Magnetic monopoles in spin ice

Magnetic monopoles in spin ice
C. Castelnovo1, R. Moessner1,2 & S. L. Sondhi
Nature 451, 42-45 (3 January 2008)


Electrically charged particles, such as the electron, are ubiquitous. In contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches (see ref. 1 for example). We pursue an alternative strategy, namely that of realizing them not as elementary but rather as emergent particles—that is, as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics2. Here we propose that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice3, 4, 5: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field6, 7, which is a liquid–gas transition of the magnetic monopoles. These monopoles can also be detected by other means, for example, in an experiment modelled after the Stanford magnetic monopole search8.

Spin-ice materials are characterized by the presence of magnetic moments.


Source: http://www.nature.com/nature/journal/v451/n7174/full/nature06433.html

Magnetism: Freedom for the poles

Magnetism: Freedom for the poles
By Oleg Tchernyshyov
Nature 451, 22-23 (3 January 2008)


Magnetic poles always come in twos, a north and a south. That received wisdom has not stopped physicists from searching for 'monopoles' in accelerators and cosmic rays. Theory now indicates a better place to look.

Despite some tantalizing clues for their existence from the realms of quantum physics, magnetic monopoles — single magnetic poles without a partner — remain elusive after decades of searching. Do they exist at all in the real world? On page 42 of this issue1, Castelnovo, Moessner and Sondhi argue yes: monopoles are alive and well in an exotic class of magnetic material known as spin ice2.

This asymmetry extends to the subatomic level. Elementary particles can carry a positive or negative electric charge, but the magnetic charge is zero without exception. Yet theory offers some hints that single magnetic poles might exist in nature. In the 1930s, Paul Dirac showed that magnetic monopoles could explain the observed quantization of electric charge. Extensions of the standard model of particle physics include particles with magnetic charge.

One environment in which monopoles might pop up is crystalline solids. In a crystal at a low temperature, excitations above the ground state often behave like elementary particles: they carry a quantized amount of energy, momentum, electric charge and spin. In their theoretical study, Castelnovo et al. find the first instance of such an excitation with a non-zero magnetic charge. Under certain conditions, these magnets behave as a gas of independent magnetic poles. There is even a phase transition at which a thin vapour of these monopoles condenses into a dense liquid.

URL:

http://www.nature.com/nature/journal/v451/n7174/full/451022b.html

See also: http://scidok.sulb.uni-saarland.de/volltexte/2009/2083/pdf/CPALPhD2009.pdf

NASA researchers find clues to a secret of life

GREENBELT, Md. - NASA scientists analyzing the dust of meteorites have discovered new clues to a long-standing mystery about how life works on its most basic, molecular level.

"We found more support for the idea that biological molecules, like amino acids, created in space and brought to Earth by meteorite impacts help explain why life is left-handed," said Dr. Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt, Md. "By that I mean why all known life uses only left-handed versions of amino acids to build proteins." Glavin is lead author of a paper on this research appearing in the Proceedings of the National Academy of Sciences March 16.

Proteins are the workhorse molecules of life, used in everything from structures like hair to enzymes, the catalysts that speed up or regulate chemical reactions. Just as the 26 letters of the alphabet are arranged in limitless combinations to make words, life uses 20 different amino acids in a huge variety of arrangements. Amino acid molecules can be built in two ways that are mirror images of each other, like your hands. Although life based on right-handed amino acids would presumably work fine, "you can't mix them," says Dr. Jason Dworkin of NASA Goddard, co-author of the study. "If you do, life turns to something resembling scrambled eggs -- it's a mess. Since life doesn't work with a mixture of left-handed and right-handed amino acids, the mystery is: how did life decide -- what made life choose left-handed amino acids over right-handed ones?"

Over the last four years, the team carefully analyzed samples of meteorites with an abundance of carbon, called carbonaceous chondrites. The researchers looked for the amino acid isovaline and discovered that three types of carbonaceous meteorites had more of the left-handed version than the right-handed variety – as much as a record 18 percent more in the often-studied Murchison meteorite. "Finding more left-handed isovaline in a variety of meteorites supports the theory that amino acids brought to the early Earth by asteroids and comets contributed to the origin of only left-handed based protein life on Earth," said Glavin.

