MIT Researchers Create the World's Coolest Stable Molecules

A team of researchers from MIT have cooled molecules in a sodium potassium (NaK) gas to a temperature of 500nK, which is slightly above absolute zero and many times colder than interstellar space.

The scientists discovered that the ultra-cold molecules were not only stable and long-lasting, but they are also capable of withstanding reactive collisions with other kinds of molecules. In addition, the molecules have strong dipole moments, which refer to powerful imbalances in electric charge that occur inside molecules and regulate forces similar to a magnet between molecules across large distances.

Air contains many disorganized molecules that zoom through space and collide with each other at many miles per hour. At ambient temperatures, such unpredictable molecular behavior is considered to be normal.

Over the years, scientists had hypothesized that if temperatures fall down to near absolute zero, all the molecules would stop their individual disordered movements and behave as a single combined body. This molecular behavior which is more organized would form unusual states of matter, which otherwise would not have been seen in the physical world.

According to Martin Zwierlein, a principal investigator in MIT's Research Laboratory of Electronics and professor of physics at MIT, normally molecules are full of energy and they vibrate, spin, and travel through space at a rapid speed. However, ultra-cold molecules developed by the research team have been successfully halted.

In other words, the molecules were cooled at a standard speed of centimeters per second and then prepared in their lowest states of rotation and vibration. Zwierlein, together with postdoc Sebastian Will and graduate student Jee Woo Park, has published the study results in the journal Physical Review Letters. All are members of the MIT-Harvard Center of Ultracold Atoms.

"We are very close to the temperature at which quantum mechanics plays a big role in the motion of molecules. So these molecules would no longer run around likebilliard balls, but move as quantum mechanical matter waves. And with ultracold molecules, you can get a huge variety of different states of matter, like superfluid crystals, which are crystalline, yet feel no friction, which is totally bizarre. This has not been observed so far, but predicted. We might not be far from seeing these effects, so we’re all excited," said Zwierlein.

Each molecule contains individual atoms, which are joined together to create a molecular structure. The most basic form of molecule looks like a dumbbell and contains a pair of atoms, which are linked by electromagnetic forces.

The researchers attempted to produce ultra-cold molecules of sodium potassium, with individual molecules containing one potassium and sodium atom. However, since molecules have many degrees of freedom such as vibration, rotation and translation, it is not easy to cool them directly. However, atoms have a simpler structure, making them easier to cool down.

The researchers first utilized lasers and then carried out evaporative cooling to cool separate atoms of potassium and sodium to almost absolute zero. After this, they applied a magnetic field to make the atoms to bond together, a mechanism called Feshbach resonance, and eventually produced ultra-cold molecules. However, the ensuing bond was rather weak and produced a fluffy molecule, which still has vibrating properties, as individual atoms are joined across a long and tenuous connection.

"It’s like tuning your radio to be in resonance with some station. These atoms start to vibrate happily together, and form a bound molecule,"Zwierlein said.

In order to produce a more stable and stronger molecule, the researchers used a unique approach that was initially reported in 2008 by scientists from the University of Innsbruck for non-polar cesium (Ce₂) molecules, and the University of Colorado for potassium rubidium (KRb) molecules.

For this method, the NaK molecules that were newly produced were exposed to a couple of lasers, the massive frequency variation of which correlated well with the energy variation between the highly vibrating state of the molecule and its least possible vibrational state.

"By way of absorption of the low-energy laser followed by emission within the high-energy laser beam, all the existing vibrational energy was lost by the molecules. Through this technique, the MIT researchers were successfully able to bring down the molecules to their lowest states of vibration and rotation. In terms of temperature, we sucked away 7,500 kelvins, just like that," Zwierlein added.

In the experiments performed in the past, the Colorado team noticed that the ultra-cold potassium rubidium molecules had a certain disadvantage. These molecules were chemically reactive and came apart upon impact with other molecules. Later, the team restricted the molecules in light crystals to prevent the occurrence of such chemical reactions.

The MIT researchers opted to produce ultra-cold molecules of sodium potassium because this molecule is not only chemically stable, but can resist reactive molecular collisions. During the experiments, the team discovered that their molecular gas was relatively stable and had a long lifespan, which lasted for approximately 2.5s.

By cooling the atoms to near absolute zero temperatures and then forming molecules, the team was able to produce an ultra-cold gas of molecules that were many times colder than would be possibly obtained through direct cooling methods.

"When two potassium rubidium molecules collide, it is more energetically favorable for the two potassium atoms and the two rubidium atoms to pair up. It turns out with our molecule, sodium potassium, this reaction is not favored energetically. It just doesn’t happen. In the case where molecules are chemically reactive, one simply doesn’t have time to study them in bulk samples: They decay away before they can be cooled further to observe interesting states. In our case, we hope our lifetime is long enough to see these novel states of matter," Zwierlein said.

According to Zwierlein says, "to achieve unusual states of matter, molecules would need to be cooled down further, freezing them in place."

"Now we’re at 500 nanokelvins, which is already fantastic, we love it. A factor of 10 colder or so, and the music starts playing," he concluded.