In physics, things exist in phases, such as solid, liquid and gas states. When
something crosses from one phase to another, we talk about a phase
transition—like water boiling into steam, turning from liquid to gas.
In our kitchens, water boils at 100 degrees C, 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 374 degrees 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 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? "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 Professor Henrik Rønnow at EPFL's School of
Basic Sciences.
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 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. The work is now
published in Nature.
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. "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 absorb energy. For example, water absorbs only
small amounts of energy at -10 degrees C, but at 0 degrees C and 100 degrees
C, 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.
"Now, when a magnetic field is applied, the problem becomes richer than
water," says Frédéric 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 Professor Philippe Corboz at the University
of Amsterdam, 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."
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."
Reference:
A quantum magnetic analogue to the critical point of water, Nature (2021).
DOI:
10.1038/s41586-021-03411-8
Tags:
Physics