Nav: Home

Ultra-cold lithium atoms shedding light on superfluid formation

May 28, 2019

  • Abrupt onset of pairing points to best theories for describing ultra-cold 'Fermi gases'
  • Implications for understanding of superconductors, superfluids in future ultra low-energy electronic systems
A FLEET/Swinburne study released this week resolves a long-standing debate about what happens at the microscopic level when matter transitions into a superconducting or superfluid state.

Correlations between pairs of atoms in an ultra-cold gas were found to grow suddenly as the system was cooled below the superfluid transition temperature, rather than appear gradually at higher temperatures, as some theories have predicted.

Experiments were carried out in Swinburne University of Technology's Ultra-cold Atomic Gas laboratory, using gases of lithium atoms cooled to temperatures below 100 nano-Kelvin (less than a millionth of a degree above Absolute Zero).

UNLOCKING PAIRING MECHANISM OF FERMI-GAS SYSTEMS

The new study unlocks key functions of a state of matter known as a 'Fermi gas', examples of which include electrons moving freely in an electrical conductor (such as in a conventional electric current), or protons and neutrons within a nucleus. Other Fermi gas systems include more-exotic states, such as electrons in superconductors, or the 'superfluid' of neutrons within a neutron star.

"One of the open questions about strongly interacting Fermi-gas systems has been the role of pairing," explains FLEET CI Prof Chris Vale. "Our study demonstrated that, at the superfluid transition temperature, pair-correlations increase abruptly, rather than gradually - as has been predicted by some theories."

This observation was quantified through measurements of a universal parameter, known as the 'contact parameter'. This parameter quantifies the likelihood of finding two atoms in very close proximity to each other, and is strongly enhanced when atoms form pairs.

A related study, by the group of Martin Zwierlein at the Massachusetts Institute of Technology and published back-to-back with the Swinburne group's paper found near identical results, using an entirely different method. The Swinburne and MIT experiments represent a key breakthrough in our understandings of pairing in Fermi superfluid systems with strong interactions between particles.

EXPERIMENTAL RESULTS POINT TO CORRECT THEORY

The Swinburne team generated a unitary Fermi gas of lithium-6 atoms and probed the system by measuring the momentum imparted to the atoms by a pair of crossed laser beams, which perturb the gas in a well-defined manner. From this data, the team extracted the contact parameter, which displayed a rapid increase of around 15% as the temperature was lowered below the superfluid transition point.

Theoretical attempts to calculate the temperature evolution of the contact parameter are notoriously difficult and have yielded very different predictions that depend on the model for interacting fermions. The Swinburne and MIT experiments support the Luttinger-Ward theory, which says that pairing turns on abruptly at the transition temperature.
-end-
THE STUDY

'Contact and sum-rules in a near-uniform Fermi gas at unitarity' was published in Physical Review Letters this week.

As well as the Australian Research Council, the authors thanks MIT's Zwierlein and Mukherjee, as well as, theoretical researchers Zwerger (Germany), Drut (USA), Goulko (Israel), Strinati (Italy) and Werner (France) for access to their data and helpful discussions.

ULTRA-COLD GAS STUDIES AT FLEET

Researchers often use ultra-cold atoms to study quantum systems, because of the ability to perfectly 'tune' atomic interactions.

Quantum gases of ultra-cold, neutral atoms are now helping unlock the fundamental physics of Fermi systems, often uncovering phenomena not readily accessible in other systems.

By increasing the interaction strength between fermionic atoms, experiments can explore the 'unitary' limit, where the atomic behaviour is expected to reveal universal features of interacting fermions that could connect our understanding of superconductivity and Bose-Einstein condensation.

"Fundamental discoveries made from experiments such as these can help guide FLEET's quest to develop 2D materials that conduct electrical current without dissipating energy," explains Chris Vale.

Chris leads FLEET's studies of quantum gases at Swinburne University of Technology, where his lab routinely cools atomic gases to temperatures approaching Absolute Zero. In this temperature range, quantum behaviours that are usually only found at the microscopic level become prominent at the macroscopic level.

