In recent years, a phenomenon called the quantum Hall effect has emerged as
a platform for hosting exotic features called quasiparticles, with
properties that could lead to exciting applications in areas like quantum
computing. When a strong magnetic field is applied to a 2D material or gas,
the electrons at the interface, unlike the ones within the bulk, are free to
move along the edges in what are called edge modes or channels—somewhat
similar to highway lanes. This edge movement, which is the essence of the
quantum Hall effect, can lead to many interesting properties depending on
the material and conditions.
For conventional electrons, the current flows only in one direction dictated
by the magnetic field ('downstream'). However, physicists have predicted
that some materials can have counter-propagating channels where some
quasiparticles can also travel in the opposite ('upstream') direction.
Although these upstream channels are of great interest to scientists because
they can host a variety of new kinds of quasiparticles, they have been
extremely difficult to identify because they do not carry any electrical
current.
In a new study, researchers from the Indian Institute of Science (IISc) and
international collaborators provide "smoking gun" evidence for the presence
of upstream modes along which certain neutral quasiparticles move in
two-layered graphene. To detect these modes or channels, the team used a
novel method employing electrical noise—fluctuations in the output signal
caused by heat dissipation.
"Though the upstream excitations are charge-neutral, they can carry heat
energy and produce a noise spot along the upstream direction," explains
Anindya Das, Associate Professor in the Department of Physics and
corresponding author of the study published in Nature Communications.
Quasiparticles are largely excitations that arise when elementary particles
like electrons interact among each other or with matter around them. They
are not truly particles but have similar particles like mass and charge. The
simplest example is a 'hole'—a vacancy where an electron is missing in a
given energy state in a semiconductor. It has an opposite charge to the
electron and can move inside a material just like the electron does. Pairs
of electrons and holes can also form quasiparticles which can propagate
along the edge of the material.
In previous studies, the researchers have shown that it might be possible to
detect emergent quasiparticles like Majorana fermions in graphene; the hope
is to harness such quasiparticles to eventually build fault-tolerant quantum
computers. For identifying and studying such particles, detecting upstream
modes that can host them is critical. Although such upstream modes have been
detected earlier in gallium-arsenide based systems, none have been
identified so far in graphene and graphene-based materials, which offer much
more promise when it comes to futuristic applications.
In the current study, when the researchers applied an electrical potential
to the edge of two-layered graphene, they found that heat was transported
only in the upstream channels and dissipated at certain "hotspots" in that
direction. At these spots, the heat generated electrical noise that could be
picked up by an electrical resonance circuit and spectrum analyser.
The authors also found that the movement of these quasiparticles in the
upstream channels was "ballistic"—heat energy flowed from one hotspot to
another without any loss—unlike the "diffusive" transport observed earlier
in gallium-arsenide based systems. Such a ballistic movement is also
indicative of the presence of exotic states and features that could help
build energy-efficient and fault-free quantum components in the future,
according to the authors.
Reference:
Kumar, R., Srivastav, S.K., Spånslätt, C. et al. Observation of ballistic
upstream modes at fractional quantum Hall edges of graphene, Nature
Communications, 13, 213 (2022).
DOI: 10.1038/s41467-021-27805-4