The hope of meltdown – an evidence of a new state of matter

Symmetry is a fundamental concept in physics. Our ‘standard
model’ of particle physics, for example, predicts that matter and
anti-matter should have been created in equal amounts at the big
bang, yet our existing universe is mostly matter. Such a
discrepancy between the symmetry of known physical laws, and
what we actually observe, are often the inspiration for realizing
that new interactions are important or that new phases of matter
can exist.
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Scientists at the RIKEN Institute in Wako, recognized that
experimentalists at The University of Tokyo had possibly discovered
a new state of matter, called a ‘chiral spin liquid’ when they reported
evidence of time-reversal symmetry breaking — a difference between
the trajectory of a particle moving along one path or its inverse — in the
oxide called Pr2Ir2O7. If a material is magnetic, or in a magnetic field,
its electrons will not obey time reversal symmetry; but in Pr2Ir2O7,
neither contribution was present to explain what the experimentalists had
observed.
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Now, Onoda and colleague Yoichi Tanaka have explained how a chiral
spin liquid could emerge from so-called ‘quantum spin fluctuations’ – the
motion of spins that occurs even at absolute zero. “The possibility of a
chiral spin liquid was first proposed twenty years ago and many
physicists had lost hope to find it,” explains Onoda. “This is a revival
of a phase that was found in a totally different system than where it was
first expected.”
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The interesting properties of Pr2Ir2O7 are rooted in its crystal structure,
called a pyrochlore lattice: four praseodymium (Pr) ions, each of which
carries a magnetic ‘spin’, form a tetrahedral cage around an oxygen (O)
ion. At low temperatures, the spins of materials with this structure
often ‘freeze’ into what is called a ‘spin ice’ (Fig. 1) because of its
similarity to the way hydrogen ions form around oxygen in water
ice.
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Onoda and Tanaka predict, however, that the quantum fluctuations
in the spins melt the spin ice structure of Pr2Ir2O7. They proposed a
realistic model of Pr spins on a pyrochlore lattice and suggested that
both the geometry of the crystal and the small size of the spin on the
Pr ion allowed the quantum fluctuations to grow so large that they
melted the spin ice into a chiral spin liquid.
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If their prediction is correct, Pr2Ir2O7 will be the first material in which
one can study this new state of matter.
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