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Tracking Upstream Modes at Quantum Hall Edges of 2D Graphene

In the last few years, an occurrence known as the quantum Hall effect has materialized as a platform for hosting exotic features known as quasiparticles, with features that could pave the way to stimulating applications in areas such as quantum computing.

Tracking Upstream Modes at Quantum Hall Edges of 2D Graphene.
Left panel: Downstream (red lines) and upstream (dashed black lines). Middle panel: Schematic for noise measurement for “upstream” mode detection. Right panel: Noise is detected for fractional quantum Hall states with “upstream” modes whereas it remains zero for only downstream modes ((Image Credit: Authors).

When a robust magnetic field is used on a 2D material or gas, the electrons at the interface — in contrast to the ones inside the bulk — are free to travel along the edges in what are known as edge modes or channels — quite like highway lanes. This edge movement, which is the core of the quantum Hall effect, can result in a number of exciting properties based on the conditions and material.

For conventional electrons, the current only travels in one direction controlled by the magnetic field (“downstream”). However, physicists have projected that some materials can possess counter-propagating channels where certain quasiparticles can also move in the opposite (“upstream”) direction.

Although these upstream channels are of significant interest to researchers as they can host a range of new types of quasiparticles, they have been very challenging to identify because they do not transport any electrical current.

In a new study, scientists from the Indian Institute of Science (IISc) and international collaborators offer “smoking gun” proof for the existence of upstream modes along which specific neutral quasiparticles travel in two-layered graphene. To identify these channels or modes, the team used a new technique using electrical noise — fluctuations in the output signal triggered by heat dissipation.

Though the upstream excitations are charge-neutral, they can carry heat energy and produce a noise spot along the upstream direction.

Anindya Das, Corresponding Study Author and Associate Professor, Department of Physics, Indian Institute of Science

The study has been published in the journal Nature Communications.

Quasiparticles are mostly excitations that develop when elementary particles such as electrons interact with each other or with matter that surrounds them. They are not really particles but have comparable particles like charge and mass.

The most basic example is a “hole,” which can be defined as a vacancy where an electron is not present in a particular energy state in a semiconductor. It comprises an opposite charge to the electron and can travel inside a material quite like the way the electron travels. Pairs of electrons and holes can also develop quasiparticles which can spread along the edge of the material.

In earlier studies, scientists have demonstrated that it could be possible to sense emergent quasiparticles such as Majorana fermions in graphene; the hope is to utilize such quasiparticles to ultimately construct fault-tolerant quantum computers.

For detecting and examining such particles, identifying upstream modes which can host them is important. Although such upstream modes have been identified in the past in gallium-arsenide-based systems, none have been detected so far in graphene and graphene-based materials, which hold a lot more promise with regard to futuristic applications.

In the present study, when the scientists applied an electrical potential to the edge of two-layered graphene, they discovered that heat was conveyed only in the upstream channels and dissipated at specific “hotspots” in that direction. At these spots, the heat produced electrical noise that could be captured by an electrical resonance circuit and spectrum analyzer.

The researchers also discovered that the travel of these quasiparticles in the upstream channels was “ballistic” — heat energy surged from one hotspot to another minus any loss — in contrast to the “diffusive” transport seen previously in gallium-arsenide based systems.

Such a ballistic movement is also suggestive of the occurrence of exotic states and features that could help construct fault-free and energy-efficient quantum components in the future, according to the researchers.

Journal Reference:

Kumar, R., et al. (2022) Observation of ballistic upstream modes at fractional quantum Hall edges of graphene. Nature Communications.


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