A step closer to explaining high-temperature superconductivity?

For years some physicists have been hoping to crack the mystery of high-temperature superconductivity—the ability of some complex materials to carry electricity without resistance at temperatures high above absolute zero—by simulating crystals with patterns of laser light and individual atoms. Now, a team has taken—almost—the next-to-last step in such "optical lattice" simulation by reproducing the pattern of magnetism seen in high-temperature superconductors from which the resistance-free flow of electricity emerges.

"It's a very big improvement over previous results," says Tilman Esslinger, an experimentalist at the Swiss Federal Institute of Technology in Zurich, who was not involved in the work. "It's very exciting to see steady progress."

An optical lattice simulation is essentially a crystal made of light. A real crystal contains a repeating 3D pattern of ions, and electrons flow from ion to ion. In the simulation, spots of laser light replace the ions, and ultracold atoms moving among spots replace the electrons. Physicists can adjust the pattern of spots, how strongly the spots attract the atoms, and how strongly the atoms repel one another. 

That makes the experiments ideal for probing physics such as high-temperature superconductivity, in which materials such as mercury barium calcium copper oxide carry electricity without resistance at temperatures up to 138 K, far higher above absolute zero than ordinary superconductors such as niobium can.

Just how the copper-and-oxygen, or cuprate, superconductors work remains unclear. The materials contain planes of copper and oxygen ions with the coppers arranged in a square pattern. Repelling one another, the electrons get stuck in a one-to-a-copper traffic jam called a Mott insulator state. They also spin like tops, and at low temperatures neighboring electrons spin in opposite directions, creating an up-down-up-down pattern of magnetism called antiferromagnetism. Superconductivity sets in when impurities soak up a few electrons and ease the traffic jam. The remaining electrons then pair to glide freely along the planes.

Theorists do not yet agree how that pairing occurs. Some think that wavelike ripples in the antiferromagnetic pattern act as a glue to attract one electron to the other. Others argue that the pairing arises, paradoxically, from the repulsion among the electrons alone. Theorists can write down a mathematical model of electrons on a checkerboard plane, known as the Fermi-Hubbard model, but it is so hard to "solve" that nobody has been able to show whether it produces superconductivity.

Experimentalists hope to reproduce the Fermi-Hubbard model in laser light and cold atoms to see if it yields superconductivity. In 2002, Immanuel Bloch, a physicist at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany, and colleagues realized a Mott insulator state in an optical lattice. Six years later, Esslinger and colleagues achieved the Mott state with atoms with the right amount of spin to mimic electrons. Now, Randall Hulet, a physicist at Rice University in Houston, Texas, and colleagues have nearly achieved the next-to-last step along the way: antiferromagnetism.

Hulet and colleagues trapped between 100,000 and 250,000 lithium-6 atoms in laser light. They then ramped up the optical lattice and ramped it back down to put them in order. Shining laser light of a specific wavelength on the atoms, they observed evidence of an emerging up-down-up-down spin pattern. The laser light was redirected, or diffracted, at a particular angle by the rows of atoms—just as x-rays diffract off the ions in a real crystal. Crucially, the light probed the spin of the atoms: The light wave flipped if it bounced off an atom spinning one way but not the other. Without that flipping, the diffraction wouldn't have occurred, so observation confirms the emergence of the up-down-up-down pattern, Hulet says.

Hulet's team solved a problem that has plagued other efforts. Usually, turning the optical lattice on heats the atoms. To avoid that, the researchers added another laser that slightly repelled the atoms, so that the most energetic ones were just barely held by the trap. Then, as the atoms heated, the most energetic ones "evaporated" like steam from hot soup to keep the other ones cool, the researchers report online this week in Nature. They didn't quite reach a full stable antiferromagnetic pattern: The temperature was 40% too high. But the technique might get there and further, 

Hulet says. "We don't have a good sense of what the limit of this method is," he says. "We could get a factor of two lower, we could get a factor of 10 lower."

"It is indeed very promising," says Tin-Lun "Jason" Ho, a theorist at Ohio State University, Columbus. Reducing the temperature by a factor of two or three might be enough to reach the superconducting state, he says. However, MPQ's Bloch cautions that it may take still other techniques to get that cold. 

