"It won’t be long before we know whether we are right or wrong,” remarked Professor Erio Tosatti when, in 2014, PNAS published a study by SISSA/ICTP carried out with Michele Fabrizio, Sandro Scandolo, and first author Yanier Crespo. Crespo—first an ICTP Diploma student and later a SISSA-ICTP PhD candidate—developed this work as his doctoral thesis.
Now, just over ten years later, experimental evidence has confirmed that theoretical study. The results of this new investigation have just been published in Physical Review Letters.
The new work, conducted at the Grenoble Synchrotron, reports the observation of a subtle structural transition in solid oxygen at about 180,000 atmospheres and focuses on the so-called “epsilon phase.”
When subjected to extremely high pressures, O₂ molecules—well known to be magnetic with “spin 1,” twice that of an electron—tend to cluster into “quartets” at around 8,000 atmospheres. At this point begins a long phase, known precisely as the “epsilon phase,” where magnetism seems to disappear. However, in the 2014 study—based on theoretical work and sophisticated simulations—the scientists from the two Trieste institutes hypothesized that reality was more complex: the epsilon phase would not be homogeneous at all, but rather composed of two physically distinct sub-phases.
According to Crespo, Fabrizio, Scandolo, and Tosatti, the first, called the “epsilon₀ phase,” occurs between 18 and 96 GPa and is indeed non-magnetic. The second, “epsilon₁ phase,” develops between about 80,000 and 180,000 times atmospheric pressure and is characterized by a “spin-1 liquid” confined within each quartet. In this state, the spins—i.e., the direction of the magnetic polarization of each molecule—oscillate continuously between “up” and “down” in a sort of perpetual dance, thereby keeping the total magnetism null. In other words, magnetism is not absent but constantly neutralized by the movement of the spins. The new research published in Physical Review Letters confirms exactly this prediction.
Erio Tosatti comments:
“Resonating spin states are quite rare but important in solid-state physics. The resonating singlet state within a quartet, realized by four spin-1 units, is absolutely unique in nature: with a bit of imagination, it can be seen as a nano-droplet of quantum liquid. What our colleagues have now observed experimentally confirms our theories, as also highlighted in Physics: it is the subtle yet unmistakable signal, at the critical pressure, of the switching-off of a pre-existing resonant state, which—following the theoretical predictions—has thus been identified for the first time. This is truly an important result and, for us, a great satisfaction. Of course, further properties of the ‘musical spin quartets’ spontaneously organized within the epsilon phase remain to be explored. The groundwork for this achievement had already been laid back then, thanks to that research carried out by the SISSA and ICTP physicists, with the fundamental contribution of a student from both institutions.”