The changing phases of quantum magnets

An example of a discontinuous phase transition is when liquid water reaches 100ºC and boils into a gas at normal atmospheric pressure. However, as pressure increases, the boiling point rises, until the critical point where the phase transition smooths out – in other words, liquid and gas merge as a single phase. 

Most work on quantum magnetic materials has focused on smooth, continuous phase transitions. Now, Henrik Rønnow at the Ecole Polytechnique Federale de Lausanne in Switzerland, together with Mohamed Zayed at the Carnegie Mellon University in Doha, Qatar, and co-workers, have demonstrated the discontinuous phase transition and critical point of a quantum magnet for the first time. 

The study builds on Zayed’s PhD project, where he used neutron scattering to determine phase transitioning in a quantum antiferromagnet called SCBO (SrCu2(BO3)2). 

“We have studied SCBO extensively using several techniques,” says Zayed. “For this study, we brought the sample to interesting regions in pressure, temperature and magnetic field to detect signals of phase transitions.” 

The team conducted four months of experiments on tiny pieces of the magnetic material to obtain precise measurements of its specific heat – the energy required to raise its temperature. They carefully controlled the pressure and magnetic field applied to the samples, while monitoring the electron spin behaviour. 

“Normally, specific heat is measured by isolating the sample in vacuum, applying a heat pulse and measuring the temperature increase,” says Rønnow. “However, in a pressure cell, you cannot thermally isolate the sample. We therefore used an oscillating heat-pulse and frequency-locked detection of the tiny temperature oscillations of the sample. This required mounting a heater and thermometer with wires just 10-20 microns thick to a small piece of single crystal, applying pressure without losing the contacts, and then cooling everything down in a dilution refrigerator.”

The resulting pressure-temperature phase diagram of the magnet showed a distinct critical point like water, separating unique quantum phases. 

The team’s findings may prove useful for making qubits, the quantum equivalent of the binary bit used in computing for storing information. However, some applications could be limited by the fact that these phase transitions suffer from hysteresis – the persistence of magnetic states that could delay switching.

“On the other hand, this hysteresis could also be exploited in situations requiring persistence of information,” notes Rønnow.

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