Chris is one of over one-hundred FLEET researchers, all motivated by one grand challenge: to reduce the energy used in information and communication technology (ICT), which already accounts for at least 8% of global electricity use, and is doubling every decade.

FLEET (the ARC Centre of Excellence in Future Low-Energy Electronics Technologies) will develop systems in which electricity flows with minimal resistance and therefore minimal wasted dissipation of energy, and devices in which this 'dissipationless' electric current can be switched on and off at will.

These devices will enable revolutionary new electronics and communications technologies with ultra-low energy consumption.

ARC Centre of Excellence in Future Low-Energy Electronics Technologies

Related Lithium Articles:

A seaweed derivative could be just what lithium-sulfur batteries need
Lithium-sulfur batteries have great potential as a low-cost, high-energy, energy source for both vehicle and grid applications.
Risk of cardiac malformations from lithium during pregnancy less significant
New research suggests there may be a more modest increased risk of cardiac defects when using lithium during the first trimester of pregnancy.
Graphene-nanotube hybrid boosts lithium metal batteries
Rice University scientists build high-capacity lithium metal batteries with anodes made of a graphene-carbon nanotube hybrid.
Better cathode materials for lithium-sulphur-batteries
A team at the Helmholtz-Zentrum Berlin (HZB) has for the first time fabricated a nanomaterial made from nanoparticles of a titanium oxide compound (Ti4O7) that is characterized by an extremely large surface area, and tested it as a cathode material in lithium-sulphur batteries.
Stabilizing molecule could pave way for lithium-air fuel cell
Lithium-oxygen fuel cells boast energy density levels comparable to fossil fuels and are thus seen as a promising candidate for future transportation-related energy needs.
Freezing lithium batteries may make them safer and bendable
Columbia Engineering Professor Yuan Yang has developed a new method that could lead to lithium batteries that are safer, have longer battery life, and are bendable, providing new possibilities such as flexible smartphones.
Electrochemical performance of lithium-ion capacitors
Pre-lithiated multiwalled carbon nanotubes and activated carbon (AC) materials were used as anode and cathode respectively for Lithium-ion capacitors (LICs).
Lighter, more efficient, safer lithium-ion batteries
Researchers from Universidad Carlos III de Madrid and the Council for Scientific Research (initialed CSIC in Spanish) have patented a method for making new ceramic electrodes for lithium-ion batteries that are more efficient, cheaper, more resistant and safer than conventional batteries.
Clarifying how lithium ions ferry around in rechargeable batteries
IBS scientists observe the real-time ultrafast bonding of lithium ions with the solvents, in the same process that happens during charging and discharging of lithium batteries, and conclude that a new theory is needed.
New gel-like coating beefs up the performance of lithium-sulfur batteries
Yale scientists have developed an ultra-thin coating material that has the potential to extend the life and improve the efficiency of lithium-sulfur batteries, one of the most promising areas of energy research today.

Related Lithium Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Digital Manipulation
Technology has reshaped our lives in amazing ways. But at what cost? This hour, TED speakers reveal how what we see, read, believe — even how we vote — can be manipulated by the technology we use. Guests include journalist Carole Cadwalladr, consumer advocate Finn Myrstad, writer and marketing professor Scott Galloway, behavioral designer Nir Eyal, and computer graphics researcher Doug Roble.
Now Playing: Science for the People

#529 Do You Really Want to Find Out Who's Your Daddy?
At least some of you by now have probably spit into a tube and mailed it off to find out who your closest relatives are, where you might be from, and what terrible diseases might await you. But what exactly did you find out? And what did you give away? In this live panel at Awesome Con we bring in science writer Tina Saey to talk about all her DNA testing, and bioethicist Debra Mathews, to determine whether Tina should have done it at all. Related links: What FamilyTreeDNA sharing genetic data with police means for you Crime solvers embraced...