Columbia University School of
Engineering and Applied Science
Researchers report that they have
observed a quantum fluid known as the fractional quantum Hall states (FQHS),
one of the most delicate phases of matter, for the first time in a monolayer 2D
semiconductor. Their findings demonstrate the excellent intrinsic quality of 2D
semiconductors and establish them as a unique test platform for future
applications in quantum computing.
"We were very surprised to observe this state in 2D
semiconductors because it has generally been assumed that they are too dirty
and disordered to host this effect," says Cory Dean, professor of physics
at Columbia University. "Moreover, the FQHS sequence in our experiment
reveals unexpected and interesting new behavior that we've never seen before,
and in fact suggests that 2D semiconductors are close-to-ideal platforms to
study FQHS further."
The fractional quantum Hall state is a collective phenomenon
that comes about when researchers confine electrons to move in a thin two-dimensional
plane, and subject them to large magnetic fields. First discovered in 1982, the
fractional quantum Hall effect has been studied for more than 40 years, yet
many fundamental questions still remain. One of the reasons for this is that
the state is very fragile and appears in only the cleanest materials.
"Observation of the FQHS is therefore often viewed as a
significant milestone for a 2D material -- one that only the very cleanest
electronic systems have reached," notes Jim Hone, Wang Fong-Jen Professor
of Mechanical Engineering at Columbia Engineering.
While graphene is the best known 2D material, a large group of
similar materials have been identified over the past 10 years, all of which can
be exfoliated down to a single layer thickness. One class of these materials is
transition metal dichalcogenides (TMD), such as WSe2, the material used in this
new study. Like graphene, they can be peeled to be atomically thin, but, unlike
graphene, their properties under magnetic fields are much simpler. The
challenge has been that the crystal quality of TMDs was not very good.
"Ever since TMD came on the stage, it was always thought of
as a dirty material with many defects," says Hone, whose group has made
significant improvement to the quality of TMDs, pushing it to a quality near to
graphene -- often considered the ultimate standard of purity among 2D
materials.
In addition to sample quality, studies of the semiconductor 2D
materials have been hindered by the difficulties to make good electrical
contact. To address this, the Columbia researchers have also been developing
the capability to measure electronic properties by capacitance, rather than the
conventional methods of flowing a current and measuring the resistance. A major
benefit of this technique is that the measurement is less sensitive both to
poor electrical contact and to impurities in the material. The measurements for
this new study were performed under very large magnetic fields -- which help to
stabilize the FQHS -- at the National High Magnetic Field Lab.
"The fractional numbers that characterize the FQHS we
observed -- the ratios of the particle to magnetic flux number -- follow a very
simple sequence," says Qianhui Shi, the paper's first author and a
postdoctoral researcher at the Columbia Nano Initiative. "The simple
sequence is consistent with generic theoretical expectations, but all previous
systems show more complex and irregular behavior. This tells us that we finally
have a nearly ideal platform for the study of FQHS, where experiments can be
directly compared to simple models."
Among the fractional numbers, one of them has an even
denominator. "Observing the fractional quantum Hall effect was itself
surprising, seeing the even-denominator state in these devices was truly
astonishing, since previously this state has only been observed in the very
best of the best devices," says Dean.
Fractional states with even denominators have received special
attention since their first discovery in the late 1980s, since they are thought
to represent a new kind of particle, one with quantum properties different from
any other known particle in the universe. "The unique properties of these
exotic particles," notes Zlatko Papic, associate professor in theoretical
physics at the University of Leeds, "could be used to design quantum
computers that are protected from many sources of errors."
So far, experimental efforts to both understand and exploit the
even denominator states have been limited by their extreme sensitivity and the
extremely small number of materials in which this state could be found.
"This makes the discovery of the even denominator state in a new -- and
different -- material platform, really very exciting," Dean adds.
The two Columbia University laboratories -- the Dean Lab and the
Hone Group -- worked in collaboration with the NIMS Japan, which supplied some
of the materials, and Papic, whose group performed computational modelling of
the experiments. Both Columbia labs are part of the university's Material
Research Science and Engineering Center. This project also used clean room
facilities at both the Columbia Nano Initiative and City College. Measurements
at large magnetic fields were made at the National High Magnetic Field
Laboratory, a user facility funded by the National Science Foundation and
headquartered at Florida State University in Tallahassee, Fl.
Now that the researchers have very clean 2D semiconductors as
well as an effective probe, they are exploring other interesting states that
emerge from these 2D platforms.
.png)
.jpg)
.gif)
