Symmetry is a fundamental concept in physics. Our \u2018standard
\n model\u2019 of particle physics, for example, predicts that matter and
\nanti-matter should have been created in equal amounts at the big
\nbang, yet our existing universe is mostly matter. Such a
\ndiscrepancy between the symmetry of known physical laws, and
\nwhat we actually observe, are often the inspiration for realizing
\n that new interactions are important or that new phases of matter
\ncan exist.
\n.
\nScientists at the RIKEN Institute in Wako, recognized that
\nexperimentalists at The University of Tokyo had possibly discovered
\na new state of matter, called a \u2018chiral spin liquid\u2019 when they reported
\n evidence of time-reversal symmetry breaking \u2014 a difference between
\nthe trajectory of a particle moving along one path or its inverse \u2014 in the
\noxide called Pr2Ir2O7. If a material is magnetic, or in a magnetic field,
\nits electrons will not obey time reversal symmetry; but in Pr2Ir2O7,
\nneither contribution was present to explain what the experimentalists had
\nobserved.
\n.
\nNow, Onoda and colleague Yoichi Tanaka have explained how a chiral
\nspin liquid could emerge from so-called \u2018quantum spin fluctuations\u2019 \u2013 the
\n motion of spins that occurs even at absolute zero. \u201cThe possibility of a
\n chiral spin liquid was first proposed twenty years ago and many
\nphysicists had lost hope to find it,\u201d explains Onoda. \u201cThis is a revival
\nof a phase that was found in a totally different system than where it was
\nfirst expected.\u201d
\n.
\nThe interesting properties of Pr2Ir2O7 are rooted in its crystal structure,
\ncalled a pyrochlore lattice: four praseodymium (Pr) ions, each of which
\ncarries a magnetic \u2018spin\u2019, form a tetrahedral cage around an oxygen (O)
\n ion. At low temperatures, the spins of materials with this structure
\noften \u2018freeze\u2019 into what is called a \u2018spin ice\u2019 (Fig. 1) because of its
\n similarity to the way hydrogen ions form around oxygen in water
\n ice.
\n.
\nOnoda and Tanaka predict, however, that the quantum fluctuations
\n in the spins melt the spin ice structure of Pr2Ir2O7. They proposed a
\n realistic model of Pr spins on a pyrochlore lattice and suggested that
\n both the geometry of the crystal and the small size of the spin on the
\nPr ion allowed the quantum fluctuations to grow so large that they
\nmelted the spin ice into a chiral spin liquid.
\n.
\nIf their prediction is correct, Pr2Ir2O7 will be the first material in which
\n one can study this new state of matter.
\n.<\/p>\n","protected":false},"excerpt":{"rendered":"
Symmetry is a fundamental concept in physics. Our \u2018standard model\u2019 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 … <\/p>\n