Ivan Bozovic |
Magnetic
studies of ultrathin slabs of copper-oxide materials reveal that at
very low temperatures, the thinnest, isolated layers lose their
long-range magnetic order and instead behave like a “quantum spin
liquid” — a state of matter where the orientations of electron spins
fluctuate wildly. This unexpected discovery by scientists at the U.S.
Department of Energy’s (DOE) Brookhaven National Laboratory and
collaborators at the Paul Scherrer Institute in Switzerland may offer
support for the idea that this novel condensed state of matter is a
precursor to the emergence of high-temperature superconductivity — the
ability to carry current with no resistance.
The
hope is that this research, just published online in Physical Review
Letters, will lead to a deeper understanding of the physics of
high-temperature superconductivity and advance the quest for new and
better superconductors for meeting the nation’s and world’s energy
needs.
The
idea of quantum spin liquids is credited to Nobel laureate Philip W.
Anderson, who also proposed the possible link to the emergence of
high-temperature superconductivity when copper-oxide, or “cuprate,”
materials are doped with mobile charge carriers — that is, when atoms
supplying additional electrons or electron vacancies are added. However,
some past experimental findings haven’t supported this proposal:
Without doping, lanthanum-copper-oxide, one of the most studied
cuprates, shows a form of long-range magnetic order known as
anti-ferromagnetism — where spin orientations on adjacent electrons
alternately point in exactly opposite directions — even at room
temperature. But the new Brookhaven Lab/Scherrer Institute experiments
suggest a different picture when one looks at thin enough layers.
“The
crystal structure of lanthanum-copper-oxide is layered; it consists of
parallel copper-oxide and lanthanum-oxide sheets,” explained Brookhaven
physicist Ivan Bozovic, one of the lead authors on the paper. “The
interaction among the spins within one copper-oxide plane is strong,
while their interaction with the spins in the nearest copper-oxide plane
(about 0.66 nanometers away) is ten thousand times weaker. Still, this
weak interaction between layers may be sufficient to suppress
fluctuations and stabilize the anti-ferromagnetic order.”
The
key to finding out if there was fluctuation-suppressing interaction
among layers was to look for magnetic order in thinner films, with fewer
layers and better insulation.
Bozovic
used a specialized atomic-layer-by-layer molecular beam epitaxy method
he’s developed to assemble lanthanum-copper-oxide samples with varying
numbers of layers. The layers were well separated and insulated to
prevent any “crosstalk.” The thickness was controlled with atomic
precision and varied digitally, down to a single copper-oxide plane.
This precision was critically important for the success of the
experiment.
These
unique samples were studied at the Paul Scherrer Institute by Elvezio
Morenzoni and his team, who had developed an exquisite diagnostic
technique called low-energy muon spin spectroscopy to detect and
investigate magnetism in such ultrathin layers.
The
magnetic measurements revealed that when the slabs contained four or
more copper-oxide layers, they showed anti-ferromagnetic ordering — just
like thick, bulk crystals of the same materials, and even up to the
same temperature. However, thinner slabs that contained just one or two
copper-oxide layers showed an unexpected result: “While the magnetic
moments, or spins, were still present and had about the same magnitude,
there was no long-range static anti-ferromagnetic order, not even on the
scale of a few nanometers. Rather, the spins were fluctuating wildly,
changing their direction very fast,” Bozovic said.
Even
more telling, this effect was stronger the lower the temperature of the
sample. “That means these fluctuations could not be of thermal origin
and must be of quantum origin — quantum objects fluctuate even at zero
temperature,” Bozovic explained.
“Altogether,
this experiment indicates that once a copper-oxide plane is well
isolated and not interacting with other such layers, it in fact seems to
behave, at low temperature, like some sort of quantum spin liquid.”
Bozovic said. So perhaps the idea that high-temperature
superconductivity emerges from this quantum spin liquid state could,
after all, be true.
“We
certainly need to do more experiments to test the implications of our
discovery and how it relates to this theoretical prediction,” Bozovic
said.
This work was supported by the DOE Office of Science.
Related links:
Exploring the Superconducting Transition in Ultra Thin Films
Fleeting Fluctuations in Superconductivity Disappear Close to Transition Temperature
Giant Proximity Effect Enhances High-Temperature Superconductivity
Pinning Down Superconductivity to a Single Layer
