Scientists have taken the clearest picture yet of electronic particles that
make up a mysterious magnetic state called quantum spin liquid (QSL).
The achievement could facilitate the development of superfast quantum
computers and energy-efficient superconductors.
The scientists are the first to capture an image of how electrons in a QSL
decompose into spin-like particles called spinons and charge-like particles
called chargons.
"Other studies have seen various footprints of this phenomenon, but we have
an actual picture of the state in which the spinon lives. This is something
new," said study leader Mike Crommie, a senior faculty scientist at Lawrence
Berkeley National Laboratory (Berkeley Lab) and physics professor at UC.
"Spinons are like ghost particles. They are like the Big Foot of quantum
physics -- people say that they've seen them, but it's hard to prove that
they exist," said co-author Sung-Kwan Mo, a staff scientist at Berkeley
Lab's Advanced Light Source. "With our method we've provided some of the
best evidence to date."
A surprise catch from a quantum wave
In a QSL, spinons freely move about carrying heat and spin -- but no
electrical charge. To detect them, most researchers have relied on
techniques that look for their heat signatures.
Now, as reported in the journal Nature Physics, Crommie, Mo, and their
research teams have demonstrated how to characterize spinons in QSLs by
directly imaging how they are distributed in a material.
To begin the study, Mo's group at Berkeley Lab's Advanced Light Source (ALS)
grew single-layer samples of tantalum diselenide (1T-TaSe2) that are only
three-atoms thick. This material is part of a class of materials called
transition metal dichalcogenides (TMDCs). The researchers in Mo's team are
experts in molecular beam epitaxy, a technique for synthesizing atomically
thin TMDC crystals from their constituent elements.
Mo's team then characterized the thin films through angle-resolved
photoemission spectroscopy, a technique that uses X-rays generated at the
ALS.
Using a microscopy technique called scanning tunneling microscopy (STM),
researchers in the Crommie lab -- including co-first authors Wei Ruan, a
postdoctoral fellow at the time, and Yi Chen, then a UC Berkeley graduate
student -- injected electrons from a metal needle into the tantalum
diselenide TMDC sample.
Images gathered by scanning tunneling spectroscopy (STS) -- an imaging
technique that measures how particles arrange themselves at a particular
energy -- revealed something quite unexpected: a layer of mysterious waves
having wavelengths larger than one nanometer (1 billionth of a meter)
blanketing the material's surface.
"The long wavelengths we saw didn't correspond to any known behavior of the
crystal," Crommie said. "We scratched our heads for a long time. What could
cause such long wavelength modulations in the crystal? We ruled out the
conventional explanations one by one. Little did we know that this was the
signature of spinon ghost particles."
How spinons take flight while chargons stand still
With help from a theoretical collaborator at MIT, the researchers realized
that when an electron is injected into a QSL from the tip of an STM, it
breaks apart into two different particles inside the QSL -- spinons (also
known as ghost particles) and chargons. This is due to the peculiar way in
which spin and charge in a QSL collectively interact with each other. The
spinon ghost particles end up separately carrying the spin while the
chargons separately bear the electrical charge.
In the current study, STM/STS images show that the chargons freeze in place,
forming what scientists call a star-of-David charge-density-wave. Meanwhile,
the spinons undergo an "out-of-body experience" as they separate from the
immobilized chargons and move freely through the material, Crommie said.
"This is unusual since in a conventional material, electrons carry both the
spin and charge combined into one particle as they move about," he
explained. "They don't usually break apart in this funny way."
Crommie added that QSLs might one day form the basis of robust quantum bits
(qubits) used for quantum computing. In conventional computing a bit encodes
information either as a zero or a one, but a qubit can hold both zero and
one at the same time, thus potentially speeding up certain types of
calculations. Understanding how spinons and chargons behave in QSLs could
help advance research in this area of next-gen computing.
Another motivation for understanding the inner workings of QSLs is that they
have been predicted to be a precursor to exotic superconductivity. Crommie
plans to test that prediction with Mo's help at the ALS.
"Part of the beauty of this topic is that all the complex interactions
within a QSL somehow combine to form a simple ghost particle that just
bounces around inside the crystal," he said. "Seeing this behavior was
pretty surprising, especially since we weren't even looking for it."
Reference:
Wei Ruan, Yi Chen, Shujie Tang, Jinwoong Hwang, Hsin-Zon Tsai, Ryan L. Lee,
Meng Wu, Hyejin Ryu, Salman Kahn, Franklin Liou, Caihong Jia, Andrew Aikawa,
Choongyu Hwang, Feng Wang, Yongseong Choi, Steven G. Louie, Patrick A. Lee,
Zhi-Xun Shen, Sung-Kwan Mo, Michael F. Crommie. Evidence for quantum spin
liquid behaviour in single-layer 1T-TaSe2 from scanning tunnelling
microscopy. Nature Physics, 2021;
DOI: 10.1038/s41567-021-01321-0
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Physics