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Recent Noteworthy Physics Papers

by ZapperZ
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ZapperZ
#109
Jun8-10, 08:14 AM
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J.T. Stewart et al., "Verification of Universal Relations in a Strongly Interacting Fermi Gas", Phys. Rev. Lett. 104, 235301 (2010).

Abstract: Many-body fermion systems are important in many branches of physics, including condensed matter, nuclear, and now cold atom physics. In many cases, the interactions between fermions can be approximated by a contact interaction. A recent theoretical advance in the study of these systems is the derivation of a number of exact universal relations that are predicted to be valid for all interaction strengths, temperatures, and spin compositions. These equations, referred to as the Tan relations, relate a microscopic quantity, namely, the amplitude of the high-momentum tail of the fermion momentum distribution, to the thermodynamics of the many-body system. In this work, we provide experimental verification of the Tan relations in a strongly interacting gas of fermionic atoms by measuring both the microscopic and macroscopic quantities in the same system.

You may read the Viewpoint article on this work here, and also get a free download of the actual publication.

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ZapperZ
#110
Jun17-10, 11:57 AM
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J.-P. Bocquet et al. "Limits on Light-Speed Anisotropies from Compton Scattering of High-Energy Electrons", Phys. Rev. Lett. v.104, p.241601 (2010).

Abstract: The possibility of anisotropies in the speed of light relative to the limiting speed of electrons is considered. The absence of sidereal variations in the energy of Compton-edge photons at the European Synchrotron Radiation Facility’s GRAAL facility constrains such anisotropies representing the first nonthreshold collision-kinematics study of Lorentz violation. When interpreted within the minimal standard-model extension, this result yields the two-sided limit of 1.6×10^-14 at 95% confidence level on a combination of the parity-violating photon and electron coefficients (κ˜o+)YZ, (κ˜o+)ZX, cTX, and cTY. This new constraint provides an improvement over previous bounds by 1 order of magnitude.

The ArXiv version can be found here.

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Count Iblis
#111
Jun17-10, 12:21 PM
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T. Goldman, Neutrino Oscillations and Energy-Momentum Conservation, Mod. Phys. Lett. A25, 479 (2010).

Abstract:

A description of neutrino oscillation phenomena is presented which is based on relativistic quantum mechanics and includes both entangled state and source dependent aspects, unlike both of the conventional approaches which use either equal energies or equal momenta for the different neutrino mass eigenstates. To second order in the neutrino masses, the standard result is recovered thus showing an absence of source dependence to this order. The time dependence of the wavefunction is found to be crucial to recovering the conventional result. An ambiguity appears at fourth order in the neutrino masses which generally leads to source dependence, but the standard formula can be promoted to this order by a plausible convention.
Preprint version.
ZapperZ
#112
Jun18-10, 07:02 AM
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T. van Zoest et al., "Bose-Einstein Condensation in Microgravity", Science v.328, p.1540 (2010).

Abstract: Albert Einstein’s insight that it is impossible to distinguish a local experiment in a "freely falling elevator" from one in free space led to the development of the theory of general relativity. The wave nature of matter manifests itself in a striking way in Bose-Einstein condensates, where millions of atoms lose their identity and can be described by a single macroscopic wave function. We combine these two topics and report the preparation and observation of a Bose-Einstein condensate during free fall in a 146-meter-tall evacuated drop tower. During the expansion over 1 second, the atoms form a giant coherent matter wave that is delocalized on a millimeter scale, which represents a promising source for matter-wave interferometry to test the universality of free fall with quantum matter.

Also see the Perspective article on this paper in the same issue of Science.

A presentation viewgraphs by one of the authors can be found here.

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ZapperZ
#113
Jul23-10, 06:55 AM
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U. Sinha et al., "Ruling Out Multi-Order Interference in Quantum Mechanics, Science v.329, p.418 (2010).

Abstract:Quantum mechanics and gravitation are two pillars of modern physics. Despite their success in describing the physical world around us, they seem to be incompatible theories. There are suggestions that one of these theories must be generalized to achieve unification. For example, Born’s rule—one of the axioms of quantum mechanics—could be violated. Born’s rule predicts that quantum interference, as shown by a double-slit diffraction experiment, occurs from pairs of paths. A generalized version of quantum mechanics might allow multipath (i.e., higher-order) interference, thus leading to a deviation from the theory. We performed a three-slit experiment with photons and bounded the magnitude of three-path interference to less than 10^–2 of the expected two-path interference, thus ruling out third- and higher-order interference and providing a bound on the accuracy of Born’s rule. Our experiment is consistent with the postulate both in semiclassical and quantum regimes.

