MIT physicists grew this pure crystal of herbertsmithite in their laboratory. This sample is 7 mm long and weighs 0.2 g. Credit: Tianheng Han |
MIT scientists have synthesized, for the
first time, a crystal they believe to be a two-dimensional quantum spin liquid:
a solid material whose atomic spins continue to have motion, even at absolute
zero temperature.
The crystal, known as herbertsmithite,
is part of a family of crystals called Zn-paratacamites, which were first
discovered in 1906. Physicists started paying more attention to quantum spin
liquids in 1987, when Nobel laureate Philip W. Anderson theorized that quantum
spin liquid theory may relate to the phenomenon of high-temperature
superconductivity, which allows materials to conduct electricity with no
resistance at temperatures above 20 degrees Kelvin (-253 degrees Celsius).
To test this theory, scientists have
been looking for materials that preserve quantum spin (a measure of angular
momentum) dynamics down to milli Kelvin temperatures (those below -273 degrees
Celsius). Almost all ordinary materials lose their spin dynamics at such low
temperatures, just as they lose all of their kinetic energy.
Identifying a quantum spin liquid, and
producing large single crystals of it, could potentially help physicists
understand the mechanisms of high-temperature superconductors, says Tianheng
Han, an MIT physics graduate student and lead author of a paper describing the
synthesis in Physical Review B.
Senior authors of the paper are associate professor of physics Young Lee and
Daniel Nocera, the Henry Dreyfus Professor of Energy and professor of
chemistry.
Herbertsmithite is an insulator, not a
superconductor, but quantum spin liquid theory predicts that doping the crystal
(such as substituting its chlorine with sulfur) could transform it into a superconductor.
Han, Lee, and their colleagues were able
to synthesize about a third of a gram of pure single crystal herbertsmithite in
their recent work. The crystal exists in nature, but in forms that are too
impure for experiments to test its physical characteristics. The researchers
plan to do more tests to determine whether the crystal really is a quantum spin
liquid. One important investigation is to search for a spinon continuum—a
signature of quantum fractionalization, a famous example of which is the
fractional quantum hall effect, which was awarded the 1998 Nobel Prize in
physics.
Having large amounts of the crystal
available should enable experiments that will reveal the unusual behavior of
herbertsmithite, says Collin Broholm, professor of physics at Johns Hopkins
Univ. “Neutron scattering experiments are now possible, and these provide very
detailed information that cannot be achieved in any other way,” he says.
