Water can freeze from liquid ice to solid ice or boil in a gas. In cooking, these “phase transitions” are not smooth, but their discontinuous character is smoothed out at high pressure. An international team of physicists led by EPFL has just discovered the same behavior in certain quantum magnets, which could have consequences for qubit technology.
In physics, things exist in “phases”, like solid, liquid, gas. When something goes from one phase to another, we talk about a “phase transition” – think of boiling water to vapor, going from liquid to gas.
In our kitchens the water boils at 100 O C, and its density changes dramatically, making a discontinuous jump from liquid to gas. However, if the pressure is increased, the boiling point of water also increases, up to a pressure of 221 atmospheres where it boils at 374 O C. Something strange is happening here: the liquid and the gas merge into a single phase. Above this “critical point” there is no phase transition at all, and thus, by controlling its pressure, water can be directed from liquid to gas without ever passing through it.
Is there a quantum version of a water-like phase transition? “The current directions of quantum magnetism and spintronics require strongly anisotropic spin interactions to produce the physics of topological phases and protected qubits, but these interactions also promote discontinuous quantum phase transitions,” explains Professor Henrik Rønnow of the EPFL Faculty of Fundamental Sciences.
Previous studies have focused on continuous and smooth phase transitions in quantum magnetic materials. Today, in an experimental and theoretical project led by Rønnow and Professor Frédéric Mila, also at the Faculty of Fundamental Sciences, physicists from EPFL and the Paul Scherrer Institute studied a discontinuous phase transition to observe the very first critical point in a quantum magnet. , similar to that of water. The work is now published in Nature.
The scientists used a “quantum antiferromagnetic”, known in the field as SCBO (after its chemical composition: SrCu 2 (BO 3) 2). Quantum antiferromagnets are particularly useful for understanding how quantum aspects of a material’s structure affect its overall properties – for example, how the spins of its electrons interact to give rise to its magnetic properties. The SCBO is also a “frustrated” magnet, which means that its electronic spins cannot stabilize in an ordered structure, and instead, they adopt unique quantum fluctuating states.
In a complex experiment, the researchers controlled both the pressure and the magnetic field applied to milligram chunks of SCBO. “This allowed us to look all around the discontinuous quantum phase transition and in this way we found the physics of critical points in a pure spin system,” says Rønnow.
The team performed high-precision measurements of the specific heat of SCBO, showing that it is ready to “absorb energy”. For example, water only absorbs small amounts of energy at -10 O C, but at 0 O C and 100 O C it can take huge amounts as each molecule is dragged through the transitions from ice to liquid and from liquid to gas. Much like water, the pressure-temperature relationship of SCBO forms a phase diagram showing a discontinuous transition line separating two quantum magnetic phases, the line ending at a critical point.
“Now, when a magnetic field is applied, the problem becomes richer than water,” explains Frédéric Mila. “No magnetic phase is strongly affected by a small field, so the line becomes a wall of discontinuities in a three-dimensional phase diagram – but then one of the phases becomes unstable and the field helps push it towards a third. phase.”
To explain this macroscopic quantum behavior, the researchers joined forces with several colleagues, including Professor Philippe Corboz of the University of Amsterdam, who have developed powerful new computing techniques.
“Previously, it was not possible to calculate the properties of ‘frustrated’ quantum magnets in a realistic two- or three-dimensional model,” says Mila. “So SCBO provides a timely example where new numerical methods meet reality to provide a quantitative explanation of a novel phenomenon for quantum magnetism. “
Henrik Rønnow concludes: “In the future, the next generation of functional quantum materials will go through discontinuous phase transitions, so a good understanding of their thermal properties will certainly include the critical point, the classic version of which has been known to science for two years. centuries. “
Reference: “A quantum magnetic analogue to the critical point of water” by J. Larrea Jiménez, S. P. G. Crone, E. Fogh, M. E. Zayed, R. Lortz, E. Pomjakushina, K. Conder, A. M. Läuchli, L. Weber, S. Wessel, A. Honecker, B. Normand, Ch. Rüegg, P. Corboz, H. M. Rønnow and F. Mila, 14 April 2021, Nature.
- University of São Paulo
- University of Amsterdam
- Carnegie Mellon University in Qatar
- Hong Kong University of Science and Technology
- University of Innsbruck
- RWTH Aachen University
- CY Cergy Paris University
- ETH Zürich
- University of Geneva
- São Paulo Research Foundation (FAPESP)
- Qatar Foundation (Carnegie Mellon University in Qatar’s Seed Research program)
- Swiss National Science Foundation (SNSF)
- European Research Council (ERC) Horizon 2020
- ERC Synergy Grant HERO
- Deutsche Forschungsgemeinschaft