Review of this work can be found at PhysOrg and PhysicsWorld.

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ultrafast
#114
Jul23-10, 05:11 PM
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I imagine APS' Physics site showcases many of these papers.
ZapperZ
#115
Jul24-10, 09:04 AM
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Quote Quote by ultrafast View Post
I imagine APS' Physics site showcases many of these papers.
That site highlights only papers published in the APS journals (Physical Review family), since they provide free access to those papers.

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ZapperZ
#116
Jul28-10, 02:15 PM
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E. Haller et al., "Pinning quantum phase transition for a Luttinger liquid of strongly interacting bosons", Nature v.466, p.597 (2010).

Abstract: Quantum many-body systems can have phase transitions even at zero temperature; fluctuations arising from Heisenberg’s uncertainty principle, as opposed to thermal effects, drive the system from one phase to another. Typically, during the transition the relative strength of two competing terms in the system’s Hamiltonian changes across a finite critical value. A well-known example is the Mott–Hubbard quantum phase transition from a superfluid to an insulating phase, which has been observed for weakly interacting bosonic atomic gases. However, for strongly interacting quantum systems confined to lower-dimensional geometry, a novel type of quantum phase transition may be induced and driven by an arbitrarily weak perturbation to the Hamiltonian. Here we observe such an effect—the sine–Gordon quantum phase transition from a superfluid Luttinger liquid to a Mott insulator, —in a one-dimensional quantum gas of bosonic caesium atoms with tunable interactions. For sufficiently strong interactions, the transition is induced by adding an arbitrarily weak optical lattice commensurate with the atomic granularity, which leads to immediate pinning of the atoms. We map out the phase diagram and find that our measurements in the strongly interacting regime agree well with a quantum field description based on the exactly solvable sine–Gordon model. We trace the phase boundary all the way to the weakly interacting regime, where we find good agreement with the predictions of the one-dimensional Bose–Hubbard model. Our results open up the experimental study of quantum phase transitions, criticality and transport phenomena beyond Hubbard-type models in the context of ultracold gases.

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ZapperZ
#117
Aug6-10, 10:50 AM
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J. Leach et al., "Quantum Correlations in Optical Angle–Orbital Angular Momentum Variables", Science v.329, p.662 (2010).

Abstract: Entanglement of the properties of two separated particles constitutes a fundamental signature of quantum mechanics and is a key resource for quantum information science. We demonstrate strong Einstein, Podolsky, and Rosen correlations between the angular position and orbital angular momentum of two photons created by the nonlinear optical process of spontaneous parametric down-conversion. The discrete nature of orbital angular momentum and the continuous but periodic nature of angular position give rise to a special sort of entanglement between these two variables. The resulting correlations are found to be an order of magnitude stronger than those allowed by the uncertainty principle for independent (nonentangled) particles. Our results suggest that angular position and orbital angular momentum may find important applications in quantum information science.

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ZapperZ
#118
Oct8-10, 07:06 AM
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O. Ben-David et al., "The Dynamics of the Onset of Frictional Slip", Science v.330, p.211 (2010).

Abstract: The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip. Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.

We get frequent questions on the origin of friction and when things start to slip. This shows that even on something that we know at the "macroscopic" level, there's still a lot to learn at the microscopic scale.

Edit: The Science webpage has a tag that says "FREE Full Text" for this paper. I don't know if you get to see this paper for free, but here's the link to it.

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ZapperZ
#119
Oct15-10, 07:57 AM
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I. Altfeder et al., "Vacuum Phonon Tunneling", Phys. Rev. Lett. v.105, p.166101 (2010).

Abstract: Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.

A Physical Review Focus article of this work can be found here.

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ZapperZ
#120
Nov3-10, 08:17 AM
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L.A. Wray et al., "Observation of topological order in a superconducting doped topological insulator", Nature Physics v.6, p.855 (2010).

