System Bits: March 3

Observing antiferromagnetic order in ultracold atoms
Rice University researchers have simulated superconducting materials and made headway on a problem that’s vexed physicists for nearly three decades using ultracold atoms as a stand-in for electrons.

The research team, led by Rice, included researchers from Ohio State University, Universidade Federal do Rio de Janeiro, University of California-Davis, San Jose State University and Princeton University believes the work could open up a new realm of unexplored science.

They pointed out that nearly 30 years have passed since physicists discovered that electrons can flow freely through certain materials — superconductors — at relatively elevated temperatures but the reasons for this 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.

However, 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.

The Rice lab that conducted the work 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 cold atoms as stand-ins for electrons and beams of laser light to mimic the crystal lattice in a real material, they were able to simulate the Hubbard model. When they did that they were able to produce antiferromagnetism in exactly the way the Hubbard model predicts, which is 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.

They believe that magnetism plays a role in this process, and know that each electron in these materials correlates with every other, in a highly complex way.With these latest findings, they’ve confirmed that the system can be cooled to the point where short-range magnetic correlations can be simulated between electrons just as they begin to develop.

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. (Source: Rice University)

This is significant because the theoretical researchers on the project were able to use a mathematical technique known as the Quantum Monte Carlo method to verify that the results match the Hubbard model.

Superconductive aluminum atom clusters
Scientists at USC may have discovered a family of materials that could make room temperature superconductors a reality.

Long thought impossible, a team led by Vitaly Kresin, professor of physics at USC, found that aluminum “superatoms” — homogenous clusters of atoms — appear to form Cooper pairs of electrons (one of the key elements of superconductivity) at temperatures around 100 Kelvin.

Though 100 Kelvin is still very cold — about -280 degrees Fahrenheit — it is an enormous increase compared to bulk aluminum metal, which turns superconductive only near 1 Kelvin (-457 degrees Fahrenheit).

They believe this may be the discovery of a new family of superconductors, and raises the possibility that other types of superatoms will be capable of superconductivity at even warmer temperatures.

Nanowires that trap superconductor-blocking electron twisters
Superconductor materials are prized for their ability to carry an electric current without resistance, but this valuable trait can be crippled or lost when electrons swirl into tiny tornado-like formations called vortices. These disruptive mini-twisters often form in the presence of magnetic fields, such as those produced by electric motors.

To keep supercurrents flowing at top speed, Johns Hopkins scientists have figured out how to constrain troublesome vortices by trapping them within extremely short, ultra-thin nanowires.

They found a way to control individual vortices to improve the performance of superconducting wires; many materials can become superconducting when cooled to a temperature of nearly 460 below zero F, which is achieved by using liquid helium.

This illustration depicts a short row of vortices held in place between the edges of a nanowire developed by Johns Hopkins scientists. (Source: Johns Hopkins)

The team noted that the new method of maintaining resistance-free current within these superconductors is important because these materials play a key role in devices such MRI medical scanners, particle accelerators, photon detectors and the radio frequency filters used in cell phone systems. In addition, superconductors are expected to become critical components in future quantum computers, which will be able to do more complex calculations than current machines.

Wider use of superconductors may hinge on stopping the nanoscopic mischief that electron vortices cause when they skitter from side to side across a conducting material, spoiling the zero-resistance current. The Johns Hopkins scientists say their nanowires keep this from happening.