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Recent Noteworthy Physics Papers

by ZapperZ
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ZapperZ
#1
Jul27-06, 01:42 PM
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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.
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actionintegral
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Jul27-06, 01:47 PM
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Why just a thread? Wouldn't this make a good category?
ZapperZ
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Jul27-06, 01:54 PM
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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 ([itex]\alpha[/itex]). 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
#4
Jul27-06, 02:00 PM
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Recent Noteworthy Physics Papers

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=...ant-ph/0603153

Not so much because I know exactly what's going on, but it sounds relevent to the recent experiments in which light was stopped. I'm reading it now...
Gokul43201
#5
Jul27-06, 02:43 PM
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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
#6
Jul28-06, 06:49 AM
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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 [itex]~9×10^{22}[/itex] 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 [itex]|g_P^e g_S^N|/(\hbar c) < 1.7 \times10^{-36}[/itex] and [itex]|g_A^e g_V^N|/(\hbar c) < 4.8 \times 10^{-56}[/itex] for [itex]\lambda > 1[/itex] AU. Our preferred-frame constraints, interpreted in the Kostelecký framework, set an upper limit on the parameter |b|<= [itex]5.0 \times10^{-21}[/itex] eV that should be compared to the benchmark value [itex]m_e^2/M_{Planck} = 2 \times 10^{-17}[/itex] 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
#7
Jul28-06, 08:32 AM
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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
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Jul31-06, 06:55 AM
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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
#9
Aug2-06, 07:06 AM
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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
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Aug5-06, 08:13 PM
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Zz, would you like to post a few comments on JC Davis' recent Nature paper on tunneling spectroscopy in BSCCO?
ZapperZ
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Aug5-06, 09:06 PM
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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.

:)

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ZapperZ
#12
Aug7-06, 07:36 AM
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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 [itex]Bi_2Sr_2CaCu_2O_{8+\delta}[/itex]

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 [itex]d^2I/dV^2[/itex] 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 [itex]d^2I/dV^2[/itex] spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report [itex]d^2I/dV^2[/itex] imaging studies of the high-Tc superconductor [itex]Bi_2Sr_2CaCu_2O_{8+\delta}[/itex]. We find intense disorder of electron–boson interaction energies at the nanometre scale, along with the expected modulations in [itex]d^2I/dV^2[/itex]. 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 [itex]^{18}O[/itex] for [itex]^{16}O[/itex] 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 [itex]d^2I/dV^2[/itex] 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 occuring), 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
#13
Aug8-06, 02:23 PM
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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
#14
Aug12-06, 06:49 AM
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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 [itex]^{133}[/itex]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 [itex]^{133}[/itex]Cs standard. At room temperatures, the values from various groups differ at the 3×10[itex]^{-15}[/itex] level, while modern clocks are aiming at 10[itex]^{-16}[/itex] 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 [itex]\beta[/itex] = -(1.710±0.006)×10[itex]^{-14}[/itex], implying 6×10[itex]^{-17}[/itex] 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 [[itex]\delta\nu/E^2[/itex] = -2.26(2)×10[itex]^{-10 }[/itex]Hz/(V/m)[itex]^2[/itex]] 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
#15
Aug13-06, 09:09 AM
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Long-Lived Bloch Oscillations with Bosonic Sr Atoms and Application to Gravity Measurement at the Micrometer Scale


Abstract:

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
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Aug29-06, 05:41 AM
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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 [itex]^4[/itex]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
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Sep12-06, 06:48 AM
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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
#18
Sep12-06, 06:57 AM
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Quote Quote by 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.
Zapper, you don't happen to know where I can find this article on-line, do you?


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