Recent Noteworthy Physics Papers

Gokul43201

Kamimara, Y. et al. "Iron-based superconductor LaO_1-xF_xFeAs (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 O^2- 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 SmFeAsO_1-xF_x", Nature 453, 761 (2008).


This is a first observation of high Tc behavior outside of cuprate systems.

Gokul43201

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 (Co_0.9Fe_0.1) 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

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.

Zz.

sygrup

yes yes :))))

ZapperZ

C.G. Camara et al. "Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape", Nature v.455, p.1089 (2008).

Abstract: Relative motion between two contacting surfaces can produce visible light, called triboluminescence. This concentration of diffuse mechanical energy into electromagnetic radiation has previously been observed to extend even to X-ray energies. Here we report that peeling common adhesive tape in a moderate vacuum produces radio and visible emission along with nanosecond, 100-mW X-ray pulses that are correlated with stick–slip peeling events. For the observed 15-keV peak in X-ray energy, various models give a competing picture of the discharge process, with the length of the gap between the separating faces of the tape being 30 or 300 mum at the moment of emission. The intensity of X-ray triboluminescence allowed us to use it as a source for X-ray imaging. The limits on energies and flash widths that can be achieved are beyond current theories of tribology.

This thing has been getting a lot of popular media coverage because the simple act of peeling an ordinary scotch tape in moderate vacuum can actually generate a small amount of short x-ray burst.

Zz.

mn4j

Event-by-Event Simulation of Einstein-Podolsky-Rosen-Bohm Experiments:

http://www.springerlink.com/content/p28v88867w7213mu/ Open Access
http://arxiv.org/pdf/0712.3693

Abstract We construct an event-based computer simulation model of the Einstein-Podolsky-Rosen-Bohm experiments with photons. The algorithm is a one-to-one copy of the data gathering and analysis procedures used in real laboratory experiments. We consider two types of experiments, those with a source emitting photons with opposite but otherwise unpredictable polarization and those with a source emitting photons with fixed polarization. In the simulation, the choice of the direction of polarization measurement for each detection event is arbitrary. We use three different procedures to identify pairs of photons and compute the frequency of coincidences by analyzing experimental data and simulation data. The model strictly satisfies Einstein’s criteria of local causality, does not rely on any concept of quantum theory and reproduces the results of quantum theory for both types of experiments. We give a rigorous proof that the probabilistic description of the simulation model yields the quantum theoretical expressions for the single- and two-particle expectation values.

ZapperZ

A. Fragner et al. "Resolving Vacuum Fluctuations in an Electrical Circuit by Measuring the Lamb Shift", Science v.322, p.1357 (2008).

Abstract: Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solid-state system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmission-line resonator leads to measurable Lamb shifts of up to 1.4% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states.

An amazing feat to detect a Lamb shift in a many-body system such an a superconducting electronic circuit.

Zz.

ZapperZ

J. S. Lundeen and A. M. Steinberg, "Experimental Joint Weak Measurement on a Photon Pair as a Probe of Hardy's Paradox", Phys. Rev. Lett. 102, 020404 (2009).

Abstract: It has been proposed that the ability to perform joint weak measurements on postselected systems would allow us to study quantum paradoxes. These measurements can investigate the history of those particles that contribute to the paradoxical outcome. Here we experimentally perform weak measurements of joint (i.e., nonlocal) observables. In an implementation of Hardy's paradox, we weakly measure the locations of two photons, the subject of the conflicting statements behind the paradox. Remarkably, the resulting weak probabilities verify all of these statements but, at the same time, resolve the paradox.

This experiment appears to be the confirmation and resolution of the Hardy's paradox.

A news article on this can be found here.

Zz.

ZapperZ

A. Cabello et al., "Proposed Bell Experiment with Genuine Energy-Time Entanglement", Phys. Rev. Lett. v.102, p.040401 (2009).

Abstract: Franson's Bell experiment with energy-time entanglement [Phys. Rev. Lett. 62, 2205 (1989)] does not rule out all local hidden variable models. This defect can be exploited to compromise the security of Bell inequality-based quantum cryptography. We introduce a novel Bell experiment using genuine energy-time entanglement, based on a novel interferometer, which rules out all local hidden variable models. The scheme is feasible with actual technology.

Zz.

hiyok

this is a recent paper addressing the origin of spin glass in hole-doped cuprate superconductors. The author attempts a new mechanism for spin glass that can live with Zhang-Rice singlet states. The paper is located at

J. Phys.: Condens. Matter 21 (2009) 075702

Abstract: To address the incompatibility of Zhang–Rice singlet formation and the observed spin glass behavior, an effective model is proposed for the electronic behavior of cuprate materials. The model includes an antiferromagnetic interaction between the spin of the hole in a Zhang–Rice orbital and the spin of the hole on the corresponding copper site. While in the large interaction limit this recovers the t–J model, in the low energy limit the Zhang–Rice singlets are deformed. It is also shown that such deformation can induce random defect ferromagnetic (FM) bonds between adjacent local spins, an effect herein referred to as unusual double exchange, and then spin glass behavior shall result in the case of localized holes. A derivation of the model is also presented.

ZapperZ

V. Moshchalkov et al., "Type-1.5 Superconductivity" Phys. Rev. Lett. 102, 117001 (2009)

Abstract: We demonstrate the existence of a novel superconducting state in high quality two-component MgB2 single crystalline superconductors where a unique combination of both type-1 (lambda1/xi1<1/sqrt(2)) and type-2 (lambda2/xi2>1/sqrt(2)) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.

