Subscribe free to our newsletters via your
. 24/7 Space News .




ENERGY TECH
Simulating superconducting materials with ultracold atoms
by Staff Writers
Houston TX (SPX) Feb 24, 2015


Rice University physicists trapped ultracold atomic gas in grids of intersecting laser beams to mimic the antiferromagnetic order observed in the parent compounds of nearly all high-temperature superconductors. Image courtesy P. Duarte/Rice University.

Using ultracold atoms as a stand-in for electrons, a Rice University-based team of physicists has simulated superconducting materials and made headway on a problem that's vexed physicists for nearly three decades.

The research was carried out by an international team of experimental and theoretical physicists and appears online this week in the journal Nature. Team leader Randy Hulet, an experimental physicist at Rice, said the work could open up a new realm of unexplored science.

Nearly 30 years have passed since physicists discovered that electrons can flow freely through certain materials -- superconductors -- at relatively elevated temperatures. The reasons for this high-temperature, or "unconventional" superconductivity are still largely unknown. One of the most promising theories to explain unconventional superconductivity -- called the Hubbard model -- is simple to express mathematically but is impossible to solve with digital computers.

"The Hubbard model is a set of mathematical equations that could hold the key to explaining high-temperature superconductivity, but they are too complex to solve -- even with the fastest supercomputer," said Hulet, Rice's Fayez Sarofim Professor of Physics and Astronomy. "That's where we come in."

Hulet's lab specializes in cooling atoms to such low temperatures that their behavior is dictated by the rules of quantum mechanics -- the same rules that electrons follow when they flow through superconductors.

"Using our cold atoms as stand-ins for electrons and beams of laser light to mimic the crystal lattice in a real material, we were able to simulate the Hubbard model," Hulet said.

"When we did that, we were able to produce antiferromagnetism in exactly the way the Hubbard model predicts. That's exciting because it's the first ultracold atomic system that's able to probe the Hubbard model in this way, and also because antiferromagnetism is known to exist in nearly all of the parent compounds of unconventional superconductors."

Hulet's team is one of many that are racing to use ultracold atomic systems to simulate the physics of high-temperature superconductors.

"Despite 30 years of effort, people have yet to develop a complete theory for high-temperature superconductivity," Hulet said. "Real electronic materials are extraordinarily complex, with impurities and lattice defects that are difficult to fully control. In fact, it has been so difficult to study the phenomenon in these materials that physicists still don't know the essential ingredients that are required to make an unconventional superconductor or how to make a material that superconducts at even greater temperature."

Hulet's system mimics the actual electronic material, but with no lattice defects or disorder.

"We believe that magnetism plays a role in this process, and we know that each electron in these materials correlates with every other, in a highly complex way," he said. "With our latest findings, we've confirmed that we can cool our system to the point where we can simulate short-range magnetic correlations between electrons just as they begin to develop.

"That's significant because our theoretical colleagues -- there were five on this paper -- were able to use a mathematical technique known as the Quantum Monte Carlo method to verify that our results match the Hubbard model," Hulet said. "It was a heroic effort, and they pushed their computer simulations as far as they could go. From here on out, as we get colder still, we'll be extending the boundaries of known physics."

Nandini Trivedi, professor of physics at Ohio State University, explained that she and her colleagues at the University of California-Davis, who formed the theoretical side of the effort, had the task of identifying just how cold the atoms had to be in the experiment.

"Some of the big questions we ask are related to the new kinds of ways in which atoms get organized at low temperatures," she said. "Because going to such low temperatures is a challenge, theory helped determine the highest temperature at which we might expect the atoms to order themselves like those of an antiferromagnet."

After high-temperature superconductivity was discovered in the 1980s, some theoretical physicists proposed that the underlying physics could be explained with the Hubbard model, a set of equations invented in the early 1960s by physicist John Hubbard to describe the magnetic and conduction properties of electrons in transition metals and transition metal oxides.

Every electron has a "spin" that behaves as a tiny magnet. Scientists in the 1950s and 1960s noticed that the spins of electrons in transition metals and transition metal oxides could become aligned in ordered patterns. In creating his model, Hubbard sought to create the simplest possible system for explaining how the electrons in these materials responded to one another.

The Hubbard model features electrons that can hop between sites in an ordered grid, or lattice. Each site in the lattice represents an ion in the crystal lattice of a material, and the electrons' behavior is dictated by just a handful of variables. First, electrons are disallowed from sharing an energy level, due to a rule known as the Pauli Exclusion Principle. Second, electrons repel one another and must pay an energy penalty when they occupy the same site.

"The Hubbard model is remarkably simple to express mathematically," Hulet said. "But because of the complexity of the solutions, we cannot calculate its properties for anything but a very small number of electrons on the lattice. There is simply too much quantum entanglement among the system's degrees of freedom."

