Water and quantum magnets share same critical physics

The study may have consequences for the technology of qubits.

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The boiling point of water is 100 degrees Celcius. Water canes its density during phase transition. Turnin up-the pressure could increase the boiling point until a pressure of 221 atmospheres where it boils at 374 degrees Celcius.

In a new study, something strange happened: the liquid and gas merge into a single phase. Above this “critical point,” there is no longer a phase transition at all, and so by controlling its pressure, water can be steered from liquid to gas without ever crossing one.

Professor Henrik Rønnow at EPFL‘s School of Basic Sciences said, “Is there a quantum version of a water-like phase transition? The current directions in quantum magnetism and spintronics require highly spin-anisotropic interactions to produce the physics of topological phases and protected qubits. Still, these interactions also favor discontinuous quantum phase transitions.”

Past studies have only determined smooth, continuous phase transitions in quantum magnetic materials. Now, in a joint experimental and theoretical project led by Rønnow and Professor Frédéric Mila, also at the School of Basic Sciences, physicists at EPFL and the Paul Scherrer Institute have studied a discontinuous phase transition to observe the first-ever critical point in a quantum magnet, similar to that of water.

Using antiferromagnet, known in the field as SCBO, scientists tended to determine how the quantum aspects of a material’s structure affect its overall properties. SCBO is also a “frustrated” magnet, meaning that its electron spins can’t stabilize in some orderly structure, and instead, they adopt some uniquely quantum fluctuating states.

In an experiment, scientists controlled both the pressure and the magnetic field applied to milligram pieces of SCBO. Doing so allowed them to observe the discontinuous quantum phase transition. In this way, scientists found critical-point physics in a pure spin system.

Scientists performed high-precision measurements of the specific heat of SCBO, which shows its readiness to “suck up energy.” Just like water, the pressure-temperature relationship of SCBO forms a phase diagram showing a discontinuous transition line separating two quantum magnetic phases, with the line ending at a critical point.

Professor Frédéric Mila said, “Now when a magnetic field is applied, the problem becomes richer than water. Neither 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.”

“Previously, it was impossible to calculate the properties of ‘frustrated’ quantum magnets in a realistic two- or a three-dimensional model. So SCBO provides a well-timed example where the new numerical methods meet reality to provide a quantitative explanation of a phenomenon new to quantum magnetism.”

Henrik Rønnow concludes“Looking forward, the next generation of functional quantum materials will be switched across discontinuous phase transitions, so a proper understanding of their thermal properties will certainly include the critical point, whose classical version has been known to science for two centuries.”

Journal Reference:
  1. J. Larrea Jiménez, et al. A quantum magnetic analogue to the critical point of water. Nature 14 April 2021. DOI: 10.1038/s41586-021-03411-8