The ‘Hall’ mark of a quantum magnet

The presence of exotic particles, called spinons, might now be detectable in a magnetic field, providing insight into quantum magnet properties

An important model to explain high-temperature superconductivity is the
so-called ‘quantum spin liquid’. Scientists are therefore interested in
understanding the low-energy excitations of this magnetic state. Now,
a theoretical study by a research team from RIKEN and the Massachusetts Institute
of Technology, USA, has explained how the properties of spin liquids could be
revealed by a simple heat-transfer experiment.

In an insulating magnetic crystal, the electronic spins are localized to the atoms
that form the crystal lattice. For most such magnets, or antiferromagnets, the chemical
bonds favor an arrangement where, at low temperatures, each spin points in a direction
opposite to that of its neighbor. However, on a triangular lattice, such as the ‘Kagome lattice’,
a spin cannot simultaneously be opposite to all of its neighbors. The spins in these magnets
never order, even at very low temperatures—giving rise to the name quantum spin liquid.

“Spin liquids have an exotic electronic state because [their] electrons can effectively
dissociate into distinguishable spin- and charge-carrying particles,” explains team-member
Naoto Nagaosa from the RIKEN Advanced Science Institute, Wako. “The spin-carrying particle
is called a spinon and determines the low-energy properties of the magnet.”

To date, however, few experiments have found spinons. Nagaosa and his collaborators
explain how a method similar to the so-called ‘Hall measurement’—an indispensible technique
for studying the properties of semiconductors—could be used to detect spinons.

In the classic version of the Hall measurement, a magnetic field is applied perpendicular
to a charge-carrying current, causing positive charges to curve one way and negative
charges the other. The deflection of the charges provides information about their properties,
including their sign.

In the ‘thermal Hall effect’ considered by Nagaosa and his colleagues, temperature serves
as the driving force to create a current—not of charges, but of magnetic excitations—that
flow in a magnetic field. For a spin liquid, these excitations are the spinons. As in the classic Hall
effect, a magnetic field will deflect these excitations, which will change the direction of the heat
flow—an effect that experimentalists should be able to measure.

Nagaosa and his colleagues showed that while there is no thermal Hall effect in most conventional
antiferromagnets, the presence of spinons in a spin liquid would result in a clear effect. This
experimental probe could therefore become an important way to identify and study excitations
of quantum magnets.