
#31
Dec5-06, 08:17 PM
P: 728
9%? How does it even keep its crystalline structure?
ZapperZ
#32
Dec20-06, 02:38 PM
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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
#33
Jan1-07, 07:21 AM
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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 the Atomic, Solid State, Comp. Physics forum, since this thread isn't meant for in-depth discussion of highlighted papers.

Zz.
FUNKER
#34
Jan18-07, 04:33 PM
P: 122
Quote Quote by actionintegral View Post
Why just a thread? Wouldn't this make a good category?
just want to say I fully AGREE!
ZapperZ
#35
Jan25-07, 12:08 PM
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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 PhysicsWeb.

Zz.
ZapperZ
#36
Feb7-07, 02:41 PM
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Naomi S. Ginsberg et al., "Coherent control of optical information with matter wave dynamics", Nature v.445, p.623 (2007).

Abstract: In recent years, significant progress has been achieved in manipulating matter with light, and light with matter1. Resonant laser fields interacting with cold, dense atom clouds provide a particularly rich system. Such light fields interact strongly with the internal electrons of the atoms, and couple directly to external atomic motion through recoil momenta imparted when photons are absorbed and emitted. Ultraslow light propagation in Bose–Einstein condensates represents an extreme example of resonant light manipulation using cold atoms. Here we demonstrate that a slow light pulse can be stopped and stored in one Bose–Einstein condensate and subsequently revived from a totally different condensate, 160 mum away; information is transferred through conversion of the optical pulse into a travelling matter wave. In the presence of an optical coupling field, a probe laser pulse is first injected into one of the condensates where it is spatially compressed to a length much shorter than the coherent extent of the condensate. The coupling field is then turned off, leaving the atoms in the first condensate in quantum superposition states that comprise a stationary component and a recoiling component in a different internal state. The amplitude and phase of the spatially localized light pulse are imprinted on the recoiling part of the wavefunction, which moves towards the second condensate. When this 'messenger' atom pulse is embedded in the second condensate, the system is re-illuminated with the coupling laser. The probe light is driven back on and the messenger pulse is coherently added to the matter field of the second condensate by way of slow-light-mediated atomic matter-wave amplification. The revived light pulse records the relative amplitude and phase between the recoiling atomic imprint and the revival condensate. Our results provide a dramatic demonstration of coherent optical information processing with matter wave dynamics. Such quantum control may find application in quantum information processing and wavefunction sculpting.

This is from Lena Hau group who, a few years ago, demonstrated that light can be stopped and then retransmitted exactly (albeit at a lower intensity). The difference in this experiment is that they used quantum mechanical property (superposition) of the spins to store light in one BEC gas and then retransmit it using a 2nd BEC gas. Very clever!

You may read an overview of this work below. Note that the Nature link works only for a short period of time.

http://www.nature.com/news/2007/0702.../070205-8.html
http://physicsweb.org/articles/news/11/2/7/1

There is also a streaming video from the Nature site:

http://www.nature.com/nature/videoar...ght/index.html

Zz.


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