A Quantum Spin Liquid
PFC-supported research at JQI has uncovered evidence for a
long-sought-after quantum state of matter, a spin liquid. You
can’t pour a spin liquid into a glass. It’s not a material at all, at
least not a material you can touch. It is more like a kind of
magnetic disorder within an ordered array of atoms.
The green laser is an tunable weak link in the
circuit that can switch off the atom current
Spin liquids arise due to “frustration,” a baffling condition
wherein neighboring atom spins try to anti-align in up-downup-down fashion, but are prevented from doing so by the
intrinsic geometry of the lattice (here, a honeycomb).
Understanding frustration and the different ways a collection
of magnetically interacting spins can arrange themselves might
help in designing new types of electronic circuitry.
One compromise that a frustrated spin system makes is to
simultaneously exist in many spin orientations . The
researchers uncovered a spin liquid when exploring frustration
in materials with a hexagonal (six-sided) unit cell lattice. They
calculated what happened when a batch of atoms interacted
among themselves in an array of hexagons consisting of 30
sites, where the spins are free to swing about in a twodimensional plane (this kind of approach is called an XY
model).
They found a “kaleidoscope” of phases that represent the
different lowest-energy states that are allowed given a range
of magnetic interactions. One of these phases turns out to be a
true quantum spin liquid having no order at all. Here, the
entire sample co-exists in millions of quantum states
simultaneously.
Figure: Above: Antiferromagnetic arrangement of spins. Below:
Arrangement of spins in a spin liquid, where there is no magnetic order.
Author Galitski describes the spins as free ranging particle-like entities
dubbed “spinons.” These spinons bob about, just as water molecules
bob about in liquid water. (Image credit: E. Edwards)
“Kaleidoscope of Exotic Quantum Phases in a Frustrated XY
Model,” Christopher N. Varney, Kai Sun, Victor Galitski, and
Marcos Rigol, Phys. Rev. Lett., 107, 077201 (2011) (also chosen
for Editor’s suggestion and Viewpoint)