Structure of LaVOSiO4. The magnetic V4+ ions occupy the centres
of the tetrahedra.
The elementary textbook view of a magnetic solid is one in which
neighbouring atomic spins are preferentially aligned parallel to
each other (ferromagnet) or antiparallel to each other (antiferromagnet)
at low temperatures. The latter is observed in the CuO2
planes of the undoped cuprate high-temperature superconductors,
for instance. The full rotational symmetry of the underlying Hamiltonian
is broken in such a state, as the system spontaneously selects a
preferred axis of orientation.
In some materials and model systems, the presence of competing
interactions can produce a transition from a state with long range
magnetic order to one with no magnetic order. Such a phase has been
termed a “spin liquid”, since there is no local rigidity
against spin fluctuations, unlike the ordered case, which might
be termed a “spin solid”. The whole picture is complicated
by the presence of “quantum fluctuations”, which make
it very difficult to compute what will happen.
Our group has been studying such problems for a number of years,
both to discover general features of frustrated low-dimensional
magnetic systems and to model particular materials. We have recently
been awarded an ARC grant of $750K over 5 years to continue this
and other work.
An archetypical model system is the spin-1/2 J1-J2
Heisenberg antiferromagnet, illustrated below. The axial interactions
with strength J1 compete with, or “frustrate”,
the diagonal interactions with strength J2. This model
has a spin-liquid phase at zero temperature in the intermediate
coupling range 0.38 J2/J1 0.6. Our work has
shown that the spin-liquid phase is not in fact structureless, but
appears to be a dimerized correlated state where local symmetry
breaking is preserved for short times, but disappears on averaging
over the system. Recent work, by our group and others, has demonstrated
the relevance of the model to the recently synthesized materials
LaVOSiO4 and Li2VOGeO4. We have
studied a somewhat similar model which appears to provide a good
description of another material SrCu2(BO3)2.
While interesting, none of these materials lies in the spin-liquid
region of the model phase diagram. Experimentalists are eagerly
searching for materials which are spin liquids or, an even more
exciting possibility, could be tuned through a quantum critical
point into a spin liquid phase.
Rob Bursill, Chris Hamer, Jaan Oitmaa,
Oleg Sushkov and Zheng Weihong