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Recent Noteworthy Physics Papers

by ZapperZ
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ZapperZ
#55
Nov9-07, 08:15 AM
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D. Akoury et al., "The Simplest Double Slit: Interference and Entanglement in Double Photoionization of H2", Science v.318, p.949 (2007)

Abstract: The wave nature of particles is rarely observed, in part because of their very short de Broglie wavelengths in most situations. However, even with wavelengths close to the size of their surroundings, the particles couple to their environment (for example, by gravity, Coulomb interaction, or thermal radiation). These couplings shift the wave phases, often in an uncontrolled way, and the resulting decoherence, or loss of phase integrity, is thought to be a main cause of the transition from quantum to classical behavior. How much interaction is needed to induce this transition? Here we show that a photoelectron and two protons form a minimum particle/slit system and that a single additional electron constitutes a minimum environment. Interference fringes observed in the angular distribution of a single electron are lost through its Coulomb interaction with a second electron, though the correlated momenta of the entangled electron pair continue to exhibit quantum interference.

Also see review of this work at the PhysicsWorld website (free registration required) and at PhysOrg.

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ZapperZ
#56
Nov26-07, 01:31 PM
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L. Ozyuzer et al., "Emission of Coherent THz Radiation from Superconductors", Science v.318, p.1291 (2007).

Abstract: Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi2Sr2CaCu2O8. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists up to ~50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

Also read the Perspective of this work in the same issue of Science. A PhysicsWorld review of this work can be found here.

I know quite a bit regarding this work, since I've done tunneling spectroscopy on superconductors and that I've worked with this Bi compound. Not only that, I personally know 2 of the authors in this paper, including the lead author. In fact, I believe I was in the lab when they were doing this work (yes, I'm nosy and tend to stick my nose into people's lab, if they let me). So I'm terribly happy that they've managed to publish in Science and get quite a bit of publicity regarding this work.

Secondly, note that this is simply another one in a long line of examples where something that appears to be esoteric and purely "physics" such as quantum tunneling, Josephson current, and the physics of superconductivity can produce a clear useful application. This is still physics, not engineering. Yet, there is a clear application of a physics principle at work here.

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ZapperZ
#57
Nov26-07, 02:45 PM
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T. Paterek et al. "Experimental Test of Non-Local Realistic Theories Without The Rotational Symmetry Assumption", Phys. Rev. Lett. 99, 210406 (2007).

Abstract: We analyze the class of nonlocal realistic theories that was originally considered by Leggett [Found. Phys. 33, 1469 (2003)] and tested by us in a recent experiment [Nature (London) 446, 871 (2007)]. We derive an incompatibility theorem that works for finite numbers of polarizer settings and that does not require the previously assumed rotational symmetry of the two-particle correlation functions. The experimentally measured case involves seven different measurement settings. Using polarization-entangled photon pairs, we exclude this broader class of nonlocal realistic models by experimentally violating a new Leggett-type inequality by 80 standard deviations.

This appears to a be a follow up to their earlier Nature paper which I had highlighted here in this thread. They claim to have excluded even a larger class of non-local realistic model.

Interestingly enough, there is ANOTHER paper right after this that presents a similar report of the violation of the Leggett inequality.

Cyril Branciard et al. " Experimental Falsification of Leggett's Nonlocal Variable Model", Phys. Rev. Lett. 99, 210407 (2007).

Abstract: Bell's theorem guarantees that no model based on local variables can reproduce quantum correlations. Also, some models based on nonlocal variables, if subject to apparently “reasonable” constraints, may fail to reproduce quantum physics. In this Letter, we introduce a family of inequalities, which use a finite number of measurement settings, and which therefore allow testing Leggett's nonlocal model versus quantum physics. Our experimental data falsify Leggett's model and are in agreement with quantum predictions.

When you have two different experiments done by two independent group coming up with the same conclusion, it makes for a very convincing argument for the validity of such a conclusion.

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ZapperZ
#58
Nov28-07, 07:51 AM
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Johannes Kofler and Časlav Brukner, " Classical World Arising out of Quantum Physics under the Restriction of Coarse-Grained Measurements", Phys. Rev. Lett. 99, 180403 (2007).

Abstract: Conceptually different from the decoherence program, we present a novel theoretical approach to macroscopic realism and classical physics within quantum theory. It focuses on the limits of observability of quantum effects of macroscopic objects, i.e., on the required precision of our measurement apparatuses such that quantum phenomena can still be observed. First, we demonstrate that for unrestricted measurement accuracy, no classical description is possible for arbitrarily large systems. Then we show for a certain time evolution that under coarse-grained measurements, not only macrorealism but even classical Newtonian laws emerge out of the Schrödinger equation and the projection postulate.

