Water and quantum magnets share critical physics

Specific heat of water and of SrCu2(BO3)2, shown together with calculated critical properties of the Shastry–Sutherland model.

In physics, things exist in “phases”, such as solid, liquid, gas. When something crosses from one phase to another, we talk about a “phase transition” – think about water boiling into steam, turning from liquid to gas.

In our kitchens water boils at 100oC, and its density changes dramatically, making a discontinuous jump from liquid to gas. However, if we turn up the pressure, the boiling point of water also increases, until a pressure of 221 atmospheres where it boils at 374oC. Here, something strange happens: 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.

Is there a quantum version of a water-like phase transition?

Previous studies have focused on smooth, continuous phase transitions in quantum magnetic materials. Now, in a joint experimental and theoretical project 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.

The scientists used a “quantum antiferromagnet”, known in the field as SCBO (from its chemical composition: SrCu2(BO3)2). Quantum antiferromagnets are especially useful for understanding how the quantum aspects of a material’s structure affect its overall properties – for example, how the spins of its electrons interact to give its magnetic 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 a complex experiment, the researchers controlled both the pressure and the magnetic field applied to milligram pieces of SCBO.

The team performed high-precision measurements of the specific heat of SCBO, which shows its readiness to “suck up energy”. For example, water absorbs only small amounts of energy at -10oC, but at 0oC and 100oC it can take up huge amounts as every molecule is driven across the transitions from ice to liquid and liquid to gas. 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.

This may have consequences for the technology of qubits.

The work has been published in Nature.

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