If the fractional quantum Hall regime were a series of highways, these highways would have two or four lanes. The flow of two- or four-stream composite fermions, like a car in this two- or four-stream composite fermion traffic scenario, naturally explains the more than 90 fractional quantum Hall states that form in a wide variety of host materials.
However, physicists at Purdue University recently discovered that fractional quantum Hall regimes are not limited to two or four streams and discovered the existence of a new type of emerging particle, which they call a six-stream composite fermion. They recently published their revolutionary discoveries in Natural communications.
Gabor Csathy, professor and chair of the Department of Physics and Astronomy in the Purdue College of Science, along with doctoral students Haoyun Huang, Waseem Hussain and recent PhD student Sean Myers, led the discovery from Purdue’s West Lafayette campus.
Csathy credits lead author Huang with design, conducting measurements, and writing much of the manuscript. All very low temperature measurements were performed in the laboratory of the Csathy Physics Building. In his laboratory, they conduct research in highly correlated electronic physics, which is sometimes called topological electronic physics.
Weak electron interactions are well established and their behavior is completely predictable. When electrons interact weakly, the electron is generally considered a natural building block of the entire system.
But when electrons interact strongly, it becomes almost impossible to interpret the behavior of the system by thinking about individual electrons.
“This happens in a very small number of cases, like in the fractional quantum Hall regime we’re studying, for example,” says Csathy. “To explain fractional quantum Hall states, the composite fermion, a very intuitive fundamental element, comes in different flavors.
They can explain the entire subset of fractional quantum Hall states. But all fully extended (i.e. topologically protected) states, fractional quantum Hall states can be explained by only two types of composite fermions: two-current and four-current composite fermions.
Here we report a new fractional quantum Hall state that cannot be explained by any of these previous ideas! Instead, we must invoke the existence of a new type of nascent particle, the six-flux composite fermions. The discovery of new fractional quantum Hall states is quite rare.
However, the discovery of a new nascent particle in condensed matter physics is indeed rare and astonishing.”
For now, these ideas will be used to expand our understanding of the order of the known fractional quantum Hall states in the “periodic system”.
Particularly significant in this process is that the nascent composite fermion particle is unique in that the electron captures six quantized quanta of magnetic flux, forming the most complex composite fermion yet known.
“The numerology of this complex physical puzzle requires some patience,” says Haoyun Huang, Csathy’s PhD student. “Let’s take the example of the fractional state nu=2/3. Since 2/3=2/(2*2-1), the state nu=2/3 belongs to the family of two flows. Similarly, for the fractional state nu=2/7, 2/7=2/(2*4-1), this state therefore belongs to the family of four flows.
In contrast, the fractional states we discovered are closely related to 2/11=2/(2*6-1). Prior to our work, a fully quantized fractional quantum Hall state that could be associated with composite six-flux fermions had not been observed.
The situation was theoretically completely different: the existence of these types of composite fermions was predicted by Jainendra Jain in his highly influential composite fermion theory published in 1989. The associated quantization was not observed during these 34 years.
The material used in this study was grown by a Princeton University team led by Loren Pfeiffer. The electrical quality of the GaAs semiconductor played a major role in the success of this research. According to Csathy, this Princeton group is a world leader in the production of the highest quality GaAs-based materials.
“The GaAs they grow is very special, because the number of imperfections is surprisingly low,” he says. “The combination of the low mess and very low temperature measurement expertise of the Csathy lab made this project possible.
One of the reasons we measured these samples is that very recently the Princeton group has significantly improved the quality of GaAs semiconductors, as measured. small amounts of present defects. These improved patterns will undoubtedly continue to provide a playground for new physics.
This exciting discovery is part of ongoing research by Csathy’s team. The team continues to push the boundaries of discovery in their relentless pursuit of topological electronic physics.
Low temperature measurements at the Csathy Laboratory were supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under grant DE-SC0006671.
The Princeton team’s sample growth efforts were supported by the Gordon and Betty Moore Foundation Grant no. GBMF 4420 and National Science Foundation MRSEC Grant no. DMR-1420541.