A review of this work can be found in Nature's Daily Science news (link may be restricted or open only for a limited time). This is an interesting and important work because they are trying to show the quantum to classical "transition" via a different approach then the standard decoherence scenario. Essentially, the coarse-grained measurement that we make causes the classical world to emerge. If we make our measurement more precise for a system that has a large number of particles, then we should start detecting random "jumps" in the system that signify the emergence of the quantum world.

So let's see some clever experimentalist design an experiment to verify this. :)

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Gokul43201
#59
Dec20-07, 04:08 PM
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This is not entirely new research as such, but today's review article in Nature is still noteworthy as a review.

P. Monthoux, D. Pines and G. G. Lonzarich, Nature 450, 1177 (2007)

Abstract: The idea of superconductivity without the mediating role of lattice vibrations (phonons) has a long history. It was realized soon after the publication of the Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity 50 years ago that a full treatment of both the charge and spin degrees of freedom of the electron predicts the existence of attractive components of the effective interaction between electrons even in the absence of lattice vibrations—a particular example is the effective interaction that depends on the relative spins of the electrons. Such attraction without phonons can lead to electronic pairing and to unconventional forms of superconductivity that can be much more sensitive than traditional (BCS) superconductivity to the precise details of the crystal structure and to the electronic and magnetic properties of a material.

The paper discusses superconductivity may be found near the onset of a magnetically (or the analogously with charge density) ordered state. The phase space for an effectively attractive e-e interaction is shown to be much wider than that traditionally assumed from phonon mediated interactions.

http://www.nature.com/nature/journal...ature06480.pdf

http://www.eurekalert.org/pub_releas...-tqf122007.php
ZapperZ
#60
Jan10-08, 07:34 AM
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C. Hertlein et al, "Direct measurement of critical Casimir forces", Nature v.451, p.172 (2008).

Abstract: When fluctuating fields are confined between two surfaces, long-range forces arise. A famous example is the quantum-electrodynamical Casimir force that results from zero-point vacuum fluctuations confined between two conducting metal plates. A thermodynamic analogue is the critical Casimir force: it acts between surfaces immersed in a binary liquid mixture close to its critical point and arises from the confinement of concentration fluctuations within the thin film of fluid separating the surfaces. So far, all experimental evidence for the existence of this effect has been indirect. Here we report the direct measurement of critical Casimir force between a single colloidal sphere and a flat silica surface immersed in a mixture of water and 2,6-lutidine near its critical point. We use total internal reflection microscopy to determine in situ the forces between the sphere and the surface, with femtonewton resolution6. Depending on whether the adsorption preferences of the sphere and the surface for water and 2,6-lutidine are identical or opposite, we measure attractive and repulsive forces, respectively, that agree quantitatively with theoretical predictions and exhibit exquisite dependence on the temperature of the system. We expect that these features of critical Casimir forces may result in novel uses of colloids as model systems.

Also read the News and Views article on this work in the same issue of Nature, and a review of this work at PhysicsWorld.

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ZapperZ
#61
Jan28-08, 08:29 AM
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I. Ferreras et al., "Necessity of Dark Matter in Modified Newtonian Dynamics within Galactic Scales", Phys. Rev. Lett. v.100, p.031302 (2008).

Abstract: To test modified Newtonian dynamics (MOND) on galactic scales, we study six strong gravitational lensing early-type galaxies from the CASTLES sample. Comparing the total mass (from lensing) with the stellar mass content (from a comparison of photometry and stellar population synthesis), we conclude that strong gravitational lensing on galactic scales requires a significant amount of dark matter, even within MOND. On such scales a 2 eV neutrino cannot explain the excess of matter in contrast with recent claims to explain the lensing data of the bullet cluster. The presence of dark matter is detected in regions with a higher acceleration than the characteristic MOND scale of ~10^-10 m/s^2. This is a serious challenge to MOND unless lensing is qualitatively different [possibly to be developed within a covariant, such as Tensor-Vector-Scalar (TeVeS), theory]

With the Bullet cluster evidence, and now this, could MOND be in serious trouble now?