"There are several cooling techniques that people are developing and interesting experiments coming up," he says.

Physicists are also exploring other systems and problems with optical lattices. The approach is still gaining steam, Hulet says: "It's an exciting time."

Article: ScienceMag

Dark matter guides growth of supermassive black holes

This illustration shows two spiral galaxies - each with supermassive black holes at their center - as they are about to collide and form an elliptical galaxy. New research shows that galaxies' dark matter halos influence these mergers and the resulting growth of supermassive black holes.
Credit: NASA/CXC/M.Weiss

Every massive galaxy has a black hole at its center, and the heftier the galaxy, the bigger its black hole. But why are the two related? After all, the black hole is millions of times smaller and less massive than its home galaxy.

A new study of football-shaped collections of stars called elliptical galaxies provides new insights into the connection between a galaxy and its black hole. It finds that the invisible hand of dark matter somehow influences black hole growth.

"There seems to be a mysterious link between the amount of dark matter a galaxy holds and the size of its central black hole, even though the two operate on vastly different scales," says lead author Akos Bogdan of the Harvard-Smithsonian Center for Astrophysics (CfA).

This new research was designed to address a controversy in the field. Previous observations had found a relationship between the mass of the central black hole and the total mass of stars in elliptical galaxies. However, more recent studies have suggested a tight correlation between the masses of the black hole and the galaxy's dark matter halo. It wasn't clear which relationship dominated.

In our universe, dark matter outweighs normal matter -- the everyday stuff we see all around us -- by a factor of 6 to 1. We know dark matter exists only from its gravitational effects. It holds together galaxies and galaxy clusters. Every galaxy is surrounded by a halo of dark matter that weighs as much as a trillion suns and extends for hundreds of thousands of light-years.

To investigate the link between dark matter halos and supermassive black holes, Bogdan and his colleague Andy Goulding (Princeton University) studied more than 3,000 elliptical galaxies. They used star motions as a tracer to weigh the galaxies' central black holes. X-ray measurements of hot gas surrounding the galaxies helped weigh the dark matter halo, because the more dark matter a galaxy has, the more hot gas it can hold onto.

They found a distinct relationship between the mass of the dark matter halo and the black hole mass -- a relationship stronger than that between a black hole and the galaxy's stars alone.
This connection is likely to be related to how elliptical galaxies grow. An elliptical galaxy is formed when smaller galaxies merge, their stars and dark matter mingling and mixing together. Because the dark matter outweighs everything else, it molds the newly formed elliptical galaxy and guides the growth of the central black hole.

"In effect, the act of merging creates a gravitational blueprint that the galaxy, the stars and the black hole will follow in order to build themselves," explains Bogdan.

Credits: ScienceDaily

Scientists still haven’t solved the mystery of the asteroid that exploded over Russia 2 years ago

Trail of the asteroid that exploded over Chelyabinsk, Russia, on February 15, 2013.

Two years after an asteroid exploded over Russia and injured more than 1,200 people, the origin of the space rock still puzzles scientists.

The 66-foot-wide (20 meters) asteroid broke up over the city of Chelyabinsk, Russia, on Feb. 15, 2013, shattering windows across the area and sending many people to the hospital with lacerations from the flying glass.

Originally, astronomers thought that the Chelyabinsk meteor came from a 1.24-mile-wide (2 kilometers) near-Earth asteroid called 1999 NC43.

But a closer look at the asteroid's orbit and likely mineral composition, gained from spectroscopy, suggests few similarities between it and the Russian meteor.

"These two bodies shared similar orbits around the sun, and initial studies suggested even similar compositions," lead study author Vishnu Reddy, a scientist with the nonprofit Planetary Science Institute in Tucson, Arizona, said in a statement.

However, "the composition of [the] Chelyabinsk meteorite that was recovered after the event is similar to a common type of meteorite called LL chondrites," he added. "The near-Earth asteroid has a composition that is distinctly different from this."

More generally, Reddy and his colleagues' work showed that it is difficult to make predictions about what particular asteroid could have shed pieces that slammed into Earth. Because most asteroids are so small and their orbits are "chaotic," it's hard to make a firm link, the authors said.



A paper based on the research appears in the journal Icarus.

