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Recent Noteworthy Physics Papers

  1. Nov 4, 2010 #121

    ZapperZ

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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 http://www.nature.com/news/2010/101103/full/news.2010.580.html" (link open for free only for a limited time).

    Zz.
     
    Last edited by a moderator: May 5, 2017
  2. Nov 4, 2010 #122

    Kurdt

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

    http://arxiv.org/abs/1010.0793
     
    Last edited by a moderator: May 5, 2017
  3. Jan 6, 2011 #123

    Cthugha

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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.
     
  4. Jan 6, 2011 #124

    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
     
  5. Jan 6, 2011 #125

    ZapperZ

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    That certainly is a very catchy ending! :)

    Zz.
     
  6. Jan 17, 2011 #126

    ZapperZ

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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/light-turns-insulator-into-a-superconductor/1006901.article

    Zz.
     
  7. Jan 20, 2011 #127

    ZapperZ

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    In case people missed it, there's a very nice review of this work in the http://focus.aps.org/story/v27/st2" [Broken] section.

    Zz.
     
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  8. Feb 1, 2011 #128

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    S. Mark et al., "Fully Electrical Read-Write Device Out of a Ferromagnetic Semiconductor", Phys. Rev. Lett. v.106, p.057204 (2011).

    Abstract: We report the realization of a read-write device out of the ferromagnetic semiconductor (Ga,Mn)As as the first step to a fundamentally new information processing paradigm. Writing the magnetic state is achieved by current-induced switching and readout of the state is done by the means of the tunneling anisotropic magnetoresistance effect. This 1 bit demonstrator device can be used to design an electrically programmable memory and logic device.

    See accompanying http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.057204" [Broken], which also gives you access to obtain the paper.

    BTW, if you encounter people who think that physics doesn't produce anything with real applications, show this!

    Zz.
     
    Last edited by a moderator: May 5, 2017
  9. Feb 1, 2011 #129
    Ok I know most people post really recent physics papers. However, this paper was done in 2006 and, as a college student myself, I think is a great teaching reasource to help student in understand angular momentum in quantum mechanics.

    "Integer Versus Half-Integer Angular Momentum", Am. J. Phys, 74, 191-192 (2006).
     
  10. Feb 2, 2011 #130

    ZapperZ

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    P. San-Jose et al, "Electron-Induced Rippling in Graphene", Phys. Rev. Lett. v.106, p.045502 (2011).

    Abstract: We show that the interaction between flexural phonons, when corrected by the exchange of electron-hole excitations, may drive the graphene sheet into a quantum critical point characterized by the vanishing of the bending rigidity of the membrane. Ripples arise then due to spontaneous symmetry breaking, following a mechanism similar to that responsible for the condensation of the Higgs field in relativistic field theories, and leading to a zero-temperature buckling transition in which the order parameter is given by the square of the gradient of the flexural phonon field.

    In other words, they think we can find hints of the Higgs field in graphene! http://physicsworld.com/cws/article/news/44994" [Broken].

    This is why condensed matter systems, such as the recently discovered topological insulators, are so interesting and important, and another example where condensed matter physics contributes to fundamental physics.

    Zz.
     
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  11. Feb 8, 2011 #131

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    A. O. Sushkov et al., "Observation of the thermal Casimir force", Nature Physics doi:10.1038/nphys1909.

    Abstract: Quantum theory predicts the existence of the Casimir force between macroscopic bodies, a force arising from the zero-point energy of electromagnetic field modes around them. A thermal Casimir force, due to thermal rather than quantum fluctuations of the electromagnetic field at finite temperature, was theoretically predicted long ago. Here we report the experimental observation of the thermal Casimir force between two gold plates. We measured the attractive force between a flat and a spherical plate for separations between 0.7 μm and 7 μm. An electrostatic force caused by potential patches on the plates’ surfaces is included in the analysis. Previous measurements of the quantum-fluctuation-induced force have been unable to clearly settle the question of whether the correct low-frequency form of the dielectric constant dispersion for calculating Casimir forces is the Drude model or the plasma model. Our experimental results are in excellent agreement (reduced χ2 of 1.04) with the Casimir force calculated using the Drude model, including the T=300 K thermal force, which dominates over the quantum fluctuation-induced force at separations greater than 3 μm. The plasma model result is excluded in the measured separation range.

    See a review of this work at http://physicsworld.com/cws/article/news/45048" [Broken].

    Zz.
     
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  12. Feb 25, 2011 #132

    Cthugha

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    S. S. Hodgman et al., "Direct Measurement of Long-Range Third-Order Coherence in Bose-Einstein Condensates", Science v.331, p.1046 (2011).