Correlated electron behaviors -- like antiferromagnetism and superconductivity -- result from feedback, as the action of every electron causes a cascade that affects all of its neighbors. Running the calculations becomes exponentially more time-consuming as the number of sites increases. To date, the best efforts to produce computer simulations of two- and three-dimensional Hubbard models involve systems with no more than a few hundred sites.

Because of these computational difficulties, it has been impossible for physicists to determine whether the Hubbard model contains the essence of unconventional superconductivity. Studies have confirmed that the model's solutions show antiferromagnetism, but it is unknown whether they also exhibit superconductivity.

In the new study, Hulet and colleagues, including postdoctoral researcher Russell Hart and graduate student Pedro Duarte, created a new experimental technique to cool the atoms in their lab to sufficiently low temperatures to begin to observe antiferromagnetic order in an optical lattice with approximately 100,000 sites. This new technique results in temperatures on the lattice that are about half of that obtained in previous experiments.

"The standard technique is to create the cold atomic gas, load it into the lattice and take measurements," Hart said. "We developed the first method for evaporative cooling of atoms that had already been loaded in a lattice. That technique, which uses what we call a 'compensated optical lattice,' also helped control the density of the sample, which becomes critical for forming antiferromagnetic order."

Hulet said a second innovation was the team's use of the optical technique called Bragg scattering to observe the symmetry planes that are characteristic of antiferromagnetic order.

He said the team will need to develop an entirely new technique to measure the electron pair correlations that cause superconductivity. And they'll also need colder samples, about 10 times colder than those used in the current study.

"We have some things in mind," Hulet said.

"I am confident we can achieve lower temperatures both by refining what we've already done and by developing new techniques. Our immediate goal is to get cold enough to get fully into the antiferromagnetic regime, and from there we'd hope to get into the d-wave pairing regime and confirm whether or not it exists in the Hubbard model."


Thanks for being here;
We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain.

With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords.

Our news coverage takes time and effort to publish 365 days a year.

If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.

SpaceDaily Contributor
$5 Billed Once


credit card or paypal
SpaceDaily Monthly Supporter
$5 Billed Monthly


paypal only

.


Related Links
Rice University
Powering The World in the 21st Century at Energy-Daily.com






Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks
del.icio.usdel.icio.us DiggDigg RedditReddit GoogleGoogle




Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News





ENERGY TECH
Insight into inner magnetic layers
Berlin, Germany (SPX) Feb 18, 2015
Measurements at BESSY II have shown how spin filters forming within magnetic sandwiches influence tunnel magnetoresistance - results that can help in designing spintronic component. In doing so, the teams enhanced our understanding of processes that are important for future TMR data storage devices and other spintronic components. Their results have now been published in Nature Communicat ... read more


ENERGY TECH
Application of laser microprobe technology to Apollo samples refines lunar impact history

NASA releases video of the far side of the Moon

US Issuing Licenses for Mineral Mining on Moon

LRO finds lunar hydrogen more abundant on Moon's pole-facing slopes

ENERGY TECH
Mars One cuts list of potential colonists to 100

Scientists fail to explain strange plumes spotted on Martian surface

NASA's Curiosity Analyzing Sample of Martian Mountain

Mars Rover Nearing Marathon Achievement

ENERGY TECH
Korean tech start-ups offer life beyond Samsung

Fast visas and dim sum: Spain seeks to attract Chinese tourists

The ISS Menu: Mayo, Espressos, Booze? Cosmonauts Reveal Their Secrets

London workshop teaches nuts and bolts behind tech

ENERGY TECH
More Astronauts for China

China launches the FY-2 08 meteorological satellite successfully

China's Long March puts satellite in orbit on 200th launch

Countdown to China's new space programs begins

ENERGY TECH
Spacesuit woes haunt NASA ahead of crucial spacewalks

Russia Launches Fresh Fruit, Oxygen to Crew on ISS

Europe destroys last space truck to ISS

NASA, Space Station Partners Announce Future Mission Crew Members

ENERGY TECH
Soyuz Installed at Baikonur, Expected to Launch Wednesday

SpaceX launches deep-space weather observatory

SpaceX cargo craft returns to Earth

High seas force SpaceX to ditch bid to recycle rocket

ENERGY TECH
Scientists predict earth-like planets around most stars

"Vulcan Planets" - Inside-Out Formation of Super-Earths

Dawn ahead!

Habitable Evaporated Cores

ENERGY TECH
Building trustworthy big data algorithms

Mighty mollusc: Limpet teeth have super strength

How iron feels the heat

Researchers glimpse distortions in atomic structure of materials




The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement All images and articles appearing on Space Media Network have been edited or digitally altered in some way. Any requests to remove copyright material will be acted upon in a timely and appropriate manner. Any attempt to extort money from Space Media Network will be ignored and reported to Australian Law Enforcement Agencies as a potential case of financial fraud involving the use of a telephonic carriage device or postal service.