A team of Japanese and US physicists has pushed thousands of Ytterbium atoms to just within a billionth of a degree above absolute zero to understand how matter behaves at these extreme temperatures. The approach treats the atoms as fermions, the type of particles like electrons and protons, that cannot end up in the so-called fifth state of matter at those extreme temperatures: a Bose-Einstein Condensate.
When fermions are actually cooled down, they do exhibit quantum properties in a way that we can’t simulate even with the most powerful supercomputer. These extremely cold atoms are placed in a lattice and they simulate a "Hubbard model" which is used to study the magnetic and superconductive behavior of materials, in particular the collective motion of electrons through them.
The symmetry of these models is known as the special unitary group, or, SU, and depends on the possible spin state. In the case of Ytterbium, that number is 6. Calculating the behavior of just 12 particles in a SU(6) Hubbard model can’t be done with computers. However, as reported in Nature Physics, the team used laser cooling to reduce the temperature of 300,000 atoms to a value almost three billion times colder than the temperature of outer space.
“Unless an alien civilization is doing experiments like these right now, anytime this experiment is running at Kyoto University it is making the coldest fermions in the universe,” co-author Rice University’s Kaden Hazzard said in a statement. “Fermions are not rare particles. They include things like electrons and are one of two types of particles that all matter is made of.”
The team reports the first observations of particle coordination in an SU(6) Hubbard model. An important step forward in understanding how these systems behave and evolve.
“Right now this coordination is short-ranged, but as the particles are cooled even further, subtler and more exotic phases of matter can appear,” he said. “One of the interesting things about some of these exotic phases is that they are not ordered in an obvious pattern, and they are also not random. There are correlations, but if you look at two atoms and ask, ‘Are they correlated?' you won't see them. They are much more subtle. You can't look at two or three or even 100 atoms. You kind of have to look at the whole system.”
The tools to measure such behaviors are still not there, but the team hopes that work to create them will soon bear fruit. By understanding the Hubbard model, one can get the basic ingredients behind the reasons why solids can be metals, insulators, magnets, or superconductors.
“One of the fascinating questions that experiments can explore is the role of symmetry,” co-author Eduardo Ibarra-García-Padilla said. “To have the capability to engineer it in a laboratory is extraordinary. If we can understand this, it may guide us to making real materials with new, desired properties.”