"Previously it was not possible to calculate the properties of 'frustrated' quantum magnets in a realistic two- or 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."Frédéric Mila, EPFL
In physics, things exist in 'phases', such as solid, liquid and 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.
When water boils at 100°C, its density changes dramatically, making a sudden, discontinuous jump from liquid to gas. If we turn up the pressure, however, the boiling point of water also increases, up to a pressure of 221 atmospheres where it boils at 374°C. Here, something strange happens: the liquid and gas merge into a single phase. Above this 'critical point', there is no longer a phase transition, and so by controlling the pressure, water can be steered from liquid to gas without ever crossing this transition.
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, but these interactions also favor discontinuous quantum phase transitions," says Henrik Rønnow, a professor in the School of Basic Sciences at the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland.
Previous studies have focused on smooth, continuous phase transitions in quantum magnetic materials. Now, in a joint experimental and theoretical project led by Rønnow and Frédéric Mila, also a professor in the School of Basic Sciences, researchers at EPFL and the Paul Scherrer Institute in Switzerland have studied a discontinuous phase transition to observe the first ever critical point in a quantum magnet, similar to that of water. The researchers report their work in a paper in Nature.
The scientists used a 'quantum antiferromagnet' known 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 give rise to its magnetic properties. SCBO is also a 'frustrated' magnet, meaning that its electron spins can't stabilize in an orderly structure, but instead 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. "This allowed us to look all around the discontinuous quantum phase transition and that way we found critical-point physics in a pure spin system," says Rønnow.
The team performed high-precision measurements of the specific heat of SCBO, which showed its readiness to 'suck up energy'. For example, water absorbs only small amounts of energy at -10°C, but at 0°C and 100°C it can take up huge amounts of energy, as every molecule is driven across the transition from ice to liquid and from 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.
"Now when a magnetic field is applied, the problem becomes richer than water," says Mila. "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."
To explain this macroscopic quantum behavior, the researchers teamed up with several colleagues, particularly Philippe Corboz at the University of Amsterdam in the Netherlands, who have been developing powerful new computer-based 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 well-timed example where the new numerical methods meet reality to provide a quantitative explanation of a phenomenon new to quantum magnetism."
"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," concludes Rønnow.
This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.