    Abstract: A major advance in understanding the behavior of light was to describe the coherence of a light source by using correlation functions that define the spatio-temporal relationship between pairs and larger groups of photons. Correlations are also a fundamental property of matter. We performed simultaneous measurement of the second- and third-order correlation functions for atoms. Atom bunching in the arrival time for pairs and triplets of thermal atoms just above the Bose-Einstein condensation (BEC) temperature was observed. At lower temperatures, we demonstrated conclusively the long-range coherence of the BEC for correlation functions to third order, which supports the prediction that like coherent light, a BEC possesses long-range coherence to all orders.
     
  13. Mar 30, 2011 #133

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    F. L. Pratt et al., "Magnetic and non-magnetic phases of a quantum spin liquid", Nature v.471, p.612 (2011).

    Abstract: A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins1. Recently, materials have become available showing persuasive experimental evidence for such a state2. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment3. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators4. The layered molecular system κ-(BEDT-TTF)2Cu2(CN)3 is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers5 that provides a prime example of a spin liquid formed in this way6, 7, 8, 9, 10, 11. Despite a high-temperature exchange coupling, J, of 250 K (ref. 6), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7, 8). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose–Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.

    Also see a review of this work in the News and Views section of the same issue of Nature on page 587.

    Zz.
     
  14. May 15, 2011 #134
    Hans De Raedt et al: "Extended Boole-Bell inequalities applicable to quantum theory"
    J. Comp. Theor. Nanosci. Vol. 8, No.6, p.1011, (2011)
    http://www.ingentaconnect.com/content/asp/jctn/2011/00000008/00000006/art00013

    Full text also in http://arxiv.org/abs/0901.2546

    Abstract:
    We address the basic meaning of apparent contradictions of quantum theory and probability frameworks as expressed by Bell's inequalities. We show that these contradictions have their origin in the incomplete considerations of the premises of the derivation of the inequalities. A careful consideration of past work, including that of Boole and Vorob'ev, has lead us to the formulation of extended Boole-Bell inequalities that are binding for both classical and quantum models. The Einstein-Podolsky-Rosen-Bohm gedanken experiment and a macroscopic quantum coherence experiment proposed by Leggett and Garg are both shown to obey the extended Boole-Bell inequalities. These examples as well as additional discussions also provide reasons for apparent violations of these inequalities.

    This paper challenges Bell's Theorem about reality and locality. Discussion thread:
    https://www.physicsforums.com/showthread.php?t=499002
     
    Last edited: May 15, 2011
  15. May 26, 2011 #135

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    J.J. Hudson et al., "Improved measurement of the shape of the electron", Nature v.473, p. 493 (2011).

    Abstract: The electron is predicted to be slightly aspheric1, with a distortion characterized by the electric dipole moment (EDM), de. No experiment has ever detected this deviation. The standard model of particle physics predicts that de is far too small to detect2, being some eleven orders of magnitude smaller than the current experimental sensitivity. However, many extensions to the standard model naturally predict much larger values of de that should be detectable3. This makes the search for the electron EDM a powerful way to search for new physics and constrain the possible extensions. In particular, the popular idea that new supersymmetric particles may exist at masses of a few hundred GeV/c2 (where c is the speed of light) is difficult to reconcile with the absence of an electron EDM at the present limit of sensitivity2, 4. The size of the EDM is also intimately related to the question of why the Universe has so little antimatter. If the reason is that some undiscovered particle interaction5 breaks the symmetry between matter and antimatter, this should result in a measurable EDM in most models of particle physics2. Here we use cold polar molecules to measure the electron EDM at the highest level of precision reported so far, providing a constraint on any possible new interactions. We obtain de = (−2.4 ± 5.7stat ± 1.5syst) × 10−28e cm, where e is the charge on the electron, which sets a new upper limit of |de| < 10.5 × 10−28e cm with 90 per cent confidence. This result, consistent with zero, indicates that the electron is spherical at this improved level of precision. Our measurement of atto-electronvolt energy shifts in a molecule probes new physics at the tera-electronvolt energy scale2.

    Read the News reports at (open for a limited time)

    http://www.nature.com/nature/journal/v473/n7348/full/nature10104.html

    A News and Views on this work written by A. Leanhardt can also also be found in the same Nature issue.

    The PhysicsWorld report on this work can be found here (you may need to register to view this later on):

    http://physicsworld.com/cws/article/news/46085

    Zz.
     
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  16. Jun 3, 2011 #136

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    S. Kocsis et al.,"Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer", Science v.332, p.1170 (2011).

    Abstract: A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.