The Russian meteor explosion has generated a great deal of interest in the search for potentially hazardous asteroids, sparking the creation of a new asteroid warning center at the European Space Agency, among other initiatives.

In a statement this week, the B612 Foundation, a nonprofit organization that seeks to reduce the threat from asteroids, urged agencies worldwide to step up their search for dangerous space rocks. The group plans to add to that effort with the asteroid-hunting Sentinel Space Telescope, which B612 hopes to launch in 2018.

"The fact of the matter is that asteroid impacts can be prevented using technology we can employ right now," B612 co-founder Ed Lu, a former space shuttle astronaut, said in a statement.

"And unlike other potentially global-scale catastrophic events, the solution is nearly purely a technical one, and with a relatively small and known cost," Lu added. "So as my friend, former Apollo 9 astronaut and co-founder of the B612 Foundation Rusty Schweickart says, 'Let’s get on with it.'"

Credits: Space.com

Five things scientists could learn with their new, improved particle accelerator

The Large Hadron Collider (LHC) is back, and it’s better than ever. The particle accelerator, located at CERN, the European particle physics lab near Geneva, Switzerland, shut down in February 2013, and since then scientists have been upgrading and repairing it and its particle detectors. The LHC will be back up to full speed this May. Yesterday, scientists discussed the new prospects for the LHC at the annual meeting of AAAS (which publishes Science).

The LHC is the world’s most powerful particle accelerator. Protons blast along its 17-mile (27-kilometer) ring at nearly light speed, colliding at the sites of several particle detectors, which sift through the resulting particle debris. In 2012, LHC’s ATLAS and CMS experiments discovered the Higgs boson with data from the LHC’s first run, thereby explaining how particles get mass. The revamped LHC will run at a 60% higher energy, with more sensitive detectors, and a higher collision rate. What might we find with the new-and-improved machine? Here are five questions scientists hope to answer:


1. Does the Higgs boson hold any surprises?

Now that we’ve found the Higgs boson, there’s still a lot we can learn from it. Thanks to the LHC’s energy boost, it will produce Higgs bosons at a rate five times higher, and scientists will be using the resulting abundance of Higgs to understand the particle in detail. How does it decay? Does it match the theoretical predictions? Anything out of the ordinary would be a boon to physicists, who are looking for evidence of new phenomena that can explain some of the unsolved mysteries of physics.


2. What is “dark matter”?

Only 15% of the matter in the universe is the kind we are familiar with. The rest is dark matter, which is invisible to us except for subtle hints, like its gravitational effects on the cosmos. Physicists are clamoring to know what it is. One likely dark matter culprit is a WIMP, or weakly interacting massive particle, which could show up in the LHC. Dark matter’s fingerprints could even be found on the Higgs boson, which may sometimes decay to dark matter. You can bet that scientists will be sifting through their data for any trace.


3. Will we ever find supersymmetry?

Supersymmetry, or SUSY, is a hugely popular theory of particle physics that would solve many unanswered questions about physics, including why the mass of the Higgs boson is lighter than naively expected if only it were true. This theory proposes a slew of exotic elementary particles that are heavier twins of known ones, but with different spin a type of intrinsic rotational momentum. Higher energies at the new LHC could boost the production of hypothetical supersymmetric particles called gluinos by a factor of 60, increasing the odds of finding it.


4. Where did all the antimatter go?

Physicists don’t know why we exist. According to theory, after the big bang the universe was equal parts matter and antimatter, which annihilate one another when they meet. This should have eventually resulted in a lifeless universe devoid of matter. But instead, our universe is full of matter, and antimatter is rare somehow, the balance between matter and antimatter tipped. With the upgraded LHC, experiments will be able to precisely test how matter might differ from antimatter, and how our universe came to be.


5. What was our infant universe like?

Just after the big bang, our universe was so hot and dense that protons and neutrons couldn’t form, and the particles that make them up—quarks and gluons—floated in a soup known as the quark-gluon plasma. To study this type of matter, the LHC produces extra-violent collisions using lead nuclei instead of protons, recreating the fireball of the primordial universe. Aided by the new LHC’s higher rate of collisions, scientists will be able to take more baby photos of our universe than ever before.

Credits: ScienceMAG