If this is true, they have found a new phase of superconductivity where both Type I and Type II properties resides in the same material (but in different bands).

You may also read a review of this work at Physics, AND, get a free copy of the exact paper.

Zz.

Cthugha

D. Gross, S.T. Flammia and J. Eisert, "Most Quantum States Are Too Entangled To Be Useful As Computational Resources" Phys. Rev. Lett. 102, 190501 (2009)

Abstract: It is often argued that entanglement is at the root of the speedup for quantum compared to classical computation, and that one needs a sufficient amount of entanglement for this speedup to be manifest. In measurement-based quantum computing, the need for a highly entangled initial state is particularly obvious. Defying this intuition, we show that quantum states can be too entangled to be useful for the purpose of computation, in that high values of the geometric measure of entanglement preclude states from offering a universal quantum computational speedup. We prove that this phenomenon occurs for a dramatic majority of all states: the fraction of useful n-qubit pure states is less than exp(-n2). This work highlights a new aspect of the role entanglement plays for quantum computational speedups.

Quantum computers are still far from realization. Usually fast decoherence and the problem of producing high degrees of entanglement between large numbers of qubits are mentioned as the first big problems, which one thinks of. Now Gross et al. show that most highly entangled quantum states will not provide a significant increase in computational speed compared to classical computers. So it might be necessary in future to identify and understand the few remaining entangled states, which are indeed useful for computation.

There is also an accompanying viewpoint to this article: http://link.aip.org/link/?&l_creator...2FPhysics.2.38

ZapperZ

M. Gu et al. "More really is different", Physica D: Nonlinear Phenomena, v.238, p.835 (2009).

Abstract: In 1972, P.W. Anderson suggested that ‘More is Different’, meaning that complex physical systems may exhibit behavior that cannot be understood only in terms of the laws governing their microscopic constituents. We strengthen this claim by proving that many macroscopic observable properties of a simple class of physical systems (the infinite periodic Ising lattice) cannot in general be derived from a microscopic description. This provides evidence that emergent behavior occurs in such systems, and indicates that even if a ‘theory of everything’ governing all microscopic interactions were discovered, the understanding of macroscopic order is likely to require additional insights.

Read the News and Views article on this paper in Nature 459, 332-334 (21 May 2009).

Edit: read the arXiv version here.

Zz.

ZapperZ

A. V. Ponomarev et al., "ac-Driven Atomic Quantum Motor", Phys. Rev. Lett. v.102, p.230601 (2009) .

Abstract: We propose an ac-driven quantum motor consisting of two different, interacting ultracold atoms placed into a ring-shaped optical lattice and submerged in a pulsating magnetic field. While the first atom carries a current, the second one serves as a quantum starter. For fixed zero-momentum initial conditions the asymptotic carrier velocity converges to a unique nonzero value. We also demonstrate that this quantum motor performs work against a constant load.

A review of this paper can also be found at the ScienceNews website.

Zz.

ZapperZ

R. Horodecki et al., "Quantum Entanglement", Rev. Mod. Phys. v.81, p865 (2009).

Abstract: From the point of view of quantum information science, entanglement is a resource that can be used to perform tasks that are impossible in a classical world. In a certain sense, the more entanglement we have, the better we can perform those tasks. Thus, one of the main goals in this field has been to identify under which conditions two or more systems are entangled, and how entangled they are. This paper reviews the main criteria to detect entanglement as well as entanglement measures and also discusses the role of entanglement in quantum communication and cryptography.

This is a HUGE, 78-page review of quantum entanglement. We get a lot of frequent questions on this topic, so it is appropriate to post a source that has a wealth of information and references.

The Arxiv version of this paper can be found here.

Zz.

ZapperZ

M. Karski et al., "Quantum Walk in Position Space with Single Optically Trapped Atoms", Science v.325, p. 174 (2009).

Abstract: The quantum walk is the quantum analog of the well-known random walk, which forms the basis for models and applications in many realms of science. Its properties are markedly different from the classical counterpart and might lead to extensive applications in quantum information science. In our experiment, we implemented a quantum walk on the line with single neutral atoms by deterministically delocalizing them over the sites of a one-dimensional spin-dependent optical lattice. With the use of site-resolved fluorescence imaging, the final wave function is characterized by local quantum state tomography, and its spatial coherence is demonstrated. Our system allows the observation of the quantum-to-classical transition and paves the way for applications, such as quantum cellular automata.

Read the ScienceNow review of this work here.

Zz.

ZapperZ

M. Aßmann et. al., "Higher-Order Photon Bunching in a Semiconductor Microcavity", Science v.325, p.297 (2009).

Abstract: Quantum mechanically indistinguishable particles such as photons may show collective behavior. Therefore, an appropriate description of a light field must consider the properties of an assembly of photons instead of independent particles. We have studied multiphoton correlations up to fourth order in the single-mode emission of a semiconductor microcavity in the weak and strong coupling regimes. The counting statistics of single photons were recorded with picosecond time resolution, allowing quantitative measurement of the few-photon bunching inside light pulses. Our results show bunching behavior in the strong coupling case, which vanishes in the weak coupling regime as the cavity starts lasing. In particular, we verify the n factorial prediction for the zero-delay correlation function of n thermal light photons.

The bunching and anti-bunching phenomena are considered to be THE strongest evidence for photons. These have no classical equivalence.

Zz.