    Press release on this work http://www.physorg.com/news/2011-06-quantum-physics-photons-two-slit-interferometer.html" [Broken].

    Astounding piece of work and the use of the weak measurement technique. This is another example where one has to understand the non-commutative operator principle of the "First Quantization" to be able to comprehend what they are doing.

    If this work holds up, it might possibly be the first indication that there's something to the Bohm-de Broglie picture of quantum mechanics.

    Zz.
     
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  17. Jun 8, 2011 #137

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    On the heels of the measurement of the average trajectory taken in a double-slit experiment using the weak measurement technique, along comes another report on a fundamental measurement in QM, also using the weak measurement. This time, they made a "direct" measurement of the QM wavefunction itself!

    J.S. Lundeen et al., "Direct measurement of the quantum wavefunction" Nature v.474, p.188 (2011).

    Abstract: The wavefunction is the complex distribution used to completely describe a quantum system, and is central to quantum theory. But despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of the wavefunction through its use to calculate measurement outcome probabilities by way of the Born rule. At present, the wavefunction is determined through tomographic methods which estimate the wavefunction most consistent with a diverse collection of measurements. The indirectness of these methods compounds the problem of defining the wavefunction. Here we show that the wavefunction can be measured directly by the sequential measurement of two complementary variables of the system. The crux of our method is that the first measurement is performed in a gentle way through weak measurement so as not to invalidate the second. The result is that the real and imaginary components of the wavefunction appear directly on our measurement apparatus. We give an experimental example by directly measuring the transverse spatial wavefunction of a single photon, a task not previously realized by any method. We show that the concept is universal, being applicable to other degrees of freedom of the photon, such as polarization or frequency, and to other quantum systems—for example, electron spins, SQUIDs (superconducting quantum interference devices) and trapped ions. Consequently, this method gives the wavefunction a straightforward and general definition in terms of a specific set of experimental operations. We expect it to expand the range of quantum systems that can be characterized and to initiate new avenues in fundamental quantum theory.

    Zz.
     
  18. Jun 16, 2011 #138

    ZapperZ

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    S. Zhang et al., "Optical Precursor of a Single Photon" Phys. Rev. Lett. v.106, p.243602 (2011).

    Abstract: We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.

    Single photons http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.243602" [Broken] in vacuum!

    Zz.
     
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  19. Jun 24, 2011 #139

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    R. Lapkiewicz et al., "Experimental non-classicality of an indivisible quantum system, Nature v.474, p.490 (2011).

    Abstract: In contrast to classical physics, quantum theory demands that not all properties can be simultaneously well defined; the Heisenberg uncertainty principle is a manifestation of this fact. Alternatives have been explored—notably theories relying on joint probability distributions or non-contextual hidden-variable models, in which the properties of a system are defined independently of their own measurement and any other measurements that are made. Various deep theoretical results imply that such theories are in conflict with quantum mechanics. Simpler cases demonstrating this conflict have been found and tested experimentally with pairs of quantum bits (qubits). Recently, an inequality satisfied by non-contextual hidden-variable models and violated by quantum mechanics for all states of two qubits was introduced and tested experimentally. A single three-state system (a qutrit) is the simplest system in which such a contradiction is possible; moreover, the contradiction cannot result from entanglement between subsystems, because such a three-state system is indivisible. Here we report an experiment with single photonic qutrits which provides evidence that no joint probability distribution describing the outcomes of all possible measurements—and, therefore, no non-contextual theory—can exist. Specifically, we observe a violation of the Bell-type inequality found by Klyachko, Can, Binicioğlu and Shumovsky. Our results illustrate a deep incompatibility between quantum mechanics and classical physics that cannot in any way result from entanglement.

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  20. Jul 12, 2011 #140

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    P. Adamson et al., "First Direct Observation of Muon Antineutrino Disappearance", Phys. Rev. Lett. v.107, p.021801 (2011).

    Abstract: This Letter reports the first direct observation of muon antineutrino disappearance. The MINOS experiment has taken data with an accelerator beam optimized for ν̅ μ production, accumulating an exposure of 1.71×1020 protons on target. In the Far Detector, 97 charged current ν̅ μ events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at 6.3σ. The best fit to oscillation yields |Δm̅2|=[3.36-0.40+0.46(stat)±0.06(syst)]×10-3  eV2, sin⁡2(2θ̅ )=0.86-0.12+0.11(stat)±0.01(syst). The MINOS νμ and ν̅ μ measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters.

    You may read a review of this at the http://physics.aps.org/articles/v4/54" [Broken] and also obtain a free download of the actual paper.

    Zz.
     
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