Contact: Bill Steigerwald
william.a.steigerwald@nasa.gov
301-286-5017
NASA/Goddard Space Flight Center

Source:
- http://www.eurekalert.org/pub_releases/2009-03/nsfc-nrf031709.php
- http://www.eurekalert.org/bysubject/space.php

Queen's scientists discover giant solar twists

Scientists at Queen's University have made a finding that will help us to understand more about the turbulent solar weather and its affect on our planet.

Along with scientists at the University of Sheffield and California State University, the researchers have detected giant twisting waves in the lower atmosphere of the Sun.

The discovery sheds some light on why the Sun's corona, the region around the Sun, has a much higher temperature than its surface - something that has always puzzled scientists.

The recent discovery by the scientists, published today in the respected journal Science, has revealed the existence of a new breed of solar wave, called the Alfvén wave. This solar wave has been shown to transport energy into the Corona or outer layer.


Contact: Lisa Mitchell
lisa.mitchell@qub.ac.uk
44-781-442-2572
Queen's University Belfast

source:
- http://www.eurekalert.org/pub_releases/2009-03/qub-qsd032009.php
- http://www.eurekalert.org/bysubject/space.php

Finding Twin Earths: Harder Than We Thought

Cambridge, MA - Does a twin Earth exist somewhere in our galaxy? Astronomers are getting closer and closer to finding an Earth-sized planet in an Earth-like orbit. NASA's Kepler spacecraft just launched to find such worlds. Once the search succeeds, the next questions driving research will be: Is that planet habitable? Does it have an Earth-like atmosphere? Answering those questions will not be easy.

In a new study, L. Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and W. Traub (Jet Propulsion Laboratory) examined the ability of JWST to characterize the atmospheres of hypothetical Earth-like planets during a transit, when part of the light of the star gets filtered through the planet's atmosphere. They found that JWST would be able to detect certain gases called biomarkers, such as ozone and methane, only for the closest Earth-size worlds.


Source:
- http://www.cfa.harvard.edu/news/2009/pr200909.html
- http://www.eurekalert.org/bysubject/space.php

AGU journal highlights -- March 12, 2009

The following highlights summarize research papers that have been published in Geophysical Research Letters (GRL).

In this release:

- Exploring how corals build their skeletons
- Earth cyclones may help explain Venusian vortices
- Model relates South Polar ozone concentrations and wind patterns
- Laboratory crystals give clues to deep Earth puzzle
- Spacecraft characterize perturbations that can affect orbiting satellites

Anyone may read the scientific abstract for these papers by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/ 2008GL036782. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions, who are registered with AGU, also may download papers cited in this release by clicking on the links below.

Contact: Maria-Jose Vinas
mjvinas@agu.org
202-777-7530
American Geophysical Union

Source:
- http://www.eurekalert.org/pub_releases/2009-03/agu-ajh031209.php
- http://www.eurekalert.org/bysubject/space.php

==========

Laboratory crystals give clues to deep Earth puzzle

Perovskite is the major mineral phase in the lower mantle—it dominates the seismic properties and viscous deformation of the deep Earth. At high pressures and temperatures, perovskite transforms into an altered crystal-packing form called postperovskite. The region where the transformation occurs, known as the D'' layer, is directly above the core-mantle boundary and is distinguished by large seismic velocity jumps. Using forms of a synthetic, solid compound containing calcium, iridium and oxygen as analogs for perovskite and postperovskite, Walte et al. conduct laboratory experiments to simulate the perovskite transitions under high temperatures and pressures. Expanding on past research that demonstrated that crystal lattice orientations in the analog postperovskite alter when deformed, the authors find that the transformation of the analog perovskite to analog postperovskite itself yields a crystal lattice structure different from postperovskite deformation textures. If the analogue between the compound used in the experiments and the perovskite crystal-packing system holds true, such lattice orientation transitions may explain the observed seismic jumps. On a more local level, the fast spikes in certain seismic velocities may be explained by downwelling material that underwent these observed crystal lattice transformations.