Two scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have discovered a new phase of matter while studying a model system of a magnetic material.
The phase is a never-before-seen pattern of electron spins—the tiny “up” and “down” magnetic moments carried by every electron. It consists of a combination of highly ordered “cold” spins and highly disordered “hot” spins, and it has thus been dubbed “half ice, half fire.” The researchers discovered the new phase while studying a one-dimensional model of a type of magnetic material called a ferrimagnet.
“Half ice, half fire” is notable not only because it has never been observed before, but also because it is able to drive extremely sharp switching between phases in the material at a reasonable, finite temperature. This phenomenon could one day result in applications in the energy and information technology industries.
The researchers, physicists Weiguo Yin and Alexei Tsvelik, describe their work in the Dec. 31, 2024, edition of the journal Physical Review Letters.
“Finding new states with exotic physical properties—and being able to understand and control the transitions between those states—are central problems in the fields of condensed matter physics and materials science,” said Yin. “Solving those problems could lead to great advances in technologies like quantum computing and spintronics.”
Tsvelik added, “We suggest that our findings may open a new door to understanding and controlling phases and phase transitions in certain materials.”
First came fire and ice
The “half-ice, half-fire” phase is the twin state of the “half-fire, half-ice” phase discovered by Yin, Tsvelik, and Christopher Roth, their 2015 undergraduate summer intern who is now a postdoc at the Flatiron Institute. They describe the discovery in a paper published in early 2024.
But the full story goes back to 2012, when Yin and Tsvelik were part of a multi-institutional collaboration, led by Brookhaven physicist John Hill, that was studying Sr3CuIrO6, a magnetic compound of strontium, copper, iridium, and oxygen. This research led to two papers, an experiment-driven study in 2012 and a theory-driven study in 2013, both published in PRL.
Yin and Tsvelik continued to look into the phase behaviors of Sr3CuIrO6 and, in 2016, found the “half-fire, half-ice” phase. In this state, which is induced by a critical external magnetic field, the “hot” spins on the copper sites are fully disordered on the atomic lattice and have smaller magnetic moments, while the “cold” spins on the iridium sites are fully ordered and have larger magnetic moments. That work was published in Physical Review B.
“But despite our extensive research, we still didn’t know how this state could be utilized, especially because it has been well known for one century that the one-dimensional Ising model, an established mathematical model of ferromagnetism that produces the half-fire, half-ice state, does not host a finite-temperature phase transition,” said Tsvelik. “We were missing pieces of the puzzle.”
A hint to the missing pieces was recently identified by Yin. In two publications for systems with and without an external magnetic field, respectively, he demonstrated that the aforementioned forbidden phase transition can be approached by ultranarrow phase crossover at fixed finite temperature.
In this current work, Yin and Tsvelik have discovered that “half fire, half ice” has an opposite, hidden state in which the hot and cold spins switch positions. That is, the hot spins become cold, and the cold spins become hot, which led them to name the phase “half ice, half fire.”
The model reveals that switching between phases takes place over an ultranarrow temperature range, and Yin and Tsvelik have already suggested possible ways that this could be used in future applications. For example, ultrasharp phase switching with a giant magnetic entropy change—offered by “half fire, half ice”—could be useful for refrigeration technologies. The phenomenon could also be used as the foundation for a new type of quantum information storage technology in which phases act as bits.
“Next, we are going to explore the fire-ice phenomenon in systems with quantum spins and with additional lattice, charge, and orbital degrees of freedom,” said Yin. “The door to new possibilities is now wide open.”
Reference: Weiguo Yin et al, Phase Switch Driven by the Hidden Half-Ice, Half-Fire State in a Ferrimagnet, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.266701.