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ZapperZ
#62
Feb6-08, 10:03 AM
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A.J. Leggett, "Realism and the physical world", Rep. Prog. Phys. v.71, p.022001 (2008)

Abstract: I consider the extent to which the applicability of the concept of classical realism is constrained, irrespective of the validity or not of the quantum formalism, by existing experiments both in the EPR–Bell setup, including recent experiments testing 'nonlocal realistic' theories, and in the area of 'macroscopic quantum coherence'. Unless we are willing to sacrifice one or more other intuitively plausible notions such as that of the conventional 'arrow of time', it appears impossible, in either context, to maintain the classical notion of realism.

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ZapperZ
#63
Mar20-08, 07:54 AM
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A. Caprez et al., "A macroscopic test of the Aharonov-Bohm effect", Phys. Rev. Lett., v99, p.210401 (2007).

Abstract: The Aharonov-Bohm (AB) effect is a purely quantum mechanical effect. The original (classified as Type-I) AB-phase shift exists in experimental conditions where the electromagnetic fields and forces are zero. It is the absence of forces that makes the AB-effect entirely quantum mechanical. Although the AB-phase shift has been demonstrated unambiguously, the absence of forces in Type-I AB-effects has never been shown. Here, we report the observation of the absence of time delays associated with forces of the magnitude needed to explain the AB-phase shift for a macroscopic system.

Also see the ArXiv preprint version.

A Perspective on this work can also be found in March 20, 2008 issue of Nature (Nature, v.452, p.298 (2008)).

Looks like the AB effect is non-local after all!

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ZapperZ
#64
Apr11-08, 08:26 AM
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A.N. Pasupathy et al., "Electronic Origin of the Inhomogeneous Pairing Interaction in the High-Tc Superconductor Bi2Sr2CaCu2O8+{delta}", Science v.320, p.196 (2008).

Abstract: Identifying the mechanism of superconductivity in the high-temperature cuprate superconductors is one of the major outstanding problems in physics. We report local measurements of the onset of superconducting pairing in the high–transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+{delta} using a lattice-tracking spectroscopy technique with a scanning tunneling microscope. We can determine the temperature dependence of the pairing energy gaps, the electronic excitations in the absence of pairing, and the effect of the local coupling of electrons to bosonic excitations. Our measurements reveal that the strength of pairing is determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 volts) electron-boson interactions are responsible for superconductivity in the cuprates.

A review of this work can be found on ScienceDaily. So the cuprates may not have a "glue" that is responsible for the superconducting mechanism? Oh my! Phil Anderson might be right after all! :)

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ZapperZ
#65
Apr11-08, 02:17 PM
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E.V. Linder, "Mapping the cosmological expansion", Rep. Prog. Phys. v.71, p.056901 (2008).

Abstract: The ability to map the cosmological expansion has developed enormously, spurred by the turning point one decade ago of the discovery of cosmic acceleration. The standard model of cosmology has shifted from a matter dominated, standard gravity, decelerating expansion to the present search for the origin of acceleration in the cosmic expansion. We present a wide ranging review of the tools, challenges and physical interpretations. The tools include direct measures of cosmic scales through Type Ia supernova luminosity distances, and angular distance scales of baryon acoustic oscillation and cosmic microwave background density perturbations, as well as indirect probes such as the effect of cosmic expansion on the growth of matter density fluctuations. Accurate mapping of the expansion requires understanding of systematic uncertainties in both the measurements and the theoretical framework, but the result will give important clues to the nature of the physics behind accelerating expansion and to the fate of the universe.

And excellent review source, especially if you're interested in how various results in cosmology are obtained.

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ZapperZ
#66
Apr21-08, 08:33 AM
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D. N. Matsukevich et al., "Bell Inequality Violation with Two Remote Atomic Qubits", Phys. Rev. Lett. v.100, p.150404 (2008).

Abstract: We observe violation of a Bell inequality between the quantum states of two remote Yb+ ions separated by a distance of about 1 m with the detection loophole closed. The heralded entanglement of two ions is established via interference and joint detection of two emitted photons, whose polarization is entangled with each ion. The entanglement of remote qubits is also characterized by full quantum state tomography.

Could we be on a clear path for a loophole-free Bell-type experiment? This report certainly is providing a convincing evidence that we are well on our way!

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ZapperZ
#67
Jun23-08, 01:48 PM
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M.T. Murphy et al. "Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe", Science v. 320, p. 1611 (2008).