Abstract: [1]Experimental observation of topological order in three-dimensional bulk solids has recently led to a flurry of research activity. Unlike the two-dimensional electron gas or quantum Hall systems, three-dimensional topological insulators can harbour superconductivity and magnetism, making it possible to study the interplay between topologically ordered phases and broken-symmetry states. One outcome of this interplay is the possible realization of Majorana fermions—quasiparticles that are their own antiparticles—on topological surfaces, which is of great interest in fundamental physics. Here we present measurements of the bulk and surface electron dynamics in Bi2Se3 doped with copper with a transition temperature Tc up to 3.8 K, observing its topological character for the first time. Our data show that superconductivity occurs in a bulk relativistic quasiparticle regime where an unusual doping mechanism causes the spin-polarized topological surface states to remain well preserved at the Fermi level of the superconductor where Cooper pairing takes place. These results suggest that the electron dynamics in superconducting Bi2Se3 are suitable for trapping non-Abelian Majorana fermions. Details of our observations constitute important clues for developing a general theory of topological superconductivity in doped topological insulators.[/i]

A review of this paper can be found here.

It won't be surprising if the Majorana fermions are first found in such a condensed matter system.

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ZapperZ
#121
Nov4-10, 07:01 AM
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D.J. Toms, "Quantum gravitational contributions to quantum electrodynamics", Nature v.468, p.56 (2010).

Abstract: Quantum electrodynamics describes the interactions of electrons and photons. Electric charge (the gauge coupling constant) is energy dependent, and there is a previous claim that charge is affected by gravity (described by general relativity) with the implication that the charge is reduced at high energies. However, that claim has been very controversial and the matter has not been settled. Here I report an analysis (free from the earlier controversies) demonstrating that quantum gravity corrections to quantum electrodynamics have a quadratic energy dependence that result in the electric charge vanishing at high energies, a result known as asymptotic freedom.

A review of this work can be found here (link open for free only for a limited time).

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Kurdt
#122
Nov4-10, 08:30 AM
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Quote Quote by ZapperZ View Post
D.J. Toms, "Quantum gravitational contributions to quantum electrodynamics", Nature v.468, p.56 (2010).

Abstract: Quantum electrodynamics describes the interactions of electrons and photons. Electric charge (the gauge coupling constant) is energy dependent, and there is a previous claim that charge is affected by gravity (described by general relativity) with the implication that the charge is reduced at high energies. However, that claim has been very controversial and the matter has not been settled. Here I report an analysis (free from the earlier controversies) demonstrating that quantum gravity corrections to quantum electrodynamics have a quadratic energy dependence that result in the electric charge vanishing at high energies, a result known as asymptotic freedom.

A review of this work can be found here (link open for free only for a limited time).

Zz.
The pre-print is on arXiv at the following link:

http://arxiv.org/abs/1010.0793
Cthugha
#123
Jan6-11, 05:11 AM
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R. Ahuja et al., "Relativity and the Lead-Acid Battery", Phys. Rev. Lett. v.106, p.018301 (2011).

Abstract: The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at nonrelativistic, scalar-relativistic, and fully relativistic levels, and using several exchange-correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7–1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

This is a good demonstration that there are devices used every day - like the lead-acid battery, which are still not understood ab-initio today and much more complex than one might imagine. In this case relativistic effects become important. Also, the concluding sentence of the paper: "Finally, we note that cars start due to relativity." is one of the funniest paper endings I have read recently.
teopze
#124
Jan6-11, 09:42 AM
P: 1
Quote Quote by Cthugha View Post
R. Ahuja et al., "Relativity and the Lead-Acid Battery", Phys. Rev. Lett. v.106, p.018301 (2011).

Abstract: The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at nonrelativistic, scalar-relativistic, and fully relativistic levels, and using several exchange-correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7–1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

This is a good demonstration that there are devices used every day - like the lead-acid battery, which are still not understood ab-initio today and much more complex than one might imagine. In this case relativistic effects become important. Also, the concluding sentence of the paper: "Finally, we note that cars start due to relativity." is one of the funniest paper endings I have read recently.

I'm glad you like the ending :)

Although we did expect relativity to have some impact on the EMF of the lead-acid battery it was a surprise to find out that relativity accounts for such a major part of the voltage. Thanks for posting the paper on Physics Forums. All the best. PZE
ZapperZ
#125
Jan6-11, 10:29 AM
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That certainly is a very catchy ending! :)

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ZapperZ
#126
Jan17-11, 07:22 AM
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D. Fausti et al., "Light-Induced Superconductivity in a Stripe-Ordered Cuprate", Science v.331, p.189 (2011).

Abstract: One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional “striped” spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting [itex]La_{1.675}Eu_{0.2}Sr_{0.125}CuO_4[/itex], into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

News report of it can be found here:

http://www.theengineer.co.uk/news/li...006901.article

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