Abstract: The Standard Model of particle physics assumes that the so-called fundamental constants are universal and unchanging. Absorption lines arising in molecular clouds along quasar sightlines offer a precise test for variations in the proton-to-electron mass ratio, µ, over cosmological time and distance scales. The inversion transitions of ammonia are particularly sensitive to µ as compared to molecular rotational transitions. Comparing the available ammonia spectra observed toward the quasar B0218+357 with new, high-quality rotational spectra, we present the first detailed measurement of µ with this technique, limiting relative deviations from the laboratory value to |{Delta}µ/µ| < 1.8 x 10–6 (95% confidence level) at approximately half the universe's current age—the strongest astrophysical constraint to date. Higher-quality ammonia observations will reduce both the statistical and systematic uncertainties in these observations.

In other words, even as far back as half of the universe's age, this ratio of the mass of the proton to the mass of electron, hasn't changed up to the accuracy limit of this measurement.

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ZapperZ
#68
Jul25-08, 03:37 PM
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L. Li et al., "Phase Transitions of Dirac Electrons in Bismuth", Science v.321, p.547 (2008).

Abstract: The Dirac Hamiltonian, which successfully describes relativistic fermions, applies equally well to electrons in solids with linear energy dispersion, for example, in bismuth and graphene. A characteristic of these materials is that a magnetic field less than 10 tesla suffices to force the Dirac electrons into the lowest Landau level, with resultant strong enhancement of the Coulomb interaction energy. Moreover, the Dirac electrons usually come with multiple flavors or valley degeneracy. These ingredients favor transitions to a collective state with novel quantum properties in large field. By using torque magnetometry, we have investigated the magnetization of bismuth to fields of 31 tesla. We report the observation of sharp field-induced phase transitions into a state with striking magnetic anisotropy, consistent with the breaking of the threefold valley degeneracy.

Read a report of this work at Science Daily.

This is another example where relativistic equations need not require some esoteric conditions to be applicable. Some of them can be found in the very material that we use in our electronics.

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Gokul43201
#69
Aug4-08, 11:45 AM
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Kamimara, Y. et al. "Iron-based superconductor LaO1-xFxFeAs (x=0.05-0.12) with Tc=26 K". J. Am. Chem. Soc. 130, 3296 (2008).

Abstract: We report that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site. The transition temperature (Tc) exhibits a trapezoid shape dependence on the F- content, with the highest Tc of ~26 K at ~11 atom %.

Full paper available here: ACS
For further developments, see:
Science Daily
Nature Asia

Related follow up:
Chen, X.H. et al. "Superconductivity at 43 K in SmFeAsO1-xFx", Nature 453, 761 (2008).


This is a first observation of high Tc behavior outside of cuprate systems.
Gokul43201
#70
Aug4-08, 12:05 PM
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L.W. Martin, et al., "Electric field control of ferromagnetism using a magnetoelectric multiferroic," Nature Mater. 7, 478 (2008)

Abstract: Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO3. The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO3 film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co0.9Fe0.1) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.

Full text and summary available here: LBL

For a review on multiferroics, see: Ying-Hao Chu et al, "Controlling magnetism with multiferroics", Materials Today 10, 16 (2007) link

The ability to control ferromagnetism using electric fields has huge potential in the area of GMR based memory storage devices.
ZapperZ
#71
Aug14-08, 08:10 AM
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D. Salart et al., "Testing the speed of 'spooky action at a distance'", Nature v.454, p.861 (2008).

Abstract: Correlations are generally described by one of two mechanisms: either a first event influences a second one by sending information encoded in bosons or other physical carriers, or the correlated events have some common causes in their shared history. Quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This reveals itself in correlations that violate Bell inequalities (implying that they cannot be described by common causes) between space-like separated events (implying that they cannot be described by classical communication). Many Bell tests have been performed, and loopholes related to locality and detection have been closed in several independent experiments. It is still possible that a first event could influence a second, but the speed of this hypothetical influence (Einstein's 'spooky action at a distance') would need to be defined in some universal privileged reference frame and be greater than the speed of light. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test over more than 24 hours between two villages separated by 18 km and approximately east–west oriented, with the source located precisely in the middle. We continuously observed two-photon interferences well above the Bell inequality threshold. Taking advantage of the Earth's rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of the influence. For example, if such a privileged reference frame exists and is such that the Earth's speed in this frame is less than 10-3 times that of the speed of light, then the speed of the influence would have to exceed that of light by at least four orders of magnitude.

Also read the News and Views article in the same issue.

Edit: A review of this work can also be found at PhysicsWorld.

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sygrup
#72
Sep28-08, 04:08 PM
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Quote Quote by FUNKER View Post
just want to say I fully AGREE!

yes yes :))))


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